Symposium EQ20—Beyond Graphene 2D Materials—Synthesis, Properties and Device Applications

Recent advances in two-dimensional (2D) materials such as graphene, molybdenum disulfide (MoS₂), boron nitride (BN), etc., have opened a new path in utilizing the atomically thin layers as oil-based additives due to their ultra-low shear strength, weak interlayer interactions, and the surface chemical stability. It is crucial to develop a fundamental understanding of the dissipation mechanisms for 2D materials in the presence of hydrocarbons to control friction using 2D materials as additives in oil-based lubrication. This work presents the load-dependent atomic force microscope (AFM) friction measurements in n-hexadecane solutions on two graphitic surfaces, i.e., highly oriented pyrolytic graphite (HOPG) and exfoliated few-layer graphene (FLG). A sublinear increase in the friction forces with load was measured on both graphitic surfaces, with FLG measuring 30% higher friction force than HOPG (at 60 nN). Further, the nanoscale friction forces exhibit hysteresis between the loading (increasing) and unloading (decreasing) normal forces. Below a specific transition load, higher friction forces were measured during unloading than the loading for a given load, while above the transition load, the friction hysteresis becomes insignificant. The origins for friction force and load-dependent friction hysteresis were explored by measuring the adhesion and roughness of the graphitic surfaces and the solvation layering behavior. Similar RMS roughness values and insignificant adhesion forces were obtained among the graphitic surface; however, higher forces were required to squeeze the solvation layers out of contact for FLG compared to HOPG surfaces. Molecular dynamics (MD) simulations assisted in understanding the impact of solvation layers of n-hexadecane on the load-bearing capacity and the friction forces of the graphitic interfaces. The load-dependent friction hysteresis originates from the irreversible reorganization of n-hexadecane solvation layers on the graphitic surfaces during the loading and unloading phenomena.

The tantalum dichalcogenides (TaS₂, TaSe₂ and their intermediate alloys) are members of the layered transition metal dichalcogenide (TMD) family of materials. The tantalum dichalcogenides present a variety of different phases at different temperatures and pressures including metallic [1], Mott insulating [2], charge density wave (CDW) [3] and superconducting [4]. Of particular interest are the low-temperature CDW phases, which display further phase transitions between different CDWs (e.g., commensurate CDW, incommensurate CDW and quasi-commensurate CDW). An understanding of the interplay of these different phases would provide a valuable insight into the nature of the electron-electron and electron-phonon interactions in these materials. Further, such properties of the tantalum dichalcogenides support their use in a variety of applications where different resistive states can be easily switched using temperature or pressure - for example, for data storage or other memory applications [5] [6].
Here, we shall present our studies on the intermediate alloy TaS_1.2Se_0.8. Large-area crystals have been grown in-house, using a chemical vapour transport method. We have characterised the material using a variety of experimental techniques, including temperature-dependent electrical transport, Raman spectroscopy and X-ray diffraction. These measurements have revealed the hysteretic properties of this alloy over a single cooling-heating cycle (320 – 4 K for the transport measurements). Furthermore, we have observed a fascinating ‘memory effect’ in this material, in which a second cooling-heating cycle yields a completely different hysteresis loop. This ‘memory effect’ has been observed in multiple crystals from several different growth batches and has been observed in both our transport and Raman measurements. The effect appears to be closely linked to the doping concentration we have used (S = 1.2, Se = 0.8), as we have not observed any such memory effect in crystals grown with slightly different stoichiometries (e.g., S = 1.3, Se = 0.7). We have further examined this effect by using a range of cooling/heating rates. These measurements have displayed a clear dependence on cooling/heating rate, which provide us with crucial insights into the mechanisms of the CDW phase transitions, in particular the nearly-commensurate to commensurate transition, in which domains of commensurability increase in size and eventually merge to form a fully-commensurate system. Meanwhile, our Raman measurements are vital in helping us to understand the CDW mechanism in this material, and the contributions of electron-electron and electron-phonon interactions in the formation and transitions of the CDWs.

[1] A. K. Geremew et al., ACS Nano 13, 6, 7231–7240 (2019)
[2] Y. D. Wang et al., Nat Commun 11, 4215 (2020)
[3] F. J. Di Salvo et al., Phys. Rev. B 12, 2220 (1975)
[4] Y. Liu et al., Appl. Phys. Lett. 102, 192602 (2013)
[5] M. Yoshida et al., Sci. Adv. 1, e1500606 (2015)
[6] I. Vaskivskyi et al., Nat. Commun. 7:11442 doi: 10.1038/ncomms11442 (2016)

WSe₂ and MoS₂, as transition metal dichalcogenides (TMDs), belong to a new class of nanomaterials with great potential for optoelectronic device applications. Thickness, changes in crystal symmetry, and growth defects are a few fundamental parameters that define the electronic, optical and thermal properties of two-dimensional (2D) crystals and for which different characterization techniques are required. In addition, the dependency of material characteristics on temperature and/or external magnetic fields is receiving increasing attention. Here we present the correlative analysis of these TMDs using Atomic Force Microscopy (AFM), Photoluminescence (PL) Imaging, Second Harmonic Generation (SHG) imaging and confocal Raman imaging under ambient and cryogenic conditions. The combination of the insights obtained by AFM, PL, SHG and Raman on WSe₂ measured under ambient conditions allow an interpretation of the cause of PL wavelength emission shifts, strain states and contaminations during the growth process. The strongly localized peak shift variations around the rim of the WSe₂ flake and their possible explanations based on the correlative results are particularly interesting. The addition of temperature-dependent PL imaging using a cryostat capable of cooling the sample down to 2K also allows the determination of the peak shift, both on the rim as well as in the center of such a flake, as a function of temperature. The newly established CryoRaman system, a combination of a WITec confocal Raman microscope and an attocube attoDry2100 closed-cycle cryostat, uses a high NA low temperature objective that allows for optical resolution close to that which is available under ambient conditions. Using this system, it was possible to discern low frequency Raman bands of WSe₂ for layering identification and observe them under varying polarization directions of the incoming laser light at cryogenic temperatures. The ability to apply high magnetic fields (up to 12T) additionally enables observation of the effect of the magnetic field on the recorded Raman spectra and reveals further properties of the TMDs.

Two-dimensional (2D) transition metal carbide and nitrides, MXenes, exhibit promising properties in different applications such as energy storage, sensing, and optoelectronics, to name a few.(1) These materials have a general formula of M_n+1X_nT_x where M is a transition metal, X is carbon and/or nitrogen, and T_x represents surface functional groups on the outer layers. Since the discovery of MXenes in 2011, research has mainly focused on Ti₃C₂T_x because of its established synthesis protocols and high chemical stability, while investigation of other experimentally made, stoichiometric MXenes has been trailing behind. Among other MXenes, M₂XT_x MXenes such as Vanadium carbide (V₂CT_x) promise exceptional theoretical properties due to the increased active surface area in M₂XT_x MXenes and the multiple available oxidation states in Vanadium systems. Despite its attractive properties, however, research related to V₂CT_x has mostly been limited to its multilayered state due to rapid oxidation after delamination. Herein, we have developed improved etching and delamination protocols through formulating new etchants and intercalants to increase the yield of synthesis and prepare high-quality and less defective V₂CT_x flakes. Furthermore, we show that through post-processing protocols the shelf life of delaminated V₂CT_x dispersions can be extended from only a few hours to multiple months with almost no degradation. Various characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis, and thermogravimetric analysis were used to investigate the synthesized materials. The combined synthesis and post-processing protocols presented here result in high quality and chemically stable V₂CT_x that shows improved electronic properties.

1. A. VahidMohammadi, J. Rosen, Y. Gogotsi, The world of two-dimensional carbides and nitrides (MXenes). Science (80-. ). 372, eabf1581 (2021).

Van der Waals (vdW) heterostructures combining dissimilar 2D materials offer a great prospect for the realization of atomically thin devices with tailored properties [1]. To achieve a high density, bottom-up integration, the synthesis of such heterostructures via vdW epitaxy is a promising alternative to mechanical transfer, which is problematic in terms of scaling and reproducibility. Nevertheless, due to the weak bonding between 2D crystals, vdW epitaxy is sensitive to various surface defects, usually leading to uncontrolled nucleation and thus non-uniform growth of polycrystalline material [2]. Hence, the control over nucleation location is considered to be one of the key challenges to achieve scalable and high-quality fabrication of 2D heterostructures [3]. In this contribution, we report on the use of focused ion beam (FIB) within a He ion microscope [4] as a novel tool to deliberately create atomic scale defects in graphene on SiC(0001), which act as nucleation sites for atomically h-BN grown via molecular beam epitaxy. Thereby, we demonstrate a mask-less, selective-area growth of h-BN/graphene heterostacks, in which nucleation yield and crystal quality of h-BN is controlled by the ion beam parameters used for the defect formation. The epitaxially grown h-BN exhibits electron tunneling characteristics comparable to those of h-BN flakes exfoliated from high-quality bulk crystals [5]. Our results open a new pathway for the scalable fabrication of not only h-BN/graphene systems, but also of other vdW heterostructures composed of 2D materials that can be synthesized via vdW epitaxy, such as layered semiconductors [3] and ferromagnets [6].
References
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[6] Lopes et al., arXiv:2104.09184 (2021).

Neutron radiation is extremely hazardous because it can induce radioactivity in most irradiated substances, including body tissues. Therefore, construction of effective nuclear reactors and repositories with appropriate neutron shielding or absorption measures to prevent exceeding nuclear criticality is necessary. B₄C is widely used as a neutron-shielding material for reactor systems because of its high neutron-absorption efficiency (i.e., neutron-absorbing cross section for ¹⁰B). Several composites such as borated stainless steel, B₄C-reinforced Al, and B₄C-reinforced polymers have been developed. However, the practical application of these composites in the fabrication of containers or casks is limited because of their low B content and insufficient workability. In addition, their relatively thick thickness (> 1 cm) required for sufficient neutron shielding severely hinder the light-weight application. Two-dimensional (2D) transition metal carbides, namely MXenes, have recently attracted considerable attention for several applications because of their large surface area, good mechanical properties, and the presence of numerous chemically active sites. The layered structure and surface functional groups of MXenes facilitate the formation of large interlayer spaces and confer high interfacial strength, respectively. In our work, 2D Ti₃C₂T_x (T refers to surface termination group) MXene flakes were chosen to cooperate with B₄C neutron absorbing powder to fabricate hierarchically layered composite films by vacuum filtration. Through the control of particle sizes and resultant electrostatic repulsion, a homogeneous dispersion of MXene flakes and B₄C powder, both with negative surface charges, was obtained. By the self-intercalation of the submicron B₄C powder between the vacuum-filtrated 2D MXene flakes with sufficiently large lateral sizes up to several micrometers, a hybrid film was successfully developed. Notably, the film showed a wide range of boron capacity (20–60 wt%) and the B₄C reinforcements were uniformly dispersed in the MXene flakes, forming multi-interfacial layers: this was facilitated by the van der Waals interactions and the hydrogen bonds formed upon the addition of a small amount of an organic binder. The free-standing hybrid film exhibited excellent neutron absorption ability (with an absorption capacity of ≈39.8 %) at an ultralow thickness (< 40 μm), leading to a macroscopic cross-section of ≈127.6 cm^-1, which is the highest value among the previous reports on boron-based neutron shielding materials to date (typically, 1-34 cm^-1). Moreover, the mechanical flexibility of 2D films and simple processing showed the possibility for shielding surfaces of any shape with reduced net weight and volume. Thus, the as-prepared film can be used as an efficient neutron-absorbing coating material not only in nuclear repositories, but also in other fields requiring accurate shielding such as spacesuit, radiation medical, and neutron transmutation doping.

2D conducting metal oxides offer outstanding optoelectronic properties for applications in sensing, microelectronics, and flexible electronics. Here, we unlock the ultra-high electron mobility of 2D oxides formed by a rapid, liquid-metal (LM) printing process that transfers ultrathin (~ 2 nm), crystalline monolayers and bilayers of In₂O₃ at low temperatures just above the melting point of indium (156 °C). We demonstrate the use of quantum confinement and low-temperature post-annealing to control the electronic density of states (DOS) of the 2D In₂O₃, fabricating thin-film transistors with ultrahigh mobility (μ₀ > 65 cm²/Vs) at linear printing speeds from 0.6 to 36 meters/min.

LM-printed monolayer and bilayer InO_x were characterized by XRD and TEM to understand their thickness-dependent crystallinity. The films exhibit a mixture of large crystalline domains (20-40 nm, laterally) of cubic In₂O₃ and amorphous regions as deposited. Post-annealing at temperatures from 150 to 250 °C further enhances the crystallinity and grain size, as observed via the (222) cubic peak as well as through TEM imaging and diffraction patterns. UV-Vis measurements indicate that this post-annealing regulates the Burstein-Moss shift in the optical bandgap, lowering the free carrier concentration and leading to positive threshold voltages in high-performance 2D transistors. To our knowledge, this is the first report of such scalable (30 cm²) device integration of these atomic-scale 2D oxides at deposition temperatures broadly suitable for high-performance flexible electronics.

Recently, transition-metal dichalcogenides (TMDs) have attracted much attention as new materials for electronics devices, electrocatalysts, photocatalysts, sensors, batteries, and bio-applications. Most TMDs are two-dimensional (2D) materials with a single layer. Bonds between each layer are made up of Van der Waals bonds, while intra-layer atoms bind together as covalent bonds.
Chemical vapor deposition (CVD) with sulfur gas is the most popular method for synthesizing large- scale 2D materials with high quality. Various MoS₂ can be obtained from this meth[od using various precursors with different properties, process temperatures, and substrate materials. Solution process methods show advantages for preparing films with large size, high throughput, low cost, thickness control, and an environmentally friendly process. Even though there is sulfur in the precursors of the solution-process synthesis methods, supplementing the sulfur that is lost in the high-temperature CVD process is unavoidable.
We developed a unique solution process to obtain large-size and uniform MoS₂ atomic layers without CVD (CVD-free). MoS₂ atomic layers could be fabricated using the S-rich precursor solution and one-step annealing. The sulfurization process is omitted to simplify the entire process, and wafer-scale and larger-scale synthesis of MoS₂ can be achieved by simple thermal treatment. The atomic layers of the synthesized MoS₂ were identified as 2 layers for 0.0070 M, 3 layers for 0.0125 M, and 5 layers for 0.0250 M of MoS₂ solution by STEM-FIB. MoS₂ patterns were made at a resolution of around 150 µm with highly uniform coverage using electrohydrodynamic (EHD)-jet printing for the first time. The EHD jet-printed MoS₂ TFTs show good electrical properties of 19.4 cm² V^-1 s^-1.

For decades we’ve been hearing about the promise of printing electronics directly onto any surface. However, despite significant progress in the development of inks and printing processes, reports on fully, direct-write printed electronics continue to rely on excessive thermal treatments and/or fabrication processes that are external from the printer. In this talk, recent progress towards print-in-place electronics will be discussed; print-in-place involves loading a substrate into a printer, printing all needed layers, then removing the substrate with electronic devices immediately ready to test. A key component of these print-in-place transistors is the use of inks from various nanomaterials, including: 2D graphene and hexagonal boron nitride, 1D carbon nanotubes, and quasi-1D silver nanowires. Using an aerosol jet printer, these mixed-dimensional inks are printed into functional 1D-2D thin-film transistors (TFTs) without ever removing the substrate from the printer and using a maximum process temperature of 80 °C with most processing occurring at room temperature. To achieve this, significant advancements were made to minimize the intermixing of printed layers, drive down sintering temperature, and achieve sufficient thin-film electrical properties. Devices are demonstrated on various substrates, including paper, and evidence of the potential for printing directly onto biological surfaces will be shown. What’s more, recent progress towards a completely recyclable printed transistor will be discussed, fabricated entirely using nanoscale carbon-based inks. These demonstrations give evidence for an electronic future involving devices with fabrication and/or function that goes beyond what is possible with silicon-based technologies.

Ferroelectricity in two-dimensional (2D) materials has been at the forefront of recent research owing to its potential application in low-powered non-volatile phase-change memory, energy harvesting, strain tuned electronics, artificial brain, neuromorphic sensors. Among the 2D materials exhibiting ferroelectricity at room temperature, α-In2Se3 stands out owing to the presence of both in- (IP) and out-of-plane (OOP) dipole polarizations. In addition, the ability to modulate IP by switching OOP and vice versa owing to their intercoupled nature make it a promising material for multimodal memory and optoelectronic applications. Herein, we experimentally demonstrate the cross-field modulation of opto- and electronic properties in α-In2Se3 based field-effect devices. Gate dependent surface potential measurements using Kelvin Probe Force Microscopy (KPFM) were extensively used in In2Se3 based devices to directly reveal the bi-directional dipole modulation. Electric field calculations obtained from the surface potential studies also show hysteretic behavior of the dipoles following high gate voltage pulses. Also, to explore the consequence of the hysteretic change in the IP electric field, photoresponse measurements following high gate pulses were performed exhibiting its functionality as a non-volatile memory switch. The multi-level photoresponse characteristics for different gate polarities in the fabricated photodetectors show a potential for their implementation and integration into non-volatile memory and electro-optical applications.

The metal chalcophosphates (MPX3) are layered van der Waals materials that have recently gained attention due to their intrinsic magnetic properties. These systems are composed of [P2X6]4- bipyramid anion units, where each P is tetrahedrally bonded to three chalcogen atoms. These anion units are surrounded in a honeycomb arrangement by metal cations, where each metal cation site is octahedrally coordinated to chalcogen atoms. By incorporating different metal cation species into the available octahedral sites different magnetic phases can be induced. Among the metal chalcophosphates, Fe2P2S6 and Co2P2S6 are known for their interesting magnetic properties. Fe2P2S6 is unique as it exhibits out-of-plane orientation magnetic moments when cooled under its Neels temperature. In contrast, Co2P2S6 exhibits in-plane alignment of magnetic moments. However, mixing these metallic cations Fe2-xCoxP2S6 has not been studied yet. This mixed metallic system can be used as a platform to study new form of magnetic states through magnetic frustration.
Here, we present a thorough investigation of the mixed Fe2-xCoxP2S6 system and study the structure, chemical states, and magnetic behavior as the Fe:Co is varied. A novel flux method is used which enables us to synthesize a series of Fe2-xCoxP2S6 with different Fe:Co (1:0, 7:1, 1:1, 1:7, and 0:1). Elemental studies have confirmed the single crystals stoichiometry of these mixed cation systems. This was further supported by structural analysis which confirms a linear expansion of the lattice from Co to Fe.All of the mixed metal cation systems exhibits a transition from a paramagnetic to antiferromagnetic state when cooled beneath each system’s Neels temperature. Interestingly, we found a dependence of the transition temperature to the ratio of Fe:Co. Furthermore, this dependence is confirmed by the electronic structural behavior which showed two distinct work functions in the Fe2-xCoxP2S6 system. This strong correlation is likely linked to a distinct ordering that is apparent from high resolution electron energy loss spectroscopy. This work provides an in-depth road map for the investigation of mixed metallic cation chalcophosphates and can be utilized as a resource for future studies on low dimensional materials in next generation electronic devices.
Acknowledgements
This work was supported by the Army Research Office (Grant W911NF1910335) and NSF Division of Material Research (NSF Grant DMR-1929356). This work made use of the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the SHyNE Resource [National Science crthe Keck Foundation, and the State of Illinois through IIN. . M.C. acknowledges support from the NSF Graduate Research Fellowship under Grant DGE-1842165.

Two dimensional materials with ferroelectricity (i.e., having a switchable spontaneous electric polarization) have recently been proposed as components for ultrathin electronic devices. However, such materials with large out-of-plane polarizations, ideal for such applications, are challenging to find. Recently, Chandrasekaran et al. proposed that the functionalized MXene Sc₂CO₂ has a metastable state with an out-of-plane electric polarization larger than other common 2D ferroelectrics. [1] In this work, we used density functional theory (DFT) calculations to investigate additional M₂CX₂ materials (M = Sc, Y, La and X = O, F). We found that (1) chemical substitution of Sc with Y and/or O with F can preferentially stabilize the ferroelectric phase as the ground state (with the dynamic stability verified with phonon calculations); (2) such substitution can also be used to systematically tune the polarization and band gap; and (3) these monolayers display large piezoelectric coefficients. We then demonstrate how these properties can be continuously tuned by the application of external strain or by alloying to create Sc₂Y_2(1-x)CO₂ monolayers. These findings demonstrate that, as with traditional bulk ferroelectrics, chemical substitution and external stimuli are powerful ways to stabilize and control the properties of low dimensional ferroelectric materials. Finally, we extend this to new families by using a high throughput DFT approach to scan through a database of 2D materials and compute their polarization to find and verify new 2D ferroelectric materials with out-of-plane polarizations.

[1] A. Chandrasekaran, A. Mishra, A. K. Singh, Nano Letters 17 3290 (2017)

T-type MoTe₂ has drawn increasing attention due to properties such as extreme magnetoresistance, exotic superconductivity, and non-trivial band topology. In bulk, MoTe₂ undergoes a layer stacking transition from the 1T’ to the T_d phase at T_c ~ 250 K, which alters the topology (higher-order topological insulator to Weyl semimetal) and induces ferroelectricity. Determining the layer stacking in thin film MoTe₂ is essential for device applications, but to date, the stacking remains poorly understood in nanoscale MoTe₂ samples. Here, we present in situ cryogenic transmission electron microscopy (cryo-TEM) measurements of exfoliated MoTe₂ flakes from 17 to 700 K, complemented by quantitative multi-slice simulations, electrical transport measurements, and Raman spectroscopy. Across the studied temperature range of 17 – 700 K, we find that the layer stacking in exfoliated MoTe₂ flakes is highly disordered, with nanoscale 1T’ and T_d domains, as well as regions with seemingly random stacking. With cooling to 17 K (230 K below the bulk T_c), there is a modest increase in T_d-type stacking, but the stacking remains mainly disordered. Through comparison with measurements of exfoliated WTe₂ flakes – which retain the same T_d stacking found in bulk WTe₂ – we argue that the observed disorder in MoTe₂ is not due to mechanical exfoliation. Instead, we argue that the observed disorder is an intrinsic effect of dimensional confinement. The observation of highly disordered stacking in exfoliated MoTe₂ has significant implications for the interpretation of superconductivity, giant nonlinear hall effect, and other exotic properties measured in thin MoTe₂ flakes.

Since monolayer graphene was discovered and its electrical transport properties were well studied [1-3]. The graphene field effect transistor (FET) was used for probing the oxygen vacancies of SrTiO₃ substrate, studying the interface with a ferroelectric substrate, or detecting the hidden localized states at the interface with h-BN [4-6]. Recently, various 2D magnetic materials (Cr₂Ge₂Te₆, CrI₃, Fe₃GeTe₂, etc.) were reported [7-9]. With van der Waals type of the material, its heterostructures have been extensively investigated, demonstrating their fundamental properties and prototypical devices [10,11]. In this study, we have investigated proximity-coupling between 2D magnet (Cr₂Ge₂Te₆, CGT) and graphene. The transport parameters such as hysteresis in back-gate voltage sweep, carrier density, and mobility were abruptly changed at a temperature around ~60 K corresponding to Curie temperature (T_C) where CGT transfer from paramagnetic to ferromagnetic phase. In addition, the quantum Hall state of graphene with filling factor n = 6 similarly shows critical occurrence near T_c, which can be attributed to the additional contribution of magnetization. These results imply that graphene can be utilized to monitor the magnetic phase transition of the adjacent CGT layer. Therefore, graphene-based vdW heterostructures is expected to become a novel electrical platform for exploring various phases in 2D magnetic insulators.
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The observation of long-range magnetic order in van der Waals layered materials down to the monolayer limit has sparked great interest for the study of low-dimensional magnetism. [1] However, the air-sensitivity of conventional magnetic layered materials like CrI₃has hindered further nanoscale investigation. Recently, CrSBr [2] has been rediscovered: it is a layered magnet that shows air stability and therefore presents exciting opportunities for atomistic measurements and manipulation. CrSBr is a semiconductor with a bandgap of approximately 1.6 eV. It has a high Néel temperature of 145 K and shows strong light-matter interaction, which makes this material interesting for the study of magneto-transport and magneto-optical excitations. [3, 4]
Here, we establish CrSBr as a material platform that allows unprecedented nanoscale control over structural and therefore magnetic properties. Layered materials in general offer a myriad of degrees of freedom to tailor the crystal structure down to the single atom scale. In the case of CrSBr, we find that controlled irradiation with electrons in a scanning transmission electron microscope induces a local phase transformation. [5] The transformed material is also a 2D layered structure, but its layers are perpendicular to those in the original material. We investigate the mechanism of the phase transformation by tracking individual atom column intensities as well as local strain, column by column. We find that the phase transformation occurs through Cr atom migration into the van der Waals gap and we show examples of twin boundaries and grain boundaries, controlled knock-out of rows of Cr atoms, and moiré twisted CrSBr. Ab-initio and spin-wave theory suggest that the individual layers in the new phase are magnetic, and the material has an energy gap and a fully spin-polarized band structure. We therefore anticipate that directing this phase transformation at the atomic scale can create spin textures with specific properties that could be tailored towards applications. Overall, we propose that CrSBr and other air-stable magnets with similar crystal structure will become unique platforms for the design, measurement, and application of nanoscale magnetic textures.

[1] Huang, B.et al. Nature 546, 270–273 (2017)
[2] Göser, O. et al. J. Magn. Magn. Mater. 92, 129 (1990)
[3] Telford, E. J. et al. Adv. Mater. 32, 1 (2020)
[4] Lee, K. et al. Nano Lett. 21, 3511 (2021)
[5] Klein, J. et al. to be submitted (2021)

The demand towards accessing one-dimensional (1D) magnetic nanostructures has grown tremendously—as driven by numerous discoveries of novel physical properties in these systems as well as their integration into next-generation data storage devices and bioengineered artificial organelles. However, with current bottom-up synthetic pathways, achieving precise control over crystallinity, long-range morphology, and surface termination in magnetic nanowires, especially as they approach the Angstrom size regime, remains challenging. In this talk, we will describe our recent efforts in developing a new material platform to access freestanding ferromagnetic nanowires from top-down routes. We realize this by exfoliating a unique class of bulk magnetic crystals comprised of quasi-1D inorganic chains that are predominantly held together by van der Waals (vdW) forces. Based on these quasi-1D vdW crystals, we establish the intrinsic magnetic anisotropy in the bulk structure via powder neutron diffraction and demonstrate that these bulk 1D vdW crystals can be readily exfoliated into 10 ± 2.8 nm thick nanowires using solution-phase exfoliation. As expected from exfoliating a vdW lattice, the resulting nanowires possess cylindrical cross-sections with negligible anisotropy and surfaces that are resistant towards oxidation in ambient conditions. We find that the reduction in size of the bulk crystal, upon exfoliation into long and thin nanowires, directly decreases the magnetic ordering temperature of these phases and results to a significant increase in both the coercive field and remanence, both of which are the hallmarks of a hard ferromagnet. Overall, these findings collectively demonstrate the adaptability of well-established exfoliation methods into new classes of functional vdW lattices, which we envision would enable unique opportunities in the creation of 1D magnetic nanostructures and the realization of new magnetic properties and applications at or near the limit of 1D confinement.

The coupling of electronic states with crystal lattice underlies a wealth of condensed matter phenomena. Coupling of electronic charge with lattice vibrations or distortions underlies superconductivity, charge density wave and metal-insulator transitions. In contrast, coupling between electronic charge and spins has been scarcely investigated, particularly in the optical domain. The recent discovery of anti-ferromagnetic (AFM) insulators of the form MPX3 (M = Fe, Mn, Ni; X= S, Se) has opened new avenues in studying spin-charge correlations via optical means. Here we report such an interaction in AFM FePS3 crystals which induces strong in-plane anisotropy, resulting in linear dichroism (LD). We identify that this LD is characteristic of the AFM spin ordering in the crystal coincident with the Neel transition temperature (~118 K). Further, we show that the LD is tunable both spectrally and in magnitude up to near-unity (~98.4%) levels when coupled to an optical cavity. Finally, through transfer-matrix simulations, we also demonstrate the dispersion of the LD in the visible-near infrared range as a function of the cavity length and the FePS3 thickness. We observe a close agreement between experiments and simulations and prove that the LD can be enhanced from 5% (in bulk) to 98.4% (in ~200nm thick samples) by virtue of cavity coupling and engineering the dielectric stack. Our findings open a new route to probing spin-charge coupling in magnetic materials and have implications in the design of novel birefringent nanophotonic elements.

Atomically thin heavy metals (Pb, Bi) are expected to exhibit strong spin orbit coupling and broken spin degeneracy, making them viable candidates for next-generation quantum applications in both spin-dependent devices (i.e. spintronics) and spin-dependent phenomena (ex. quantum spin Hall effects). However, most ultrathin, mono-elemental metals are extremely sensitive to oxidation and degradation in ambient conditions. To answer this, our group has developed a synthesis route known as confinement heteroepitaxy (CHet), through which we have created large-area, atomically thin 2D-Pb, 2D-Bi, and 2D-PbGa capped by a quasi-freestanding epitaxial graphene which provides air-stability. Through our ex-situ characterization work, we have demonstrated strong spin splitting (~500meV) and a unique out-of-plane spin polarization for 2D-Pb, superconductivity up to 3K in 2D-PbGa, and strong photoluminescence in 2D-Bi, implying their potential as precursors for next-generation, quantum spin technologies.

Confinement heteroepitaxy (CHet) is a promising route to create robust 2D allotropes of traditionally 3D materials that could have significant technological impacts on next generation technologies such as topological quantum computing and high frequency electronics. In CHet, we exploit the epitaxial graphene (EG)/SiC interface as a platform to synthesis atomically thin, air stable metals, metal-alloys, oxides, and nitrides. Unique properties, such as controlled tunability, non-centrosymmetric bonding, novel atomic structures, enhanced superconductivity, extreme optical properties, and long-term stability make such system highly attractive for device applications.
In this talk, we will discuss CHet of atomically thin, InxGa1-x alloys and how quantization impacts physical, electronic, and optical properties. We find the 2D metal alloy composition is readily tunable by changing the elemental concentration in the precursor, confirmed through high resolution x-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) mapping. Cross-sectional scanning transmission electron microscopy (STEM) and azimuthal reflection high-energy electron diffraction (ARHEED) confirm the metals are epitaxial to the SiC substrate, and electron energy loss spectroscopy (EELS) demonstrates metal confinement between graphene and SiC. First-principles density functional theory (DFT) predicts and STEM verifies that the layers are randomly mixed with a nominal thickness of two atoms. The optical and electronic properties directly correlate with alloy composition, where the dielectric response, electronic band structure, superconductivity, and charge transfer from metal to graphene all vary in controlled ways with respect to the indium concentration in the 2D layer.

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers such as MoS₂, WS₂ and WSe₂ is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal monolayers over large areas via coalescence of TMD domains with the same crystallographic direction. Sapphire has emerged as a promising substrate for wafer-scale epitaxy due to its commensurability with the hexagonal lattice of TMDs and its good thermal and chemical stability. However, the three-fold symmetry of TMDs typically results in inversion domains, also referred to as 60^o twins, on substrates such as c-plane sapphire forming twin boundaries upon coalescence that are generally undesired.

In this study, we demonstrate the epitaxial growth of TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire are shown to break the surface symmetry giving rise to a preferred domain orientation. Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe₂ and WS₂ monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850^oC to 1000^oC, was used to achieve epitaxial films using W(CO)₆, H₂Se and H₂S as precursors in a H₂ carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries and good circular helicity. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.

Doping is the cornerstone of modern semiconductor technology where extrinsic semiconductors are the fundamental building blocks of everyday electronic devices. Recently emerged two-dimensional (2D) transition metal dichalcogenides (TMDs) are actively considered as one of the potential materials system for next-generation electronics, where significant progress has been made in the wafer-scale synthesis of pristine TMDs as large as 300 mm. Even though electronic-grade, industrial-scale intrinsic materials exist, to date, similar success does not hold for scalable doping of TMDs, which remains at the proof-of-concept stage.
In this study, we present scalable rhenium (n-type) and vanadium doping (p-type) of 2D WSe₂ films at front-end-of-line (FEOL) and back-end-of-line (BEOL) compatible temperatures via gas source chemical vapor deposition (CVD). Detailed experimental and theoretical studies demonstrate that Re and V substitutionally replace W atoms in the WSe₂ lattice and introduce discrete energy states that lie close to the conduction band and valance band, respectively. The concentration of the dopants can be precisely controlled down to <0.0001 at % and confirmed with a combination of x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Incorporated Re and V atoms break the translational symmetry of the host lattice resulting in increased density of defect-activated Raman modes. Moreover, the density of positive and negative trions is increased as a function of V and Re atom concentration leading to the complete quench of the photoluminescence spectra due to the non-radiative recombination nature of the trions. Transport measurements verify the n- and p-type nature of the rhenium and vanadium dopants, respectively. Moreover, we demonstrate that the dopants are not efficiently ionized in monolayer WSe₂ due to ineffective Coulomb screening and a strong dependence of dopant ionization energy on the thickness of the 2D films. Our findings are the first demonstration of extrinsic 2D semiconductors that are fully compatible with state-of-the-art Si processing which will be indispensable in constructing a heterogeneous platform for “Beyond Moore” technology.

The International Roadmap for Semiconductors expressed the urgent need for ultrathin (~1nm) diffusion barrier materials for copper interconnects. Recently, 2D transition metal dichalcogenides (TMD) such as MoS₂ have been proposed as a possible solution to this technological need as they can offer excellent diffusion blocking capabilities even at low thickness unlike conventional barrier materials. The challenge with 2D TMD integration as barrier materials is that synthesis of high-quality layers is typically achieved via vapor-phase techniques like chemical vapor deposition (CVD) at temperatures much higher than the back-end-of line (BEOL) limit (<500C). In this study we demonstrate the growth of nanometer think Re-doped MoS₂ via metal organic chemical vapor deposition (MOCVD) at BEOL-compatible temperatures with excellent Cu diffusion barrier performance. Employing Re₂(CO)₁₀ as a gas-source dopant precursor within the MOCVD process, we exhibit unprecedented control over Mo-substitutional doping concentration even up to degenerate levels confirmed by a combination of X-ray photoelectron spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements. With this precise regulation of dopant source flux and resulting Re concentration, we show how both diffusion blocking capabilities and carrier mobility of the grown MoS₂ films change as a function of dopant concentration. Time dependent dielectric breakdown (TDDB) tests are employed to measure the breakdown time and benchmark the Re-doped MoS₂ films against industry standard barrier materials (Ta/TaN) as well as other 2D nanosheet-based diffusion barriers. Carrier mobility on the other hand is extracted by fabricating back-gated and electrolyte double layer gated field-effect transistors (FETs). These results show that degenerately doped MoS₂ films are a promising candidate for ultrathin next-generation Cu/TMD hybrid interconnects.

Semiconducting transition metal dichalcogenides (TMDCs), such as MoS₂, exhibit vast potential in a variety of applications due to their unique and favorable electronic, optical, and mechanical properties. However, the commonly used gas-phase TMDC deposition methods require high temperatures that are incompatible with many applications and substrates, in particular the emerging field of flexible electronics.¹

In this contribution, we demonstrate wafer-scale deposition of polycrystalline MoS₂ thin films at very low temperatures down to 100 °C using plasma-enhanced (PE) atomic layer deposition (ALD). ALD is a scalable, semiconductor industry compatible gas-phase method producing uniform, high-quality thin films with accurately controlled thickness. These attributes have led to rapidly growing interest in ALD for preparing TMDCs.^2,3

Our PEALD process is based on self-limiting, alternating surface reactions of a metalorganic molybdenum precursor Mo(N^tBu)₂(NMe₂)₂ and mixed H₂S/H₂/Ar plasma.⁴ We have identified the critical role of hydrogen during the plasma step in controlling the composition and properties of molybdenum sulfide films. By increasing the H₂/H₂S ratio, we are able to deposit polycrystalline MoS₂ films at temperatures as low as 100 °C. To the best of our knowledge, this represents the lowest temperature for crystalline MoS₂ films prepared by any chemical gas-phase method.

Based on thorough film characterization, we find that adding hydrogen in the plasma helps avoid excess sulfur incorporation at low temperatures. The correct stoichiometry, in turn, enables crystallization of MoS₂. Furthermore, the H₂/H₂S ratio is found to control growth rate, film morphology, and electrical properties. For example, electrical conductivity can be increased by at least four orders of magnitude by increasing the H₂/H₂S ratio. Thus, the PEALD process enables tailoring molybdenum sulfide films to meet the requirements of different applications. We are currently investigating the electronic transport properties of our MoS₂ films prepared at conditions that are compatible with flexible electronics as well as other applications and substrates with a low thermal budget.

1 Yao and Gang, J. Appl. Phys. 2020, 127, 030902
2 Mattinen et al., Adv. Mater. Interfaces 2021, 8, 2001677
3 Kim et al., Adv. Mater., 2021, 2005907
4 Sharma et al., Nanoscale, 2018, 10, 8615

Semiconducting 2D transition metal dichalcogenides (TMDs) such as MoS₂ and WS₂ have many unique properties such as stable excitons at room temperature and direct-gap luminescence at monolayer thickness, making them promising materials for various applications such as photovoltaics, optical sensing and catalysis. In particular, alloys of 2D TMDs hold great potential due to their composition-controlled properties, making them more versatile than pure TMDs. Hence, there is a clear need for a synthesis method of 2D TMD alloys that can exert excellent control over the alloy composition. While TMD alloys have been synthesized by chemical vapor deposition (CVD), the control of this method is limited, which is exemplified by the lack of control over atomic ordering (i.e. mixing or clustering of the alloy’s constituents).

In this work we employ plasma-enhanced atomic layer deposition (ALD) to synthesize the 2D TMD alloys Mo_xW_1-xS₂ and Nb_xW_1-xS₂. As MoS₂ and WS₂ are both semiconductors, Mo_xW_1-xS₂ is a good case study for a tunable-bandgap semiconductor. The growth of the nanocrystalline Mo_xW_1-xS₂ alloys is shown to be exceptionally well-behaved, as their composition follows the ideal rule of mixtures. Beyond resulting in excellent control over fundamental properties such as the bandgap (evidenced by optical absorption spectroscopy), this ideal alloying behavior enables us to control the atomic ordering (i.e. mixing or clustering) of the alloys in the deposition process. Specifically, it is shown by atomic resolution HAADF-STEM that the atomic ordering can be tuned from a randomly mixed Mo_xW_1-xS₂ alloy to an alloy that exhibits clusters of MoS₂ and WS₂. When a sub-monolayer film with clustered atomic ordering is grown, the resulting film effectively consists of 2D core-shell nanoparticles (MoS₂ islands epitaxially bordered by WS₂) which are expected to have fascinating novel properties.

On the other hand, Nb is a group-VB transition metal whereas W is a group-VIB transition metal, making Nb_xW_1-xS₂ a relevant case study for electronic applications where well-controlled doping is desired. For the NbS₂ – WS₂ alloy system, we observe similarly well-behaved rule-of-mixtures growth. The electronic properties of the Nb_xW_1-xS₂ alloy are studied based on Hall measurements and their suitability for transistors is evaluated. Furthermore, the atomic arrangement of the alloy is studied using HAADF-STEM to gain fundamental insight into the alloying behavior of the NbS₂ – WS₂ system.

Our results demonstrate excellent control over the composition and atomic ordering of 2D TMD alloys by ALD, thus enabling fine tuning of the material properties as well as the emergence of novel properties, thereby opening up many possibilities for next-generation devices based on 2D materials.

This talk will present our latest research adventures on 2D atomic nanomaterials towards greater scientific understanding and applications. In particular, the talk will highlight our work on non-volatile memory (atomristors) that can be employed in various applications including information storage, brain-inspired computing, and zero-power RF/THz switches. Non-volatile memory device based on atomically-thin materials is an application of defects, and is a rapidly advancing field with rich physics that can be attributed to metal ion binding to native defects. The promising 2D materials for neuromorphic computing include a library of TMDs (e.g. MoS2), and hBN. In particular, hBN memory devices feature very low switching energy, a relatively wide range of stable programmable resistance states, and good endurance and retention. Recent progress has also demonstrated wafer-scale integration of 2D memory devices with a prospect to leverage silicon electronics for fully integrated systems.

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures and making outstanding progress on fundamental studies, practical applications of 2D materials will require a broader integration strategy. I will present our ongoing and recent work on integration of 2D materials with 3D electronic materials to realize logic switches and memory devices with novel functionality that can potentially augment the performance and functionality of Silicon technology. First, I will present our recent work on gate-tunable diode¹ as well as tunnel junction devices based on integration of 2D chalcogenides with Si and GaN. I will show that combining 2D semiconductors with 3D semiconductors allows highly-tunable diode devices with applications in logic. Following this I will present our recent work on non-volatile memories based on Ferroelectric Field Effect Transistors (FE-FETs) made using a heterostructure of MoS₂/AlScN² and I also will present our work on Ferroelectric Diode devices based on thin AlScN and ultrafast switching in them.^{3, 4} Finally, I will also present our recent and ongoing work on probing buried 2D semiconductor/3D metal interfaces⁵ and their impact on contact polarity and resistance engineering in electronic and opto-electronic devices.
I will end by giving a broad perspective on future opportunities of 2D semiconductors in basic science and applied microelectronics technology.

References:
1. Miao, J.; Liu, X.; Jo, K.; He, K.; Saxena, R.; Song, B.; Zhang, H.; He, J.; Han, M.-G.; Hu, W.; Jariwala, D. Nano Letters 2020, 20, (4), 2907-2915.
2. Liu, X.; Wang, D.; Kim, K.-H.; Katti, K.; Zheng, J.; Musavigharavi, P.; Miao, J.; Stach, E. A.; Olsson, R. H.; Jariwala, D. Nano Letters 2021, 21, (9), 3753-3761.
3. Liu, X.; Zheng, J.; Wang, D.; Musavigharavi, P.; Stach, E. A.; Olsson III, R.; Jariwala, D. Applied Physics Letters 2021, 118, (20), 202901.
4. Wang, D.; Musavigharavi, P.; Zheng, J.; Esteves, G.; Liu, X.; Fiagbenu, M. M. A.; Stach, E. A.; Jariwala, D.; Olsson III, R. H. physica status solidi (RRL)–Rapid Research Letters 2021, 10.1002/pssr.202000575.
5. Jo, K.; Kumar, P.; Orr, J.; Anantharaman, S. B.; Miao, J.; Motala, M. J.; Bandyopadhyay, A.; Kisslinger, K.; Muratore, C.; Shenoy, V. B.; Stach, E. A.; Glavin, N. R.; Jariwala, D. ACS Nano 2021, 15, (3), 5618-5630.

In this work, a scaling law is demonstrated in the conductivity of gated two-dimensional (2D) materials with tunable concentration of impurity scatterers, such as from shallow capacitively charged traps or adsorbed gas molecules. For the same device at different concentration of scatterers, a single scaled conductivity curve is observed upon rescaling the non-linear gate-dependent conductivity by a mobility scaling factor r to represent different strength of impurity-induced scattering potential while simultaneously shifting the gate voltage by an amount ΔV_g to represent different concentrations of impurity-induced doping. The scaling effect is demonstrated first by cooling an encapsulated black phosphorus (BP) 2D multilayer at T_L = 100 K with charge trap scatterers that are populated by cooling down from room temperature under a gate bias. A second case of mobility scaling is observed in a Bi₂Se₃ 2D monolayer at room temperature exposed to gas adsorbate scatterers. The 2D nature of these samples weakly screens the charge of the adjustable density of point-like scatterers. The observed scaling in the conductance curve can be explained with a simple mobility model whereby the screened potential of individual scatterers determines the scaled conductivity versus density curve.
In the case of BP multilayers, the sample is tested under a series of bias cooling process which appears to remotely charge a trap layer with a tunable concentration of impurity scatterers. A gate bias is first applied on the back gate at room temperature. Then, the sample is cooled down to T_L = 100 K under the same gate voltage, namely the bias cooling voltage V_BC. Once the sample reaches T_L, the gate bias is then swept. For each V_BC, a distinct field-effect conductivity curve is obtained at T_L. The lineshapes of these curves have little in common. Remarkably, however, these curves can be empirically aligned onto a single curve if one artificially scales the conductivity by a factor r and shifts the gate voltage by ΔV_g, where a distinct pair of parameters r and ΔV_g is associated with each V_BC. The model is validated by taking Hall measurements for each bias cooling voltage V_BC, whereby the carrier densities are also observed to collapse onto a single curve upon shifting the gate voltage by ΔV_g.
These observations are interpreted with a charge-trap model. It is commonly known that the BP is susceptible to surface oxidization. The oxidization sites host charge traps that can be capacitively loaded. During the bias cooling process, the charge traps that have deep energy barriers are effectively frozen out, thereby determining the strength of the scattering potential and the concentration of doping in the device at T_L. Further, our analysis suggests that the mobility-limiting factors are charged impurity scattering and surface roughness scattering in the positive and negative limits of gate bias, respectively. The shallow charge trap model described here shows how naively estimated field-effect mobilities may be commonly over-estimated in 2D FET devices.
The Bi₂Se₃ monolayer device is studied under a gaseous environment from vacuum to forming gas to air. Various field-effect conductivity curves are obtained in each environment. Following the conductivity scaling analysis, the parameters indicate that the impurities are positively charged and scattering strength increases irrespective to the type of gas molecules present.
The scaled 2D conductivity analysis introduced in this work can serve as a fingerprint for a given material over an extended gate voltage range, allowing one to determine the characteristic field-effect conductivity with any impurity density and gate voltage for a given type of impurity. From the slope of the mobility scaling factor r plotted versus gate shift ΔV_g, one can further determine whether the impurities are acceptor-like or donor-like.

Emerging 2D and topological quantum materials have gained increasing attention due to their unique electronic and optical properties, and have shown promise in sensing applications. The realization of sensing devices using these materials still faces several challenges. For example, it is critical to gain clear understandings of (1) the fundamental light-matter interactions and their relations to the atomic structures, which govern many key material properties and device performances; and (2) the coupling with other nanostructures and molecules, which is a required structure for sensing devices and systems. This talk introduces new discoveries and pioneering works on these critical challenges, and novel applications of these materials in biochemical sensing. The first part of this talk presents light-matter interactions and techniques to augment material performance, including 3D Weyl semimetals, 2D Janus transition metal dichalcogenides, and 1D nanoscrolls. The characterization techniques employed, including low-frequency Raman spectroscopy, polarization- and time-resolved spectroscopy, and ultrafast electron diffraction, can be widely used in other material systems. The second part of this talk focuses on the interaction of 2D materials with organic molecules and related sensing applications. In particular, a novel enhancement effect of molecular Raman signals on 2D surface was discovered, which offers a new paradigm of biochemical sensing with high specificity, high multiplexity, and low noise. The selection rule for the 2D material substrates has been revealed, which is critical for device design. Two sensing applications for Alzheimer’s disease and respiratory viruses will also be discussed. Overall, the works presented in this talk are significant in fundamental quantum science, and offer important guidelines for practical applications in sensing and quantum technologies. The methodologies used here also provide a framework for the future study of many emerging materials and sensing scenarios.

Sensors are ubiquitous in today's world starting from healthcare to security and forensic industries. Especially, 2D materials are highly promising for sensing applications because of their large surface to volume ratio, possibility to obtain pristine interfaces and opprtunites for unique device design leveraging the ability form Van der Waal's heterostructures. Here, we will discuss the advantages of 2D material based Field effect Transistors (FETs) for highly sensitive and label-free electrical detection of biomolecules. Going beyond conventional FETs, we will discuss how novel quantum mechanical transistors can overcome the fundamental limitations in sensitivity of conventional electrical biosensors. We will also present how 2D materials can particularly enable the develpment of such novel quantum mechanical devices, which can open up new avenues in point-of-care applications.

The exponentially improving performance of conventional digital computers has slowed in recent years due to the speed and power consumption issues that are largely attributable to the von Neumann bottleneck (i.e., the need to transfer data between spatially separate processor and memory blocks). In contrast, neuromorphic (i.e., brain-like) computing aims to circumvent the limitations of von Neumann architectures by spatially co-locating processor and memory blocks or even combining logic and data storage functions within the same device. In addition to reducing power consumption in conventional computing, neuromorphic devices also provide efficient architectures for emerging applications such as image recognition, machine learning, and artificial intelligence [1]. With this motivation in mind, this talk will explore how the reduced dielectric screening in 2D nanoelectronic materials enables opportunities for novel gate-tunable neuromorphic devices [2]. For example, by utilizing self-aligned, atomically thin p-n heterojunctions, dual-gated Gaussian heterojunction transistors have been realized, which show fully tunable anti-ambipolar transfer characteristics that are suitable for artificial neurons, competitive learning, and spiking circuits [3]. In addition, by exploiting field-driven defect motion in monolayer transition metal dichalcogenides, gate-tunable memristive phenomena have been achieved, which serve as the basis of hybrid memristor/transistor devices (i.e., ‘memtransistors’) that concurrently provide logic and data storage functions [4]. The planar geometry of memtransistors further allows multiple contacts to the channel region and dual gating that mimic the behavior of biological systems such as heterosynaptic responses [5]. Overall, this work introduces new foundational circuit elements for neuromorphic computing in addition to providing alternative pathways for studying and utilizing the unique quantum characteristics of 2D nanoelectronic materials [6].

[1] V. K. Sangwan, et al., Nature Nanotechnology, 15, 517 (2020).
[2] M. E. Beck, et al., ACS Nano, 14, 6498 (2020).
[3] M. E. Beck, et al., Nature Communications, 11, 1565 (2020).
[4] V. K. Sangwan, et al., Nature, 554, 500 (2018).
[5] H.-S. Lee, et al., Advanced Functional Materials, 30, 2003683 (2020).
[6] X. Liu et al., Nature Reviews Materials, 4, 669 (2019).

Metal-organic frameworks (MOFs) have emerged as a class of structurally diverse materials with immense potential for many applications. Their often fascinating properties, apart from composition, depend strongly on the structure. An important subclass – MOFs based on naturally abundant and generally non-toxic alkaline metals – have recently offered a sustainable alternative to conventional rare-earth and transition metal complexes. Among the elements of the alkaline metals group, strontium (Sr) currently remains the most exotic and least studied in context of metal-organic complexes. However, few recent reports have suggested Sr-based complexes’ suitability as semiconducting materials, which is rare for the class of MOFs. In this work two structurally different Sr²⁺ MOFs of the same composition were prepared by introducing slight changes into the synthesis conditions. Single crystal diffraction data was obtained and used to determine the structures of both complexes. The discovered structural differences have shown that control of crystalline structure and topology by small changes in preparation method is possible for similar alkaline metal complexes. High stability to temperatures up to 250 C^o was confirmed by thermogravimetric analysis. Optical properties were studied and bandgap values determined. With facile synthesis method, synthetic tunability and remarkable properties both complexes exhibit high promise in semiconducting technology.

Tetragonal iron sulfide and selenide, FeQ (Q = S, Se), adopt the simplest crystal structures of the iron-based superconductors. Their layered structures are suitable for development of the hybrid magnetic and superconducting materials by the intercalation of the host species in the interlayer space. Low-temperature solution-assisted intercalations provides an access to rich phase space of intercalates based on C,N,O,H-containing molecular species. A complete description of the Fe-Q layer as well as the intercalate is required to accurately link these structural aspects to a material’s bulk properties. While interplanar van der Waals stacking is ideal for diverse intercalation it can limit crystal quality and in turn complicate the structure-properties analysis. To determine exact composition and challenging complex crystal structures of two pseudo-polymorphic Fe-S-en (en = ethylenediamine) layered hybrid phases we applied a synergistic combination of elemental composition, decomposition behavior, high-resolution synchrotron X-ray diffraction and total scattering, ⁵⁷Fe Mössbauer spectroscopy, and electron diffraction methods. The intricate interplay between Fe vacancies, puckering of the Fe-s layers, number and charge of the intercalated species, structural distortion, and magnetic properties will be discussed. Our work shows an importance of detailed characterization of the crystal structure of intercalated compounds to understand the origin of the observed properties and develop proper structure-properties relationships.

Transition metal trichalcogenides (TMTs) are quasi-one-dimensional semiconductors with modest band gaps that offer an opportunity to fabricate transistors with widths less than 10 nm, as they lack undesirable edge effects that plague other well-studied two-dimensional materials (namely: graphene-based materials and transition metal dichalcogenides). Among the family of TMTs, TiS₃, ZrS₃, and In₄Se₃ are n-type semiconductors, while HfS₃ is a p-type semiconducting trichalcogenide with high spin-orbit coupling. Besides, in addition to their electronic properties and adequate band gaps, their opto-electronic properties project them as promising candidates for photodetector applications. That is to say, potential of TiS₃, ZrS₃ and In₄Se₃ for photodetector applications is now known owing to successful experimental demonstrations of working light polarization-dependent phototransistors made from these TMTs; and this is consistent with appreciable photo-response of HfS₃ reported elsewhere.¹ Furthermore, temperature-dependent X-ray photoemission spectroscopy (XPS) and Raman spectroscopy have been employed to investigate and quantify the extent of electron-phonon scattering in these transition metal trichalcogenides. The Debye–Waller factor plots (based on the temperature-dependent XPS) for S 2p and Hf 4f core-levels of HfS₃ suggest that there exists a phase transition between 225 K and 240 K; however, no such phenomenon is seen for TiS₃, ZrS₃, and In₄Se₃. Thus, HfS₃ exhibits higher electron-phonon scattering at temperatures above 240 K than at temperatures below 225 K. Generally, it is clear that phonon scattering affects carrier mobilities of transition metal trichalcogenides. Nonetheless, the interaction between the Au metal contacts and the various transition metal trichalcogenides, which differs substantially, also affects the measured carrier mobilities. The Au thickness-dependent XPS measurements confirm Schottky-barrier formation at the Au/HfS₃ and Au/In₄Se₃ interfaces, which is different from what is seen at the Au/TiS₃ and Au/ZrS₃ interfaces. This, in turn, implies that the work function of Au is less than that of HfS₃ because the formation of a Schottky-barrier at the metal/p-type semiconductor interface is only possible if the work function of the p-type semiconductor is higher than that of the metal. Nevertheless, noteworthy in this regard is that for other Au/TMT interfaces, some complex interfacial chemistry also plays a big role. Yet, there are good reasons to believe that the fabrication of transition metal trichalcogenides-based Schottky transistors, Schottky diodes, and phototransistors are all possible. These results are important as they open new pathways for efficient integration of logic and sensing, which is key to robotics applications.

1. W. -W. Xiong, J. -Q. Chen, X. -C. Wu, J. -J. Zhu, J. Mater. Chem. C 2, 7392 (2014). DOI:10.1039/C4TC01039F

For optoelectronic applications, graphene is usually growth on metallic substrates then transferred into a dielectric. This process leaves large amounts of residual chemicals and a high risk of wrinkling or breakage of the film. A solution is to growth graphene directly on the dielectric, but this still is very challenging.
In this work, we show the direct synthesis of graphene on SiO₂/Si and SiC by hot-filament chemical vapor deposition. The graphene deposition was conducted at low pressures (35 Torr) with a mixture methane in hydrogen and a substrate temperature ~ 900°C followed by abrupt cooling to room temperature. A thin strip of copper layer was sputtered in the middle of the SiO₂/Si substrates as a catalytic material. The structural properties of the graphene films were analyzed using Raman Spectroscopy, Atomic Force Microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Raman mapping and AFM measurements indicate the growth of few-layer graphene films in all cases. X-ray photoelectron spectroscopy confirmed the presence of graphene deposition on SiO₂/Si and SiC substrates. The results show that the effect of the surface catalytic of copper allows the graphene growth beyond the copper strip and all over the SiO₂/Si substrates.

As modern transistor technologies reach their physical limits, new materials are being actively explored for device scaling strategies, novel device structures, or integration strategies. Two-dimensional (2D) transition metal dichalcogenides (TMDs) show promise because their atomic thickness enables aggressive channel length scaling and they are compatible with low thermal budget fabrication into transistors. For TMDs to be practical for logic transistors, complementary (both n- and p-type) devices need to be realized. Typically, semiconductors tend to show either p- or n-type behavior in transistors, depending on how their energy bands align with the Fermi level of the source/drain contact metals. Theoretically, metals can be carefully chosen by considering their work functions to give n- or p-type devices based on preferential carrier injection to the conduction or valence band, respectively. However, in real devices, Fermi-level pinning and related effects limit the effectiveness of this approach. In previous works, modification of the metal-TMD interface with an Ar⁺ ion beam has been shown to enhance device performance in MoS₂ transistors, with increased on-current and on/off-current ratio [1-2]. In this study, we demonstrate the effect of this in-situ Ar⁺ ion beam modification of the metal-TMD contact interface for Pd-contacted WS₂ transistors. The ion beam was used to modify the inert surface of the TMD (selectively in the contact regions) to generate dangling bonds for promoting improving carrier injection with the Pd. Having the ion beam in the same high vacuum chamber as the electron beam evaporator for metal deposition enabled a fully in-situ process, which reduced contaminantion from ambient exposure before contact deposition. To eliminate flake-to-flake variation, multiple modified and unmodified devices were fabricated on each flake of the mechanically exfoliated WS₂. Raman spectroscopy was used to study the impact of the ion beam modification on the WS₂ surface. An unanticipated polarity switch in these WS₂ transistors was driven by the ion beam modification, which enhanced the p-branch current, causing a polarity switch compared to the devices with unmodified Pd-WS₂ contacts. The p-branch on-current in the modified devices showed an enhancement of two to three times, on average, over the n-branch on-current of the unmodified devices. This discovery presents a unique path towards complementary WS₂ devices to be easily fabricated with the addition of an ion beam modification step to the fabrication process. Based on these findings, ion beam effects should be studied more extensively on other TMDs using a multitude of different metals to thoroughly characterize the enhancement and polarity switching effects that may occur.

There are serious safety concerns with materials that are used in high-voltage applications because electric arc represents one of the most sever thermal plasma energy discharges involving extremely high temperature (>25kK)¹. Therefore, the need for electrical discharge resistant dielectrics is imperative for safe high voltage design. A milestone of the 2D materials that are still using in this kind of applications is mica, of which the best properties are retained in the form as tape insulation. On the other hand, modern applications require conformal coating or injection molded insulation with superior discharge resistance and mica does not fulfill this requirement. Also, high layer thickness of mica insulation system behaves as a barrier for heat conduction dissipation. Therefore, the development of a novel polymer-based coating with self-assembling, inorganic 2D nanofiller montmorillonite (MMT) appears to be a promising way to overcome these problems. In this study, spray coating is applied to Kapton substrates to significantly increase discharge resistance. The electric performance of 2D nanoclay coated dielectrics is extensively characterized via voltage endurance and breakdown strength testing. The samples are further characterized using microscopy, x-ray diffraction, and broadband dielectric spectroscopy.

References
1. Xia, J.; Li, Z.; Nasreen, S.; Ronzello, J.; Teng, H.; Jacobs, L.; Cao, Y., Discharge Resistant Nano-Coatings. In 2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), 2018; pp 183-186.

Monolayers of bismuth and tin arranged in a honeycomb lattice hold great promise for next-generation electronics. These materials, so-called “bismuthene” and “stanene”, could host a quantum spin Hall effect near room-temperature. Unlike graphene, however, these materials cannot be readily exfoliated from a bulk crystal and must be stabilized in the thin film form. Here we use molecular-beam epitaxy to construct these metastable structures. We discuss the preparation of the Ge(111) surface as a platform for our epitaxial deposition.

The high contact resistance of transition-metal dichalcogenide (TMD)-based devices is one of the bottlenecks that limit the application of TMDs in various domains. The contact resistance of TMD-based devices is strongly related to the interface chemistry and band alignment at the contact metal/TMD interfaces. To understand the metal/MoS₂ interface chemistry and band alignment, Ni and Ag metal contacts are deposited on MoS₂ bulk and chemical vapor deposition bilayer MoS₂ (2L-MoS₂) film samples under ultrahigh vacuum (UHV, ∼3 × 10^-11 mbar) and high vacuum (∼3 × 10^-6 mbar) conditions. X-ray photoelectron spectroscopy (XPS) is used to characterize the interface chemistry and band alignment of the metal/MoS₂ stacks. Ni forms covalent contact on MoS₂ bulk and 2L- MoS₂ film by reducing MoS₂ to form interfacial metal sulfides. In contrast, van der Waals gaps form at the Ag/MoS₂ bulk and Ag/2L-MoS₂ film interfaces, proved by the absence of an additional metal sulfide chemical state and the detection of Ag islands on the surface. Different from other metal/MoS₂ systems studied in this work, Ag shows potential to form an Ohmic contact on MoS₂ bulk regardless of the deposition ambient. Fermi levels (E_F’s) are pinned near the intrinsic E_F of the 2L-MoS₂ film with high defect density regardless of the work function of the metal, which highlights the impact of substrate defect density on the E_F pinning effect and contact resistance.
To achieve reliable MoS₂ based devices with a higher thermal budget, it is imperative in understanding the impact of thermal treatment on the interface chemistry of the Ni/MoS₂ interface. 1 nm Ni films were deposited on CVD 1L-MoS₂/SiO₂/Si and 2L-MoS₂/SiO₂/Si wafers, and exfoliated MoS₂ bulk crystal in UHV. In-situ XPS was employed to study the interface chemistry and band alignment after Ni deposition and subsequent stepwise UHV annealing up to 400 °C. Ni in-diffusion is detected for all the Ni/MoS₂ systems after annealing at 300 °C in UHV. The defects generated by Ni reduction of MoS₂ and Ni in-diffusion cause the interfacial reaction at the MoS₂/SiO₂ interface after annealing at 400 °C for the CVD MoS₂ film systems. This work highlights the influence of the processing condition on the interface chemistry and band alignment of metal contacts/TMD/oxide stacks and their implications on the device performance. The interface reaction between CVD few-layer MoS₂ film and SiO₂/Si wafer necessitates the careful processing of the transferred or deposited TMD thin films on oxide substrates.
This work was supported in part by NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST through award number 70NANB17H041.

Although chemical vapor deposition (CVD) enables large graphene sheets, the polymer-free transfer techniques of this type of graphene are still challenging. The main reason that complicates graphene's current direct transfer techniques is the non-understood effect of high surface tension (ST) of water on graphene during this type of transfer. For instance, it is widely accepted that ST of water (72 dyne/cm) would break graphene sheets during metal etching or solution replacement. In this work, we have achieved several experiments and calculations to unveil the actual effect of high ST on graphene and to provide a simple, effective, and clean method to achieve graphene transfer. We designed a specific way that allows tuning surface tension of liquids underneath graphene. We successfully transferred graphene at ST lower than the initial ST (ST of the water) and provided a wide range of liquids that can be used to replace water underneath graphene. Leveraging the high ST of water, a graphene water membrane (GWM) would be formed, providing a new understanding of the graphene-water molecules dynamics. GWM was functioned successfully to deliver graphene to varied substrates. Our work removes the long believed idea that high ST has a damaging effect on graphene. Additionally, it expands the graphene applications in several fields, such as sensors and actuators.

Structural polymorphs of two-dimensional 2D layered Van der Waal solids like MoS₂ hold great potential for major applications in the advanced electronics and energy sectors. However, in contrast to the facile formation of the most stable 2H polymorph, it is difficult to stabilize the metastable 1T or 1T' polymorph of MoS₂ in spite of their exciting application-worthy properties, due to their higher formation energies and hence metastability. Moreover for applications in device technology it is essential to stabilize these on thin film platform. While strain is known to play wonders in stabilizing metastable phases, it is not clear how this can play out for a Van der Waal solid comprised of loosely assembled 2D layers on a typical 3D inorganic solid substrate. Herein we tackle this question via growth and analysis of MoS₂ on different single crystal metal oxide substrates (SrLaAlO₄, c-Al₂O₃, SrTiO₃, LaAlO₃) deposited by Pulsed Laser Deposition (PLD). We observe that on the structurally compatible tetragonal SrLaAlO₄ (001) substrate mixed 1T'+2H phase MoS₂ grows with a dominant (~75%) 1T' phase, while on hexagonal c-Al₂O₃ (0001) substrate pure 2H phase grows. On the SrTiO₃ and LaAlO₃ substrates also the 1T'+2H mixed phase grows, but the 1T' phase contribution is lesser in the order SLAO>STO>LAO. The fact that 1T' contribution is higher in SLAO and STO, both containing strontium, points to its chemical role in the early growth. X-ray diffraction, Raman spectroscopy, core-level X-ray photoelectron spectroscopy, valence band spectroscopy, and electrostatic force microscopy are used to confirm and quantify the presence, contributions and properties of these films and phases. Remarkably, with increased film thickness on SrLaAlO₄, the contribution of pure 2H phase is enhanced, indicating the pivotal role of lattice strain in stabilizing the initial layer(s). The mixed 1T'+2H phase in ultrathin film shows high room temperature resistivity (~2 mW cm) with respect to that of pure 2H phase (~14 mW cm). Density functional theory reveals that to attain higher % of 1T' phase, not only the structural compatibility is important but also the surface chemistry. This substrate selective polymorphism with distinct electronic features provides an approach to stabilization of metastable phases of even soft Van der Waal solids on hard substrates for integration into novel device functions.

References:
Swati Parmar, Abhijit Biswas, Bishakha Ray, Suresh Gosavi, Suwarna Datar, and Satishchandra Ogale, Stabilizing metastable polymorphs of Van der Waal solid MoS₂ on single crystal oxide substrates: Exploring the possible role of surface chemistry and structure, J. Phys. Chem. C 125, 20, 11216–11224 (2021).

Graphene isolation in 2004 initiated the research era into layer separation and subsequent analyses of various 2D materials that exhibit many exciting properties.¹ Among the widely growing 2D layered materials, transition metal dichalcogenides (TMDCs) has attracted significant attention.¹ This family of materials possesses a sizable bandgap, and several reports indicate that they could be suitable as active layers in logic electronics and optoelectronics in addition to electrode components in a variety of energy-related applications. Layered TMDCs have the generic formula MX₂, where M stands for metal, and X represents a chalcogen, and their interatomic interaction within each layer is covalent.² In contrast, all these individual layers are held together by weak van der Waals (vdW) forces (for example, MoS₂, MoSe₂, WS₂, WSe₂, WTe₂, NbSe₂, etc.), just as in graphene. These interlayer vdW forces are responsible for their anisotropy and allow cleaving of the layers.³
Since TMDCs are unique layered materials with exotic properties, examining their structure holds tremendous importance. 2H-MoSe₂ (analogous to MoS₂; Gr. 6 TMDC) is a crucial optoelectronic material studied extensively using Raman Spectroscopy.⁴ In this regard, Low-Frequency Raman (LFR) can probe this material's structure as it reveals distinct vibration modes. Here, we focus on understanding the microstructural evolution of different 2H-MoSe₂ morphologies and their layers using LFR. We grew phase-pure 2H-MoSe₂ (with variable microstructure) directly on Mo foil using a two furnace ambient pressure chemical vapor deposition (CVD) by carefully controlling the process parameters. We analyzed the layers of exfoliated flakes after ultrasonication and drop cast different layer thicknesses of 2H-MoSe₂ by choosing different concentrations of 2H-MoSe₂ solutions. Analyses of the respective LFR regions confirm the presence of newly identified Raman signals for the 2H-MoSe₂ nanosheets drop cast on Raman-grade CaF₂. Our results show that CaF₂ is an appropriate Raman-enhancing substrate compared to Si/SiO2 as it presents new LFR modes of 2H-MoSe₂. Therefore CaF₂ substrates are a promising medium to characterize in detail other TMDCs using LFR.
References
(1) Yu, X.; Sivula, K. Toward Large-Area Solar Energy Conversion with Semiconducting 2D Transition Metal Dichalcogenides. ACS Energy Lett. 2016, 1 (1), 315–322. https://doi.org/10.1021/acsenergylett.6b00114.
(2) Liang, L.; Zhang, J.; Sumpter, B. G.; Tan, Q. H.; Tan, P. H.; Meunier, V. Low-Frequency Shear and Layer-Breathing Modes in Raman Scattering of Two-Dimensional Materials. ACS Nano 2017, 11 (12), 11777–11802. https://doi.org/10.1021/acsnano.7b06551.
(3) O’Brien, M.; McEvoy, N.; Hanlon, D.; Hallam, T.; Coleman, J. N.; Duesberg, G. S. Mapping of Low-Frequency Raman Modes in CVD-Grown Transition Metal Dichalcogenides: Layer Number, Stacking Orientation and Resonant Effects. Sci. Rep. 2016, 6 (1), 19476. https://doi.org/10.1038/srep19476.
(4) Alessandri, I.; Lombardi, J. R. Enhanced Raman Scattering with Dielectrics. Chem. Rev. 2016, 116 (24), 14921–14981. https://doi.org/10.1021/acs.chemrev.6b00365.

Fossil fuel combustion and automotive emissions always result in highly toxic emissions. Some of the most commonly known pollutants are nitrogen dioxide (NO₂), hydrogen disulfide (H₂S), ammonia (NH₃), and acetone, to name a few.¹ These gases inflict severe detrimental effects on human health and are the primary reason behind lung diseases, such as respiratory irritation syndrome, emphysema, and chronic bronchitis. So, developing susceptible and selective gas sensors for air monitoring and human health protection becomes an important issue. Among the various gas sensors, resistance-type gas sensors are the most attractive and practical for sensing toxic analytes and explosive gases due to their facile fabrication, ease of operation, low cost, and miniaturization.¹ In this regard, 2D materials (2DMs) and especially transition-metal chalcogenides (TMCs) or transition-metal di-chalcogenides (TMDCs) specify a massive range of unique properties that prove their usefulness in applications toward gas sensing.² Semiconducting two-dimensional (2D) TMDCs of the type MX₂, where M = Mo, W and X = S, Se, are encouraging materials to be explored in areas of gas-sensing because they are susceptible to the ambient conditions.³ We report a facile and robust room-temperature NO₂ sensor fabricated using bi- and a multi-layered 2H variant of tungsten di-selenide (2H-WSe₂) nanosheets, exhibiting high sensing characteristics. ⁴A simple liquid-assisted exfoliation of 2H-WSe₂, prepared using ambient pressure chemical vapor deposition, allows smooth integration of these nanosheets on transducers. Three sensor batches are fabricated by modulating the total number of layers (L) obtained from the total number of droplets from a homogeneous 2H-WSe₂ dispersion, such as ∼2L, ∼5–6L, and ∼13–17L, respectively. Room temperature (RT) experiments show that these devices are specifically tailored for NO₂ detection. 2L WSe₂ nanosheets deliver the best rapid response compared to ∼5–6L or ∼13–17L. The response of 2L WSe₂ at RT is 250, 328, and 361% to 2, 4, and 6 ppm NO₂, respectively. The sensor showed nearly the same response toward low NO₂ concentration even after 9 months of testing, confirming its remarkable long-term stability. A selectivity study, performed at three working temperatures (RT, 100, and 150 °C), shows high selectivity at 150 and 100 °C. Full selectivity toward NO₂ at RT confirms that 2H-WSe₂ nanosheet-based sensors are ideal candidates for NO₂ gas detection.
References
(1) Kumar, R.; Goel, N.; Hojamberdiev, M.; Kumar, M. Transition Metal Dichalcogenides-Based Flexible Gas Sensors. Sensors Actuators, A Phys. 2020, 303, 111875. https://doi.org/10.1016/j.sna.2020.111875.
(2) Kumar, R.; Liu, X.; Zhang, J.; Kumar, M. Room-Temperature Gas Sensors Under Photoactivation: From Metal Oxides to 2D Materials; Springer Singapore, 2020; Vol. 12. https://doi.org/10.1007/s40820-020-00503-4.
(3) Yuan, S.; Zhang, S. Recent Progress on Gas Sensors Based on Graphene-like 2D/2D Nanocomposites. J. Semicond. 2019, 40 (11). https://doi.org/10.1088/1674-4926/40/11/111608.
(4) Moumen, A.; Konar, R.; Zappa, D.; Teblum, E.; Perelshtein, I.; Lavi, R.; Ruthstein, S.; Nessim, G. D.; Comini, E. Robust Room-Temperature NO 2 Sensors from Exfoliated 2D Few-Layered CVD-Grown Bulk Tungsten Di-Selenide (2H-WSe 2 ). ACS Appl. Mater. Interfaces 2021, 13 (3), 4316–4329. https://doi.org/10.1021/acsami.0c17924.
Rajashree Konar and Abderrahim Moumen have equal contribution

Type-II heterostructures (HSs) are building blocks of next generation electronic and optoelectronic devices. Till date, studies have shown that the interlayer charge transfer (CT) mechanism is the dominating carrier relaxation pathway in type-II transition metal dichalcogenide (TMD) HSs. This work shows in a type-II HS formed between monolayers of group-6 and group-7 TMD materials, nonradiative energy transfer (ET) dominates over the traditional CT process even without a charge-blocking interlayer. Without charge-blocking interlayer, the measured ET efficiency is 70%, which leads to 3.6 times photoluminescence (PL) enhancement from the acceptor material in HS area. By completely blocking the interlayer CT process we achieved more than one order magnitude higher PL emission from the acceptor material in the HS area. The PL enhancement factor (I/I_o) shows a distance (d) dependency of 1/d², revealing the interaction between the TMD layers via 2D-to-2D dipole coupling. This study not only provides significant insight into the competing interlayer processes, but also shows an innovative way to increase the PL quantum yield of the desired TMD material using ET process.

Recently, transition metal dichalcogenide materials have attracted a lot of attention because they have extraordinary structural and electrical properties. We report the optical properties and electrical properties of double-atomic-layer molybdenum disulfide (MoS₂) films, monolayer Tungsten diselenide (WSe₂) films and tungsten disurfide (WS₂) films by using Raman spectroscopy, photoluminescence (PL), field effect transports and transmission-line-method (TLM). MoS₂, WSe₂ and WS₂ films are grown on SiO2/Si substrates using MOCVD and CVD.
The PL measurement of MoS₂, WSe₂ and WS₂ films reveals strong peak at 685.1 nm, 748.6 nm and 624 nm corresponding to a bandgap energy of 1.8 eV, 1.65 eV and 1.98 eV, attributed to highly efficient radiative transition. Quantitative contact resistance of MoS₂, WSe₂ and WS₂-based field-effect transistors (FETs) with Ti/Au (5/50 nm) electrodes are obtained by using the TLM. The monolayer MoS₂/Ti/Au, WSe₂/Ti/Au, WS₂/Ti/Au yields a contact resistance of 0.1, 0.495 and 7.63 ΜΩµm, respectively. Moreover, quantitative Schottky barrier heights are determined by analyzing the low-temperature transport properties. The MoS₂/Ti/Au, WSe₂/Ti/Au and WS₂/Ti/Au yields a Schottky barrier heights of 32 meV, 38 meV and 48 meV, respectively. Fabricated FET, based on MoS₂, WSe₂ and WS₂ films, device shown an electron mobility of 1.3×10^-4, 0.2 and 1.97 cm²/Vs, respectively and an on/off ratio of 6.5×10², 6×10⁵ and 1.51×10⁵, respectively.

Recently, two-dimensional (2D) transition metal dichalcogenides (TMD) based on atomically thin films of layered semiconductors are attractive for band-to-band tunneling devices. In this study, we have transferred molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂) few-atomic-layer films grown by chemical vapor deposition on SiO₂/Si substrate for building vertical heterostructure. Characteristics of transferred MoS₂, WSe₂ and MoS₂/WSe₂ heterostructure were investigated by Raman spectroscopy and photoluminescence. Heterostructure of intrinsic MoS₂ and WSe₂ having n-type and p-type conductivity, respectively, is suitable for band-to-band tunneling field effect transistor (BTBT-FET) application. Thus, we measure electrical properties of MoS₂/WSe₂ heterostructures using ion-gel top gate. The characteristic rectifying behavior was observed in I-V output curve of heterojunction MoS₂/WSe₂. BTBT-FET is achieved subthreshold swing under ~60mV/dec to tunable band alignment of the MoS₂/WSe₂ heterojunction with ion-gel dielectric through control top gate voltage.. Furthermore, ion-gel with high capacitance can form electric-double-layer (EDL) at interface, which leads to large carrier density and allows the transistor operating under a low voltage. This work provides considerable promise for the next-generation low-power electronic devices.

Graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) are novel nanofillers possessing attractive characteristics, including robust compatibility with most polymers, high absolute strength, and cost-effectiveness. In this study, an outstanding synergetic effect on the graphene nanoplatelet (GNP) and multiwalled carbon nanotube (MWCNT) hybrids were using a simple dispersing method to reinforce epoxy/glass fiber composite laminates via vacuum-assisted resin transfer molding (VARTM) to enhance their mechanical properties. The GNP/CNT hybrids (0.1, 0.25, 0.5 and 1.0 wt%) with different mixing ratios (i.e., 9:1, 5:5, and 1:9) were dispersed in epoxy resin to prepare GNP/CNT/GFRP laminates. The mechanical properties such as ultimate tensile strength, flexural strength, flexural modulus, and interlaminar shear strength (ILSS) of these nanocomposite laminates were investigated. Additionally, the fracture surfaces of the specimens examined using field-emission scanning electron microscopy to determine the dispersal mechanisms and reinforce behavior of the GNP/CNT hybrids in the GFRP laminate. The experimental results indicate that the mechanical properties of GNP/CNT/GFRP laminates are optimized by reinforcement through the addition of GNP/CNT hybrids.

In 2D materials, each constituent is permanently in contact with the environment. While this is beneficial for applications such as catalysis or sensors, such exposure to its surrounding makes 2D materials susceptible to mechanical stress, chemical modification and degradation. Clearly, the prospective application of 2D materials determines whether degradation or long-term stability is required. Yet, the degradation behavior of 2D materials under different environmental conditions remains largely unexplored.
Here, we use the Langmuir monolayer technique to study the preparation and degradation of 2D materials, formed in situ by the reaction of monolayers of hydrophilic polymers with suitable cross-linkers. In particular, we are interested in the influence of acidic, alkaline and oxidative environments on the mass loss and the mechanical properties of the 2D sheets. We show how functional groups that are particularly susceptible to distinct environmental factors enable a rapid loss of mass and mechanical strength. The experimental results are compared to existing theoretical models for the degradation of 2D polymer networks, as a first step towards a prediction of the environmental fate of these fascinating materials.
[1] S. Saretia, R. Machatschek, B. Schulz, B., A. Lendlein, Reversible 2D networks of oligo (ε–caprolactone) at the air–water interface. Biomedical Materials, 14(3), 034103 (2019).
[2] S. Saretia, R. Machatschek, T. Bhuvanesh, A. Lendlein. Effect of Water on Crystallization and Melting of Telechelic Oligo (ε-caprolactone) s in Ultrathin Films. Advanced Materials Interfaces, 8(7), 2001940 (2021).

Transition metal dichalcogenides (TMDs) are intriguing due to their unique properties and potentials for applications in next-generation electronic devices. However, strong Fermi level (E_F) pinning manifests at the metal/TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the interface chemistry and band alignment of Ni and Ag on the Molybdenum and Tungsten based (Mo, W-based) TMDs. Ni reduction of all the TMD substrates and the formation of intermetallic products are detected by X-ray photoelectron spectroscopy (XPS) after in-situ ultra-high vacuum (UHV, ~ 3 × 10^-11 mbar) and ex-situ high vacuum (HV, ~ 3 × 10^-6 mbar) metal depositions. E_F’s are pinned close to the middle of the bandgap for Ni contacts with MoSe₂ and MoTe₂. In contrast, Ni tends to form n-type and p-type Schottky contacts with WS₂ and WSe₂, respectively. Van der Waals interfaces form between Ag and TMDs with E_F’s pinned close to the intrinsic E_F’s of the TMDs, indicating the impact of the natural defects of the TMD substrates. The oxidation of metal contacts and the metal/TMD interfaces are observed for most of the ex-situ HV samples. The metal deposition mechanism is characterized by atomic force microscopy and the surface morphology is correlated to the interface chemistry. The implications of interface chemistry on the true band alignment of Ni and Ag contacts on Mo and W-based TMDs will be discussed. This work improves the understanding of the possible E_F pinning origins at the metal/TMD interfaces.
This work was supported in part by NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST through award number 70NANB17H041.

Transition metal dichalcogenides (TMDs) exhibit a variety of electronic behaviors depending on the number of layers and width. Therefore, developing facile methods for their controllable synthesis is of central importance. We present a way to control the width by employing metal nanoparticles and the number of layers by substrates. We found that on some types of substrates metal nanoparticle promotes both heterogenous nucleation of the first layer of TMDs (e.g., MoS₂) and simultaneously homoepitaxial tip growth of a second layer via vapor-liquid-solid (VLS) mechanism, resulting in bilayer nanoribbons with the width down to sub-10 nm as controlled by the nanoparticle diameter; while on some other types of substrates single layer of MoS₂ nanoribbons are directly grown via the VLS mechanism through the metal nanoparticle. Simulations further confirm the VLS growth mechanism towards nanoribbons and its orders of magnitude faster growth speed compared to the conventional non-catalytic growth of flakes. Width-dependent photoluminescence and Coulomb-blockade oscillation observed in the transfer characteristics of the nanoribbons at temperatures up to 60 K demonstrate the width evinces the value of this proposed synthesis strategy for future nanoelectronics.

Hydrodynamic electron flow in condensed matters has been one of the most active research areas recently. While progress from both theory and experimental techniques are made, open questions regarding the underlying mechanisms in layered quantum materials still remain. We utilize ab initio techniques to treat the electron scattering events explicitly, and show in combination with the Boltzmann transport equation a generic metric of hydrodynamic transport taking into account temperature, channel width, and impurity length, which can be directly verified by various experimental techniques.
We start by investigating different electron scattering mean free paths in WTe2, and show that that phonon mediated electron-electron interaction could lead to much shorter momentum conserving mean free path (lmc) than momentum relaxing lmr, facilitating hydrodynamic behavior in systems where the direct Coulomb interaction is largely screened [Vool 2021].
We then present the transport regime limits in PdCoO2, and identify the finite size effects in both the quasi-hydrodynamic and quasi-ballistic transport regimes, which leads to the the in-plane conductivity anisotropy observed recently [Maja 2019, Maja 2021]. We further discuss the dynamics of phonons through the lifetime hierarchy from interactions with electrons and the lattice, and confirm that phonons do play an important role in the electronic transport phenomena [Varnavides, Wang 2021].
Finally, we predict prominent hydrodynamic effects in other layered semimetals ZrSiS and TaAs2, and propose a few key ingredients including high carrier mobility, large electron-phonon matrix elements, and slow phonon-phonon scattering. The plethora of low symmetry crystals whose Fermi surfaces are composed of d orbitals from the transition metal and p orbitals from the metalloids provides a much larger pool for further study. This work provides ab initio signatures of material-specifics to explore hydrodynamic electron flow in a much larger family of condensed matter systems, and thus offers insights into further study of electron interactions through transport phenomena.

Important for all (quantum) optical technologies is the manipulation of the light-matter interaction to achieve a high level of control, particularly in technologically relevant solid-state nanomaterials. Atomically thin two-dimensional layered materials receive great interest because of their unique properties. A novel class of atomically thin materials such as 2D polar metals such as 2D gallium or 2D indium and their ternary alloys that exhibit fascinating properties like superconductivity [1,2] strong nonlinear optical properties emerging by giant second harmonic generation [3] and epsilon near zero behavior in the visible and NIR range [4]. The layer dependence of the energies of localized plasmons due to interband transition strongly suggest quantum confined 2D metal films. In contrast, monolayers and hetero-bilayers of semiconducting transition metal dichalcogenides (SC-TMDCs) excel due to their strong exciton dominated light matter interaction with the possibility to study and manipulate dense and correlated exciton ensemble [5-7] and to realize quantum light sources [8].
In this talk, we will introduce the rather new class of 2D materials, 2D polar metals and focus on their linear optical response measured by imaging spectroscopic ellipsometry in dependence of the temperature covering room temperature to 800mK. At low temperatures emergent behavior such as superconductivity occurs such that we are able to monitor the dielectric functions across the phase transitions.

We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC 2089/1 – 390776260 and projects Wu 637/4- 1, 4 – 2, 7-1 and SPP 2244.

References
[1] N. Briggs et al. Nature Materials 19 (6), 637-643 (2020).
[2] S. Rajabpour et al. arXiv:2106.00117 (2021).
[2] M. A. Steves et al. Nano Letters 20, 11, 8312–8318 (2020).
[3] K. Nisi et al. Advanced Functional Materials 2005977, 1-11 (2020)
[4] B. Miller, et al., Nano Lett. 17(9), 5229–5237 (2017).
[5] J. Kiemle et al. Phys. Rev. B 101, 121404(R) (2020).
[6] L. Sigl et al. Physical Review Research 2, 042044(R) (2020)
[7] J. Klein et al. ACS Photonics 8, 2, 669–677 (2021).

Intercalation of lithium into van der Waals gaps of two-dimensional (2D) materials has been exploited to induce useful electronic and structural phase transitions enabled by its high electron doping power on the order of 10¹⁴ e/cm². For example, semiconducting 2H-MoS₂ can be transformed into metallic 1T’-MoS₂ with lithium intercalation. Despite the extensive studies of lithium intercalation in MoS₂ and WS₂, the effects of lithium intercalation in other 2D transition metal dichalcogenides have rarely been explored. Herein, we report a reversible structural phase transition in 2D semi-metallic Td-WTe₂ by electrochemical lithium intercalation. The phase transition dynamics were systematically investigated with a combination of in situ characterization tools to monitor the changes in the optical, structural, and electrical properties of WTe₂ as a function of intercalation driving voltage in real-time. The intercalation-induced new phase was probed structurally by in situ polarization angle-dependent Raman spectroscopy, which revealed new Raman peaks as well as a distinct symmetry for the high-frequency intralayer and low-frequency interlayer vibration modes at the phase transition point. Surprisingly, the new phase shows an increase of longitudinal resistance by two orders of magnitude and a significant reduction of carrier density at the phase transition point, probed by in situ Hall measurements as a function of intercalation. The in situ transport data suggests that the new phase has a distinct electronic band structure from the T_d phase. Based on our observations and ab initio calculations, we suggest possible crystal structures and corresponding electronic band structures for the intercalation-induced phase in WTe₂.

Strong evidence suggests that transformative correlated electron behavior may exist only in unrealized clean 2D materials such as 1T-TaS₂ [1]. Unfortunately, experiment [2, 3] and theory [4] suggest that extrinsic disorder in free standing 2D layers impedes correlation-driven quantum behavior. Here we demonstrate a new route to realizing fragile 2D quantum states through epitaxial polytype engineering of van der Waals materials [5]. The isolation of truly 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate (tC-) CDW reported herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces epitaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication.

CDWs are an emergent periodic modulation of the electron density that permeates a crystal with strong electron-lattice coupling [6, 7]. TaS₂ and TaS_xSe_2-x host several CDWs that spontaneously break crystal symmetries, mediate metal–insulator transitions [6, 8], and compete with superconductivity [9–11]. These quantum states are promising candidates for novel devices [12–15], efficient ultrafast non-volatile switching [16, 17], and suggest elusive chiral superconductivity [18, 19]. Unfortunately, extrinsic and thermal disorder in free standing 2D layers degrades correlation-driven quantum behavior and clean 2D CDWs or superconductivity are near absent. Room temperature access to spatially coherent CDWs and clean 2D confinement could enable a paradigm shift toward device logic and quantum computing.

Here we show the critical temperature for spatially-coherent, commensurate (C-) CDW in 1T-TaS₂ can be raised to well above room temperature (~150 K above the expected transition) by synthesizing clean interleaved 2D polytypic heterostructures [5]. This stabilizes a collective insulating ground state (i.e. C-CDW) not expected to exist at room temperature. We show the formation of these spatially coherent states occurs when 2D CDWs are confined between metallic prismatic polytypes. Metallic layers screen impurity potentials to suppress the disordered nearly-commensurate (NC-CDW) phase. At the same time, interleaving disables interlayer coupling between CDWs . This raises the critical temperature of the C-CDW and forms out-of-plane twinned commensurate (tC) CDWs as revealed by scanned nanobeam electron diffraction. These results demonstrate polytype engineering as a route to isolating 2D collective quantum states in a well-defined extrinsic environment with identical chemistry but distinct band structure.

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Zeolitic imidazole frameworks (ZIFs) are a type of metal organic framework (MOF), where Zn metal atoms are tetrahedrally coordinated to imidazole linkers.¹ ZIFs are widely studied as potential materials of interest in electronic devices and sensor applications due to their low-dielectric constants. For device applications, high precision patterning of the ZIFs is essential, which has been demonstrated by using an electron and X-ray beam. To optimize these patterning techniques, it is necessary to understand the structural and chemical changes happening in the MOF framework upon electron-beam irradiation. This has been widely studied in other inorganic crystalline materials like zeolites, but is limited for MOFs.² Here, we study the transition of a ZIF-L MOF from an ordered crystalline state to a disordered amorphous state using dose-controlled transmission electron microscopy (TEM) coupled with electron energy-loss spectroscopy (EELS) to simultaneously monitor chemical changes during the transition.
First, structural changes in ZIF-L were monitored using dose-dependent electron diffraction. It was observed that at an accumulated dose of ~ 100 e/Ų, there is a complete loss in diffraction intensity along with the appearance of broad amorphous like peaks indicating the formation of an amorphous ZIF-L phase. Further, shifts in diffraction peaks with dose indicates a decrease in the lattice parameters, which is also observed as particle shrinkage in the high-angle annular dark-field scanning TEM (HAADF-STEM) images. Thus, in the initial stages of beam exposure, it can be concluded that there is a collapse of the porous framework resulting in the formation of a “disordered” amorphous ZIF-L phase, retaining the short-range order of the parent framework.
To study the chemical changes in ZIF-L during this transformation, core-level EELS spectra were obtained from C, N and Zn as a function of dose to monitor changes in the fine-structure which are sensitive to the local chemical environment. At doses < 100 e/Ų, changes in the fine-structure were minor indicating no changes in the local bonding of the amorphous ZIF-L. At higher doses > 500 e/Ų, considerable changes were observed in the core-level EELS fine-structure, resulting from the radiolytic modification of the imidazole linkers by the loss of methyl groups and ring opening reactions, degrading the linker in the second stage of the structural modification. These results point to a completely radiolytic behavior of the electron-beam-ZIF interaction. Further, these structural modifications impact the dielectric properties of the material by shifting the energy for the optical transitions, which are important for device applications.³
References:
1. Park K.S., Ni Z., Cote A. P., Choi J. Y., Huang R., Uribe-Romo F.J., Chae H. K., O’Keeffe M., Yaghi O. M., PNAS July 2006, 103(27) 10186-10191.
2. Kumar P., Kim D. W., Rangnekar N., Xu H., Fetisov E. O., Ghosh S., Zhang H., Xia Q., Shete M., Siepmann J. I., Dumitrica T., McCool B., Tsapatsis M., Mkhoyan K. A., Nat. Mater. 2020, 19, 443-449.
3. Ghosh S., Yun H., Kumar P., Conrad S., Tsapatsis M., Mkhoyan K. A., Chem, Mater., 33, 2021.

In general, the progress in the field of condensed matter comes from the discovery of new materials with novel properties. For instance, the discovery of two dimensional (2D) materials such as graphene in 2004 and transition metal dichalcogenides (TMDs) in 2005 had great impact on the scientific community, giving rise to tremendous and ongoing activity in both theoretical and experimental fields. Following that, van der Waals (vdW) heterostructures have gained attention, where two or more atomically thin layers interact with each other through weak vdW forces, and induced effects due to the proximity of the layers lead to intriguing properties that can be engineered for several practical applications. Recently, with the advance on experimental techniques to fabricate and isolate new materials, as well as to probe their features, other novel properties have also been reported intrinsically in 2D materials: spontaneous magnetization, quantum Hall effect and superconductivity. Nonetheless, the rational design of new heterostructures combining these materials, and thus their properties, allows the development of new quantum phases of matter that have not been observed in stand-alone materials, such as the topological superconductivity, which has aroused great interest in recent years due to the potential applications in topological quantum computers. Here we present results obtained for the 2D magnetic CrBr₃ monolayer deposited on top of the layered superconductor NbSe₂, as well as the metal-organic framework M-DCA (with M=Cu, Co and Ni) deposited either on graphene or NbSe₂, focusing mainly on their electronic and structural properties obtained through density functional theory (DFT) and tight-binding (TB) calculations. We compare these directly to state-of-the-art Scanning Tunnelling Microscopy experiments and develop a framework for the systematic design of quantum materials.

Two-dimensional (2D) materials derived from van der Waals (vdW)-bonded layered crystals have been the subject of considerable research focus, but analogous one-dimensional (1D) materials have received less attention while potentially also exfoliable and useful for optical or electronic applications. These bulk crystals consist of covalently bonded multi-atom atomic chains with weak van der Waals bonds between adjacent chains. Using density-functional-theory-based methods, we find the binding energies of several 1D families of materials to be within typical exfoliation ranges possible for 2D materials. In addition, we compute the electronic properties of a variety of insulating, semiconducting, and metallic individual wires and find differences that could enable the identification of and distinction between 1D, 2D, and 3D forms during mechanical exfoliation onto a substrate. We find 1D wires from chemical families of the forms PdBr₂, SbSeI, and GePdS₃ are likely to be distinguishable from bulk materials via photoluminescence. Like 2D vdW materials, we find some of these 1D vdW materials have the potential to retain their bulk properties down to nearly atomic film thicknesses, including the structural families of HfI₃ and PNF₂, a useful property for some applications including electronic interconnects.

Machine learning methods are also used to predict previously undiscovered low-dimensional van der Waals materials, targeting compositions with conductive and potentially magnetic properties. We find different models trained on different subsets of the data recover similar predicted unsynthesized 1D materials, illustrating model robustness. A particular composition, MoI₃,which is predicted by the model to be 1D, is indeed confirmed to exist with wire-like subcomponents. We train on classes of materials tailored for potential experimental synthesizability, specifically for chalcogen, halogen, and pnictogen-containing compounds due to their ease of synthesis using chemical vapour deposition (CVD) and chemical vapour transport (CVT).

Single layer Transition Metal Dichalcogenides (LTMDs) have demonstrated extraordinary device and catalytic performance due to their extreme surface area and exceptional optoelectronic properties. Integration of these two-dimensional materials into low-cost, flexible coatings and devices (e.g. wearables, filters, energy harvesting, energy storage, environmental sensors, etc.) requires highly stable formulations with oxidative stability for ambient manufacturing technologies (i.e. slot die coating; spray coating; inkjet printing) via techniques that preserve their novel atomic-scale properties. Absorbed interface modifiers, such as surfactants, struggle to simultaneously deliver the compositional flexibility necessary for formulations, while enabling control of local morphology and LTMD’s optoelectronic characteristics (high refractive indices, large optical cross-sections, and environmentally sensitive band structure, excitons, and charge carriers). Here in, we summarize new surface functionalization chemistries afforded by new exfoliation approaches that provide dispersions in a wide range of oxygen-free and water-free solvents. In general, surface derivatization, bonding, doping, and orbital hybridization is achieved via targeting valence states of Group VI (MoX₂, WX₂) and Group V (NbX₂, TaX₂) LTMDs. For example, high electronegative groups (R-MgBr; R-NH^-) result in tethering of organic moieties to the basal surface of Group VI LTMDs. This method results in a structured organic canopy, preservation of the semi-conductor character, and stable dispersion of Group VI LTMDs in non-polar media. For Group V LTMDs, Lewis acid-base chemistry enables similar basal surface modification, as well as stoichiometric tunability of the valence electronic states, shifting Group V semi-metals to semiconductors. These results will enable rapid development of hybrid LTMD-organic heterostructures that can be integrated into composites, electronics, and optical filters.

Pumping charges into nanostructures by contact electrification is important for controlling and manipulating a device functionality in tribotronics. This study demonstrates a controlled charge transfer by contact electrification on a few-layered MoS₂ nanosheet obtained by a chemical vapor deposition process. Nucleation sites present in the MoS₂ nanosheets with sharp-tip features are used to transfer the charge between conducting tip and MoS₂ nanosheets during contact electrification. The biased tip touching nucleation site pumps charges on the nanosheets, resulting in sharp change in contrast during dynamic electrostatic force microscope (EFM) scanning. A systematic analysis of charge transfer is performed by studying the phase and amplitude changes in EFM mode and surface potential change in Kelvin probe force microscope (KPFM) mode. Charge pumping is achieved by controlling tip bias, tip lift height, and scan rate. The charge density on the nanosheets is increased by raising the tip bias. On the contrary, the charge density decreases as the tip lift height is increased. The tip proximity to the nucleation site is used to control the transfer of charges pumped into the MoS₂ nanosheet. The sustenance of pumped charges in the nanosheet is important for potential applications of 2D materials. Charge diffusion between MoS₂ nanosheets and the Si/SiO₂ substrate after charge pumping is studied by monitoring the time-dependent phase shift in EFM and surface potential in KPFM modes, and charge decay times of 6.9 and 7.5 h, respectively, are estimated. Controlled charge pumping through contact electrification in MoS₂ can provide ample opportunity to build a new class of two-dimensional (2D) MoS₂-based tribotronic devices.

To improve the properties of MXenes and explore new applications, it is of paramount importance to study the etching mechanism, in addition to obtaining direct insight on the impact of different etchants on the structure, such as defects, and the properties, such as electrical conductivity. Despite its importance, this has been very challenging due to the atomic layer of A-elements being etched and the elaborateness required in investigating the inner structural changes of individual MAX particles. Here, we try to observe the etching behavior of Ti₃AlC₂ MAX phase at an atomic-scale and correlate the structural changes of Ti₃AlC₂ MAX phase and we suggest etching mechanism of atomic Al layers in MAX phase during MXene synthesis. For this purpose, we tried to observe etching state of atomic Al layers in etched MAX phases depending on Al etchants and etching times. At first, we confirmed etching state of Al layers after etching with LiF and HCl according to specific etching time (3, 6 and 24 hours). For all etchants, we show that etching starts from the edges and propagates along the basal planes, leading to the stepwise etching of Al layers. However, the etching mechanism is very different for the grain boundary depending on the types of etchants. HF and NH₄HF₂ etchants can break poly-crystals at their grain boundaries into single-crystals, while poly-crystal particles remain after LiF/HCl etching. Meanwhile, in earlier stage, etching of Al atoms occurred at relatively weak edge layers, which are exposed to etchant including inside of MAX phase like voids, in MAX particle. In these weak edge layers, one side boned Ti₃C₂-Al layers are existed dominantly, and they can have relatively lower bonding energy than both side bonded Al-Ti₃C₂-Al layers, which are in inside of MAX phase particle. Then, etching process are proceeded to inner layers and next layers sequentially from starting points. We expect that our results can open the ways to develop MXene synthesis methods for high quality and mass production, and give us possibility for tuning the properties of synthesized MXene to their proper application.

In nanomaterial synthesis, there is a tradeoff between scalability, material quality and uniformity. The tradeoff is especially evident in the case of two dimensional (2D) materials, which are layered materials with single or few atoms thick layers that are held together by weak van der Waals forces. Semiconducting transition metal dichalcogenides (TMDC) from the family of 2D materials offer opportunities including exploring strongly bound excitons and engineering highly deformable, high mobility electronics. However, progress in translating these materials from scientific curiosities to applications is largely limited by the availability of synthesis techniques that are both scalable and yet lead to high quality materials. A new approach gaining momentum is to use large area exfoliation through selective adhesion of bulk 2D materials to metal adlayers to get monolayers with the quality and lateral sizes similar to the starting bulk crystal. These techniques rely on the stronger adhesion of gold to the TMDC layers compared to the interlayer van der Waals adhesion in the bulk crystal[1-6].

In this work, we demonstrate a modified gold mediated technique for large area exfoliation of high quality TMDC monolayers over millimeter scales which leads to much higher yield and uniformity compared with previous approaches. With the higher uniformity from the exfoliation, we discover a surprising inhomogeneity in the quality of the starting bulk crystal at ~ 100 um length scales which was not evident through traditional exfoliation techniques.

Our technique adds an additional step by relaxing the gold adhesion layer when laminating onto the 2D bulk surface, leading to better conformation at the Au-TMDC interface. As a result, we were able exfoliate monolayers with lateral sizes up to 5 mm, limited only by the size of the starting bulk crystal. To make direct quantitative comparison, we exfoliated large monolayers of WSe₂ from the same bulk crystal using multiple large area exfoliation techniques. On average, the yield was 75% using our approach compared with 45% for other similar approaches. In addition, our approach leads to fewer cracks, higher reproducibility, and higher optical uniformity from other approaches.

Intriguingly, by comparing optical maps of sequential transfers from the same bulk crystal, we observe inhomogeneities in the optical response on the scale of 100 μm that are in the same relative positions on each monolayer. By comparing the photoluminescence peak positions and intensities, we relate these variations to discrete changes in the defect density or material doping present in the bulk crystals.

These results show that the uniformity of exfoliation is now limited not by the uniformity of transfer, but the quality of the starting material. The variations even within bulk crystals show that care must be taken when comparing properties based on defect density – highly pertinent to many device applications relevant to properties including doping, contact engineering, excitonic states, and mobility.

References:
[1] Liu, F., et al. Disassembling 2D van der Waals crystals into macroscopic monolayers and reassembling into artificial lattices. Science 2020, 367, 903–906
[2] Moon, J.Y., et al. Layer-engineered large-area exfoliation of graphene. Science Advances 2020, 6(44), eabc6601
[3] Desai, S.B., et al. Gold-mediated exfoliation of ultralarge optoelectronically-perfect monolayers. Advanced Materials 2016, 28(21), 4053-4058
[4] Huang, Y., et al. Universal mechanical exfoliation of large-area 2D crystals. Nature Communications 2020, 11(1), 1-9
[5] Velicky, M., et al. Mechanism of gold-assisted exfoliation of centimeter-sized transition-metal dichalcogenide monolayers. ACS Nano 2018, 12(10), 10463-10472
[6] Magda, G.Z., et al., Exfoliation of large-area transition metal chalcogenide single layers. Scientific Reports 2015, 5(1), 1-5

The experimental realization of 3D quasicrystals (QC) opened new pathways in materials science [1], due to their unusual structures and properties. QC are complex intermediate structures with long-range translational symmetry and forbidden rotational symmetry but no periodicity [1]. The concept of QC in lower dimensions is even more appealing. It was recently [2] reported that 2D quasicrystals can be realized using ultrasonication-assisted exfoliation techniques. Combining extensive experimental characterization techniques (XRD, XPS, AFM, SEM, and FEI) and ab initio (DFT) simulations [2] it was demonstrated that the threefold symmetry plane with the highest atomic density plane is the most stable configuration in comparison to twofold and fivefold planes in the icosahedral Al–Pd–Mn quasicrystals.
In this work [3], using similar approaches we show that 2D aluminum alloys can be obtained from 3D decagonal quasicrystals. We used the aluminum-based decagonal QC Al66Co17Cu17 alloy to extract the corresponding 2D alloy structure.
In order to gain further insights into these processes, we have carried out fully atomistic molecular dynamics simulations using the reactive force field embedded-atom method (EAM) [4] to address the structural stability of different 2D QCs and also the energetics of hydrogen evolution. Our results from MD simulations on the structural stability show that some proposed initial configurations converge to the structures with the decagonal symmetry domains. This suggests that the decagonal configurations are the more stable structures, which are consistent with the experimental data [3]. The 2D alloys were also combined with WS2 layers forming stacked heterostructures, which worked as a stable and efficient catalyst for hydrogen evolution reactions.

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Non-magnetic delafossite metals, such as PdCoO₂ and PtCoO₂, exhibit single-band, quasi-two dimensional Fermi surfaces (FS) which can host extremely long carrier mean free paths, despite their high carrier-density^1,2. This provides opportunities to investigate unconventional carrier transport where the intrinsic scattering mechanisms additionally compete with the micro-scale device's length-scale, resulting in finite-size effects such as the recently observed crystal symmetry-forbidden directional ``quasi-ballistic'' transport^3,4. Interestingly, the hexagonal FS of these delafossite metals deviates strongly from a circular FS - with pronounced facets, leading to an anisotropic Fermi velocity distribution^3,4.

Here, we parametrize the electronic 2D Boltzmann transport equation (BTE) to account for the in-plane FS anisotropy and include both momentum-relaxing and momentum-conserving interactions⁵. This allows us to investigate unconventional electronic transport in these systems by numerically solving the anisotropic BTE, across a range of momentum-relaxing and momentum-conserving interaction lengthscales, to create transport regime plots for each FS orientation. Using first principles calculations of the (momentum-relaxing) electron-phonon and (momentum-conserving) phonon-mediated electron-electron interactions, we populate the resulting transport regime plots and classify electronic transport in PdCoO₂ between the "quasi-diffusive'', "quasi-ballistic'', and "quasi-hydrodynamic'' transport limits as a function of temperature.

Consistent with experimental observations we find the directional dependence of electron flow, which gives rise to crystal symmetry-forbidden in-plane resistivity anisotropy, is strongest at low temperature in the "quasi-ballistic'' regime, i.e. in the absence of momentum-conserving interactions⁵. As momentum-conserving interactions increase, leading to collective flow which acts to homogenize the current distribution, the effect gets weaker in the "quasi-hydrodynamic'' regime identified between 10-25 K, albeit still expected to be measurable⁵. This work provides insights into understanding electron transport from microscopic scattering mechanisms and Fermi surface geometries, paving the way for designing finite-size devices from high carrier-density quasi two-dimensional metals.

¹ C.W. Hicks, A.S. Gibbs, A.P. Mackenzie, et al., Phys. Rev. Lett. 109, 116401 (2012)
² R. Daou, R. Fresard, V. Eyert and A. Maignan, Science and Technology of Advanced Materials 18, 919 (2017)
³ M.D. Bachmann, A.L. Sharpe, A.W. Barnard, et al., Nature Communications 10, 1 (2019)
⁴ M.D. Bachmann, A.L. Sharpe, A.W. Barnard, et al., arXiv:2103.01332 (2021)
⁵ G. Varnavides, Y. Wang, P.J.W. Moll, P. Anikeeva, and P. Narang, arXiv:2106.00697 (2021)

Recently, the first synthesis of a free-standing monolayer amorphous carbon (MAC) was reported [1]. MAC is a pure carbon structure composed of randomly distributed five, six, seven, and eight atom rings and has many structural differences from a random network. Its electronic properties are similar to boron nitride. Also, several nanostructures, such as nanotubes and nanoscrolls, build from MACs were found to be stable [2]. In this work, we have investigated, through fully atomistic reactive molecular dynamics simulations the structural and thermal MAC properties. Our results [3] show that MAC and graphene (G) sheets of similar dimensions have quite distinct mechanical behavior. In general, G exhibits one stage of the elastic regime, followed by abrupt fracture (brittle), while MAC exhibits several structural stages where the stress values drop, but with no abrupt fractures (ductile). These stages present structural reconstructions followed by the appearance of linear atomic chains (LAC) in the final fracture stages. LAC are commonly observed in fractured carbon-based nanostructures [4]. These extensive MAC reconstructions are possible due to the large distribution of bond lengths and bond angle values, and the presence of different types of rings, which creates multiple stress release channels. The G and MAC fracture dynamics are also different, while PG presents abrupt fractures and fast crack propagation, MAC exhibits more localized fractures that prevent fast crack propagation and works as efficient stress release mechanisms. MAC is softer than G, their corresponding Young’s modulus values are 881 and 587 GPa, respectively. MAC also exhibits remarkable thermal stability with a melting point (5368K) very close to that of G (5643K).

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Graphene's unique thermal and electrical conductivity has been known to falloff due to the loss of its pristine crystal structure, combined with the reduction of its grain boundaries/grain sizes. This property often limits graphene in achieving some of its highly desired properties as a transformative material system. We propose a novel method to mechanochemical modify graphene in a scalable way, such to adapt these surface structures for a superior surface conductivity. By selectively choosing single system and multisystem transition metal powders with in-situ developed graphene, we were able to create newly formed transition metal graphene-carbides. The kinetics of carbide formation were resolved using High Temperature Calorimetry, X-Ray Diffraction, and Raman Spectroscopy. Using Electron Microscopy, we were able to detect surface morphologies displaying our materials system’s ability to grow graphene on top of a graphene-carbide substructure. This work brings us closer to improving inexpensive graphene properties and further develop an ideal interconnected graphene network.

Ternary copper indium selenide (CIS), as a novel two-dimensional (2D) material, has a high external quantum efficiency and a sufficiently narrow bandgap (1.1 eV), making it a promising candidate for the development of ultra-thin and flexible current photodetectors. Since CIS can exhibit a variety of structures depending on the ratio between Cu and In, here, we investigate the optoelectronic response characteristics of the new 2D atomically layered ternary compound that was synthesized with the ratio between Cu and In was 1:7 (CuIn₇Se₁₁), consistent with the range where the layered phase forms. We discovered that, unlike conventional 2D materials which have the blue-shifted phenomena in photoconductivity spectra as the thickness decreases, the CuIn₇Se₁₁ shows an opposite trend of red-shift. This unique feature makes it an ideal candidate for a red light or near-infrared photodetector material. Additionally, we unveil a strong linear polarization dependence in its photoconductivity response, which also exhibits an intriguing relationship with the sample thickness and working temperature. This may indicate a phase transition occurs at low temperature and induced the variation in dipolar transition law. Our study will provide an alternative material system for the fabrication of highly-effective polarization-sensitive visible and near-infrared photodetector.

While the permanent porosity and crystalline structure of metal-organic frameworks (MOFs) has been exploited for gas sequestration and catalysis, recent attention has focused on the potential for MOFs to enable a new generation of sensors, actuators, and low-power consumption electronics. To do so, chemical design of MOFs and other molecular frameworks that exhibit stimuli-responsive properties is necessary. Previous work from the Kempa group demonstrated the gas-phase assembly of 2D MOFs comprised of redox-active, stimuli-responsive Mo₂(isonicotinate)₄ paddlewheels. Here we present and investigate the tunable properties of MOFs assembled from derivatives of this di-Mo paddlewheel. A notable example is comprised of Mo₂(isonicotinamide)₄ clusters, which we show can be assembled using solution- and gas-phase methods into MOFs with 2D and quasi-1D topologies. X-Ray diffraction (XRD) data collected on Mo₂(isonicotinate)₄ and on Mo₂(isonicotinamide)₄ MOFs at various partial pressures of CO₂ reveal the structural origins of interlayer gas uptake anomalies. These data suggest that the commensurately stacked 2D monolayers can dynamically and reversibly re-order to assume new phases in response to external stimuli. Device transport measurements can reveal not only the significant modulation of framework conductivity corresponding to the induced phase changes, but also the role of the metal-ligand coordination environment. These efforts lay the groundwork for implementation of these designer molecular frameworks as switches and dynamically tunable membranes.

Two dimensional materials are promising building blocks for various optoelectronic application such as ultra-high-mobility transistors, tunable p-n junction diodes, solar cells, light-emitting diodes and their unique properties have attracted attention. However, in the case of monolayer materials, strongly bound excitons generated by Coulomb interaction inhibit the formation of free carriers due to the absence of internal potential. Here, we demonstrate p-n junction phototransistor, which has a novel hybrid photonic crystal structure for improve the optical properties of the photo transistor in active area. To understand enhanced optical properties and optoelectronic interaction with our novel hybrid structure, we have performed density functional theory calculations as well as Raman spectroscopy, Photoluminescence (PL), Time-Resolved Photoluminescence (TR-PL) and measurement of electrical and optical transport properties for device characteristics. The characteristics of the heterojunction devices with PANS reveal enhanced photodetector figures-of-merit compared with pristine heterojunction devices. The phototransistor with novel hybrid photonic crystal structure shows dramatically enhanced photo current (A), responsivity (A/W) and detectivity (Jones). Therefore, it is important advantage in using our novel hybrid nanopores structure in optoelectronic application.

Confronted by the inherent physical limitations in scaling down Si technology, transition metal dichalcogenides (TMDCs) as alternatives are being tremendously researched and paid attention to. However, mature counter doping technology for TMDCs is still elusive, and thus, a controllable and reversible charge enhancer is adopted for acceptor (or donor)-like doping via octadecyltrichlorosilane (ODTS) (or poly-L-lysine (PLL)) treatment. Furthermore, multiple counter doping for TMDC field-effect transistors (FETs), combined with a threshold voltage (V_th) freezing scheme, renders the V_th modulation controllable, with negligible degradation and decent sustainability of FETs even after each treatment of a representative charge enhancer. In parallel, the counter doping mechanism is systematically investigated via photoluminescence spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy (AFM), surface energy characterization, and measurement of optoelectronic properties under illumination with light of various wavelength. More impressively, complementary inverters, composed of type-converted molybdenum ditelluride (MoTe₂) FETs and hetero-TMDC FETs in enhancement mode, are demonstrated via respective ODTS/PLL treatment. Herein, driving backplane application for micro-light-emitting diode (µ-LED) displays and physical validation of a corresponding counter doping scheme even for flexible polyethylene terephthalate (PET) substrates could be leveraged to relieve daunting challenges in the application of nanoscale Si-based 3D stacked systems, with potential adoption of ultralow power and monolithic optical interconnection technology.

Recently, transition metal dichalcogenide (TMDC) has gained tremendous interests in electronic/optoelectronic applications because it has prominent optical/electrical properties, a controllable bandgap, superior electrostatic gate coupling, and immunity to a short channel effect. Despite these outstanding merits, the study of emerging devices incorporated with TMDC thin film transistors (TFTs) and quantum dot light-emitting diodes (QLEDs) has been rarely reported. In this sense, TMDCs have one of the excellent candidates to realize the switching device for QLED displays in lieu of conventional Si technology. So, we adopted a molybdenum ditelluride (MoTe₂) as a switching component exhibiting a negligible Fermi-level pinning phenomenon, light insensitive properties, and the availability of type conversion. In this study, molecular doping by Poly-L-lysine (PLL) is adopted for a type conversion of MoTe₂ TFTs and surface ligand modification is utilized for the improvement of QLED performance. Based on these methodologies, we firstly demonstrate active-matrixed light-emitting devices, composed of ultra-thin MoTe₂ TFTs and highly efficient QLEDs. As a result, the driving capability of MoTe₂ TFTs for the operation of QLEDs display provides the successful and advanced applications as next generation display backplane transistors of QLEDs.

Two-dimensional (2D) materials, crystalline solids consisting of atomically thin sheets, have become the focus of intense research in the past decade¹. Incorporation of molecular motifs into covalent 2D networks offers immense potential for the systematic design of new structures². Fullerenes (C₆₀), a spheroidal carbon cage, can be polymerized by solid state synthesis under high-pressure, photoirradiation, or by reaction with alkali and alkaline earth metals at elevated temperatures³. Varying the polymerization conditions leads to various fullerene polymer topologies and stoichiometries, however, nearly all known synthetic routes produce weakly polycrystalline powders due to the uncontrolled nature of these polymerizations³.
Recently, we have synthesized a previously unknown covalent 2D fullerene-based polymer, Mg₄C₆₀, by the reduction-driven covalent cross-linking of C₆₀ using solid state reaction with Mg. Our synthesis yields semiconducting crystals of sizes sufficient for precise crystal structure determination, exfoliation and device fabrication. The 2D structure of these crystals, bridges the gap between molecular carbon structures and extended carbon materials.


References
1. Novoselov, K.; Mishchenko, o. A.; Carvalho, o. A.; Neto, A. C., 2D materials and van der Waals heterostructures. Science 2016, 353 (6298), aac9439-1-aac9439-11.
2. Bisbey, R. P.; Dichtel, W. R., Covalent organic frameworks as a platform for multidimensional polymerization. ACS Cent. Sci. 2017, 3 (6), 533-543.
3. Forro, L.; Mihaly, L., Electronic properties of doped fullerenes. Rep. Prog. Phys. 2001, 64 (5), 649-699.

MXenes are a family of two-dimensional (2D) nanomaterial. Titanium Carbide MXene (Ti₃C₂-MXene) is the first inorganic compound discovered among the MXene family in 2011. Ti₃C₂-MXenes have been studied extensively for their physical properties and chemical stability. There have also been many applications of these MXenes for sensing applications. In this present work, we discuss the response of Ti₃C₂-MXene nanomaterials to low-temperature. The Ti₃C₂-MXene samples are tested for their change in electrical resistivity for a wide range of temperatures (3K – 300 K). The unique behaviors observed in Ti₃C₂-MXene as well as their precursor MAX (Ti₃AlC₂-MAX) phase material are presented. Then the magnetoresistance behavior of Ti₃C₂-MXene and Ti₃C₂-MAX phase materials are studied at various temperatures leading to their analysis and reporting. These unique behaviors of Ti₃C₂-MXenes have the capability for sensor development and thus extensive repeatability tests are conducted. The sensing applications of Ti₃C₂-MXene at low temperatures with their magnetoresistance behavior opens avenues for the application of these sensors for space technology and explorations. Finally, the possibilities of various sensors using the magnetoresistance behavior are envisaged.

The real use of graphene in electronic devices, such as field-effect transistors (FET), meets several principal complications. Opening of graphene band gap usually leads to significant drop of electron mobility; limitation of Klein tunneling paradox can prevent graphene-based devices from switching into the OFF state. Some of the complications can be overcome by combining graphene with insulating layers in two-dimensional heterostructures that allow for fabrication of a graphene-based FET with a high ON and OFF switching ratio ¹, and precise control over graphene doping. However, fabrication of such heterostructures is not trivial. The best performing heterostructures are made by manual assembly, which is not very efficient method for their mass production.
We have prepared two-dimensional heterostructures more efficiently, by growing graphene from a nitrogen-carbon precursor inside the confined space of a layered silicate. This approach promises simultaneous band-gap opening in graphene and formation of final heterostructures in one step. The resulting structure has the form of a multi-layered sandwich composed of nitrogen-doped (N-doped) graphene-like layers and the single layer sheets of synthetic layered silicate, sodium fluorohectorite.
Can such a large-scale production method generate well-defined and homogeneous clean system useable for FET or other electronic applications of graphene? To answer this question, we need to examine the layered heterostructure itself, to reveal the nature of the N-doped graphene-like layers. As the initial point, it is of high benefit to know how are the nitrogen atoms distributed within the graphene lattice. Electronic transport properties of sandwich multi-layers, which are important parameters for device performance, can be then explained in the context of geometry of individual layers.
Nitrogen doping of graphene is the efficient way of band-gap modification, as it allows band-gap opening and its transformation to an n or a p-type semiconductor ². Nitrogen can be incorporated into the graphene lattice in three different configurations ^2,3, with planar sp² (pyridinic and pyrrolic) and tetrahedral sp³ (quaternary N) hybridizations. Depending on the level of the sp²-N doping and the geometry of the N-doped graphene lattice, band gap can vary from 0.14 eV up to 0.7 eV⁴. Characteristic C-C, N-C and N-N bond lengths depend on the lattice configuration and differ from the un-doped graphene^2,4.
Growth of the N-doped graphene in confined space ³ is a promising method superior to the other preparation strategies, as it provides selective formation of pyridinic and pyrrolic groups in the graphene lattice and suppress formation of quaternary N. Carbon lattice doped with planar N-sites can serve as useful building block for use in the supercapacitors, batteries, transistors, fuel cells and other semiconductor applications² , and also possess high activity in the oxidation reduction reactions ³ in contrast to the lattice containing quaternary N. In all cases, types of nitrogen doping, lattice geometry and defects together with the lattice distortions significantly alter both properties, performance and the suitable application potential of the N-doped graphene². Its combination with layered silicate is a promising step towards the real applications.

References.
1. Britnell, L., Gorbachev, R. V. Jalil, R. Science 286, 947–951 (2012). 2. Wang, H., Maiyalagan, T. & Wang, X. ACS Catal. 2, 781–794 (2012). 3. Ding, W. et al. Angew. Chemie - Int. Ed. 52, 11755–11759 (2013). 4. Rani, P. & Jindal, V. K. RSC Adv. 3, 802–812 (2013). 5. Breu, J., Seidl, W., Stoll, A. J., Lange, K. G. & Probst, T. U. Chem. Mater. 13, 4213–4220 (2001). 6. Rojas, W. Y. et al. Langmuir 34, 1783-94 (2017). 7. Chuang, C.-H. et al. Sci. Rep. 7, 42235 (2017). 8. Ehlert, C. et al. Phys. Chem. Chem. Phys. 16, 14083–95 (2014).

Transition metal dichalcogenide monolayers are atomically thin semiconductors that can host tightly bound excitons. Recent advances in materials growth and fabrication have enabled the preparation of high-quality van der Waals heterostructures incorporating these two-dimensional materials. In this presentation, I will describe our efforts to use these heterostructures as a “canvas” to realize new quantum optoelectronic devices for excitons and electrons. I will discuss how we improve the spectral/spatial uniformity and coherence of excitons and realize atomically thin mirrors and active metasurfaces. I will also describe our recent observation of long-sought electron Wigner crystal phases in these heterostructures without a magnetic field or moiré potential. Our studies illustrate that the heterostructures made of atomically thin semiconductors are an attractive solid-state platform for exploring novel excitonic and correlate-electron phenomena.

Low dimensional systems, such as monolayer TMDCs, host strong non-hydrogenic Rydberg-series excitonic resonances that dominate the optical spectrum. Here, we investigated the dynamical radiative properties of the excitonic Rydberg series-driven photoluminescence in monolayer MoTe₂. High quality hBN-encapsulated MoTe₂ monolayer heterostructures with electrical contacts and back gate, which enabled modulation of the doping density in MoTe₂, were fabricated to perform low temperature (~4.5K) photoluminescence measurements. We observed bright, narrow emission from the first three states of the excitonic Rydberg series, namely A1s, 2s and 3s with record linewidths of ~3, 6 and 10 meV, respectively. All these states exhibit linear scaling with incident excitation pump fluence. Upon injecting excess free charges on either electron or hole side in MoTe₂, dipole oscillator strengths were rapidly transferred from the excitonic to the corresponding trionic (charged exciton/attractive polaron) states. Energy shifts between the neutral and dressed excitonic states were observed as a function of gate voltage, which strongly depend on the Rydberg state of the exciton – indicating strong excitonic wavefunction dependent screening properties and electron-exciton interaction. Our findings provide a route to realize exotic excitonic phases in monolayer or twisted bilayer MoTe₂.

Dynamic control of the optical scattering properties in resonant nanostructures is central to the realization of active nanophotonics, but concepts thus far have primarily used dielectric or plasmonic Mie resonances which have limited electrical tunability. Here, we report over 200% modulation in both the real and imaginary part of the complex refractive index in hBN-encapsulated monolayer molybdenum diselenide (MoSe₂) by tuning the exciton resonances. Gate modulated control of the Fermi level allows us to explore the doping dependence of the A and B exciton resonances for temperatures between 4 K to 150 K. We observe both a temperature and carrier density dependent variation in the permittivity and transition from metallic to dielectric response in the epsilon-near-zero wavelength range. We attribute the change in the refractive index to the interplay between radiative and nonradiative channels that are dependent on carrier density and temperature. Our findings of large electrical tunability of the complex refractive index is accompanied by large changes in reflectance amplitude and phase, suggesting the potential of monolayer MoSe₂ as an active material for emerging photonics applications.

In this work we investigate theoretically and experimentally light guiding in nanometer-scale waveguides made of transition metal dichalcogenides (TMDCs). We explore the limits of light guiding and show that in certain configurations owing to unique material properties light can be squeezed and guided in nanometer thick channels. In our experimental effort we discuss the fabrication approach and the preliminary measurements examining performance of such waveguides.
TMDCs are an emerging type of layered van der Waals semiconductors possessing unique electronic and optical properties [1,2], which make them promising candidates for photonic applications. Specifically, bulk TMDCs are of great interest for a range of optical applications, including metasurfaces [3], gratings [4], and nano-resonators [5,]. Recently promise of bulk TMDCS for integrated nanophotonics [6] was also explored. Here we show that owing to their unique optical properties, including high refractive index, room temperature excitonic resonance and strong anisotropy, bulk TMDCs are capable of guiding light in a deeply subwavelength regime.
We begin our analysis with a study of dispersion relations and influence of materials anisotropy on optical field confinements in slab waveguides. We show that owing to high refractive index light can be efficiently guided in 120 nm thick TMDC flakes. We then examine the limits of light guidance in structures made of bulk TMDCs, especially in nanometer-size void waveguides, which are formed within a bulk of MoS₂ material. In such systems we find that the light can be efficiently confined in very narrow air channels, down to even sub-nanometer scale. We show that by selecting appropriate waveguide dimensions over 75% of the power can be squeezed into a waveguide with less than 35 nm air gap across a broad spectral range (visible to near infrared). We then study how material anisotropy influences the light confinement and guidance and show that interlayer voids provide a simpler approach to device fabrication and experimental study of the phenomenon. Finally we discuss the influence of losses and show that such waveguides can be low loss in the sub-band gap regime. Our theoretical findings are of direct interest to highly confined optical interconnects and to light materials coupling in atomic size channels of 2D materials.
Lastly, we discuss pathways for experimental demonstration of light guiding in such nanometer scale devices. We discuss our approach to fabricating such nanoscale waveguides, in which we have optimized processes to obtaining structures made of ~100 nm MoS₂. Next we discuss design and fabrication of grating coupler excitation of bulk MoS₂ waveguides. We then perform far-field measurements in 800 nm-1100 nm band that indicate that light can be efficiently guided in such subwavelength structures.
In conclusion, our work paves the way to a new type of highly confined low loss optical waveguides, which will find use in a broad range of applications including among others integrated photonics, quantum optics and sensing.

References
1. Wang, Qing Hua, et al. "Electronics and optoelectronics of two-dimensional transition metal dichalcogenides." Nature nanotechnology 7.11 (2012): 699-712.
2. Mak, Kin Fai, and Jie Shan. "Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides." Nature Photonics 10.4 (2016): 216-226.
3. Liu, Chang-Hua, et al. "Ultrathin van der Waals metalenses." Nano letters 18.11 (2018): 6961-6966.
4. Zhang, Huiqin, et al. "Hybrid exciton-plasmon-polaritons in van der Waals semiconductor gratings." Nature communications 11.1 (2020): 1-9.
5. Verre, Ruggero, et al. "Transition metal dichalcogenide nanodisks as high-index dielectric Mie nanoresonators." Nature nanotechnology 14.7 (2019): 679-683.
6. Ling, Haonan, Renjie Li, and Artur R. Davoyan. "All van der Waals Integrated Nanophotonics with Bulk Transition Metal Dichalcogenides." ACS Photonics 8.3 (2021): 721-730.

Nanophotonics is expected to play a key role in the next decades. The miniaturization and integration of these components has, however, an additional challenge with respect to nanoelectronics devices, namely the diffraction limit. If dielectric materials are used, light cannot be confined and guided if the size of the waveguides and components is below a critical size which is determined by the wavelength and the refractive index of the used dielectric material.
However, this limit can be circumvented if materials supporting polaritons are used instead of dielectrics. Polaritons consist in a strong interaction between light and a dipole-carrying excitation (e.g. plasmons, phonons and excitons).
Phonon polaritons in van der Waals (vdW) materials such as hexagonal boron nitride (h-BN) and molybdenum trioxide (MoO3) have been extensively studied with techniques including near-field scanning optical microscopy (NSOM) and are considered as particularly good candidates for miniaturized photonics in the mid-infrared [1-6].
In this contribution we will first present some recent results from our group. We first demonstrate high quality factor phonon polaritons resonators (with Q reaching almost 500) based on isotopically pure h-BN and MoO3 [6]. Compared to previous devices based on non-isotopically-pure hBN [3], these resonators can achieve a Q factor up to 3 time larger due to the intrinsic higher quality of the phonon in the material [4]. The resonators (arrays of disks with thickness of tens of nanometers and diameters of hundreds of nanometers) have been fabricated with electron beam lithography followed by reactive ion etching of the vdW materials. Using FTIR spectroscopy and NSOM we observed and studied the resonance in both the restrahlen bands for hBN and in all 3 restrahlen bands in MoO3, finding high quality factors in all hBN bands and in the third band of MoO3. Strikingly, the resonance is maintained even if the volume of the resonator is 10^6 times smaller than the cube of the wavelength in free space, and thanks to the hyperbolic properties of hBN and MoO3 it can be pushed further down.
We then determined a theoretical upper bound on the quality factor of the resonators working for polaritonic resonators in the sub-wavelength limit. Through rigorous manipulation of Maxwell's equations, we found that the linewidth of the resonator can never be smaller than the intrinsic linewidth of the polaritonic resonance.
Second, we will show flat reconfigurable photonic devices based on a heterostructure of isotopically pure hBN deposited on a thin layer of the phase-change material GeSbTe[5]. With a laser diode we can induce crystallization of GST ("write" process) and we can restore the amorphous state ("erase" process). Because the crystalline form has much larger refractive index, we can induce waveguides for hBN phonon polaritons via substrate effects. We demonstrated fully programmable and reprogrammable lenses, metalenses and prisms for bidimensional polaritons. We also created guided wave resonators with periodic substrate patterning, which are akin to photonic crystals but with phonon polaritons.
Finally, we will discuss applications of these devices for bio-chemical sensing and photodetectors.

[1] D. N. Basov et al, "Polaritons in van der Waals materials." Science 354.6309 (2016).
[2] T. Low, et al. "Polaritons in layered two-dimensional materials." Nature materials 16.2, 182-194 (2017).
[3] M. Tamagnone, et al. "Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures." Science advances 4.6, eaat7189 (2018).
[4] A. J. Giles et al. "Ultralow-loss polaritons in isotopically pure boron nitride." Nature Materials 17.2, 134-139 (2018).
[5] K. Chaudhary et al. "Polariton nanophotonics using phase-change materials." Nature Communications 10.1, 4487 (2019).
[6] M. Tamagnone et al. "High quality factor polariton resonators using van der Waals materials." arXiv preprint 1905.02177 (2019).

Atomically-thin van der Waals semiconductor materials have attracted much attention because of their superior and unique material properties. However, the precise control of carrier doping levels is still a challenge due to their intrinsic high surface-to-volume ratio [1]. In fact, MoS₂ can be p-doped in bulk by Nb doping but show p- to n-type transition as the number of layers decreases [2]. In addition, the performance of the devices using these materials is often affected by a semiconductor-substrate interface and the doping levels may also be changed through various treatments in device process.
Imaging microscopic carrier distributions will help the understanding the mechanism of the doping level changes. As a key imaging technology, here we demonstrate that scanning nonlinear dielectric microscopy (SNDM) permits the imaging of dominant carrier distribution of atomically-thin layers [3]. SNDM normally measures the voltage derivative of local capacitance (dC/dV) below the tip. SNDM imaging is useful because dC/dV images reflect the dominant carrier distributions. The polarity of dC/dV is inverted depending on the polarity of dominant carriers and its magnitude varies with dominant carrier concentration. It is also notable that, because of capacitance sensing using near-field microwave electric fields, this microscopy can image atomically-thin materials on a substrate without preparing contact electrodes to the sample or fabricating test device structures.
Here we report the details of dominant carrier distributions in Nb-doped MoS₂ layers mechanically exfoliated on a SiO₂/Si substrate [4]. The sample had a bulk p-doping level of 5×10¹⁹ /cm³. By dC/dV imaging of a sample with various thickness, we visualized p- to n-type transition of Nb-doped MoS₂. As expected, thick layers were actually p-doped but we found that, as the layer number (L) decreased, hole concentration became lower. In more detail, for layers with L > 6, dC/dV signals did not depend on L significantly, while for L ≤ 6, they significantly varied with the decrease of L. The signals became zero at L = 3 once, the polarity was finally inverted for L = 2, 1. This implies that a counter n-doping effect called surface electron accumulation [5] manifested for L ≤ 6 and dominated the doping levels for L = 1 and 2. We could estimate surface level to be 1×10¹³ /cm². In addition, from our analysis, we suggest that a large part of p-type carriers supplied from Nb accepters were trapped at the interface states. The density of trapped carriers were estimated to be on the order of 10¹² /cm². These results are actually consistent with the previous literatures [1, 2]. Furthermore, we also examined the impact of ultraviolet-ozone (UV/O₃) treatment on p-doping level. UV/O₃ treatment can be used prior to the growth of gate dielectric films to make a nucleation layer on the dangling bond-free materials [5]. We performed UV/O₃ treatment of the sample in air (~10 mW/cm², 185 nm and 254 nm). We found that the signal polarity was inverted even on the thicker layers after the treatment. This shows that UV/O₃ treatment has n-type doping effect, which is strong enough to cause p- to n-type transition even on the thicker layers. The results here demonstrate that SNDM provides microscopic key information for understanding and controlling the doping levels in atomically-thin van der Waals semiconductors.
References:
[1] M. D. Siao et al., Nature Commun. 9, 1442 (2018).
[2] N. Fang et al., Adv. Funct. Mater. 29, 1904465 (2019).
[3] Y. Cho, Scanning Nonlinear Dielectric Microscopy: Investigation of Ferroelectric, Dielectric, and Semiconductor Materials and Devices. Elsevier, 2020, ISBN 9780128172469
[4] K. Yamasue and Y. Cho, J. Appl. Phys. 128, 074301 (2020).
[5] Azcatl et al., Appl. Phys. Lett. 104, 111601 (2014).

Two dimensional (2D) materials with magnetic properties open exciting new design opportunities for nanoscale magneto-electric devices. The layered magnet CrSBr is particularly interesting because of its stability in air, giving this material a key advantage for device integration compared to other 2D magnets such as CrI₃. Since practical devices based on 2D materials often involve contacting, patterning or modifying the layers, it is important to develop methods for controlling the structure and properties of CrSBr at the local, nanoscale level. Here we discuss strategies for achieving this goal, making use of approaches that have proved promising for other 2D materials. We first discuss local control of structure. We find that electron beam irradiation in a scanning transmission electron microscope induces a surprising structural change, where certain atoms migrate into the van der Waals gap to create a new phase with layer direction (and, in theory, magnetization) perpendicular to the initial layers. The ability to modulate the magnetization direction deterministically is of great interest for device applications. We also study local chemical modulation in solid-solution alloys such as CrSCl_xBr_1-x. In these materials we investigate the distribution of the alloying elements, aiming to understand magnetism and phase transformations in such tailored 2D magnets. We finally discuss strategies for contacts, focusing on the opportunities for epitaxial growth of metals and other 3D crystals onto the CrSBr surface. We have developed an approach to grow single crystal or heterostructured metals on graphene, hBN and transition metal dichalcogenides; the 2D/3D contact resistance is known to improve for larger crystals with fewer grain boundaries. We will explore how nucleation and epitaxy phenomena play out for pristine and patterned CrSBr with its distinctive crystal symmetry. We believe that atomistic level structural and chemical modification and control are crucial for understanding properties and designing novel low-dimensional magnetic devices that use these fascinating materials.

Layered two-dimensional (2D) magnetic materials have intriguing low-dimensional magnetic physics and promising applications in spintronic and magneto-electronic devices. CrI₃ has emerged as a well-studied 2D magnet. [1] However, its poor stability in ambient conditions has been a significant hindrance towards its integration in devices, heterostructures, and further experiments. In contrast, the layered van der Waals antiferromagnet CrSBr [2] is suggested to be air-stable in the mono- and few-layer limit, making it a promising alternative for magneto-transport and magneto-optical studies and device applications. [3, 4] In addition, CrSBr is a semiconductor with a bandgap of ~1.6 eV, a Néel temperature of 145 K, and a strong light-matter interaction.
In this work, we investigate the crystal stability and impact of defects on mono-, bi- and few-layer CrSBr using a combination of Raman spectroscopy, scanning transmission electron microscopy (STEM) and helium ion irradiation in a helium ion microscope (HIM). We study two types of samples: (i) CrSBr exfoliated via the scotch tape method and stored at ambient conditions over intervals up to months, and (ii) CrSBr exfoliated and irradiated with helium ions. We find that in pristine, mono-, and few-layer CrSBr, Raman peak shifts are a direct fingerprint of the layer number. By varying the excitation laser energy, we additionally observe resonant Raman modes due to enhanced electron-phonon coupling. Studying Raman line shapes of mono-, bi-, and few-layer CrSBr over long time periods, we find that this material is stable under ambient conditions, with only slight changes in peak shape over time. We also compare CrSBr flakes before and after annealing. At the monolayer level, changes between the annealed and unannealed spectra are most pronounced. This agrees with STEM images of annealed monolayers that display significant degradation and defect formation, whereas bilayers appear to be stable. To better understand the influence of defects on the vibrational properties, we intentionally introduce defects through controlled helium ion irradiation. With increasing helium ion dose, Raman peaks shift and broaden significantly, suggesting increased defect concentration and phonon confinement, as further manifested by our STEM studies. We further corroborate our results by ab-initio calculations of the phonon dispersion. Overall, we demonstrate how Raman spectroscopy is useful towards assessing the stability of CrSBr and its suitability in device applications.

Atomically thin or two-dimensional (2D) materials can be assembled into bespoke heterostructures to produce some extraordinary physical phenomena. Likewise, these highly manipulatable materials are useful platforms for exploring fundamental questions of interfacial chemical/electrochemical reactivity. In this talk, I will discuss how spontaneous mechanical deformations (atomic reconstruction) and resultant intralayer strain fields at twisted bilayer graphene have been quantitatively imaged using Bragg interferometry, based on four-dimensional scanning transmission electron microscopy, and the impact of these mechanical deformations to the electronic band structure of these moiré superlattices. The talk will then explore how various degrees of freedom that are unique to 2D materials may be used to tailor interfacial charge transfer at well defined mesoscopic electrodes and the outlook for new paradigms of functional materials for energy conversion and low-power electronic devices.

Presented by Nadire Nayir
Mechanical Engineering, The Pennsylvania State University, State College, PA, United States.
2D Crystal Consortium, The Pennsylvania State University, State college, PA, United States.
Physics, Karamanoglu Mehmetbey University, Karaman, Turkey.

We introduce new reactive potentials (ReaxFF)^1,2,3,4 for large-scale molecular dynamics simulations of the synthesis, processing, and characterization of two-dimensional transition metal dichalcogenides (MoS₂, WSe₂, MoSe₂, and WS₂), guided by an extensive quantum mechanical dataset on both periodic and non-periodic systems and validated against HAADF/STEM experiments. These new potentials are designed to capture the most essential features of TMD thin films and elucidate the structural transition from metallic to semiconducting phases, the energetics of various defects, and the Se-vacancy migration barrier. The new ReaxFF descriptions provide valuable insights into nucleation of the 1T-phase on the 2H basal plane or edges, rotational and translational grain boundaries, and the coupled effect of chemical potential and edge stability on the formation of S- and W-oriented grain boundaries. Since controllable edge-mediated growth kinetics of 2D-MoSe₂ are of great interest to the 2D community, these potentials are trained further against the edge formation energies of MoSe₂ nanoribbons with different Se coverages, we believe they will be a powerful complementary tool to experimental studies by simulating the edge-growth kinetics of 2D-MoSe₂ at high speed and low cost.

[1] Ostadhossein, A.; Rahnamoun, A.; Wang, Y.; Zhao, P.; Zhang, S.; Crespi, V. H.; van Duin, A. C. T. ReaxFF Reactive Force-Field Study of Molybdenum Disulfide (MoS2). J. Phys. Chem. Lett. 2017, 8 (3), 631–640. https://doi.org/10.1021/acs.jpclett.6b02902.
[2] N. Nayir, Y. Wang, S. Shabnam, D.R. Hi ckey, L. Miao, X. Zhang, S. Bachu, N. Alem, J. Redwing, V.H. Cr espi, A.C. T. van Duin, Modeling for structural engineering and synthesis of two-dimensional WSe2 using a newly developed ReaxFF reactive force field, J. Phys. Chem. C. 124 (51) (2020) 28285–28297, https: //doi.org/10.1021/acs. jpcc.0c09155.
[3] N. Nayir; Y. Wang; Y. Ji; T. H. Choudhury; J. M. Redwing; L.-Q. Chen; V. H. Crespi; A. C. T. van Duin. Theoretical Modeling of Edge-ControlledGrowth Kinetics and Structural Engineering of 2D-MoSe2. MatSciEng B Vol. 271, Sep. 2021, 115263, DOI:10.1016/j.mseb.2021.115263
[4] N. Nayir; Y. Shin; Y. Wang; M. Sengul; D. R. Hickey; M. Chubarow; T. Choudhury; N. Alem; J. Redwing; V. Crespi; A. C. T. van Duin, Development of a ReaxFF Force Field for 2D-WS 2 and Its Interaction with Sapphire (under review at JPCC).

MoTe₂ is a 2D transition metal dichalcogenide with small energy differences between its polymorphs, allowing access to a wide range of electronic states including the room-temperature semiconducting 2H state and semimetallic 1T′ state, and low-temperature topological T_d state. Due to this, MoTe₂ is a model material for studying phase transformations and structure-property relationships and has potential applications in low-power, non-volatile phase change memory.
These phase changes can be induced through application of strain and charge doping, and are affected greatly by the choice of substrate. Thus, understanding the interfacial effects between MoTe₂ films and substrate is important for selective synthesis and control over different crystalline phases of MoTe₂. Further, while there are many growth studies to investigate the effects of substrate for 2D sulfides and selenides, there is a lack of fundamental understanding of the growth mechanisms for the tellurides.
Here, we examined the effect of the substrate on the morphology and crystalline phase of MoTe₂ by tracking the nucleation and growth of MoTe₂ on different substrates (CrystEngComm 2021, doi:10.1039/d1ce00275a). We grew monolayer MoTe₂ by converting MoO_x thin films deposited on three different substrates: sapphire Al₂O₃ (0001), amorphous SiO₂, and amorphous AlO_x, following the synthesis method we developed for large-area MoTe₂ thin films (ACS Nano 2021, 15, 1, 410–418). We systematically examined the early stages of the conversion reaction occurring on the three substrates to elucidate the substrate effects on the nucleation of MoTe₂. We observe that the chemical composition of the substrate is more important than the surface topography and crystallinity of the substrate, with high quality monolayer 2H MoTe₂ formed on both Al₂O₃ (0001) and AlO_x in contrast to mixed phase 2H/1T′ MoTe₂ formed on SiO₂, as determined by Raman spectroscopy, x-ray photoelectron spectroscopy, cross-sectional transmission electron microscopy, and atomic force microscopy. We attribute these results to increased wetting between Te or Mo atoms on AlO_x vs. SiO₂, which allows for improved lateral mobility and reconfiguration. Our work highlights the role of chemistry regarding substrate interactions during the synthesis of 2D materials and particularly the diffusion of metal species on different oxide films.

Monolayers of transition metal dichalcogenides (TMDs) are promising materials for electronic, optoelectronic, spintronic and valleytronic devices. Furthermore, wafer-scale epitaxial growth of TMD monolayers has been demonstrated^1,2; however, fundamental understanding of these processes is limited. In this work, we demonstrate and compare the epitaxial growth of MoS₂, WS₂ and WSe₂ on 2” c-plane sapphire wafers using metal-organic chemical vapor deposition (MOCVD). Heat and mass transport within the reactor was modelled with computational fluid dynamics (CFD) to help predict and understand the effect of process parameters such as substrate rotation on the growth rate and uniformity of the TMD films. The films displayed uniform thickness and topography across the wafer, as measured via atomic force microscopy. X-ray diffraction measurements confirmed the films were epitaxial with respect to the sapphire substrate, while dark field transmission electron microscopy was used to image microstructural features such as translational and anti-phase grain boundaries in the coalesced monolayers. Different excitonic species such as trions and defect-bound excitons were observed with variable-temperature photoluminescence (PL) spectroscopy, while optical properties measured by spectroscopic ellipsometry closely matched predictions from theoretical models. The electronic properties of the films were measured using FETs, with the sulfide and selenide films having carrier mobilities of ~20 cm²/Vs and ~2.5 cm²/Vs, respectively. The lower carrier mobility in the WSe₂ films correlates with defect-related exciton emission which was observed in the PL spectrum.

We also systematically studied how the transition metal and chalcogen species affected the growth of the monolayers. For example, under similar growth conditions, WSe₂ exhibited larger domains and lower nucleation densities compared to WS₂ films, which suggests that surface diffusivity is higher on sapphire under a selenium ambient compared to sulfur. For W-containing TMDs, a three-step nucleation-ripening-lateral growth process was more effective in achieving uniform domain size compared to a single-step growth process. In contrast, no such difference was observed for MoS₂; several possible causes are currently under investigation. Understanding these trends will prove useful in the rational selection of growth parameters for TMD films.

(1) Zhang, X.; Choudhury, T. H.; Chubarov, M.; Xiang, Y.; Jariwala, B.; Zhang, F.; Alem, N.; Wang, G.-C.; Robinson, J. A.; Redwing, J. M. Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire. Nano Lett. 2018, 18 (2), 1049–1056.
(2) Chubarov, M.; Choudhury, T. H.; Hickey, D. R.; Bachu, S.; Zhang, T.; Sebastian, A.; Bansal, A.; Zhu, H.; Trainor, N.; Das, S. Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire. ACS Nano 2021, 15 (2), 2532–2541.

Among transition metal dichalcogenides (TMDs), molybdenum ditelluride (MoTe₂) shows significant attention due to its polymorphic nature from semiconductor to topological insulator and Weyl-semimetal. To date, considerable efforts have been devoted for synthesizing large area MoTe₂ suitable for the fabrication of devices to be integrated in different applications. As compared to other synthesis methods, chemical vapor deposition (CVD) is a facile and promising way for synthesizing large scale MoTe₂ nanosheets.
Here we report the CVD growth and controlled phase tuning strategies for obtaining 1T’(metallic) and 2H(semiconducting) molybdenum ditelluride (MoTe₂) nanosheets via two ways: 1) co-deposition of tellurium and MoO₃ powder precursors and 2) tellurization of pre-deposited Mo thin film. In first case, we employed CVD through powder precursors targeting large area growth of thin MoTe₂ nanosheets with controlled allotropic 1T’ and 2H phases. We investigated how the local tellurium vapor concentrations determines the MoTe₂ phase (2H and 1T′) depending on the flow rate of the Ar carrier gas and with the insertion of a physical barrier in the reaction chamber. The effect of the physical barrier is also reflected in the shape of the crystallite population and in their log-normal areal distribution pointing out to a homogeneous nucleation mode of the MoTe₂ crystals [1]. On the other hand, in tellurization we show a detailed study of the heterogeneous vapor-solid reaction between an e-beam pre-deposited molybdenum film and tellurium vapor, thus yielding a large area growth of MoTe₂ onto SiO₂/Si substrate [2]. MoTe₂ growth is mainly influenced by involved kinetics, tellurium concentration and geometry configurations inside the CVD furnace. As grown MoTe₂ nanosheets were characterized by several techniques such as Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). This study is a crucial step for enabling 1T’ and 2H-MoTe₂ integration in numerous potential applications in novel micro- and nano-electronics, quantum technologies, integrated photonics, and thermoelectric devices.

References:
C. Martella, Alessio Quadrelli, Pinaka Pani Tummala, Cristina Lenardi, Roberto Mantovan, Alessio Lamperti, Alessandro Molle, Cryst. Growth Des. 2021, 21, 5, 2970–2976.
Jin Cheol Park, Seok Joon Yun, Hyun Kim, Ji-Hoon Park ,Sang Hoon Chae, Sung-Jin An, Jeong-Gyun Kim, Soo Min Kim, Ki Kang Kim, Young Hee Lee, ACS Nano 2015, 9, 6, 6548–6554

Abstract
In this study, a newly-proposed oxide scale sublimation chemical vapor deposition (OSSCVD) technique is demonstrated for the controlled growth of two-dimensional (2D) monolayer MoS₂ crystals. With the new reactor configuration, MoO₃, which is generated in situ by flowing O₂ over Mo metal surface, can be sublimated by heating and supplied separately from the H₂S source in a controlled manner through mass flow controllers and pneumatic switching valves. A series of proof-of-concept growth experiments with varied gas flow rate and growth time was carried out using high-temperature resistant alkali-aluminosilicate glass, Dragontrail (DT-glass), as catalytic substrate. We confirmed that OSSCVD enables the growth of MoS₂ with significantly improved flexibility, controllability, and reproducibility that are absent in the conventional powder-source CVD. As a result, single-crystalline MoS₂ triangular domains as large as 25 µm was observed reproducibly, followed by the fully-covered continuous MoS₂ layer within 25 minutes. Photoluminescence and Raman spectroscopies revealed that both the achieved domains and the continuous film possess monolayer MoS₂ features and good optical quality. Meanwhile, the characteristics of fabricated field effect transistors showed maximum electron mobility of 13 cm²/Vs, indicating a good electrical property as well. Utilizing DT-glass as substrate, the MoS₂ growth process is strongly affected by suppressed nucleation density and enhanced lateral growth, probably due to the catalytic effect of sodium and/or potassium in the glass. Particularly, we found that before growth in situ DT-glass substrate treatment by H₂S enables a remarkably enlarged domain size with reduced domain density. Intriguingly, these growth improvements might be primarily related to the alkali metal elements extracted from DT-glass to the reactor inner walls rather than the modification on the glass surface during the H₂S pretreatment process. The results achieved in this work open new avenues for growing high-quality MoS₂ and other 2D transition metal dichalcogenides using the simple OSSCVD and the cost-effective catalytic glass substrate. More details will be presented at the conference.

Acknowledgments
This work was partly supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan, and JSPS KAKENHI (No. JP17H03241)

Monolayer molybdenum disulfide (ML-MoS₂) with excellent exotic electronic and optical properties has shown great potential for applications in transistors, sensors, energy harvesting, light-emitting diodes, catalysis, flexible devices, etc. However, it is a great challenge to achieve wafer-scale growth of continuous monolayer for MoS₂ and to control the doping because of its intrinsic n-type conductivity. Substitution doping (such as Nb, Mn, N atoms) has been proved to be an effective approach to tune their native properties toward p-type and enhance device performance. Herein, we report the growth of sulfur-rich large-area monolayer MoS₂ by a one-step chemical vapor deposition method. Much attention will be paid to the electric properties and the atomic structures of the sulfur-rich MoS₂ using Kelvin Probe Force Microscopy (KPFM, Bruker ICON) and X-ray photoelectron spectroscopy (XPS), respectively. Further studies of electrical measurements indicate that the p-type transport behavior can be observed in sulfur-rich MoS₂ field-effect transistor (FET) devices. The results of these studies will have a considerable impact on the tuning the electronic structure of two-dimensional TMDs, and create new direction for fundamental study and their novel device applications.

Ultrathin transition metal dichalcogenides (TMD) are two-dimensional semiconductors exhibiting exceptional optical, mechanical and chemical properties. Hence, they can complement other layered materials like graphene in various next-generation nanodevices.
High-quality TMD monolayers can be obtained by mechanical exfoliation - a process that is extremely time-consuming, non-deterministic and provides flakes of only a few-micrometers in size. Here, we present a heteroepitaxial method to grow single layer MoSe₂ directly on hBN, allowing to overcome the before mentioned problems.
In our approach, we use Metalorganic Vapour Phase Epitaxy (MOVPE) to grow few-nanometres thick layers of hexagonal boron nitride (h-BN) on two-inch sapphire substrates [1,2]. We present thorough optical and structural studies, demonstrating a high structural quality of the samples. h-BN possesses a wide bandgap (~6 eV) that makes it a perfect insulating barrier in heterostructures and is also known to improve optical properties of other layered materials due to the lack of dangling bonds on its surface [3]. Therefore, we use the epitaxial h-BN as a substrate for a subsequent growth of a monolayer of molybdenum diselenide (MoSe₂) using Molecular Beam Epitaxy (MBE) [4].
We studied the optical quality of the obtained TMD layers as a function of h-BN substrate thickness by performing Raman scattering and photoluminescence measurements. For the samples from well-optimized processes, we observe excitonic lines that can be resolved into two peaks corresponding to the neutral exciton A and trion at low temperatures (4 K), indicating that the material is of excellent optical quality.
Further characterization includes optical mapping of the whole two-inch samples, which proves a high homogeneity of the material. Such measurements additionally demonstrate the formation of a single atomic layer of MoSe₂.
Our results constitute a large step towards the wafer-scale growth of van der Waals heterostructures, which is of crucial importance for future applications. Our work also demonstrates for the first time, that using epitaxial h-BN, MoSe₂ of excellent optical quality can be scaled up to the size of 2 inch wafers. Moreover, the presented method can be easily applied to grow other TMD which can be used to construct optoelectronic devices such as photodetectors, LEDs and phototransistors.

[1] K. Pakula, A. Dabrowska, M. Tokarczyk, R. Bozek, J. Binder, G. Kowalski, A. Wysmolek, R. Stepniewski ”Fundamental mechanisms of hBN growth by MOVPE” arXiv: 1906.05319 (2019)
[2] A.K. Dabrowska, M. Tokarczyk, G. Kowalski, J. Binder, R. Bozek, J. Borysiuk, R. Stepniewski, A.Wysmolek, ”Two stage epitaxial growth of wafer-size multilayer h-BN by metal-organic vapor phase epitaxy – a homoepitaxial approach”, 2D Mater., 8, 015017 (2021)
[3] F. Cadiz, E. Courtade, C. Robert, G. Wang, Y. Shen, H. Cai, T. Taniguchi, K. Watanabe, H. Carrere, D. Lagarde, M. Manca, T. Amand, P. Renucci, S. Tongay, X. Marie, and B. Urbaszek, “Excitonic Linewidth Approaching the Homogeneous Limit in MoS2-Based van der Waals Heterostructures” Phys. Rev. X 7, 021026
[4] W. Pacuski, M. Grzeszczyk, K. Nogajewski, A. Bogucki, K. Oreszczuk, J. Kucharek, K. E. Polczynska, B. Seredynski, A. Rodek, R. Bozek, T. Taniguchi, K. Watanabe, S. Kret, J. Sadowski, T. Kazimierczuk, M. Potemski, and P. Kossacki, “Narrow Excitonic Lines and Large-Scale Homogeneity of Transition-Metal Dichalcogenide Monolayers Grown by Molecular Beam Epitaxy on Hexagonal Boron Nitride”, Nano Lett. 20, 3058–3066 (2020)

Vanadium disulfide (VS₂) has recently attracted much attention due to their intriguing properties, such as charge density wave (CDW), ferromagnetism, and surface catalytic behavior. However, VS₂ has received relatively little experimental research compared to other vanadium chalcogenides (VSe₂, VTe₂), so physical properties of VS₂ such as an origin of CDW, and ferromagnetism are experimentally unsettled. Here, we report MBE growth of VS₂ with different thicknesses. We investigated temperature dependent electronic structure studies by using angle-resolved photoemission spectroscopy (ARPES) measurements and show the signature of CDW transition above 300 K. Furthermore, we investigated enhanced water-dissociation properties of few layer VS2 layers by using ambient pressure X-ray photoemission spectroscopy (APXPS) measurements. These results suggest promising properties of VS₂ and related metallic sulfides.

InSe was recently reported as another candidate in the family of layered semiconductor material with tremendous application potential because of its high charge carrier mobility of above 1000 cm²/Vs at room temperature, a high photoresponsivity and sizeable principal optical bandgap of 1.2 eV in the bulk that can be tuned by the number of InSe layers up to 1.9 eV in a bilayer structure, a natural ultra-thin body form factor suitable to ensure excellent electrostatic control over the channel and synthesis parameters compatible with the stringent back-end-of-line requirements.
In this talk the results of InSe thin films grown by molecular beam epitaxy (MBE) on Si(111) will be presented. By taking advantage of excellent in-situ control and monitoring capability of MBE, specifically a precise temperature control using IR cameras as well as employing the approach of controlled flux gradients across the wafer, we have identified suitable growth conditions to synthesize single phase InSe, thus demonstrating the capability of large scale high-quality InSe film on 3 inch Si(111) wafers despite the very narrow growth parameter window. We will discuss how different growth parameters affect the phase formation and surface morphology of the films. We have mapped out the growth parameter space for which single phase InSe can be grown in the temperature range between 350°C to 500°C. The single phased InSe films are confirmed by reflection high energy electron diffraction (RHEED) and X-ray diffraction. The films surface morphology is found to be affected by the chemistry composition of the growth front and growth temperature. In-rich growth fronts at lower temperatures promote the formation of pyramid-like triangles, while lower growth rates and higher temperature suppressed the pyramid formation, giving rise to smooth film surfaces with atomic terraces. Raman and photoluminescence (PL) measurements as well as X-ray photoelectron spectroscopy results will be correlated with the growth conditions and transport measurements to provide a comprehensive understanding towards wafer scale high quality growth of this promising semiconductor.

Semiconducting van der Waals materials, including transition metal dichalcogenides (TMDCs) and tetradymites are fascinating platforms for strong light-matter interactions. Thanks to their weak anisotropic dielectric screening, their excitonic landscape include bright and dark-exciton with large binding energies. Therefore, excitons can be excited, for example by optical means, at the room temperature. Normally, exciton-photon interactions are in the form of weak interactions, as revealed from luminescence spectroscopy. Strong exciton-photon couplings though could lead to the emergence of exciton polaritons allowing for coherent transfer of the electromagnetic energy with applications in optoelectronic devices.
Strategies for enhancing the coupling between excitons and photons include the exploitation of photonic resonators or plasmonic effects. Such strong-coupling effects thus normally incorporate optical means for excitations and detections. Here, we use cathodoluminescence spectroscopy technique that is based on the spontaneous interaction of electron beams with the optical modes, hence could be a signature of the spontaneous coherence caused by collective polaritonic polarizations in the material [1, 2]. Our talk thus constitute three explorations: First, we investigate exciton-photon couplings in thin Wse₂ films [3]. Thanks to the Fabry-Perot-like resonances in thin films, the guided photonic modes pursue a longer interaction time with exciton A in the material and hence undergo a strong interaction with exciton A. We will show that the strong-coupling effect could be traced out by observing the splitting in the eigen energies and be detected spectroscopically, whereas the near-field interference effects demonstrate the excitation of exciton polaritons with their dispersions perfectly matching our theoretical results. Second, the interaction between exciton polaritons and the optical modes of a plasmonic lattice is investigated. We will show that depending on the thickness of the WSe₂ film, the excitons luminescence response is either quenched or enhanced, signifying the interplay between the strong-coupling and screening effects. Third, we investigate exciton polaritons in Bi₂Se₃ thin films [4]. Thanks to the hyperbolic response of the material, hyperbolic exciton polaritons are excited that in contrast with the excitons polaritons in WSe₂ – that are bulk polaritons – are bound to the surfaces and edges of the materials. We particularly show that hyperbolic edge exciton-polariton sustain a long propagation length with promising applications in optics-electronics interconnects.


References:
[1] N. van Nielen, M. Hentschel, N. Schilder, H. Giessen, A. Polman, N. Talebi, “Electrons generate self-complementary broadband vortex light beams using chiral photon sieves,” Nano Letters 20 (8), 5975-5981 (2020)
[2] N Talebi, S Meuret, S Guo, M Hentschel, A Polman, H Giessen, P. A. van Aken, “Merging transformation optics with electron-driven photon sources,” Nature communications 10 (1), 1-8 (2019)
[3] M. Taleb, F. Davoodi, F. Diekmann, K. Rossnagel, N. Talebi, “Charting the Exciton-Polariton Landscape in WSe Thin Flakes by Cathodoluminescence Spectroscopy,”
arXiv preprint arXiv:2101.01465 (2021)
[4] R. Lingstädt, N. Talebi, M. Hentschel, S. Mashhadi, B Gompf, M Burghard, “Interaction of edge exciton polaritons with engineered defects in the hyperbolic material Bi₂Se₃,” Communications Materials 2 (1), 1-11 (2021)

One of the characteristic features of 2D semiconductors is the strong role of many-body electronic effects that result from the reduced dimensionality and reduced dielectric screening in these systems. In particular, the optically excited states of 2D materials and heterostructures are dominated by exciton transitions, giving rise to many distinctive properties, such as sharp, strong features and the ability to tune transition energies by external dielectric screening.

Despite our deepening understanding of excitons in 2D semiconductors and their heterostructures through advances in optical spectroscopy, some fundamental aspects of the properties of excitons in 2D semiconductors have been difficult to address directly by experiment. These include a determination of the location in momentum space of the electron and hole forming an exciton and a determination of the actual excitonic wavefunction. In this talk, we will describe recent advances in the application of time-resolved ARPES (angularly resolved photoemission spectroscopy) based on femtosecond laser sources [1,2]. This technique allows one to investigate the nature of excited states of 2D materials directly in momentum space.

We will present results demonstrating how the method allows us to identify the valley characteristics of both the electron and hole forming excitons in both 2D semiconductors and 2D heterostructures in the transition metal dichalcogenide family. The dynamics of exciton relaxation and scattering can also be captured in ultrafast pump-probe measurements. Further, the measured dispersion characteristics of the electrons photoemitted from an exciton, as opposed to a photoemission of excited free carriers, show a distinctive dispersion relation reflecting the characteristics of the remaining hole. A particularly important possibility of this approach is the direct imaging of the excitonic wavefunction through its mapping into the momentum distribution of the photoemitted electrons. We will demonstrate how this can be applied not only to isolated monolayers, but also to interlayer excitons in semiconductor heterostructures.

This work was performed in collaboration with the group of Keshav Dani at OIST, Japan, with theoretical contributions for Ting Cao at the University of Washington and Felipe da Jornada at Stanford University.

[1] J. Madéo et al., Science 370, 1199-1204 (2020).
[2] M. K. L. Man et al., Science Adv. 7, eabg0192 (2021).

Non-linear optical spectroscopy of atomically thin crystals gives crucial insights into their physical properties and investigates potential applications. Second harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. SHG is vital both as a spectroscopic tool and also for applications of a large class of materials such as II-VI and III-V semiconductors, nanotubes, magnetic- and non-magnetic layered materials, perovskites and antiferromagnetic oxides.

Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS2, that shows strong optical oscillator strength for the intra- but also interlayer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations [1]. Since interlayer exciton transitions are highly tunable also by choosing twist angle and material combination in heterostructures our results open up new approaches for designing the SHG response of layered materials.

[1] Shivangi Shree et al, arXiv 2104.01225

Scattering in a device often causes performance degradation. Here we experimentally demonstrate a counter-intuitive enhancement of device performance induced by scattering. In monolayer MoS2, we show that scattering can markedly improve its excitonic valley polarization. We report seven- and twelve-folds of valley polarization enhancement due to thermally activated and charge doping induced scattering respectively. This interesting effect is attributed to the disruption of valley pseudospin precession arising from rapid modulation of exciton momentum and concomitant exchange interaction field, which is analogous to motional narrowing in nuclear magnetic resonance spectroscopy. Our work advances understanding of valley depolarization mechanisms in transition metal dichalcogenide atomic layers and illustrates a novel approach for controlling and improving valleytronic devices.

With the recent advances in synthesis and characterization of two-dimensional materials, there is great interest in exploiting their favorable electronic properties to create compact devices. In particular, materials which lack inversion symmetry can break degeneracy between otherwise equivalent crystal momentum valleys if the time-reversal symmetry is also removed. This allows the generation of valley-polarized carrier populations, because each valley preferentially absorbs a certain polarization of light and the lack of degeneracy changes the absorption frequency for each valley. Traditionally, the symmetry has been broken with an external magnetic field, but the proximity effect of a magnetic substrate layer can potentially induce stronger Zeeman splitting of the valleys without the need for a bulky magnet. A number of computational studies have identified valleytronic/substrate material pairs which break the degeneracy between valleys, and a few have been experimentally verified. However, a systematic understanding is absent because most studies only test one or two interfaces, and differing methods make it hard to compare results.
For this reason, we have done a larger computational search of eight different materials pairs in three possible stackings, with the goal of identifying the design principles which generate much greater valley splitting in certain interfaces than in others. The valleytronic layers are transition metal dichalcogenides (Mo,W)(S,Se)₂, while we use two antiferromagnetic (Mn, Ni)PSe₃ substrates or the ferromagnets CrBr₃ and CrGeTe₃ to generate the proximity effect; we choose the specific pairs according to their predicted strain and band alignment, as well as their experimental relevance. While there are several common definitions of valley splitting, we select the difference between same-spin excitation energies at K vs. K’. Our first-principles calculations find a wide range of valley splittings from -26.7 to 9.8 meV, with strong dependence on both chemistry and stacking pattern. The largest valley splitting is obtained in a ferromagnetic interface which has unusually strong band hybridization between the two layers, and we analyze the possible effects of this hybridization. We identify several factors which can influence the sign and magnitude of the valley splitting.
“Strong Zeeman Splitting in Orbital-Hybridized Valleytronic Interfaces,” Steven T. Hartman and Ghanshyam Pilania (in preparation).

Monolayer transition metal dichalcogenides (TMDCs) have attracted attention as direct band-gap semiconductor materials consisting of three atomic thin-layer (0.7 nm thick). A near-unity photoluminescence (PL) quantum yield of a monolayer MoS₂ was demonstrated via treatment with a superacid molecular (bis(trifuloromethane)sulfonimide (TFSI)) under excitation laser intensity of 10^-4~10^-2 W cm^-2 [1]. Recently, the superacid treatment followed by UV irradiation leads to strong photoluminescence with high yield [2]. In this study, we examined the same treatment on tungsten disulfide (WS₂), which has sulfur in its structure similar to MoS₂ and investigated the relationship between the superacid molecular treatment and UV light irradiation, as well as the dependence on the solvent to prepare the superacid solution. Furthermore, we achieved the air-stability of the superacid-treated WS₂ by coating with an organic polymer for more than 30 days.
A mechanically exfoliated monolayer WS₂ was immersed in 1 mg/mL superacid solution followed by UV light irradiation. PL measurements were performed for the samples as-exfoliated, TFSI-treated, and UV light irradiated after the TFSI-treatment. The as-exfoliated WS₂ shows strong PL intensity compared to MoS₂ cases, and the magnitude of the intensity is more than 300 times. The PL intensities of a monolayer WS₂ after the TFSI-treatment and the following UV light-irradiation were 7 times and 11 times enhancement, respectively, from the as-exfoliated sample. The magnitude of the enhancement was not as large compared to the previous MoS₂ cases [2], however, the PL intensity of the monolayer WS₂ of the treatment showed about 10 times stronger than that of MoS₂.
To further investigate the modulation of PL intensity of the monolayer WS₂, the solvent dependency of TFSI solution was investigated by comparing the hydrophobic 1,2-dichloroethane (DCE) and the hydrophilic acetonitrile. As a result, it was confirmed that the PL intensity of the hydrophilic acetonitrile was almost twice that of the hydrophobic DCE on average. This behavior would be due to the wettability of the acetonitrile solution of TFSI on WS₂.
We further achieved the stabilization of the strong photoluminescence of the superacid-treated WS₂ for more than 30 days in the air by coating with organic polymers. This behavior would be due to the physical separation of the TFSI molecules from the adsorption of H₂O molecules.
According to the above results, the combination of TFSI treatment and UV light irradiation results in strong photoluminescence enhancement in WS₂. In addition, the TFSI solution was prepared using hydrophilic acetonitrile as a solvent, which enabled us to investigate the optimal method for increasing the PL intensity in this system. We developed the stabilization of the strong photoluminescence of the superacid-treated WS₂ by coating with an organic polymer for more than 30 days in the air. In the presentation, we will discuss the detailed mechanism of air stability as well.

[Reference]
[1] M. Amani, D. H. Lien, D. Kiriya, et al., Science,350, 1065,2015.
[2] Y. Yamada, K. Shinokita, Y. Okajima, et al., ACS Appl. Mater. Interfaces, 2020, 12, 36496-36504.

Two-dimensional crystals that are overlaid with a difference in lattice constant or a relative twist form a moiré pattern. In semiconductors and semimetals, the low-energy electronic properties of these systems are described by Hamiltonians with the periodicity of the moiré pattern, opening up a strategy to make artificial two-dimensional crystals with lattice constants on the ten nm scale. I refer to these artificial crystals as moiré materials. Because of their large lattice constants, the band filling factors of moiré materials can be electrically tuned over large ranges without introducing chemical dopants. Moiré materials, can be used to flexibly simulate the physics of real atomic scale crystals, and to create new states of matter. I will overview progress that has been made in understanding the low-temperature properties of twisted graphene and transition-metal dichalcogenide multilayers, the first moiré materials, and provide a perspective on prospects for future room-temperature applications. My talk will emphasize the physics of broken spin/valley flavor symmetries including the frequent emergence of Chern insulator states, and moire material ferromagnetism and superconductivity.

Moiré superlattices in transition metal dichalcogenide (TMD) heterostructures host novel correlated quantum phenomena due to the interplay of narrow moiré flat bands and strong, long-range Coulomb interactions. One such phenomenon is the Wigner crystal, a 2D crystallization of electrons. Recent optical spectroscopy has provided evidence of generalized Wigner crystal states in TMD moiré superlattices (a generalized Wigner crystal is one in which the electrons can fractionally fill a lattice). Direct observation of 2D Wigner crystals in real space, however, has remained an outstanding challenge. Scanning tunneling microscopy (STM) in principle has sufficient spatial resolution to image a Wigner crystal, but conventional STM measurements perturb fragile Wigner crystal states in the process of measurement. Here I will discuss recent work where we have performed real-space imaging of 2D Wigner crystals in a WSe₂/WS₂ moiré heterostructure using a new non-invasive STM spectroscopy technique. This involves the use of a graphene sensing layer placed in close proximity to an occupied WSe₂/WS₂ moiré superlattice. The STM tunnel current into the graphene sensing layer is modified by the underlying electron lattice of the Wigner crystal in the WSe₂/WS₂ heterostructure. Our measurement directly visualizes different lattice configurations associated with Wigner crystal states at fractional electron fillings of n = 1/3, 1/2, and 2/3, where n is the electron number per site (n is controlled by voltages that can be separately applied to both a top and a bottom gate). The n=1/3 and n=2/3 Wigner crystal states are observed to exhibit triangular and a honeycomb lattices, respectively, in order to minimize nearest-neighbor Coulomb interactions. The n = 1/2 state, on the other hand, spontaneously breaks the original C3 symmetry and forms a stripe structure in real space.

The quantum anomalous Hall effect (QAHE) stems from the interplay between magnetism and the energy band structure of topological materials. The hallmark of QAHE is the precise quantization of Hall resistance at ±h/e² (h = Planck’s constant, e = charge of an electron) without an external magnetic field. Notably, QAHE features topologically protected, dissipation-less current channels along the edge of the sample, and thus can be of great interest for emerging electronic devices. However, there is no clear pathway to utilize QAHE at the device/circuit/system level. Recently, intrinsic QAHE has been observed in twisted bilayer graphene (tBLG) moiré heterostructure ¹ and tri-layer graphene ², where the well-defined QAH states can be tuned by applying a bias current. Leveraging this aspect, we propose and construct a cryogenic non-volatile memory system using tBLG moiré heterostructure ³. We define the memory states (‘1’ and ‘0’) with the two Hall resistance states (±h/e²). We utilize the hysteretic dependence of Hall resistance on the bias current to write into the accessed cell. During a read operation, the Hall voltage of the accessed cell shows different polarities depending on the stored memory state. Finally, the polarity of the Hall voltage is sensed using a voltage comparator to decide the memory state stored in the accessed cell. The designed memory system can operate with ultra-low power because of the nano-ampere level of bias current requirement for switching between two Hall resistance states. To scale up, we design a memory array (similar to a 3D Cross-point array) with novel peripherals (accessing and sensing mechanism) to ensure the highest scalability, minimum leakage power, and to avoid accidental data manipulation. Thanks to its small cell size and simple peripherals, the designed QAHE based memory array promises to solve the scalability issue faced by the state-of-the-art superconducting memories. We believe, this work will provide a framework for realizing memory systems based on the emerging magic-angle Van der Waals materials for cryogenic applications.

References:
1. Serlin, M. et al. Intrinsic quantized anomalous Hall effect in a moiré heterostructure. Science 367, 900–903. doi:10.1126/science.aay5533 (2020).
2. Chen, S. et al. Electrically tunable correlated and topological states in twisted monolayer–bilayer graphene. Nat. Phys. doi:10.1038/s41567-020-01062-6 (2020).
3. Alam, S. et Al. A non-volatile cryogenic random-access memory based on the quantum anomalous Hall effect. Sci. Rep. doi:10.1038/s41598-021-87056-7 (2021).

Transition metal dichalcogenides (TMDs) of semiconductor band structures have recently emerged as a new family of two-dimensional (2D) materials. TMDs can have a tunable bandgap in the range of 1 − 3 eV, which enables their application in ultrathin field-effect transistors, sensors, and optoelectronic devices. Tungsten-based TMDCs such as WS2 and WSe2 have electronic states that support a strong interlayer coupling, and they exhibit an indirect-to-direct band gap transition when the number of layers decreases. After the discovery of magic angles in twisted graphene, it was found that electronic flat bands appeared for relatively small angles close to 0° and 60° in twisted bilayers of TMDs. We propose to explore the linear and nonlinear optical properties at these relatively small twist angles so that we could understand the optical physics behind the singularities appearing at these twist angles. At small relative twist angles, the stacking can generate the so-called Moiré excitons with Moiré patterns. The transitions of the Moiré excitons can be measured by photoluminescence spectroscopy, which enables the study of interlayer coupling effects.The electrical and optical properties of these stacks are significantly influenced by the twist angle, reflecting the modulated electronic band structure due to angle-dependent interlayer coupling. In our work, we use density functional theory (DFT) calculations on the electronic and optical properties of multilayer WS2 and WSe2. Calculations of the band structures, frequency-dependent dielectric function, and optical absorbance for mono, bi, and trilayers of WS2 and WSe2 shows that the optical properties strongly depend on the twist angles between two or more stacked layers. Also, many possible combinations of the crystal axes in bilayer TMDs, which cause a change in the symmetry, enable us to control the nonlinear properties. Based on DFT calculations, we propose to study the nonlinear optical properties of TMDs, such as second and third harmonic generations as a function of the number of layers and stacking orientation.

Interlayer coupling in van der Waals heterostacks governs a variety of optical and electronic properties. Janus transition metal dichalcogenides (TMDs), a newly emerged TMD with out-of-plane asymmetry, offers a simple and versatile approach to tune the interlayer coupling. Our previous work has demonstrated that Janus MoSSe can enhance the interlayer coupling with MoS₂ by up to 13% compared to the corresponding TMD bilayers. Here, we employed Janus MoSSe to tune the interlayer phononic and intralayer excitonic properties of Janus MoSSe/MoS₂ by forming heterostructures with S/S and Se/S interfaces, in which the sulfur or selenium side of MoSSe is in contact with bottom MoS₂, and with twist angles between the in-plane crystalline axes. The S/S heterobilayer exhibited a stronger interlayer coupling than the Se/S heterobilayer as shown by low-frequency Raman modes. The stronger interlayer coupling was in agreement with the density-functional theory calculations and was explained by a smaller interlayer distance in S/S heterostructures. Besides, the intrinsic dipole at S/S and Se/S interfaces led to enhanced and reduced charge transfer and thus different intralayer exciton relaxation, respectively. Our work opens the avenue to manipulate the asymmetry of Janus TMD to tailor the van der Waals interfacial interactions.

We present density functional theory (DFT) calculations of MoSe₂/WSe₂ bilayers twisted a small angle (~ 3^o-5^o) away from the commensurate 2H stacking. As the twist angle decreases, our calculations show the emergence of flat bands in both conduction and valence bands. The degeneracies of the flat bands suggest their origins in features of the 2H band structure. We also analyze the atomic relaxation of the twisted MoSe₂/WSe₂ bilayer away from the rigid moiré structure. While full reconstruction is not achieved at the angles we study, we begin to see the increase in area of regions of low energy stacking. We show that the states associated with the conduction and valence band flat bands are localized in different regions of local stacking. The localization of states is pronounced even though reconstruction is incomplete.

In recent years, phase-change materials (PCMs) have received increasing attention because of their unique properties, exploited in data storage/processing applications. PCMs require pronounced contrast in optical and/or electrical properties between two different structural state. Alternative PCMs to conventional chalcogenide alloys, containing Ge, Sb, and Te, (GST) are being extensively studied.
In the rising family of two-dimensional, 2D, materials, group (III) chalcogenides such as, GaS, Ga2S3, GaSe, GaTe, gallium sulfide, GaS is interesting as phase-change material in the optical range due to its wide bandgap of approximately 2.4 eV in the bulk form. GaS is a layered van der Waals semiconductor consisting in the stacking of covalently bond S-Ga-Ga-S atomic planes, where the successive tetralayers are held together via van der Waals forces. Few layers of GaS have successfully been produced by mechanical exfoliation, liquid exfoliation⁴, chemical vapor deposition (CVD), Atomic Layer Deposition (ALD), and used in field effect transistors (FET), near-blue light emitting devices, photodetectors², energy storage, gas sensing³, and in hydrogen evolution catalysis.
Here we present characteristics of few layers GaS grown by CVD. We establish the interplay between number of layers, from monolayers up to 10 layers, of GaS and its structural, optical and electrical properties. Those characteristics have been determined through an extensive characterization by Raman spectroscopy, run as a function of laser wavelength and number of GaS layers, scanning electron microscopy (SEM) used to image the morphology of the layers and count the number of layers, X-ray photoelectron spectroscopy to determine the chemical Ga:S ratio and hence, stoichiometry of the layers, spectroscopic ellipsometry used to determine the dielectric function and optical gap as a function of number of layers, as well as photoconductivity measurements also depending on number of layers.
The effect of thermal processing on those structural, compositional, opto-electronic properties is also thoroughly discussed for the first time to the best of our knowledge.
We also present results on 2D heterojunctions of GaS/graphene for blue-light photodetector, exploring different configurations of GaS/Graphene coupling. We dalso iscuss the role of graphene electron transfer on the phase change dynamics of GaS. This study represents a step forward 2D phase change heterostructures.

Acknowledgement: This work has been supported by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 899598 – PHEMTRONICS.

Intercalation of lithium ions is one of the most effective methods to realize structural transformation and to tune the optical and electrical properties of two-dimensional transition metal dichalcogenides (2D TMDs). Numerous studies have focused on the phase transition from semiconducting 2H phase to metallic 1T (or 1T’) phase in MoS₂ and WS₂ induced by the intercalation of lithium ions. However, few reports explore the effects of lithium intercalation in other TMDs, such as Mo- or W- ditellurides. In particular, novel electronic and energy devices can be achieved using the lithium-intercalated MoTe₂ with its intriguing electrical, topological and catalytic properties. Density-functional theory (DFT) calculations suggest that lithium-adsorbed T’-MoTe₂ is thermodynamically more stable than H-MoTe₂ at the same electron-doped level and the energy barrier to trigger the H to T’ phase transition for monolayer MoTe₂ should be less than that for monolayer MoS₂ or MoSe₂ by lithium intercalation. [1]
Here, we report electrochemical lithium intercalation into 2H- and 1T’- MoTe₂ flakes. We observe that intercalated lithium dopes electrons to 2H-MoTe₂, as evidenced by the downshift of the peak position and broadening of the full-width at half maximum (FWHM) of A_1g peak in Raman spectroscopy. Further, the n-type doping of intercalated lithium to 2H-MoTe₂ is confirmed by the increase in conductivity of 2H-MoTe₂ with lithium intercalation. However, in situ Raman spectroscopy shows that 2H-MoTe₂ does not covert to 1T’ phase by the intercalation of lithium ions, but directly breaks down to Mo metal clusters and lithium tellurides at an applied electrochemical voltage of 0.7 – 0.8 V. For 1T’-MoTe₂, the 1T’ phase is stable down to 1.2 V of the applied electrochemical voltage, and a new phase is observed with no prominent Raman peaks between 0.8 – 1.1 V before becoming amorphous at 0.7 V. Our study thus suggests that, while calculations indicate that lithium-intercalated 1T’-MoTe₂ might be thermodynamically more stable than 2H phase, experimentally this is not realized. We hypothesize that this discrepancy is due to a high nucleation barrier of the 1T’ phase, where the nucleation barrier cannot be overcome by the applied electrochemical voltage at room temperature. Our results thus highlight the importance of phase transition pathways, in addition to the final phases at thermodynamic equilibrium, in intercalation-induced phase transitions in 2D TMDs.

[1] Journal of Materials Chemistry C 8, 4432-4440

Layered As₂P₃ compounds were synthesized from red phosphorus and gray arsenic by chemical vapor transport growth method. A systematic study on electrochemical intercalation in As₂P₃ was carried out by both in-situ Raman scattering and ex-situ X-ray diffraction (XRD). A dedicated electrochemical cell under Galvanostatic discharge for in-situ Raman spectroscopy studies was used to study time evolution of vibrational modes under lithiation. During Lithium intercalation in As₂P₃, an abrupt shift in characteristic Raman peak positions of As₂P₃ after x = 0.06 (in Li_x (As₂P₃)_1-x) was observed. At this degree of lithiation (x), an emergence of new peaks ~ 200 cm^-1 and ~ 406 cm^-1 were also observed. It is believed that an intercalation induced phase transition taking place at this specific intercalation level. Ex-situ XRD spectra also show evidence for different structural phases in the pristine alloy (before intercalation) and after intercalation beyond x=0.06. Raman shifts indicate competing effects of strain and charge transfer at the initial intercalation levels followed by structural reorganization before the phase transition. In the final stages of the intercalation, the entire process is governed by charge transfer.

Phase transitions of two-dimensional (2D) materials and their heterostructures enable many applications including electrochemical energy storage, catalysis, and memory. These phase transitions are initiated by nucleation, and the transition pathways between phases are complex and heavily influenced by factors such as nanoscale confinement and interfaces. To effectively exploit these phase transitions for device applications, understanding the thermodynamics and kinetics of the nucleation pathways is essential; however, nucleation pathways remain virtually unexplored for many 2D transition metal dichalcogenides (TMDs) that have been demonstrated to undergo phase transitions.

Here we demonstrate that the lithium intercalation-induced 2H-1T' phase transition in the TMD MoS₂ proceeds via nucleation of the 1T' phase at a heterointerface by monitoring the phase transition of MoS₂/graphene and MoS₂/hexagonal boron nitride (hBN) heterostructures during intercalation with Raman spectroscopy in situ. We observe that graphene-MoS₂ heterointerfaces require an increase of 0.8 V in applied electrochemical potential to nucleate the 1T' phase in MoS₂, while hBN-MoS₂ heterointerfaces do not. The increased nucleation barrier at graphene-MoS₂ heterointerfaces is due to the reduced doping power of lithium to MoS₂ at the heterointerface as lithium also dopes graphene based on ab initio calculations. Further, we show that the growth of the 1T' domain propagates along the heterointerface, rather than through the interior of MoS₂. Our results provide the first experimental observations of the heterogeneous nucleation and growth of intercalation-induced phase transitions in two-dimensional materials and heterointerface effects on the phase transition.

A methodical density functional theory calculation has been performed to understand the effect of alloying in stabilizing black As_xP_1-x system and the role of Li intercalation on black As_xP_1-x alloy. The structures of black As_xP_1-x alloy were constructed by randomly substituting phosphorus (P) atoms with arsenic (As) atoms in the framework of the black phosphorus. Various distributions of As atoms for a given As concentration were considered and the constructed alloy systems were optimized within a full relaxation process which allows all the atoms fully relaxed with no restriction on the lattice symmetry and volume. It was found that in the energetically most stable black As_xP_1-x alloys, both As and P atoms prefer to forming armchair As-As and P-P bonds, aligning along zigzag direction. Due to the larger atomic size of As, a local distortion was found after the release of the local strain. The in-plane lattice constants (a and c) and the volume approximately follow the Vegard’s law, but the vertical lattice constant (b) shows a fluctuation as the function of As concentration. The Li interaction has been performed on Li_x(As_0.375P_0.625) system. Our results show that a structural transformation occurs under the Li intercalation. With increasing Li concentration, the AB stacking of black As₀.₃₇₅P_0.625 flipped from symmetric to antisymmetric ordering, then the lattice transferred from orthorhombic to rhombohedral, and a bond-breaking happened with an assembly of buckled rhombohedral nanoribbons and zigzag chains. This study pointed out that Li atoms intercalated in black As_0.375P_0.625 could act as ‘catalyst’ in a local structural transformation, indicating a possible pass way for structural phase transition and even a discovery of novel materials.

Charge density wave (CDW) is one of the most common, yet least understood, quantum phenomena in condensed matter physics. Occurring in metallic systems, it forms a new ordered state with a distorted lattice structure and modulated charge density distribution. Astonishingly, these modulations emerge in various patterns with disparate dimensional charactersitics, even within the same family of materials. In this talk, I propose a general framework [1], that helps us to unify distinct trends of CDW instability. To show this, I focus on an isoelectronic group of transition metal dichalcogenide, 2H-MX2 (M=Nb, Ta and X=S, Se) which are best known for their puzzling CDW orderings. For example, while NbSe2 exhibits a strongly enhanced CDW order in two dimensions, TaSe2 and TaS2 behave oppositely, with CDW being absent in NbS2 entirely. Combining Raman scattering spectroscopy with first-principles calculations, I show that such a disparity arises from a competition of ionic charge transfer, electron-phonon coupling, and electron correlation. Despite its simplicity, the approach presented here, in principle, explain dimensional dependence of CDW in any material, thereby shedding new light on this intriguing quantum phenomenon and its underlying mechanisms.
[1] D. Lin, S. Li, J. Wen, H. Berger, L. Forró, H. Zhou, S. Jia, T. Taniguchi, K. Watanabe, X. Xi, M. S. Bahramy, Nature Communications 11, 2406 (2020).

We explore the effect of charge density wave (CDW) phase transition on the in-plane and cross-plane properties of (PbSe)_1+δ(VSe₂)₁ and (PbSe)_1+δ(VSe₂)₂ heterostructures. For in-plane transport measurements of (PbSe)_1+δ(VSe₂)₁, we observe an abrupt 86% increase in the Seebeck coefficient, 245% increase in the power factor, and a slight decrease in resistivity before and after the CDW transition. This phenomenon is not observed in (PbSe)_1+δ(VSe₂)₂ and is highly unusual compared to the general trend observed in other materials. The abrupt transition shows a deviation from the Mott relationship through electron-electron and electron-phonon interactions. Raman spectra analysis of the (PbSe)_1+δ(VSe₂)₁ material shows the emergence of additional peaks associated with CDW transition in VSe₂ material. Temperature-dependent in-plane X-ray diffraction (XRD) spectra show a change in the in-plane thermal expansion coefficient of VSe₂ layers in (PbSe)_1+δ(VSe₂)₁ due to lattice distortion. The increase in the power factor and a slight decrease in the resistivity due to CDW suggest a potential mechanism for enhancing the thermoelectric performance at the low temperature region. Anisotropic transport measurements of (PbSe)_1+δ(VSe₂)₁ show 4 orders of magnitude differences between cross-plane and in-plane resistivities over the entire measured temperature range (77 - 300 K). There is a 30% change for cross-plane resistivity of (PbSe)_1+δ(VSe₂)₁ during the measured temperature range, while the change of in-plane resistivity is only less than 8%. The designed measurement platform can be generalized to explore anisotropic properties of various 2D layered heterostructures.

Among the bulk of post-graphene 2D layered materials, black phosphorous (BP) has attracted much attention since the isolation of its layers in 2014 because of its exotic properties which include: high carrier mobility, in-plane anisotropy, and thickness-dependent bandgap. Intercalation has been used to expand the versatility of BP by tuning its properties and inducing structural phase transitions for diverse applications. This happens by introducing foreign atoms (intercalants) into the van der Waals (vdW) gaps of BP. There have been past attempts to understand the underlying mechanism of intercalation in BP using different intercalants and intercalation strategies. Herein, we carry out an ex-situ study of the structural evolution and an in-situ study of the electrical transport properties of BP as it is intercalated with cesium (Cs) in the vapor phase. The Raman studies of Cs-intercalated and pristine BP indicates a redshift of its characteristic vibrational modes (A¹_g,B_2g and A²_g) relative to the pristine ones where B_2g and A²g modes shifted 1.3 times faster than A¹g mode; reinforcing the established preference of alkali ions to intercalate in the in-plane direction (arm-chair and zig-zag) of BP rather than the out-of-plane direction while the x-ray diffraction (XRD) patterns of Cs-intercalated BP suggest weakening vdW interactions between BP layers. Scanning electron microscopy (SEM) was done to quantify the amount of Cs intercalated into BP while x-ray photoelectron spectroscopy (XPS) was used to analyze the chemistry of these samples. Density functional theory (DFT) calculations were also carried out to help explain experimental findings.

Layered van der Waals materials, in particular transition metal dichalcogenides (TMDs) exhibit a wide variety of exotic electronic and photophysical phenomena, such as non-trivial topology and valleytronics [1]. These desirable properties, combined with their quasi-two-dimensional nature, make TMDs particularly attractive as candidate materials for nanoscale non-volatile logic and memory devices. The TMD 1T-TaS2 is particularly promising for device applications due to its rich phase diagram, including multiple distinct charge density wave (CDW) phases, all of which can be manipulated by electromagnetic perturbations. Specifically, 1T-TaS2 exhibits multiple distinct charge density wave (CDW) phases. We focus on the low-temperature, commensurate CDW phase which manifests as an in-plane Star of David-type deformation of the tantalum sublattice. An additional degree of freedom arises from numerous metastable stacking configurations of the adjacent CDW layers. The onset of the Star of David CDW transforms the material’s electronic structure at low temperatures [2] in a way that enables a metal-insulator transition upon cooling through a critical temperature. Within this lowtemperature, insulating CDW phase, recent experiments have demonstrated that 1T-TaS2 can be switched between states of high and low resistance by application of orthogonal current pulses. While anisotropic switching has been observed in thin-film flakes in the insulating state at low temperatures [3], this behavior arises even in the 3D limit, although the role played by the interlayer stacking on the nature of the CDW phase and switching is unknown. Motivated by previous calculations demonstrating the dramatic effect of CDW stacking on the electronic structure of 1T-TaS2, we investigate the hypothesis that the resistivity switching may be caused by a current-induced change in the CDW stacking sequence using first-principles density functional theory calculations. For a number of stacking sequences, we examine the computed band structure’s anisotropy in connection with the possible dynamics and mechanisms of CDW movement, and we propose a model for the nature of 2D CDW sliding, identifying low-energy pathways for CDW motion. Using this detailed information, we construct a quantitative hypothesis to explain the switching behavior observed experimentally [4]. Our work leads to new understanding of CDWs in layered van der Waals materials and their dynamics, and could reveal novel energy-efficient routes for the electric field control of logic and memory for the next generation of switching devices. [1] Manzeli, S., Ovchinnikov, D., Pasquier, D. et al. 2D transition metal dichalcogenides. Nat Rev Mater 2, 17033 (2017). https://doi.org/10.1038/natrevmats.2017.33 [2] Ritschel, T., Trinckauf, J., Koepernik, K. et al. Orbital textures and charge density waves in transition metal dichalcogenides. Nature Phys 11, 328–331 (2015). https://doi.org/10.1038/nphys3267 [3] Svetin, D., Vaskivskyi, I., Brazovskii, S. et al. Three-dimensional resistivity and switching between correlated electronic states in 1T-TaS2. Sci Rep 7, 46048 (2017). https://doi.org/10.1038/srep46048 [4] Maniv, E., Haley, S., Analytis, J. G., Metastable Slidetronic Switching in Bulk 1T-TaS2, unpublished work.

Ferromagnetism in two-dimensional materials presents a promising platform for the development of novel ultrathin spintronic devices with new functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI₃, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. In this talk, we report the observation of a new air stable and layer dependent ferromagnetic (FM) van der Waals crystal, CrPS₄, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about half a day, opening a door to the practical applications of van der Waals spintronics.

Two-dimensional (2D) layered magnetic materials have attracted increasing attention after the 2D ferromagnetic order was first discovered in chromium compounds such as CrI₃ and Cr₂Ge₂Te₆. The family of 2D magnets continues to grow larger and larger: in addition to insulating CrI₃ and Cr₂Ge₂Te₆, Fe₃GeTe₂ as a 2D magnetic metal has also joined the family. The outstanding physical properties in Fe₃GeTe₂ demonstrate its potential values in fundamental research and future applications. Before any practical applications of 2D magnets, the lattice vibrations (i.e., phonons), the magnetic orders (i.e., spins), and their coupling need to be understood first. Raman spectroscopy is among the most fast and reliable experimental techniques that can probe both phonons and spins in 2D magnetic materials. To explain and guide experimental Raman measurements, first-principles modeling of Raman scattering based on density functional theory (DFT) is often required. Here, I will highlight a recent Raman modeling-driven project on Fe₃GeTe₂ [1]. Its phonon vibrations and Raman intensities were calculated from the bulk to the monolayer. The spin-phonon coupling effect was investigated by considering different interlayer magnetic orders: ferromagnetic and antiferromagnetic. The frequencies of Raman modes in Fe₃GeTe₂ were found to exhibit considerable dependence on both the layer number and spin order. The results not only reveal the notable spin-phonon interactions in Fe₃GeTe₂, but also demonstrate that Raman modes can be utilized for characterizing the sample thickness and interlayer magnetic order in Fe₃GeTe₂.
References
[1] X. Kong, T. Berlijn, L. Liang*, “Thickness and spin dependence of Raman modes in magnetic layered Fe₃GeTe₂”, Advanced Electronic Materials, 2001159 (2021).

Recently, ferroelectricity in layered materials “beyond graphene” has attracted a lot of attention because of its applications in high-density nonvolatile memory devices. However, very few layered materials demonstrate switchable out-of-plane polarization. Of them, the transition metal thiophosphates are a promising family of beyond graphene materials that host out-of-plane ferrielectricity (FiE) with large values of polarization [1-2]. Further, research efforts in these materials have reported a plethora of properties including negative piezoelectricity, negative electrostriction, large dielectric tunability, and unconventional field-induced switching behavior, all of which depend strongly on the polarization [3-5]. Despite the growing interest, a clear understanding of the origin and stabilization of the polar phase is still lacking in transition metal thiophosphate.

Using first-principles calculations and group-theoretical methods, we study the origin and stabilization of FiE in CuInP₂Se₆ [6]. We find that the polar distortions of metal atoms create most of the polarization in the FiE phase. Surprisingly, the stabilization of the FiE phase comes from an anharmonic coupling between the polar mode and a fully symmetric Raman-active mode comprising primarily of the Se atoms. This coupling is large, with the degree of anharmonicity is comparable to improper ferroelectrics. Thus, the origin of polarization is different from the factors that stabilizes the FiE phase in CuInP₂Se₆, unlike conventional ferroelectrics. Our results open possibilities for dynamical control of the single-step ferroelectric switching barrier by directly tuning the Raman-active mode. These finding has important implications not only for designing next-generation microelectronic devices that can overcome the voltage-time dilemma [7] but also in understanding the emergent responses in these materials.

[1] V. Maisonneuve et al., Phys. Rev. B 56, 10860 (1997).
[2] F. Liu et al., Nat. Comm. 7, 12357 (2016).
[3] A. Dziaugys et al., “Piezoelectric domain walls in van der Waals antiferroelectric CuInP₂Se₆”, Nat. Comm. 11, 3623 (2020).
[4] J. A. Brehm et al., “Tunable quadruple-well ferroelectric van der Waals crystals”, Nat. Mat. 19, 43 (2020)
[5] Y. Qi and A. M. Rappe, Phys. Rev. Lett. 126, 217601 (2021).
[6] N. Sivadas et al., “Origin and stabilization of ferrielectricity in CuInP₂S_e6”, arXiv:2106.08783
[7] T. Schenk et al., Reports on Progress in Phy. 83, 086501 (2020).

Owing to dimensionality constraints, two-dimensional materials are of great interest for a number of different applications ranging from nanophotonics and plasmonics to mechanics and nanoelectronics.^1,2 Much focus has been on the ubiquitous graphene, as well as transition metal dichalcogenides,³ where several applications and unique physics have been uncovered.⁴ Recently, the emergent field of twistronics has enabled tremendous functionality based on twist angle between two single atomic layers.^5,6 Consequently, a great number of alternative materials have been overlooked and is arguably a source of many missed opportunities.
Here we explore the electronic and structural properties of MnPS3 using scanning transmission electron microscopy (STEM) and monochromated electron energy loss spectroscopy (EELS). Previous work on MnPS3 has largely considered it as a candidate for an antiferromagnetic 2D magnet by studying its magnetic ordering.^7,8 Through electron microscopy, we find that compared to other layered materials, MnPS3 is a host for several unique characteristics. We focus on two of these phenomena that we have recently discovered by STEM-EELS in which both great utility and distinct physics reside.
One of the most striking behaviors of MnPS3 is its sensitivity to electron beam irradiation. The effect this has on the crystal structure of MnPS3, however, is remarkable, and to our knowledge, is the first time such an effect has been observed. By electron beam irradiation, we observe the spontaneous formation of Moiré patterns with varying length scales. In effect, the electron beam in the STEM provides a modification tool that selectively induces the formation of Moiré patterns. Moreover, several degrees of freedom are present in this capability and consequently several Moiré wavelengths can be produced by the electron beam. We consider different mechanisms of Moiré pattern formation, explore the electronic property changes of the modified material as measured by STEM-EELS, as well as discuss possible applications this holds.
A second property of MnPS3 of potential great interest to the nanophotonics community lies in its support of localized plasmon resonances. Particularly, we find through hyperspectral STEM-EELS analysis that an edge plasmon near 5 eV is supported in varying number of layers. Crucially, the localization length of the edge plasmon is on the order of two-unit cells – i.e., it is extremely confined, and at the same time is relatively strong. Moreover, due to the electron beam sensitivity of MnPS3, edges can be engineered at will by controlling the position and dose of the electron beam, paving the way toward quantum nanophotonic and plasmonic applications. We discuss the plasmonic behavior of the system and suggest its candidacy as a nanophotonic waveguide and emitter.

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This effort is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division.

The ability to isolate atomically thin crystals, such as graphene, boron nitride, and the transition metal dichalcogenides, and then mechanically layer them to form new heterostructures has driven a recent revolution in materials design. These layered structures are held together by weak van der waals forces, rather than chemical bonding making it possible to readily mix and match 2D materials spanning a wide range of properties, virtually at will. More recently it has been demonstrated that the weak interlayer coupling also makes it possible to arbitrarily tune the rotation angle between adjacent layers and that varying this twist angle can lead to dramatic transformations in electronic, optical and magnetic of the combined heterostructure. Exploiting this new degree of freedom, which has been termed twistronics, has already revealed remarkable new opportunities with twisted layered structures demonstrating the ability to tune through metallic, insulating, semiconducting, semimetal, magnetic, superconducting, and topological behaviour by simply varying the twist angle. So far the vast majority of studies have reported static devices, where the twist angle is established during the fabrication process such that each device represents a single twist angle. However the weak interlayer bonding means that it is possible to realize devices where the twist angle can be dynamically varied post-fabrication. Here I will discuss our ongoing efforts to develop improved in-situ control with the goal towards realizing fully tunable devices structures consisting of just two atomic layers. I will discuss recent progress in terms of mechanical rotation of individual monolayers, improved understanding of interfacial dissipation mechanisms, implementation of in-situ feedback of twist angle and new approaches towards developing non-mechanical control of the heterostructure geometry at the nanoscale. Prospects for realizing functional twistronic devices with real-time in-situ control will be discussed.

Two dimensional materials (2D) have desirable properties such as high mobility at an atomic thickness, superior electrostatic control, and potential for low-power electronics. Forming a low resistance contacts to 2D such as MoS₂ is one key challenge for realizing 2D electronics. The presence of Schottky barrier height (SBH), associated with the metal-induced gap states (MIGS), limits the contact performance. In a top contact geometry, we find that the semimetallic bismuth contacted to MoS2 monolayers lead to a pinning-free Ohmic contact. A record-low contact resistance (RC) of 123 Ω μm is achieved. For the edge contact geometry, Fermi-level pinning-free Ni-MoS₂ edge contact transistor devices are obtained using in-situ etching and metal deposition. We conclude that the edge contact fabrication processes strongly affect the electrical characteristics such as Schottky barrier height where the in-situ process gives the best performance and pinning-free characteristics.

A big chunk of the price tag of electronic components is due to the cost of silicon wafer substrates. Although silicon is a highly abundant and cheap element, the transformation and processing from the raw material into high quality silicon wafers results very costly. In fact, the cost of silicon substrates constitutes ~1/3^rd of the total cost of a memory chip and about ~1/10^th of the cost of a high-end state of the art micro-processor. The societal, industrial and technological demands of ultra-low-cost electronic components has spurred the quests towards lower cost substrates. This has motivated a surge of works on paper-based electronics in the last years. In fact, paper substrates cost (~0.1 €/m²) is orders of magnitude lower than that of polymer substrates (PET ~2 €/m² and PI ~30 €/m²) and crystalline silicon (~1000 €/m²).

Despite the promises of paper-based electronics, there are several challenges to be solved. One of the major challenges is that the rough, fiber-based structure of paper makes it impossible to fabricate devices using conventional lithographic techniques. In this talk I will discuss our last works to integrate different van der Waals materials onto standard paper substrates [1-4].

[1] J Azpeitia et al. Materials Advances (2021)
[2] W Zhang et al. Applied Materials Today (2021) 23, 101012
[3] M Lee et al. Nanoscale (2020) 12 (43), 22091-22096
[4] A Mazaheri et al. Nanoscale (2020) 12 (37), 19068-19074

Devices based on 2-Dimensional materials have been extensively investigated in recent years. Reasons such as superior electrical, optical, thermal, and mechanical properties have made these materials attractive for the next generation of devices. Yet, the realization of these 2D devices is still hindered by many challenges including a comprehensive understanding of the electrical and optical characteristics of 2D materials. In this talk, I will discuss our recent work carried out on different 2-D devices. First, I will discuss our recent development of our new pulsed thermal annealing technique to produce a controlled and tunable light emission from black phosphorus nanosheet. This tunable light emission exhibits a wide bandwidth reaching up to110nm. This wide bandwidth has been found to be caused by the formation of black phosphorus oxide, which exhibit a direct bandgap that can cause radiative processes at room temperature.

Second, I will demonstrate our recent work on Hafnium Diselenide (HfSe₂), an emerging 2D material that exhibit a bandgap of ~1.1eV which is analogous to silicon. I will show how HfSe₂ can exhibit conductivity switch using deterministic laser induced methods. This conductivity switch has been observed in both phases of HfSe₂ (1T and 2H). This conductivity switch has been attributed to an increase in the Schottky barrier height at the contacts, which pushes the Fermi level closer to the valance band, causing the observation of this conductivity switch.

Moreover, I will discuss our recent results obtained on Germanium Sulfide devices. Due to its similar structure to black phosphorus, GeS can be a promising material for device applications. Yet, GeS based devices are facing a major challenge due to the high contact resistance obtained. I will demonstrate our new method of 2D doping of GeS that exhibit an intentional conductivity switch from p-type to n-type with a conductivity enhancement reaching more than 10 folds. Our technique relies on deterministic deposition of degradable black phosphorus nanosheets which chemically interacts with GeS channel. We also observe a striking Negative Differential Resistance (NDR) accompanied during the transient doping of GeS devices. We characterize these devices and show that this NDR behavior is caused by partial doping in the transient state, which creates resonant tunneling paths causing the observation of this remarkable NDR behavior.

Finally, I will discuss our recent fabrication work on exfoliated Tellurene nanosheets for device applications. This material which exhibits superior thermoelectric and optoelectronic properties, is facing a major exfoliation challenge. Previous work has demonstrated solution processing technique to obtain few layers Te nanosheets . Yet, additive chemicals during liquid exfoliation may impose a potential risk in altering the pristine nature of Tellurene. I will demonstrate Tellurene thinning using specific oxygen annealing technique. Morphology measurements show a favorable thinning direction which is attributed to the anisotropy of the material. This thinning method can be a stepping milestone towards pristine few layers Tellurene devices. Our results demonstrate promising future for 2D materials to be used in device applications.

Understanding the underlying physical mechanisms that govern charge transport in two-dimensional (2D) covalent organic frameworks (COFs) will facilitate the development of novel COF-based devices for optoelectronic and thermoelectric applications. In this context, the low energy mid-infrared absorption contains valuable information about the structure-property relationships and the extent of intra- and inter-framework “hole” polaron delocalization in doped and undoped COF structures. A theory describing the spatial coherence length of polarons in disordered COFs is presented, revealing a simple relationship between the oscillator strength of the mid infrared absorption band and the polaron coherence function. Emphasis is placed on the fundamental nature and origin of the components polarized along the intra- and inter-framework directions and their dependence on the spatial distribution of disorder as well as the position of the dopant counter-anion relative to the framework. The Hamiltonian, represented in a multiarticle basis set,¹ has been successful in quantitatively reproducing recently measured spectra recorded in doped COF films and our analysis provides conclusive evidence of why iodine doped COFs have so far shown lower bulk conductivity compared to doped polythiophenes.² Finally, we propose new research directions to address existing limitations and improve charge transport in COFs for applications in functional molecular electronic devices.

1. Raja Ghosh and Frank C. Spano, “Excitons and Polarons in Organic Materials”, Acc. Chem. Res. 2020, 53, 10, 2201–2211
2. Raja Ghosh and Francesco Paesani, “Unraveling the Effect of Defects, Domain Size, and Chemical Doping on Photophysics and Charge Transport in Covalent Organic Frameworks”, Chem. Sci. 2021, 10.1039/d1sc01262b

MXenes are a rapidly developing class of two-dimensional materials with suitable properties for use in emerging electrochemical energy storage devices, including batteries and supercapacitors. The chemical synthesis of MXene leads to the surfaces being functionalized with different functional groups (O, OH, and F). Since MXene layers are only atomically thin, such surface functionalities can play an essential role in the electrical conductivity of MXene. Conductivity is a key for both the transport and storage of ions in MXene during their energy applications. While there are some experimental studies on the electrical conductivity of MXene, theoretical conductivity studies are rare. Investigations of charge carrier transport in MXene nanowires necessarily involves calculations with hundreds of atoms at the quantum level, which typically requires extraordinary computational resources. Complications due to the surface structure variability increases with model system size and increasing numbers of functional groups. The density-functional tight-binding (DFTB) approach enables the treatment of complex systems with several hundred to thousands of atoms at the quantum level at reduced computational cost with good accuracy compared to DFT. In this study, we use DFTB calculations with the nonequilibrium Green’s functions (NEGF) technique to calculate in-plane transport in the MXenes (i.e., within the MXene layers) as a function of composition. All MXene compositions were found to show a linear relationship between current and voltage at lower potentials, indicating their metallic character. However, their I-V curves, i.e., conductivity, show different trends among different compositions. MXenes without any surface terminations (Ti₃C₂) show higher conductivity compared to MXenes with surface functionalization (Ti₃C₂O₂ and Ti₃C₂(OH)₂). Among the MXenes with O and OH termination, MXenes with O surface termination have lower conductivity than OH surface termination. Conductivity changes with the ratio of O and OH on the MXene surface. I-V curves and their conductivities correlate with transmission functions and the electronic density of states (DOS) around the Fermi level. The surface composition-dependent conductivity of the MXenes provides a path to tune their conductivity for enhanced pseudocapacitive performance.

Acknowledgment: This material is based upon work supported by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. DOE Office of Science. The research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231.

Transition metal chalcogenides (TMDCs) are two-dimensional semiconductor materials with a thickness of ~0.7 nm at the monolayer limit. A carrier modulation method of TMDCs is surface charge transfer doping by a junction with other materials such as potassium, Cs₂CO₃, and polyvinyl alcohol, etc. [1] One of the strongest modulating agents is the molecular benzyl viologen (BV) [1,2]. In previous work, BV-molecular treated MoS₂ showed degenerate semiconducting behavior without electrostatic gate-dependence in the metal-oxide-semiconductor field-effect-transistors (MOSFETs). However, the device characteristics of the organic BV-doped TMDC MOSFET have not been characterized sufficiently, and the metallic behavior by the molecular-doped has not been examined yet. The metallic behavior should be analyzed from the temperature-dependent transport behavior. In this study, we demonstrate the temperature and magnetic-field dependence of the monolayer and multilayer MoS₂ MOSFETs and characterize the behavior of the organic BV-treated MoS₂ as the metallic state and discuss the conduction mechanism of the device.
We fabricated back-gated MOSFET using monolayer and multilayer (~10 nm thick) MoS₂ flakes on Si/SiO₂ substrate and carefully measured the transport properties in the temperature range from cryogenic (~2K) to room temperature. The conductance of the device increased with decreasing the temperature in both monolayer and multilayer devices without applying gate-voltage. Therefore, the BV-doped MoS₂ obviously transformed from a semiconductor to a metallic situation.
As expected from the temperature-dependency of the BV-doped MoS₂ MOSFET, the transfer characteristic curves of BV-treated devices showed a gate-independent behavior with an ON/OFF ratio of ~10¹ in both monolayer and multilayer devices at room temperature. We further analyzed the gate- and temperature-dependency of the MOSFETs of monolayer and multilayer MoS₂ MOSFETs. In the case of monolayer MoS₂ MOSFET, the conductance just increases with decreasing the temperature even applying negative gate-voltage. On the other hand, the conductivity in the multilayer sample decreases under the negative gate-voltage with decreasing the temperature, therefore, the transport mechanism would be different in the monolayer and multilayer samples. To discuss further the details of the transport behavior, we focused on the contact resistance of the BV-doped devices. Monolayer MoS₂ MOSFET shows small gate-dependency of the contact resistance, therefore, the direct current path between the electrode and MoS₂ channel would be constructed. On the other hand, the multilayer MoS₂ device showed a large dependence of the gate voltage, this may be attributed to the additional carrier conduction under the multilayer MoS₂. In the presentation, we will discuss the details of the transport behaviors.

[Reference]
[1] P. Luo et al. Nanoscale Horiz., 4, 26-51 (2019).
[2] D. Kiriya et al. J. Am. Chem. Soc. 136, 7853-7856 (2014).
[3] K. Matsuyama et al. ChemistryOpen, 8, 908-914 (2019).

Graphene/diamond (carbon sp²-sp³) interfaces have attracted much attention because the interfaces become sources of various interesting electronic phenomena. Recently, we found that vertically-aligned graphene (VG)/diamond heterojunctions (C sp²-sp³ interfaces) became photo-controllable memristors [1], which are optically controllable memory-resistors with nonvolatile memory and switching functions. One of the most important application of the photo-controllable memristors is thought to be optoelectronic memory, which possess both photo-sensing and data storage function. In this study, we demonstrate that VG/diamond heterojunctions can be used as special types of optoelectronic memory, that is, brain mimic optoelectronic memory, that exhibit some of the basic functions of the human brain, such as an excitatory postsynaptic current (EPSC) and transition from short (STM) to long-term memory (LTM) states.
VG/diamond bilayers were grown in situ by using microwave plasma CVD, and VG/diamond junctions with diameters of 80-160 µm were fabricated using the bilayers [1, 2]. The I-V characteristics of these junctions in response to photoirradiation were measured using blue light emitting diodes (LEDs) or a fluorescent lamp in air at room temperature.
Raman, SEM and TEM results suggest that VG/diamond heterostructures with sharp interfaces were successfully formed. The VG/diamond junctions exhibited hysteretic I-V characteristics. The resistance of the junctions was changed to low or high resistance states by the light irradiation with bias voltage application and the resistance states was maintained in nonvolatile manner. The resistance difference ratio between the HRS and LRS (= R_HRS/R_LRS) was approximately 10² and was maintained for 2 days.
The VG/diamond junctions also exhibited clear responses to optical pulses and brain-like optoelectronic memory properties were observed. The junctions’ current, which depended on the conductivity of the junctions, increased stepwise in response to each optical pulse, and a value of approximately 11 nA was obtained after applying 10 optical pulses. The current was subsequently maintained without significant deterioration. This generation of a current after frequent stimulation (via optical pulses) is analogous to the generation of the EPSC at biological synapses. Thus, the present VG/diamond junctions can be said to have produced an EPSC in response to optical stimulation. The magnitude of the EPSC generated by the junctions could be tuned by varying the intensity, width, frequency and numbers of the optical pulses, suggesting good controllability of the EPSC based on applying optical spikes. The energy consumption per event under these conditions was estimated to be approximately 440 pJ. This energy value is quite low and suggests the possibility of fabricating devices that mimic brain functions with very low energy consumption.
The memory retention time of the junctions was changed from shorter to longer by increasing the irradiated numbers of optical pulses. The behaviors of conductance decay can be described well by using the following decay function, G(t)/G₀ = exp[-(t/τ)^β], which was often used for analysis of forgetting characters of the human brain. The relaxation time τ for above 7 pulses became ~2 orders higher than those for below 4 pulses. The memory character of the junctions was changed by optical pulses, and transition from STM to LTM states was caused by stronger optical stimulation. These kind of STM-LTM transition was also caused by controlling frequency of optical pulses. These results indicate that the VG/diamond junctions can be used as novel brain mimic optoelectronic memory devices, where weakly stimulated information was lost immediately and only strongly stimulated information was kept for a long time.
Ref.: [1] K. Ueda et al., J. Mater. Res. 34 (2019) 626, [2] K. Ueda et al., Appl. Phys. Lett. 117 (2020) 092103.

The emergence of two-dimensional (2D) magnets offers exciting opportunities to discover new physical phenomena. Theoretical studies predicted that substitutional doping of magnetic atoms in transition metal dichalcogenide (TMD) monolayers would enable 2D ferromagnets at room temperature, which is a critical requirement for practical applications [1]. Experiential observations of magnetism have been made in bulk TMDs or transition metal-doped few-layer TMDs, using dopants, including V, Mn, Co, Ni, Cu, Nb and Re [2, 3]. Recently, the RT ferromagnetism was successfully demonstrated in in-situ Fe-doped MoS₂ (Fe:MoS₂) and V-doped WSe₂ (V:WSe₂) via chemical vapor deposition (CVD) [4, 5]. These 2D magnets can be used as electrodes or barriers in vdW magnetic tunnel junctions (MTJs). Previous reports include an MTJ using MoS₂, CrI₃, or graphene as a tunneling barrier and using Fe₃GeTe₂ or Cr₂Ge₂Te₆ as magnetic electrodes [6–8]. The magnetic switching in the MTJ can be achieved using spin-transfer torque (STT), where the orientation of the Fe layer can be fliped using a spin-polarized current. A fast the current-induced magnetization switching (CIMS) is important for high-speed and low power next-generation spintronics devices [9], which we expect to achieve using 2D magnets. Here, we present the layer-transfer of graphene and Fe-doped MoS₂ (Fe:MoS₂) monolayers on hBN toward fabricating an MTJ consisting of a Fe/hBN/Fe:MoS₂ heterostructure. This Fe:MoS₂ monolayers were synthesized via low-pressure chemical vapor deposition to induce magnetic properties at a room temperature. Here, hBN is used as the tunneling barrier, Fe:MoS₂ as the free layer, and Fe thin film as the fixed layer. As a next step, we will characterize the junction magnetoresistance inside an ultra-low vibration cryogen-free cooling closed-cycle cryostat with a temperature range from 4 K to 300 K. We will measure the CIMS by applying short current pulses over the junction. This work is expected to pave the way to developing van der Waals spintronics based on various magnetic transitional metal dichalcogenides.
References:
[1] Y. Wang, S. Li, and J. Yi, Scientific Reports, 6 (1), 24153, (2016).
[2] Y.-C. Lin, Advanced Science, 8 (9), 2004249, (2021).
[3] T. Zhang, ACS nano, 14 (4), 4326–4335, (2020).
[4] F. Zhang, Advancved Science, 7 (24), 2001174, (2020).
[5] S. Fu, Nature Communications, 11 (1), 2034, (2020).
[6] Z. Fei, B. Huang, Nature Materials, 17 (9), 778–782, (2018).
[7] K. Shayan, Nano Letters, 19 (10), 7301–7308, (2019).
[8] S. Jiang, Nature Materials, 17 (5), 406–410, (2018).
[9] S. Ikeda, Nature Materials, 9 (9), 721–724, (2010).

Localized graphitization of aromatic polyimides (PIs) can be achieved by laser direct write process owing to the high absorptance in the infrared wavelength range of commercial CO₂ laser cutting/scribing machines. This one-step photothermal process allows the formation of unique hierarchical nano-/micro-structured graphene patterns on the surface of polyimide film. Nevertheless, controlling chemical doping in this laser-induced graphene, has been limited thus far. Herein, we present a facile approach for manufacturing patternable, heteroatom-doped, hierarchically porous graphene from molecularly engineered PI films using a continuous-wave 10.6 μm laser in ambient air. Conventional two-step polymerization of PIs was carried out from 4,4'-oxydianiline and three different commercially available tetracarboxylic dianhydrides with different internal linkages such as phenylene, trifluoromethyl or sulfone groups. We show that the chemical structure of PIs and laser fluence of the incident radiation affect the gas evolution, inducing different structure rearrangements, as interesting heteroatom self-doping (N-doping, F-doping, S-doping) and consequently different electrical and electrochemical properties. Impressively, the electrical resistivity of F-doped laser-induced graphene can be precisely tuned to lower than ~13 Ω sq^–1 Moreover, these fabricated grapehene electrodes are capable of detecting nanomolar concentrations of neurotransmitter, which is promising for implantable neural probes. Hence, the proposed laser one-step synthesis of graphitic structures can be readily extended for the fabrication of various advanced materials including early diagnosis of neurological disorders.

A novel process to synthesize plasmonic MoO_3-X nanosheets is demonstrated, in which MoS₂ powders suspended in ethanol/water are irradiated with pulses from a femtosecond laser, resulting in simultaneous Coulomb explosion, photoexfoliation, and oxidation. The oxidation process is found to start with the formation of hydrogen-bonded molybdenum oxide (H_XMoO₃), followed by the release of -OH₂ groups to create oxygen vacancies, and finally the MoO_3-X is oxidized to MoO₃ after extended irradiation. The formation of H_XMoO₃ is the critical step to create enough oxygen vacancies for localized surface plasmon resonance (LSPR), and this step is attributed to H₃⁺ dissociated from ethanol under femtosecond laser irradiation. It is found that 80-90% ethanol is the optimal concentration to synthesize plasmonic MoO_3-X, where the balance of water facilitates the release of the -OH₂ groups to create the required vacancies. It is shown that different organic solvents like methanol, 1-propanol and isopropyl alcohol that were reported to generate large amounts of H₃⁺ under femtosecond laser irradiation can also oxidize MoS₂ into plasmonic MoO_3-X. The LSPR properties of the synthesized MoO_3-X are evaluated by UV-vis spectroscopy and photothermal conversion measurements.

Graphene has a transmittance higher than 85% for extreme ultraviolet (EUV) light (13.5 nm) and high in-plane Young’s modulus. Graphene can satisfy all characteristic indicators as a material of EUV pellicles. Among the many techniques developed for synthesizing high-quality graphene, metal induced crystallization (MIC) of amorphous carbon is a unique technique for fabricating uniform multilayer graphene (MLG) that has an advantage in pellicle application due to no additional transfer process. In this study, we investigated Ni catalyzed crystallization of amorphous carbon on SiN_x/Si substrate, and further on diffusion mechanism between amorphous carbon and Ni. We found the optimal conditions to form a highly uniform MLG as a material for EUV pellicle. We observed the effect of carbon density and thickness ratio of Ni and carbon layer(t_a-C/t_Ni) on the MIC and succeeded in forming uniform graphene on 8 inch wafer with short annealing of 1 h at 500^oC. The density of amorphous carbon was tuned by changing sputtering conditions such as power and gas flow rate. The formation of MLG on SiN_x/Si was confirmed by Raman spectroscopy, atomic force microscopy, cross-section transmission electron microscopy. The kinetic model between amorphous carbon and nickel was substantiated by experimental findings based on amorphous carbon/Ni stacked structures of various thicknesses and densities with different heat treatment conditions.

Mesoporous spherical graphene oxide (MSGO) that is transformed from two-dimensional (2D) GO sheets can not only avoid the undesirable restacking of 2D nanomaterials, but also inherit the unique properties of the GO building blocks such as good water dispersibility, easy access for reduction and chemical functionalization due to the existence of abundant surface oxygen-containing functional groups, showing attractive application potential in batteries, nanoreactors, catalysis, adsorption, supercapacitors and biomedical fields. However, to date facile synthesis of nanoscale MSGO particles with controllable morphology is still a grand challenge.
Kirigami, one type of ancient creative paper-cutting art, has recently emerged as an attractive way to construct flexible 3D curved structures. By introducing cut-outs into the paper plane, the flat sheets can accommodate local in-plane stretch and transform into curved surfaces. Inspired by such Kirigami technique, a facile way to synthesize nanoscale MSGO through spray drying of holey GO (HGO) sheets is provided. By increasing pore size and porosity of the holey GO sheets, the spray-dried particles underwent the transition from rough crumpled structure to smooth spherical shape.
Similar to the Kirigami process, after introducing the in-plane holes, both the elastic modulus and effective stiffness of the GO sheets decline and the ductility improves. The introduced holes allow for the in-plane stretch, and the part between adjacent holes can serve as a hinge to accommodate the out-plane bending and twisting, which can neutralize the in-plane stress concentration. The obtained soft HGO sheets can keep compliant with the contracting spherical water aerosol droplets during the spray drying process. By contrast, the pristine GO sheets with high stiffness can only relax the stress concentration through crumpling and folding the surface. During the spray drying process, the evaporation-induced capillary forces acting on the pristine GO sheets and HGO sheets are different. For the pristine GO sheets, the water molecules can only escape from the narrow gap between the stacked GO sheets. In this case, the GO sheets are clung and dragged by the evaporation-induced capillary force, nonuniform deformation including wrinkles and folds are generated. For the HGO sheets, the etched nanopores facilitate evaporation of water molecules through the introduced holes, thus generating isotropic capillary forces that compress the self-assembled HGO shells into MSGO particles with smooth surfaces and abundant pores.
Expect for the morphology, the particle size, porosity and specific surface area of MSGO particles can all be turned through tailoring the etching time and precursor concentration. The prepared MSGO particles have tunable diameters smaller than 200 nm and a high specific surface area of 503 m²/g, which enable consistent high-efficiency physical adsorption removal of organic dyes such as Rhodamine B from polluted water with a high adsorption rate of 335.6 mg/(g min).
Moreover, the spray drying synthetic approach also enables easy incorporation of other particles and elements within MSGO particles to prepare multifunctional composite particles. For example, CuO/MSGO hybrid particles were synthesized by combining the aerosol drying of the HGO solution loading with CuSO₄ precursors and subsequent microwave heating of the dried particles. In another case, N-doped MSGO particles can be prepared by annealing the as-synthesized MSGO particles together with melamine under argon atmosphere at 850 °C for 3 h. Both the spherical structure and particle size of the MSGO particles can be maintained.
It is expected that the reported Kirigami-inspired spray drying is also applicable for converting other holey 2D sheets into mesoporous nanospheres, thus opening up new opportunities for synthesis functional mesoporous nanomaterials, which may enable a broad range of promising applications.

Carbon nanostructures, such as graphene and carbon nanotubes incorporated in metals have recently gained great interest for the possibility of enhancing the electrical and thermal conductivities as well as mechanical strength of the metal. This is possible because of the combination of excellent charge carrier mobility, thermal conductivity and mechanical strength of the carbon nanostructures and the high density of electrons in the metal. The improved properties make these nanocarbon metal composites unique candidates in applications such as high power transmission lines, interconnects, heat exchangers, motors, photovoltaic cells and transparent electrodes, among others. We developed a process to synthesize graphitic nanostructures in metals by the application of a high electrical current to liquid metal containing particles of activated carbon. The process called electrocharging assisted process transforms the activated carbon particles from amorphous to crystalline graphitic nanoribbons in the liquid metal. Upon solidification of the metal the nanoribbons self-assemble with an epitaxial relation with the metal. We have used Al 1350 and activated carbon particles of ~100 nm and obtained samples with global electrical conductivities more than 5% higher than the parent alloy as well as enhanced local stiffness, measured by nanoindentation. The conductivity enhancement is correlated with increasing crystallite size and concentration of the produced graphitic nanostructures. We have investigated different parameters in the process such as applied current, duration of the current, stirring speed of the liquid metal (with and without Argon bubbler) to optimize the process. The approach is scalable to large scale manufacturing.
Supported by DOE EERE Award EE0008313.

The limitless quest for building low cost and sustainable batteries to effectively harvest renewable energy from intermittent sources is ever-increasing. The research on Na-ion technology has gained vital interest due to low-cost and broad availability of precursor materials. Till now, various Na-ion battery anodes have been explored including carbonaceous materials such hard carbon, alloy-based materials such as Bi, Pb, and Sb based systems and conversion materials ¹. Along this line, the recent chase on two-dimensional (2D) materials such as chalcogenides, MXenes and van der Waals types oxyhalide heterostructures, have gained recent attention due to their excellent electrical and structural properties ². Two-dimensional (2D) materials are considered as more effective players regarding ionic intercalation compared to their bulk counterparts. This property makes 2D materials attractive for applications such as energy storage. Among them, 2D Sillén-type metal oxyhalide heterostructures (MOX), due to their unique layered structure consisting of anionic sheets interleaved with cationic layers, have gain particular interest as electrode materials. BiOX is one such candidate with fluorite-like layers, [Bi₂O₂]²⁺ intergrown with halide X^– anions, have been explored as anode in rechargeable batteries. For instance, the 2D-(Bi_1-xFe_x)OCl have increased insertion kinetics in Li-ion storage performances ³ compared to the unsubstituted BiOCl, whereas 2D-(Bi_1-xSb_x)OCl heterostructures have shown synergetic effect of (Bi/Sb) alloys on with improved K-ion storage capacity ⁴. However, in continuation, the further assessment of other mixed cationic Sillén heterostructures with the crystal structures of [Bi_xM_1-xO₂]ⁿ⁺ (M = Na⁺/Ca²⁺/Pb²⁺/Fe³⁺/Sb³⁺; n = +1, +2 and +3) layers intergrown with single/double/triple halide anionic layers, remains unexplored. This is mainly challenged by their current synthesis strategies demanding high temperature, longer reaction durations, toxic chemicals, and complex reaction setup.

In this work, 2D Sillén-type metal oxyhalide 2D-(Bi_1-xM_x)O_xCl, M= Pb i.e., mixed lead bismuth heterostructure nanosheets were synthesized by new rapid room temperature, facile, rapid, and scalable chemical precipitation route ⁵. The stepwise reaction intermediates were harvested and investigated further with XRD and SEM mechanistic studies. The formation proceeds via topotactic ion exchange route from analogous intermediate phases. When assessed as an Na-ion battery anode, this 2D heterostructure exhibited stable sodium storage performance (~250 mAh g^–1 for 250 cycles at 100 mA g^–1). Furthermore, its electrochemical mechanism was examined with the combined in-operando XRD and ex-situ TEM, XPS studies, which reveals a distinct (de)sodiation process of the heterostructure anode in comparison with reported phase diagrams. Henceforth, the detailed understanding of formation process of 2D-(Bi_1-xM_x)O_xCl heterostructures at room temperature using non-operando material characterizations and electrochemical studies combined with operando findings will be discussed to comprehend the reversible storage mechanism of 2D- Sillén-type metal oxyhalide heterostructures as anodes in SIBs.
References:
1. J.Y. Hwang, S.T. Myung, Y.K. Sun, Chem. Soc. Rev. 2017, 46, 3529–3614.
2. M. Zeng, Y. Xiao, J. Liu, K. Yang, L. Fu, Chem. Rev. 2018, 118, 6236–6296.
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5. V. Ahuja, S. Singh, R. Vengarathody, P. Senguttuvan, J. Phys. Chem. C 2021 (Under revision)

Two-dimensional (2D) materials gather attention because of their large carrier mobility, transparency, and high-optoelectronic characters in a thin-confined structure. Transition metal chalcogenides (TMDCs) with the formula of MX₂ (M = transition metals, X = chalcogen atoms) are a family of 2D materials; MoS₂ is a representative material of TMDCs, which shows direct bandgap in a single layer. The single-layer MoS₂ further shows distinctive physical properties such as piezoelectric effect and second harmonic generation due to its symmetry-breaking structure.
It would be important to obtain thin layers selectively on the substrate to study 2D materials effectively. Usually, samples are prepared via a mechanical exfoliation method because the method is simple, and the exfoliated crystals usually show good crystallinity. Although the exfoliation method can transfer monolayers on the substrate, a large number of bulk flakes are simultaneously obtained. The bulk flakes cause trouble to make devices because the electric lines sometimes may disconnect by overwrapping the thick flakes. Once transfer the flakes, it is very hard to remove the bulk flakes selectively. In this study, we show our method of selective removal of bulk flakes from the substrate and maintain thin layers including monolayers by applying sonication in organic solvents.
MoS₂ flakes are transferred on a Si/SiO₂ substrate using a general mechanical exfoliation method with scotch tape. Various thick flakes cover the substrate. Then, the substrate is sonicated under an organic solvent for 5 minutes at most. After the sonication, the bulk flakes are selectively removed, and the thin layers, including monolayers, remain on the substrate.
In the process of flake-removal process, the solvent is critical. In some solvents, the selectivity is very low, and a lot of bulk flakes remained on the surface. This removal process is highly solution-dependent. The relationship between the solvent and residual rate is analyzed in terms of polar dispersion factors of the solvents. In the presentation, we will show the details of the mechanism of the selective removal of the sonication method.

Two dimensional (2D) layered materials emerged enormously in the past decade, largely being investigated fundamentally and practically. The unique layered structure and atomic-scale thickness make them attractive to have exclusive electrical and optical properties than their bulk counterparts. 2D layered materials easily integrated and formed heterostructure due to the dangling bond free surfaces. Heterostructure can access more functioning ability beyond its individual constituent. However, for novel electronic/optoelectronic device applications, it is very critical to understand the charge carrier dynamics at the interface of heterostructures. Here, we report a highly sensitive self-powered MoS₂/ReS₂ broadband photodetector, covering visible (400 nm) to NIR (1100 nm) regime with a high value of responsivity 42.61 A/W at 1V under the illumination of 800 nm, which is approximately 16 times of the reference pristine MoS₂ photodetector. The favourable energy band alignment of n-n hetero-junction is responsible for enhanced photo-responsivity and self-driven feature of the detector. Moreover, fast rise/decay transient photoresponse (20/19 ms) strongly advocate the spatial separation of charge carrier across the interface. This work facilitates the understanding of fundamental interfacial study of a new growing van der waals heterostructures for the development of novel device applications.

Two-dimensional (2D) layered materials exhibit an intrinsic attitude to sustain high curvature angle without rupture. ^[1]Therefore, shaping 2D layers into complex curved geometries is emerging as an exciting way to access unprecedented physical properties driven by two different responses to the strain-induced morphological modifications. On one hand, deformations of the atomic lattice positions lead to energetic bandstructure modifications with dramatic impacts on the opto-electronic properties of the 2D materials.^[2] On the other hand, accommodation of strain through slip-and-shear mechanisms of the layers leads to superlubricity .^[3]
Here we show that shape transformations in atomically thin MoS₂ layers can be controlled in the first stage of their formation by means of chemical vapor deposition on pre-patterned substrates by design. Aiming at harnessing the local curvature of mono- and few-layers MoS₂, we functionalize the pre-patterned substrates via aromatic perylene molecules serving as nucleation seeding promoters.^[4] Based on the analysis of atomically resolved transmission electron microscope images, we demonstrated that the so-grown MoS₂ layers bend conformally without rupture and detachment from the substrate even at the extreme geometric features (asperities, edges, etc..) only when the seeding promoter is used. Moreover, theoretical energetic arguments allow us to quantitative derive the role of the perylene molecules to increase the interfacial adhesion energy between MoS₂ and SiO₂ substrate by balancing the strain and bending energy contributions.
References
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[2] A. Castellanos-Gomez, R. Roldán, E. Cappelluti, M. Buscema, F. Guinea, H. S. J. van der Zant, G. A. Steele, Nano Lett. 2013, 13, 5361.
[3] J. Yu, E. Han, M. A. Hossain, K. Watanabe, T. Taniguchi, E. Ertekin, A. M. Zande, P. Y. Huang, Adv. Mater. 2021, 33, 2007269.
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Developing sensors that can be operated at room-temperature and ambient conditions has been an important goal for a wide range of applications. Coupled with this goal is the desire to have sensors that are small, low-power, fast and able to detect small concentrations of a molecular species. Here, we demonstrate a room-temperature, highly sensitive, selective, stable, and reversible chemical sensor based on single layer and few layers of the transition-metal dichalcogenide alloy Re_0.5Nb_0.5S₂. This alloy exhibits short-range ordering of atomic species due to the geometric frustration in the triangular sublattice that corresponds to the transition metal atoms. The resulting sensing device exhibits a thickness-dependent carrier type due to the band alignment changing with thickness. Upon exposure to NO₂ molecules, its electrical resistance considerably increases or decreases depending on thickness. The sensor is highly selective to NO₂ with only minimal response to other gases such as NH₃, CH₂O, and CO₂. Surprisingly, in the presence of humidity, the sensor shows complete reversibility with fast recovery at room temperature, in contrast with other similar sensors whose performance is lowered with humidity. In this presentation, we focus on the theoretical analysis of the sensing platform and identify the atomically sensitive transduction mechanism. We also explain the structural properties of the Re_0.5Nb_0.5S₂ and its electronic properties.

Acknowledgments: This work was primarily supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05-CH11231, within the sp2-bonded Materials Program (KC2207), which provided for materials synthesis, chemical sensitivity tests, and atomic structure calculations. Additional support was provided by the National Science Foundation under grant no. DMR-1807233, which provided for STEM measurements, and under grant no. DMR-1926004, which provided for calculations of precise electronic structures. Computational resources were provided by the DOE at Lawrence Berkeley National Laboratory’s NERSC facility and the NSF through XSEDE resources at NICS.

Lanthanum nickelates (La_n+1Ni_nO_3n+1) with two-dimensional perovskite structure, so called Ruddlesden-Popper phases, have attracted interests for their electronic properties derived from their layered structure and mixed valence Ni-ions ^[1]. These two-dimensional perovskite layers are also expected as precursors to develop infinite-layer nickelate such as Sr-doped NdNiO₃ for which superconducting behavior was reported recently^[2]. Epitaxy of multilayered La_n+1Ni_nO_3n+1 thin films and characterizing their structure, electronic state, and properties according to interfacial strain and aliovalent doping would contribute to further comprehension of their solid-state physics. There are also reports of La_n+1Ni_nO_3n+1 for its metal-insulator-transition according to oxygen deficiency, and applicability to SOFC cathodes and gas sensors ^[1,3]. However, aliovalent doping in La₄Ni₃O₁₀ epitaxial thin films similar to La₂NiO₄ and La₃Ni₂O₇ still requires further researches to understand its effect on structure and properties ^[5,6]. In this study, epitaxial growth of triple-layered La₄Ni₃O₁₀ thin films and the effect of tetravalent dopants substituting La³⁺ on the growth, structure, and electric properties was investigated.
The lanthanum nickelate thin films were grown on single crystal substrates such as SrTiO₃, NdGaO₃, and LaAlO₃ by pulsed laser deposition technique equipped with a KrF excimer laser (λ=248 nm, d~20 ns, and E~1.0 J/cm²). The films were deposited at substrate temperature range from room-temperature (not heated intentionally) to 750°C in ~10 Pa O₂-flowed ambience (base pressure ~3×10^–6 Pa) using sintered targets of pure and (Hf⁴⁺, Sn⁴⁺) co-doped La₄Ni₃O₁₀. The thin films were subsequently post-annealed in high-purity O₂ atmosphere (1 atm, 200 sccm) at temperature up to 1100°C to improve crystallinity as well as to modify the crystal phases and the valence of Ni-ions. The epitaxy of La₄Ni₃O₁₀ (001) thin films after annealing in O₂ was verified by {001} diffraction and four-fold planar symmetry detected in XRD 2θ/ω and φ scans, while epitaxial growth of La₂NiO₄ (100) was observed for the as-grown films. The Hf and Sn-doped La₄Ni₃O₁₀ (001) thin films with thickness of ~100 nm demonstrated low resistivity of ~5×10^–3 Ωcm at room-temperature. The obtained epitaxial thin films would allow us further formation and analyses of aliovalent doped and multi-layered infinite layer nickalates reduced from the Ruddlesden-Popper phase. Additional detailed structural analyses of the doped La₄Ni₃O₁₀ thin films, and the effect of the tetravalent doping on the electronic property would also be discussed.
[1] K.-T. Wu et al., J. Mater. Chem. A, 5 (2017) 9003–9013.
[2] D. Li et al., Nature, 572 (2019) 624–627.
[3] V. Pardo et al., Phys. Rev. B, 83 (2011) 245128.
[4] S. Yoo et al., RSC Advances, 2 (2012) 4648–4655.
[5] R. J. CaVa et al., Phys. Rev. B, 43 (1991) 1229–1232.
[6] S. A. Nedilko et al., J. Alloys Compd., 367 (2004) 251–254.

Recent developments have shown that chemical synthesis is a powerful tool for controlling the size, shape, and composition of two-dimensional (2D) crystals with broad implications for their implementation in optoelectronic and quantum devices. However, the gas-phase reactions carried out by most researchers are not conducive to precise manipulation of the 2D crystal edge structure. Here we demonstrate a salt-assisted low-pressure chemical vapor deposition method, which enables growth of 2D WSe₂ monolayers whose edge morphology can be tuned from straight, to sawtooth, to fractal-like by adjusting the ratio of WO₃ to NaCl. We discuss the volatility of the metal precursor and its role in dictating not only the edge structure, but also the defect density within the as-grown crystals. These studies provide important insight into new 2D crystal growth modes and synthetic strategies for producing crystals with unique structures and optoelectronic properties.

Layered 2D sheets of Transition Metal Dichalcogenides (TMDCs) exhibit interesting optical and electronic properties such as indirect to direct bandgap transition and valley-spin coupling when scaling from bulk to a few layers. TMDCs are also flexible and versatile for constructing a wide range of heterostructures with atomic-level control over their layer thickness, devoid of issues pertaining to lattice mismatch. Hence layered TMDCs have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. Therefore, a detailed structural study on these materials is important to develop novel electronic and optoelectronic devices. Low-Frequency Raman (LFR) Spectroscopy is an ideal tool to investigate the signature structural information and the layer-to-layer dependency of TMDCs. LFR is a highly efficient technique for material analysis and layer identification with significant advantages over other vibrational or fluorescence spectroscopy methods. This technique remains to date one of the most popular characterization tools because of its non-invasive nature and ultra-fast detection limits. With LFR measurements, crucial information such as the tilt and number of layers can be determined. For instance, enhancement in optical properties can be understood by monitoring the evolution of the in-plane (shear- E’/E_g) and the out-of-plane ((A’₁/A_1g)) vibration modes with the number of layers.
In this work, we explore a combination of metal/dielectric substrates to enhance the LFR scattering of different 2H-TMDC layers of WSe₂, WS₂, MoSe₂, and thereby to understand the structure of these nanoscale films. Phase-pure 2H-TMDCs were grown via an in-built two furnace ambient pressure CVD and the layers of exfoliated flakes were studied. Layer thickness and number of layers of 2H-MoSe₂, WSe_2, and WS₂ were varied. We used different substrates such as coatings of Al₂O₃ with different thickness on top of Al mirror layer, Al coated glass and Raman-grade CaF₂ among other substrates, to study bi-layers and a few layers of the above-mentioned TMDCs. We observe an enhancement of 30 times in LFR signal corresponding to WSe₂ and WS₂ on an Al₂O₃ coated Al substrate with oxide thickness of 80 nm and Al thickness of 200 nm.

References:

1. X. Yu and K. Sivula, ACS Energy Lett., 2016, 1, 315–322.
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5. Rajashree Konar, Bharathi Rajeswaran, Hagit Aviv, Eti Teblum, Ilya Grinberg, Yaakov Raphael Tischler, Gilbert Daniel Nessim (JMC C, submitted)

Nowadays, flexible electronics demands new materials for integrated circuits of high flexibility and stretchability [1] . Single-walled carbon nanotubes (SWCNTs) are one of the most promising materials for utilization in the flexible integrated circuits owing to the outstanding electrical and mechanical properties [2]. However, the electronic and optoelectronic applications demand an advanced control of the nanotubes properties such as: length distribution, defectiveness, and most importantly ratio of semiconducting/metallic SWCNTs [3].

Most synthesis methods yield in 1/3 of metallic SWCNTs, thus, obtaining purely semiconducting species is a complex multistep procedure. Common strategies include either precise chirality distribution tuning during SWCNTs synthesis, post-synthesis etching of SWCNTs powders and films by oxidizing agents, or chirality separation. However, despite being well-established, these methods are suffering from multistage treatments, induced damage of SWCNTs [4], and irreversible attachment of surfactants that diminish performance of semiconducting devices [5].

Herein we report a simple one-step method for selective etching of metallic SWCNTs with nitrous oxide in aerosol phase. Our approach bases on faster oxidization rate of metallic SWCNTs compared to semiconducting ones, with advantage of aerosol chemical vapor deposition method, providing flow of individual SWCNTs in gas phase. We show a scalable method for obtaining semiconducting enriched SWCNT films with selectivity of 97% by treatment of pristine SWCNT aerosols for 6 sec at 600°C in the atmosphere of 30% N₂O. We assess structural changes in the nanotubes structure using comprehensive set of methods: UV-vis-NIR and Raman spectroscopies, differential mobility of aerosol particles, X-ray photoelectron spectroscopy, scanning and transmission electron microscopies and investigate the mechanism employing the DFT studies. We verify optimal etching conditions by equivalent sheet resistance, and temperature dependence of resistance dependencies, and the performance of SWCNT thin films as a channel material for thin film field-effect transistors, which showed an average improvement of on/off current ratio from 10⁰-10¹to 10³- 10⁴ and open state resistance from 10²to 10⁹ Ω/μm for statistics over 9000 devices.

[1] Q. Huang and Y. Zhu, “Printing Conductive Nanomaterials for Flexible and Stretchable Electronics: A Review of Materials, Processes, and Applications,” Adv. Mater. Technol., vol. 4, no. 5, pp. 1–41, 2019, doi: 10.1002/admt.201800546.

[2] Jariwala, D., Sangwan, V. K., Lauhon, L. J., Marks, T. J., & Hersam, M. C. (2013). Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing. Chemical Society Reviews, 42, 2824–2860. https://doi.org/10.1039/c2cs35335k

[3] Rao, R., Pint, C. L., Islam, A. E., Weatherup, R. S., Hofmann, S., Meshot, E. R., Hart, A. J. (2018). Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications. ACS Nano,12(12), 11756–11784. https://doi.org/10.1021/acsnano.8b06511

[4] Hu, H., Zhao, B., Itkis, M. E., & Haddon, R. C. (2003). Nitric Acid Purification of Single-Walled Carbon Nanotubes. Journal of Physical Chemistry B, 107(50), 13838–13842. https://doi.org/10.1021/jp035719i

[5] Wei, N., Laiho, P., Khan, A. T., Hussain, A., Lyuleeva, A., Ahmed, S., Kauppinen, E. I. (2020). Fast and Ultraclean Approach for Measuring the Transport Properties of Carbon Nanotubes. Advanced Functional Materials, 30(5), 1–9. https://doi.org/10.1002/adfm.201907150

Financed by: Russian Science Foundation № 17-19-01787

Two-dimensional materials such as graphene, transition metal dichalcogenide(TMD), h-BN have been studied in various fields and their stacking heterostructures have also received great attention. However, devices fabricated by stacking two-dimensional materials have an issue about ideality factor that cannot reach unity compared to the PN junction diode.
To solve this problem, we fabricated a two-terminal diode without forming the heterojunction or doping, just simply connecting the gate and drain. We used graphene as a bottom gate and TMD such as MoS₂ and WSe₂ as a conducting channel. Our gate-drain connected diode obtained ideality factor near unity. When drain-gate voltage is negative, which is reverse bias, Schottky barrier height is high so that current flow can be suppressed. In contrast, when drain-gate voltage is positive, which is forward bias, Schottky barrier height is low so that current can flow easily. This is further confirmed by Schottky barrier height modulation with varying drain-gate voltage by low-temperature measurement. The Schottky barrier height difference of 0.2eV was observed between the reverse bias and forward bias. The important role of the drain-connected bottom gate was also confirmed with varying carrier density with different drain-gate voltage, by hall-bar measurement. Due to the drain-gate connected structure, when a reverse bias is applied to the drain, the bottom gate is also negatively charged and a negative field is applied to the channel, thereby lowering the carrier concentration. Similarly, when it is forward bias, the carrier concentration in the channel increases due to the positively charged bottom gate.

Molybdenum disulfide (MoS₂) is a transition metal dichalcogenide known for its exceptional optoelectronic properties in its few- and monolayer form. It has a band gap in the range of 1.2 to 1.9 eV, depending on the layer thickness. This, along with other optoelectronic properties such as high absorptivity, makes MoS₂ a promising absorber layer in photovoltaic (PV) applications, like PV windows. Through optical simulation and Raman and photoluminescence (PL) spectroscopy we investigate the potential of MoS₂ in combination with the transparent carrier selective contacts molybdenum oxide and titanium oxide. The optical simulations show that this combination of layers has the potential to be semi-transparent and color neutral with possible current densities of 1 to 5 mA/cm² and average visible transmittance of 42% and 32% for device stacks with monolayer and few layer (< 5nm) MoS₂, respectively. Results from Raman and PL spectroscopy indicate that molybdenum oxide and titanium oxide influence the contribution of excitonic charge carriers in the MoS₂ absorber. This is observed in the form of spectral shifts in both Raman and PL. This can be explained by doping and screening effects induced by the metal-oxide layers. Both metal oxides can induce charge carrier extraction from the MoS₂, observed as quenching of the PL spectra. We compare these effects in exfoliated single crystalline MoS₂ flakes and transferred centimeter scale atomic layer deposition grown MoS₂ films and find that they are the same. This indicates that the knowledge gained from studying single crystalline flakes can be transferred to large area MoS₂ films which are a possible route to realize larger scale applications. The results from our optical simulation and optical spectroscopy, we will show the potential of this layer stack for applications such as transparent photovoltaic windows.

The fabrication of two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), in geometries beyond the standard platelet-like configuration exhibits significant challenges which severely limit the range of available morphologies. These challenges arise due to the anisotropic character of their bonding, van der Waals out-of-plane while covalent in-plane. Furthermore, industrial applications based on TMDs nanostructures with non-standard morphologies require full control on the size-, morphology- and position in the wafer scale. Such a precise control remains an open problem whose solution would lead to the opening of novel directions in terms of electronic and optoelectronic applications. Here, we report on a novel strategy to fabricate vertical position-controlled Mo/MoS₂ core-shell nanopillars (NPs). Metal-molybdenum (Mo) NPs are first patterned in a silicon wafer using lithography and cryo-etching. These Mo NPs are then used as scaffolds for the synthesis of Mo/MoS₂ core/shell NPs by exposing them in a rich sulfur environment. Cross-sectional transmission electron microscopy (TEM) analysis reveals the well-defined morphologies and the Mo/MoS₂ core/shell nature of the NPs. We demonstrate that individual Mo/MoS₂ core/shell NPs exhibit enhanced second order nonlinear optical processes, with an intensity increase up to a factor 40 as compared with regular MoS₂ flakes. Our results, and specifically the high degree of morphology- and position-control achieved on the core/shell NPs, represent an important step towards realising one-dimensional TMD-based nanostructures as building blocks of a new generation of sensors, nanophotonic devices, and hydrogen evolution reactions.

Importance of two-dimensional transition metal dichalcogenide (2D TMDC) has been well known due to their novel optical and electrical properties but their growth mechanism has still been unclear. In this study, we have accomplished 1×1 cm² monolayer WS₂ film on sapphire substrate using single zone atmospheric pressure chemical vapor deposition (APCVD) method. The work is centered upon the detailed explanation on the growth of large area film by employing the importance of alkali halides and other growth parameters like gas flow rate, growth time, quantity of sulfur powder with respect to WO₃ powder, and substrate temperature. Structural and optical characterization were done using optical microscopy, Raman spectroscopy, Raman mapping, photoluminescence, UV-Visible spectroscopy, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM). Herein, optical microscopic images represent different stages of the growth of thin film, UV-Visible absorption spectra show the presence of A, B, and C excitons indicating its broad range absorption capability, Raman and PL spectra show peaks characteristic to the monolayer WS₂, and the thickness is 1 nm as measured from the height profile image which further confirms it to be monolayer. Moreover, single crystal nature of the as-grown film is deduced from the HRTEM images and SAED pattern which depict only single set of diffraction planes. Chemical composition of the film is confirmed from XPS spectra which exhibit peak characteristic to W⁺⁴ and S^2- ions in bulk WS₂ revealing its complete sulfurization.
Following the high quality of the as-grown films as deduced from the afore mentioned characterizations, photodetector was fabricated by using standard e-beam lithography technique to study its photo response over a wide range of wavelengths from 380 nm to 660 nm. Responsivity and detectivity spectra indicate broad range photo-response from UV to visible region with highest value of 847 mA/W and 6.27× 10¹² Jones at 380 nm, respectively.

Molybdenum disulfide (MoS₂), having excellent optical and electrical properties and tunable bandgap, offers new opportunities for next-generation electronic devices. It is difficult to grow MoS₂ onto most substrates (specially on flexible substrates) due to growth limitations such as temperature, growth promoters, precursors, etc. In the present work, we have first grown large-area (centimeter-scale) trilayer MoS₂/SiO₂ via CVD and then transferred onto various substrates (sapphire, SiO₂, flexible substrates) without degradation of film quality. To transfer the MoS₂, a water-soluble layer (Na₂S/Na₂SO₄) was grown on the growth substrate during the growth of MoS₂ film, such that the MoS₂ lies on top of the water-soluble layer. Upon treating with DI water, the water-soluble film dissolves, causing the MoS₂ film to lift off and float to the surface. The film is then cleaned and transferred onto the desired substrate. Optical, morphological, and XPS results show that transferred film is highly clean and damage-free. A broadband (NIR-Vis-UV) photodetector was also fabricated onto transferred 3L MoS₂/sapphire to show the compatibility of the transfer process with the nanodevice fabrication. The photocurrent was enhanced 100 times and 10 times to dark current in UV and visible region, respectively. The maximum responsivity was found to be 8.6 mA/W. The proposed work offers enormous potential for the industrial implementation of 2D material technology platforms. Also, this process may be used to transfer other 2D materials like WS₂, MoSe_2, etc.

In this study, in-plane, and out-of-plane growth of WS₂ on SiO₂/Si substrate has been achieved using pulsed laser deposition technique. We have systematically studied the effect on the growth of the thin films at different sets of pressure (30 mTorr, 50 mTorr, and 70 mTorr) and temperature (400 °C, 500 °C, and 600 °C). Samples grown at the combination of low temperature and low vacuum showed a vertical flake-like growth, whereas at high temperature and high vacuum horizontal growth was achieved. XRD and Raman spectra show the formation of the 2H-WS₂ phase with (002) dominant plane which reflects the layered growth. FESEM, AFM, and HRTEM depict the growth evolution at different deposition parameters and thickness of the thin film. XPS compares the chemical state and composition of the as-grown films. Deposition pressure and substrate temperature were found crucial for controlling the stoichiometry and texture of WS₂ thin film. Furthermore, the gas sensing performance of the as-grown horizontal and vertical film has been compared. Later showed an enhanced response and selectivity towards NO₂ gas with a LOD of 100ppb at room temperature. The reason might be ascribed to the presence of a comparatively large number of active edge sites which are more reactive as compared to the flat basal planes present in the horizontal film.

2D transition metal dichalcogenides (TMDs) have been increasingly studied due to their wide range of optical, mechanical, chemical, and electric properties, from metal to insulator. 2D TMDs has graphene-like layered structure but with a controllable wide-bandgap by their atomic arrangement. Structural phase control in TMDs provides a variety of material phenomena that can be applied to tune the intrinsic properties. In general, group VIB TMDs can have thermodynamically stable semiconducting 2H phases and metastable metallic 1T phases. Recently, S vacancy-driven atomic diffusion has been studied to be an efficient approach for polymorphic transition from 2H-MoS₂ to 1T-MoS₂ by using carbon monoxide (CO)-mediated gas-solid reaction. Gas-solid reaction behaviors in nanomaterials can be predicted by their thermodynamic Gibbs free energy. The processing parameters of gas-solid reactions, such as the temperature and pressure, can be predicted using thermodynamic calculations that can be applied to drive targeted reactions. By using thermodynamic predictions and the properties of gas-solid reactions, the targeted reaction can be selectively induced among various competing reactions in multicomponent systems.
In this study, phase engineering of group VIB TMDs: WS₂ and MoS₂ was performed via CO-based gas-solid reactions with thermodynamic prediction. We predicted the thermal stability of chemical compounds corresponding to the process conditions and designed a process window map with the temperature and CO ratio. Each predicted phases appeared as oxides, sulfides, carbides and metals. In the synthesis of TMDs with precursors in a CO-mediated reaction environment, the metallic species exhibited a variety of phases within the quaternary-element system with Sulfur, Oxygen and Carbon. The trend of experimental fabrication was consistent with the prediction by the thermodynamic model. Synthesized TMDs exhibited 2H- to 1T-phase transition. 1T transition ratios was compared in two different TMDs according to the process parameters. The reaction between CO and S atoms became more active with increasing temperature, CO ratio, and calcination time that promote considerable S vacancy formation and cause phase transition. This revealed that gas-solid reactions-induced S vacancy formation triggered T-phase formation in not only MoS₂ but also WS_2.
This work presents the possibility of phase engineering TMDs through thermodynamic prediction, thereby helping to improve the synthesis efficiency of targeted materials and promoting expanded phase engineering of a variety of 2D nanomaterials as well.

Two-dimensional (2D) WS₂ is among the most intriguing representatives of the semiconducting transition metal dichalcogenide (TMD) family with a direct bandgap ~2 eV. WS₂ material shows extraordinary properties at the monolayer limit, such as strong light absorption, one of the lowest electron effective masses, high (>6.4%) photoluminescence yield (which can be further enhanced via doping), excellent thermal stability, mechanical flexibility and access to valley degree of freedom. Strong light absorption and emission at visible wavelengths opens great opportunities for its integration in ultrathin optoelectronic devices, such as light-emitting diodes, photodetectors, photovoltaic cells, and microcavity lasers.
However, for any practical application of WS₂, it is required to find a method to grow uniform and highly oriented WS₂ monolayers with a precise control over the layers number on a large scale. Tremendous efforts were devoted to study 2D TMD growth and examine their electronic and structural properties. Recently, several approaches were suggested to obtain a wafer-scale production of TMD monolayers. These methods include chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD)¹, magnetron sputtering, and molecular beam epitaxy (MBE), etc. Pulsed laser deposition (PLD) is one of the prominent ways to synthesize TMD monolayers, which has demonstrated its potential for 2D materials synthesis both via direct growth² and two-step process³.
In this paper, we report on two-step synthesis of WS₂ mono- and multilayers by high-temperature sulfurization of oxygen-deficient tungsten oxide films obtained by PLD on sapphire. PLD is an established method to grow high-quality oxides with sharp interfaces, and it offers great opportunities to control crystalline quality of the material and its composition. This is achieved by deposition of precursors at different substrate temperatures and background gases (oxygen and argon). The X-ray photoelectron spectroscopy (XPS) results reveal that by changing the growth conditions in the PLD process, the properties of WO_x precursors can be significantly varied.
Next, we unravel how the presence of intrinsic oxygen vacancies in the tungsten suboxide (WO_x) precursor leads to a more facile conversion from WO_x to WS₂ films in a CVD process. Our study suggests that native oxygen vacancies in the PLD-grown precursors can serve as niches through which sulfur atoms enters the lattice and facilitates the growth of WS₂ crystals with high photoluminescence (PL) emission and large domain size. Indeed, the overall PL emission increases ~40 fold while the full width of half maximum (FWHM) of the PL peak decreases significantly for WS₂ crystals synthesized from precursors with a high content of oxygen vacancies as compared to that obtained from nearly stoichiometric counterpart. Based on atomic resolution images we will discuss intrinsic, grain boundaries, bilayer WS₂ crystal orientation evolve from nearly stoichiometric oxides to highly reduced oxide precursors.
Our studies reveals how the epitaxial WO_x precursors with tunable properties grown by PLD can be used to independently control the nucleation, lateral growth and ultimately the WS₂ domain size.

1. Cun, H. et al. Wafer-scale MOCVD growth of monolayer MoS2 on sapphire and SiO2. Nano Res 12, 2646–2652 (2019).
2. Bertoldo, F. et al. Intrinsic Defects in MoS2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers. ACS Nano 15, 2858–2868 (2021).
3. Xu, X., Wang, Z., Lopatin, S., Quevedo-Lopez, M. A. & Alshareef, H. N. Wafer scale quasi single crystalline MoS2 realized by epitaxial phase conversion. 2D Mater 6, (2019).

Hexagonal boron nitride (h-BN) is promising alternative wide bandgap material for the p-type doping in UV-LEDs ^[1] and UV detectors ^[2]. Experimental work reports control of p-type conductivity of h-BN by means of in-situ magnesium, with hole concentration of 1x10¹⁸cm^-3 and with Mg activation energy of 30-300 meV ^[3-7]. However, density functional theory (DFT) studies predict that p-type doping in h-BN is very difficult due to the formation of small hole polarons, which increases the acceptor ionization energy to more than 1 eV ^[8].
In the present investigation, we systematically study Mg incorporation in h-BN as well as Mg doped h-BN grown on n-type Al rich AlGaN (58% Al content) heterostructures. Mg doped h-BN layers with characteristics wrinkles were obtained, showing that Mg doping did not alter the 2D nature of the h-BN layers for a large range of Cp2Mg molar flow. 2θ-ω XRD scans clearly evidence a single peak, corresponding to the (002) reflection plane of h-BN with a full-width half maximum increasing with Mg incorporation in h-BN. Secondary ion mass spectrometry analysis gave Mg incorporation, up to 4x10¹⁸ /cm³ in h-BN. The current-voltage characterisctics of heterostructures are rectifying for temperatures from 123K to 423K. Under ultraviolet illumination photocurrent was generated, as is typical for p-n diodes. C-V measurements evidence a built-in potential of 3.89 V. These encouraging results can indicate p-type behavior, opening a pathway for a new class of wide bandgap p-type layers.
References :

[1] K. Watanabe, et al, Nat. Photonics, 2009, 3, 591.
[2] H. X. Jiang, et al, ECS J. Solid State Sci. Technol., 2017, 6, 2.
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[7] B. He, et al, Appl. Phys. Lett., 2009, 95, 252106.
[8] L. Weston, et al, Phys. Rev. B, 2017, 96, 100102.

Atomically thin (2D) nanoporous membranes are an excellent platform for a broad scope of academic research. Their thickness and intrinsic ion selectivity (demonstrated for example, in molybdenum disulfide MoS2) make them particularly attractive for single molecule biosensing experiments and osmotic energy harvesting membranes. Currently, one of the significant challenges associated with the research progress and industrial development of 2D nanopore membrane devices is small scale thin film growth and small area transfer methods. This challenge has been recently tackled. Beyond large area growth and integration into devices, several other essential aspects such as mechanical stability and pore geometry, pore shape stability and its chemical termination require close attention from the community working with the 2D nanoporous membrane.

Indium Selenide (InSe) is a remarkable two-dimensional quantum material whose characteristic properties include a bandgap that increases with fewer layers and a high controllability of p- and n-type doping. Furthermore, its bandgap can lie in the near infrared region, depending on the specific crystalline phase adopted. Indium Selenide is known to crystallize in either the β(2H)-, γ(3R)- or the ε(2H)-phase. Of these three crystalline phases, only the β(2H) and γ(3R) ones exhibit a direct bandgap, which makes them particularly suitable for optoelectronic applications. However, while the β(2H)-phase can be easily disentangled from the other, the γ(3R)- and ε(2H)-phases are indistinguishable from the in-plane point of view and appear also very similar from the out-of-plane one. An attractive possibility to tailor the optoelectronic properties of InSe is based on the introduction of dopants, which leads to structural defects and can modify the crystalline structure and symmetry properties of the material. In this work, we investigate p- and n-doped (as well as undoped) Indium Selenide by means of Transmission Electron Microscopy and related techniques. We determine the crystalline phase present in these Indium Selenide specimens with High Resolution TEM by means of a systematic investigation of both in- and out-of-plane cross-sections, and compare the structural differences arising from different types and amounts of doping. We further assess the impact of dopants on the local electronic properties using Electron Energy-Loss Spectroscopy (EELS) by comparing how relevant features in the spectra (including surface- and edge-related properties) depend on the doping. Finally, we deploy Machine Learning techniques for a model-independent subtraction of the Zero Loss Peak, which makes possible identifying in an unbiased manner the impact of dopants in InSe in the ultra-low-loss region of the EELS spectra.

2D materials have been under the spotlight for over a decade. Given the multitude of promising properties demonstrated at lab-scale, a host of disruptive technologies with exponentially improved performances were expected to revolutionise markets ranging from integrated electronics to catalysis. In reality, however, the emergence of such game-changers has been significantly delayed. One major reason is the aging and failure of 2D materials in real working environments [1,2], the dynamics of which remain poorly characterised.
Here, we present an in situ study of the thermal oxidation of transition metal dichalcogenide monolayers to foster a fundamental understanding of the corrosion and aging dynamics in 2D materials. The full corrosion process is monitored in real-time, from the nanometre to sub-millimetre scale, utilising a custom in situ environmental SEM (ESEM), permitting conventional theories of materials corrosion to be tested in the 2D limit. We focus on the emergence of corrosion pits in WS₂ monolayers during oxidation, leveraging the ESEM’s large field of view to simultaneously track up to 10³ to 10⁴ pits. The corrosion kinetics are quantified by analysing the growth kinetics of individual pits, based on which the corrosion kinetics are found to transition from a reaction-limited to a diffusion-limited regime as a function of oxidation temperature. Moreover, an acceleration of corrosion is found to occur when adjacent corrosion pits coalesce with each other. All results are compared with phase field modelling, which provide a predictable model for the 2D thermal oxidation process. Our study demonstrate the power of the in situ ESEM in elucidating material aging mechanisms 2D materials at the nanoscale.

References:
[1] Gao et al., ACS Nano, 2016, 10(2), 2628
[2] Li et al., J. Mater. Chem. A, 2019, 7(9), 4291

In nature, most of the visible black materials have high absorption/emission in the mid-IR region (2.5-20 μm). Visible black materials with low infrared absorption/emission (or IR white) are rare but highly desired in numerous areas, such as solar energy harvesting, multispectral camouflage, thermal insulation, and information security management. Due to the lack of wavelength selectivity in intrinsic materials, such counter-intuitive properties are generally realized by constructing complicated subwavelength metamaterials such as plasmonic nanostructures and photonic crystals with costly nanofabrication techniques. Here, for the first time, the intrinsically low mid-IR emissivity (down to 10%) of the black 2D Ti₃C₂T_x MXene is reported. Associated with a high absorptance of 90% in average over the solar spectrun , it embraces the best wavelength selectivity among the reported intrinsic black solar absorbing materials. Its appealing potentials in multispectral camouflage, smart textiles, and anti-counterfeiting applications are experimentally demonstrated. First-principles calculations reveal that the IR emissivity of MXene relies on both the nanoflake orientations and terminal groups, indicating great tunability. The calculations also suggest more potential low-emissivity MXenes including Ti₂CT_x, Nb₂CT_x, and V₂CT_x. This work opens the avenue to further exploration of a family of intrinsically low-emissivity materials with over 70 members.

Feedback-controlled electroburning of graphene nanoribbons has been shown to be effective at creating nanogaps within the ribbon.^1–3Molecules introduced into these nanogaps form tunnel junctions which can form the basis for molecular rectifiers, switches, transistors, or sensors.³ Secondary electron e-beam induced current (SEEBIC) is a technique that can be used to reveal sample conductivity and connectivity with high spatial resolution in a scanning transmission electron microscope (STEM).^4,5 Here, we use feedback-controlled electroburning in situ to observe the evolution of the sample while it is being burned. We leverage the SEEBIC technique to image conductive, connected regions of the sample and diagnose device failures. When a nanogap is formed, producing a non-ohmic device response, the SEEBIC signal is observed to display dynamic contrast variation. A significant component of this contrast variation can be attributed to conductance switching in the high resistance region of the graphene nanodevice as the beam differentially charges the substrate. These results suggest that the SEEBIC imaging technique can be leveraged to provide information about charge flow and dynamic variations in conductance at the nanometer scale.⁶
References
(1) Sarwat, S. G.; Gehring, P.; Rodriguez Hernandez, G.; Warner, J. H.; Briggs, G. A. D.; Mol, J. A.; Bhaskaran, H. Scaling Limits of Graphene Nanoelectrodes. Nano Lett. 2017, 17 (6), 3688–3693. https://doi.org/10.1021/acs.nanolett.7b00909.
(2) S. Lau, C.; A. Mol, J.; H. Warner, J.; D. Briggs, G. A. Nanoscale Control of Graphene Electrodes. Phys. Chem. Chem. Phys. 2014, 16 (38), 20398–20401. https://doi.org/10.1039/C4CP03257H.
(3) Prins, F.; Barreiro, A.; Ruitenberg, J. W.; Seldenthuis, J. S.; Aliaga-Alcalde, N.; Vandersypen, L. M. K.; van der Zant, H. S. J. Room-Temperature Gating of Molecular Junctions Using Few-Layer Graphene Nanogap Electrodes. Nano Lett. 2011, 11 (11), 4607–4611. https://doi.org/10.1021/nl202065x.
(4) Hubbard, W. A.; Mecklenburg, M.; Chan, H. L.; Regan, B. C. STEM Imaging with Beam-Induced Hole and Secondary Electron Currents. Phys. Rev. Appl. 2018, 10 (4), 044066. https://doi.org/10.1103/PhysRevApplied.10.044066.
(5) Dyck, O.; Swett, J. L.; Lupini, A. R.; Mol, J. A.; Jesse, S. Imaging Secondary Electron Emission from a Single Atomic Layer. Small Methods 2021, 5 (4), 2000950. https://doi.org/10.1002/smtd.202000950.
(6) This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (A.R.L., S.J., O.D.). The authors acknowledge use of characterization facilities within the David Cockayne Centre for Electron Microscopy, Department of Materials, University of Oxford, alongside financial support provided by the Henry Royce Institute (Grant ref EP/R010145/1).

We report the results of a study of the temperature-dependent infrared spectroscopy and magnetism of single crystals of the antiferromagnetic semiconductor, Co₂P₂S₆. For comparison, we also report results on a second crystal substituted with 20% non-magnetic, closed-shell Zn. Concurrent with other, recent reports we observe a transition from a paramagnetic to antiferromagnetic state with a Neel temperature at T_N ~ 120 K. The magnetic transition is strongly reflected in differences between the infrared spectra taken above and below T_N. At 300 K we identify 4 relatively narrow phonon modes and a very broad mid-IR absorption while below T_N we observe that several new phonons appear, and a number alter their oscillator strength. The lowest energy mode exhibits significant asymmetry. With Zn doping significant changes occur at low energies where the motion is dominated by the transition metal atoms while the high energy modes, where motion is largely restricted to the sulphur atoms, are less affected.

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are extensively used to characterize chemical vapor deposition (CVD)-grown transition metal dichalcogenides (TMDs). To prepare the specimens for such an imaging, a polymethyl methacrylate (PMMA) assists wet-etching transfer technique is often used ^[1]. Here, PMMA residues can affect the TEM image quality, especially under high-magnifications. While thermal annealing under vacuum is typically performed to reduce the residue problem, the process can still damage TMD monolayers due to their poor in-air stability ^[2]. Here, we present an enhanced sample preparation technique for the TEM imaging of CVD-grown TMD monolayers, and the sample attachment onto a TEM grid, reducing TMD oxidation of monolayers. The new steps incorporated in this process include a thorough PMMA removal using acetone at 60 C followed by annealing at 350 C in 0.01 mTorr for 4 hours, which was corroborated by measuring Energy-dispersive X-ray spectroscopy (EDS). In addition, we dropped PMMA in a liquid form onto the TEM grid to fully cover the sample with the grid, ensuring a firm attachment of TMDs onto the grid during acetone washing. The sample was checked by scanning electron microscope (SEM), which confirmed that the TMD monolayers were successfully transferred onto TEM grids. Further, combining the EDS and EELS measurements, a significantly reduced carbon signal was observed. The high-resolution TEM (HRTEM) showed no polymer residues over the imaged regions, while the atomic structures were clearly seen. Several STEM images were also collected, and no e-beam radiation damage was observed.

Reference
[1] L. Zhang, Nanoscale 2017, 9, 19124.
[2] K. Kang, Adv. Mater. 2017, 29, 1603898.
[3] J. Lin, APL Mater. 2016, 4, 116108.

Two-dimensional materials cover a large range of high-interest properties such as high mobility, piezo- and ferroelectricity, scalable band gap, and topological non-trivial band structures to name a few. The key to access and study those properties lies in high-quality materials synthesis, which in turn requires precise control over relevant growth parameters. The complex interplay of rate and relative flux ratio of the supplied constituents as well as substrate temperature at the growth front determines film growth rate, and stoichiometry. However, to date, there is no unifying method to measure these parameters in real time. Commonly used methods to monitor flux rates or film thickness and temperature differ in accuracy and applicability. Consequently, rather large unintentional deviations exist in reported values which presents a sizeable problem in the reproducibility of growth conditions, in particular for the growth temperature.
With our work, we propose in-situ spectroscopic ellipsometry (SE) as an alternative method that offers the potential for absolute temperature measurements and simultaneous determination of physical film thicknesses in-operando and non-destructively which furthermore can be used by any deposition technique. Using in-situ SE during molecular beam epitaxy (MBE) growth, we have developed a precise model for the dielectric function of Bi₂Se₃ thin films grown on Al₂O₃ substrates that allows to extract and thus monitor film thickness and growth temperature simultaneously. Throughout our work, we used an infrared (IR) camera system and X-ray reflectivity (XRR) to calibrate our SE measurements against absolute temperature in-situ and against physical film thickness ex-situ for which we capped Bi₂Se₃ films with an amorphous Se layer to protect them against oxidation under ambient pressure conditions.
In the first part we demonstrate the dielectric model for the Al₂O₃ substrate and the thickness dependence for the Bi₂Se₃ thin film model at the fixed growth temperature of 225 °C for 5 samples. The dielectric model obtained by in-situ SE proofed to be very sensitive to film thickness changes with a precision of about 1 %.
In the second part we show in-situ SE measurements performed on the same samples at different temperatures to demonstrate temperature dependent changes in the dielectric function of Bi₂Se₃ for fixed film thickness. Having thus decoupled thickness and temperature related changes in the dielectric function, we included temperature effects in our model, which allowed us to monitor the film temperature with a precision of about 10 K.
Based on the presented data, in-situ SE bears the potential to serve as an ultimate baseline for thickness and temperature calibration for two-dimensional materials, thus enhancing reproducibility and advancing material quality.

Nanostructured materials have demonstrated profound potentials in applications ranging from robotics, energy, medicines, to electronics. The as-fabricate nanomaterials, however, often exhibit remarkable heterogeneity in physical properties that greatly limit their performance when applied in devices. It is highly desirable to be able to evaluate the properties of nanostructured materials via a facile and high-throughput approach. Herein, we report the utilization of mechanical rotation in an electric field that allows rapid characterization of the electronic properties of longitudinal nanostructures, e.g., MoS₂ nanoribbons and the by-products, i.e., MoO₃ and MoS₂/MoO₂ created during the fabrication process. The MoS₂ nanoribbons rotate both clockwise and counterclockwise in an aqueous solution depending on the frequency of the applied rotating electric field. With the understanding of theoretical modeling based on both Maxwell-Wagner polarization and induced electrical double layer, the measured electro-rotation spectrum can be readily analyzed to extract the electrical conductivity of the MoS₂ and the range of electric conductivities of MoO₃, and MoS₂/MoO₂ nanoribbons. Traditional four-probe measurements also are carried out, the results of which agree with the obtained results from electrorotation. Differently, the reported approach is non-destructive and only takes minutes for a test without the need for access to elaborate facilities. This work could be applied to the study of electric types of various nanostructures and determine the close range of the electric conductivity of dielectric structures less than 100 S/m.

The lacking control of large scale and homogeneous preparation of well-known 2D van der Waals materials MS₂ (M= Mo^IV, W^IV, Ti^IV, Sn^IV), MS (M=Sn^II, Ge^II) for commercial application, motivated us to develop a unique synthetic approach to molecular precursors for layered 2D van der Waals materials.
A uniform synthesis route of molecular building blocks for controlled formation of stable precursor class [M(SEtN(Me)EtS)_x] (M = Mo^IV, W^IV, Ti^IV, x = 2; M = Ge^II, Sn^II, x = 1) was observed by using chelating SNS ligand systems and suitable metal precursors, which have been fully characterized by NMR, TG-DSC, mass spectroscopic and single crystal analysis. Following the simple synthetic protocol of new metal chalcogenide complexes isolation of (air)stable molecular precursors have been performed. These complexes enabled the targeted preparation of homogeneous crystalline 2D MoS₂, WS₂, TiS₂, SnS₂ and SnS by varying thermal based decomposition experiments as well as using wet chemical decomposition routs, resulting in SnS and SnS₂ particles. The as prepared 2D materials have been identified by XRD, XPS, SEM and TEM analysis.
The presented molecular building blocks provide an extraordinary synthetic approach to a unique molecular precursor class for 2D material preparation.

Two dimensional van der Waals "heterostructures", which consist of two dimensional materials such as pristine graphene, hexagonal boron nitride and transition-metal dichalcogenides, have attracted much attention because they have various interesting electronic properties. The heterostructures are expected to be promising device materials in the next generation.Up to now, two dimensional materials without intentional defects have been used. However, periodically modified graphene with various types of defects, "graphene antidot lattices", are also expected to be the components of the heterostructures in the near future. Graphene antidot lattices have been studied theoretically for the last two decades and it is known that they can be semiconductors with the band gap values of 0 to 2 eV. We have studied the electronic properties of the graphene antidot lattices with the triangular defects in the super honeycomb arrangement. It is found that the band gap value can be tuned between 0.1 and 1.3 eV and the eigenvalue at the conduction band edge is nearly independent of the size of the defects while the eigenvalue at the valence band edge is strongly dependent on the size of the defects with the super-honeycomb-lattice-constant fixed. We should design both Type1 and Type2 quantum well structures by stacking the graphene antidot lattices with the triangular defects in the super honeycomb arrangement. From this background, we investigate the electronic properties of the trilayer systems consisting of the graphene antidot lattices with the triangular defects in the super honeycomb arrangement by performing first-principles electronic structure calculations within the framework of the density-functional theory. We study the stabilities, the electronic structures and the electronic states of the valence and conduction band edges. A graphene sheet with smaller band gap is sandwiched between two sheets with larger band gaps. It is found that electronic structures near the valence edge are independent of the stacking sequence while the electronic structures near the conduction band edge strongly depend on the stacking sequence. Interestingly, it is also found that both electrons and holes are expected to be confined in the middle layer for some cases. In other words, these types of trilayer graphene systems have Type1 quantum well structure. This result opens a new way of realizing a very thin quantum well laser of graphene antidot lattices. We will also report on the effect of the chemical doping and the electronic field.

We will discuss the fabrication of mixed dimensional van der Walls heterostructures using monolayer MoS₂ as one of the components. Due to the 2D nature of MoS₂, Coulomb screening is less effective and the electronic and optoelectronic properties of these heterostructures depend strongly on the local environment. We found that with increasing dielectric constant of the environment, photogenerated carriers become easily separable and the photoresponse increases nearly three orders of magnitude. We used various characterization methods to get a better understanding of the charge transport mechanism in these systems. These findings demonstrate a pathway of using dielectric engineering for the development of 2D materials-based high-efficiency optoelectronic devices.

N,N'-dimethylformamide (DMF) is one of the industrially important solvents. DMF is easily absorbed into the human body by inhalation or skin absorption. DMF is metabolized to generate N-methylformamide (NMF), which is highly toxic for the human body^[1]. In addition, DMF molecule shows cytotoxicity, and 50% inhibitive concentration is about 1 v/v% in a culture fluid^[2]. Therefore, it is important to monitor DMF concentration at a scale of 0.1 % in a situation of contaminated solution. However, it is so far not easy to monitor DMF in solutions because of the low reactivity of DMF molecules, and the coexistence of polar molecules such as water prohibits the detection of DMF molecules. In this study, we examine the interaction between DMF molecules and transition metal dichalcogenides and found a specific interaction with DMF molecules on the surface. This specific interaction enables to generate DMF sensor to monitor the concentration even in a contaminated solution such as a cell culture fluid.
A metal-oxide-semiconductor field-effect transistor (MOSFET) was fabricated by a standard lithography process. The channel material is MoS₂ in this study. Transfer characteristic curves of MoS₂ MOSFET show usual n-type behavior with an average ON/OFF ratio of 10⁵. By treating DMF solution, the drain current dramatically increases, and the MOSFET shows small electrostatic modulation by applying gate voltage. From the MOSFET characteristics, DMF molecules show n-type donor property to MoS₂ with achieving degenerate doping level. By using the strong donor ability of DMF molecules for MoS₂, we examined the DMF sensor in contaminated solutions. A microfluidic chamber was fabricated over the MoS₂-MOSFET device, and DMF-mixed solution in 0.15 M NaCl_aq was introduced in the microchannel. MOSFET monitors the MoS₂ surface state and we found that the drain current can successfully distinguish the DMF concentration in NaCl solution at the concentration level of 0.1 v/v%.
To further investigate the mechanism of DMF-MoS₂ interaction, we calculated the frontier orbitals of DMF and band alignment with MoS₂ by DFT (B3LYP) method. From the calculation, we found that the location of the HOMO energy level of DMF molecule is under the conduction band of MoS₂, therefore, the n-type behavior would not be a usual electron transfer process. By further DFT calculation with the structural optimization of the interaction between DMF and MoS₂ and we found that the DMF molecules would be specifically oriented on the oxygen adatoms on MoS₂.

[1] N. Sun et al., Sensors & Actuators: B. Chemical, 261, 153-160 (2018)
[2] Jamalzadeh. L et al., Avicenna J Med Biochem., 4(1):e33453. (2016)

Early detection of cancer can noticeably increase the survival chance of many cancer patients. Quantifying cancer biomarkers detectable from blood is an efficient way for the early detection of cancer diseases. In recent years, the application of synthetic DNA or RNA-based bio-recognizers (i.e., aptamers) in cancer biomarker sensor development has been vastly investigated. Electrochemical label-free aptamer-based biosensors (also known as aptasensors) are highly suitable for point-of-care applications. In this study, the application of bipolar exfoliated (BPE) graphene for developing cancer biomarker label-free aptasensors is explored. Graphene as a single, two-dimensional layer of carbon atoms has very remarkable features suitable for biosensor applications. The common graphene synthesis methods require complicated and costly processes and excessive use of harsh chemicals, as well as complex subsequent deposition procedures. In this study, a single setup has been used for exfoliation, reduction, and deposition of graphene nanosheets on a conductive electrode based on the principle of bipolar electrochemistry of graphite in deionized water. We investigated the properties of aptasensors based on graphene oxide (GO) deposited on a positive electrode feeding electrode and reduced-GO (rGO) deposited on a negative electrode feeding electrode. The PDGF-BB affinity aptamers were covalently immobilized by binding amino-tag terminated aptamers and carboxyl groups of GO and rGO surfaces. In this study, Fourier-transform infrared spectroscopy (FTIR) was used to study the surface characteristics of synthesized graphene and developed aptasensors. The scanning electron microscopy (SEM) was used to study the morphology of the bipolar exfoliated GO and rGO. The SEM study demonstrated that the rGO has a porous vertically aligned structure with pore sizes of around 100 nm, while GO has bulky flattened plates with random cracks. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used for characterizing the BPE graphene-based aptasensors in different stages of development and their sensing performances. The turn-off sensing strategy was implemented by measuring the peak-currents from DPV plots. The optimized aptasensor based on rGO showed a wide linear range of 0.75 pM-10 nM, high sensitivity of 7.83 A Logc^-1 (unit of c, pM), and a low detection limit of 0.53 pM. However, the optimized aptasensor based on GO reached its saturation point around 150 pM. This study demonstrated that bipolar electrochemistry is a simple yet efficient technique that could provide high-quality graphene for biosensing applications. Considering the BPE technique's simplicity and efficiency, this technique is highly promising for developing feasible and affordable lab-on-chip and point-of-care cancer diagnosis technologies. <!--![endif]---->

This work represents an electrochemical sensor based on flower-like metallic 1T phase tungsten disulfide (WS₂) for the detection of trace levels of mercury ions (Hg²⁺) with high sensitivity and selectivity. 1T-WS₂ microflowers were synthesized through a facile and cost-effective hydrothermal method and applied to modify the glassy carbon electrode using drop-cast. Under the optimized experimental conditions, the sensor showed excellent sensitivities of ~ 15.9 µA/µM, 2.54 µA/µM, 13.84 µA/µM, and 0.04646 µA/µM for Hg²⁺ with wide linear detection ranges (LDRs) of 1 - 90 nM, 0.1 - 0.4 µM, 0.5 - 1.0 µM, and 0.1 - 1.0 mM, respectively. In addition to that, the limit of detection of the sensor toward Hg²⁺ was 0.0798 nM or 79.8 pM, which is well below the guideline value recommended by the World Health Organization and the United States Environmental Protection Agency. The sensor exhibited excellent selectivity for Hg²⁺ against other heavy metal ions including Cu²⁺, Fe³⁺, Ni²⁺, Pb²⁺, Cr³⁺, K⁺, Na⁺, Ag⁺, Sn²⁺, and Cd²⁺. The excellent sensitivity and selectivity with wide LDRs can be attributed to the high conductivity, large surface area microflower structured 1T-WS₂, and the complexation of Hg²⁺ ions with sulfur (S^2-). In addition to good repeatability, reproducibility, and stability, this sensor showed the practical feasibility of Hg²⁺ detection in tap water suggesting a promising device for real applications.

Laser induced graphene (LIG) is a rapid fabrication technique for producing passive sensors that are lightweight, flexible, and tailored for applications ranging from biometric analysis to chemical detection. Through laser irradiation of a substrate surface, typically polyimide, a porous graphene can be formed whose electrical properties depend on the laser fluence, substrate composition, and atmosphere used. Though less conductive than traditional printed electronics, the porous nature has previously been leveraged to create piezoresistive sensors that change their conductivity in response to temperature, strain, or pressure. Additionally, the inherent robustness of polyimide substrates allows for application to extreme environments including space and ocean exploration. In this work, we expand on previous efforts to create LIG sensors for pressure measurement by correlating laser fluence and resultant graphene porosity to sensor hydrostatic response. The impact of laser fluence is investigated through changing speeds, powers, and density (pulses per inch) of the laser beam, and characterizing the porous structures formed with scanning electron microscopy. Hydrostatic testing at pressures up to 2,000 psi was also performed with supporting electronics to determine sensor response and validate the repeatability of the fabrication method. Through this work, we aim to demonstrate that by controlling the porosity of the graphene formed, and characterizing its piezoresistive response with respect to pressure, on demand passive pressure sensors can be fabricated for targeted pressure ranges and applications.

The advancement of ion transport applications will require the development of new materials with a high ionic conductivity that is stable, scalable, and micro-patternable. We report unusually high ionic conductivity of Li⁺, Na⁺, and K⁺ in 2D MoS₂ nanofilm exceeding 1 S/cm, which is more than two orders of magnitude when compared to that of conventional solid ionic materials. The high ion conductivity of different cations can be explained by the mitigated activation energy via percolative ion channels in 2H-MoS_2, including the 1D ion channel at the grain boundary, as confirmed by modeling and analysis. We obtain field-effect modulation of ion transport with a high on/off ratio. The ion channel is large-scale patternable by conventional lithography, and the thickness can be tuned down to a single atomic layer. The findings yield insight into the ion transport mechanism of van der Waals solid materials and guide the development of future ionic devices owing to the facile and scalable device fabrication with superionic conductivity.

Two-dimensional layered materials that harvest or transduce light energy are of paramount interest. In this context, two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS₂) are attractive for harnessing optoelectronics. In this work, we report the integration of MoS₂ with a strong electron accepting graphene yielding novel MoS₂ (1-2L)/graphene (1-2L) heterostructure ensemble by facile and efficient dry transfer method. We also found additional hetero-interfaces due to folded flakes such as MoS₂/MoS₂/graphene and MoS₂/MoS₂/Au hetero-interfaces. We characterized the heterointerfaces using correlated micro-Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy, Kelvin probe microscopy and conducting AFM, tip-enhanced Raman spectroscopy (TERS), and tip-enhanced photoluminescence (TEPL) techniques to gain insights into the structural quality, hetero-interfaces, and excitonic effects to elucidate the role of a number of graphene layers which are crucial in modulating the light emission. We studied the charge transfer in these vertical heterostructures unraveling the interlayer electronic coupling. We demonstrate strong evidence that the excitonic properties of MoS₂ are most effective for two layers of graphene layers underneath leading to a significant enhancement of the PL response as compared to MoS₂ supported on one-layer graphene and directly on the gold-coated substrate. We observed a marginal increase in work function with increasing MoS₂ layer as anticipated and for heterointerfaces which are indicative of electron injection. We measured C-AFM without light (dark) and with light in view of photoconducting properties of heterointerfaces and found a rectifying behavior with an apparent increase in current by almost twice for MoS₂/graphene heterostructures in contrast to MoS₂/Au which shows ohmic behavior. Our findings signify the importance of substrate engineering when constructing 2D multilayered architectures for harvesting or storing light energy. This work is financially supported in parts by NSF and NSF-MRI Grants.

The dielectric environment strongly influences the photoluminescence (PL) of transition metal dichalcogenide (TMD) monolayers. The PL spectra are usually modified by the defect states present in the monolayer and substrate interface. To study the effect of substrate defects on PL, we have gradually increased the separation between substrate and WS₂ by placing hBN of various thicknesses in between. Further dielectric engineering was done by patterning the substrate with holes and pillars. In this way, we were able to tune the contribution of trion and exciton in the PL of WS₂. An excitation power-dependent study was also done to understand the tuning mechanism of PL in various kinds of engineered substrates. The charge of the substrate defects is also important in determining the nature of the PL spectra in monolayer WS₂. This effect was also studied by treating the substrate with APTES which changes the nature of the charge carried by a SiO₂/Si substrate. This study will be important for the fabrication of valleytronic and excitonic interconnects.

Recently, a diverse range of memristive phenomena have been demonstrated in 2D transition metal dichalcogenides (TMD) in vertical and lateral device architectures. TMD-based memristors can exhibit unipolar and bipolar resistive switching, tunable set voltage, a resistance ratio as high as 10⁵, endurance up to 10⁷, thus, presenting a new opportunity for ultra-scaled memory cells [1-3]. The lateral architecture is implemented as a gate tunable memtransistor and relies on bias-induced Schottky barrier modulation, which either originates from defect migration or charge trapping/detrapping processes [4]. High performance is achieved in the lateral memtransistor architecture across an unusually wide defect density range up to ~50% [3]. Therefore, due to its inherent defect tolerance and ultrathin body [5], the TMD memtransistor may provide superior durability of in-memory computation in harsh radiation environments.

In this presentation, I will discuss the effects of 48 keV Au⁺ irradiation and 532 nm laser irradiation on the physical and electrical properties of the MoS₂ memtransistor as a function of photon exposure or 48 keV Au⁺ fluence. We employ X-ray photoelectron spectroscopy, Atomic Force Microscopy, Raman spectroscopy, and transmission electron microscopy to elucidate the structural and chemical effects of the radiation species and dose on polycrystalline MoS₂ grown by chemical vapor deposition. The V_G-dependent I_D-V_D response and gate-contact capacitance are measured ex-situ as a function of radiation dose to evaluate the radiation tolerance of the memtransistor and deconvolute the radiation effects in the gate oxide, respectively.

The S:Mo ratio of the MoS₂ decreases from 1.9 to 0.3 and the relative concentration of MoO_x increases from 0.1 to 0.5 after an ion fluence of 9E15 cm^-2. The E_2g mode exhibits a ~6 cm^-1 red shift and the intensities of the LO(M) and TO(M) defect modes increase after an ion fluence of 1E14 cm^-2, which corroborates the significant increase in defect concentration in the MoS₂ with increasing ion fluence. The ON/OFF ratio of the memtransistor devices is reduced from ~10 to ~0 after serial exposure to an ion fluence of 1E13 cm^-2 and a 532 nm laser. Interestingly, some memtransistor devices that are only ion irradiated retain initial electrical performance despite a significant increase in gate-contact capacitance. Our preliminary results suggest the MoS₂ memtransistor exhibits impressive tolerance to ionizing radiation.

This work was supported by a Laboratory Directed Research & Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science.

References:
[1] V. K. Sangwan et al. Nature Nanotechnol. 2015, 10, 403-406
[2] R. Ge et al. Nano Lett. 2018, 18, 434-441
[3] M. Wang et al. Nature Electronics 2018, 1, 130-136
[4] V. K. Sangwan et al. Nature 2018, 554, 500-504
[5] A. J. Arnold et al. ACS Appl. Mater. Interfaces 2019, 11, 8391-8399

Xenes, the two-dimensional (2D) monoelemental graphene-like materials, promise to open new research frontiers in the technological exploitation of atomically thin materials because of their scalability and of their rich physics varying from one member to another.^1,2 However, significant challenges are at stake when considering the Xenes synthesis, stabilization and manipulation. Because of their artificial nature, Xene deposition usually requires an extreme control of the experimental conditions including growth temperature and substrate choice, in particular when an epitaxial matching is necessary. Moreover, strategies for overcoming the stabilization issues related to their manipulation in ambient condition are to be implemented. Taking the silicene case, namely the 2D buckled form of silicon, as paradigmatic of the Xenes family, here we show that meeting these requirements is often a matter of adopting smart experimental solutions. For example, the use of single crystal metal films grown on cleavable mica substrates allows for the development of manipulation schemes enabling portability and transfer of the Xene layers by means of mechanical and chemical delamination.³In addition, the in-situ encapsulation of the silicene enables us to preserve the structural integrity against aging and degradation.⁴ Here we demonstrate that, when disassembled from the substrate, silicene of different forms, i.e. single layer, multilayer or heterostructured layers,⁵ can be bent by reshaping or rolling it around any macroscopic surface or object showing the emergence of a strain feature in its Raman response. Bendable silicene and Xenes may thus hold high potential for strain sensor devices. The work is within the ERC-COG 2017 Grant N0. 772261 “XFab”.

[1] A. Molle, J. Goldberger, M. Houssa, Y. Xu, S.-C. Zhang, D. Akinwande, Nat. Mater. 2017, 16, 163.
[2] C. Grazianetti, C. Martella, A. Molle, Phys. status solidi – Rapid Res. Lett. 2020, 14, 1900439.
[3] C. Martella, G. Faraone, M. H. Alam, D. Taneja, L. Tao, G. Scavia, E. Bonera, C. Grazianetti, D. Akinwande, A. Molle, Adv. Funct. Mater. 2020, 30, 2004546.
[4] A. Molle, G. Faraone, A. Lamperti, D. Chiappe, E. Cinquanta, C. Martella, E. Bonera, E. Scalise, C. Grazianetti, Faraday Discuss. 2020, DOI 10.1039/c9fd00121b.
[5] D. S. Dhungana, C. Grazianetti, C. Martella, S. Achilli, G. Fratesi, A. Molle, Adv. Funct. Mater. 2021, 2102797.

Two-dimensional transition metal dichalcogenides (TMDCs) are promising candidates for ultrathin photovoltaics due to their high absorption coefficients and intrinsically passivated surfaces.¹ However, careful selection of carrier-selective contacts for efficient TMDC solar cells has never been seriously considered. Here, we demonstrate that open-circuit voltages greater than 500 mV are achievable in ultrathin (total cell thickness less than 150 nm) TMDC solar cells modeled on inverted perovskite device architectures. Currently, the highest efficiency inverted (p-i-n) perovskite solar cells employ the polymer hole-transporting layer PTAA as a bottom contact and the organic electron-transporting layer C60 as a top contact.^2,3 Following this inverted perovskite geometry, we fabricate carrier-selective contact TMDC solar cells. We start with a template-stripped gold substrate, spin coat 10 nm of PTAA, exfoliate 15 nm of tungsten disulfide, thermally evaporate 10 nm of C60, and finally thermally evaporate 20 nm of silver with a bathocuproine interlayer. These devices achieve open-circuit voltages greater than 500 mV even with nonideal optical design. Using transfer matrix calculations, we demonstrate that short-circuit current greater than 20 mA/cm² is possible in a similar electronic architecture illuminated from the bottom through an ITO-coated glass substrate, with a 100-nm silver top contact as a back-reflector. We fabricate these devices and characterize them for their optoelectronic properties using one-sun illumination in a solar simulator, absorption and quantum efficiency measurements, photocurrent mapping, and power-dependent current-voltage sweeps. Finally, we discuss pathways to greater than 10% one-sun power conversion efficiencies. Our work demonstrates the transferability of perovskite carrier-selective contacts to TMDC solar cells, opening up the possibility for TMDCs to become a new low-cost, high-efficiency photovoltaic technology.

References:
1. D. Jariwala, A. R. Davoyan, J. Wong, H. A. Atwater, Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook. ACS Photonics 4, 2962–2970 (2017).
2. X. Zheng et al., Managing grains and interfaces via ligand anchoring enables 22.3%-efficiency inverted perovskite solar cells. Nat. Energy 5, 131–140 (2020)
3. F. Li, et al., Regulating Surface Termination for Efficient Inverted Perovskite Solar Cells with Greater Than 23% Efficiency. J. Am. Chem. Soc. 142, 20134–20142 (2020).

Engineering the electronic and physical properties, such as work function, band gaps and transport properties for two-dimensional (2D) materials is essential for their promising applications in advanced electronic and optoelectronic devices. In this work, we study the effect of experimentally self-assembled organic monolayers on the electronic properties of 2D materials using first-principles density functional theory calculations. Unlike surfaces of bulk materials, we find that 2D materials are strongly impacted by near-field electrostatic effects resulting from high multipoles of the organic layer electronic density. We show this effect can lead to significant (~0.5V) modulation of the in-plane potential experienced by the electrons within 4 Å of the molecular layer, with a transition between near- and far-field. We develop an electrostatic theory of this effect, showing that the effect of the molecular layer is akin to the one of a discretized planar charge density, which can be derived from the multipole moments of the molecules. Solving this model computationally and analytically, we propose implementations of this effect to generate novel electronic properties for electrons in 2D materials. As a proof of principle, we show that the near-field potential of phthalocyanine molecular monolayers deposited on graphene leads to bandgap opening.

Surface functionalization of 2D materials with organic electron donors (OEDs) is a powerful pathway to modulate the electronic properties of 2D materials. However, understanding of the doping mechanism is largely limited to categorization of molecular dopants as n- or p-type based on the relative position of the molecule’s redox potential to the Fermi level of 2D host. Systematic studies to examine factors beyond redox potential for molecular doping are critically lacking. In our previous work, we showed that an organic dopant based on 4,4 bipyridine (DMAP-OED) has a doping power of 0.63-1.26 electrons per molecule to MoS₂ at maximum functionalization conditions.^[1] We calculated the molecular doping by measuring the carrier density of functionalized MoS₂ field effect transistors and estimating the molecular surface coverage of DMAP-OED by atomic force microscopy. We also observed that the molecular arrangement and surface coverage greatly affects the doping power of the molecule.

In this study, we use monolayer MoS₂ as a model 2D system and two molecular dopants, Me- and ^tBu-OED, which have the same redox potential but different ligand groups to probe the effects of molecular size and ligand chemistry on the molecular surface coverage and doping power to MoS₂. We show that, for the same functionalization conditions, the doping powers of Me- and ^tBu-OED are 0.22-0.44 and 0.11 electrons per molecule, respectively, demonstrating that the molecular size and ligand chemistry critically affect molecular doping. Using the stronger dopant, Me-OED, a carrier density of 1.10 x 10¹⁴ cm^-2 is achieved in MoS₂, the highest doping level to date in MoS₂ by surface functionalization. We thus show that proper tuning of the molecular ligand chemistry and size is essential in the rational design of powerful molecular dopants.


[1] M. Yarali, Y. Zhong, S. N. Reed, J. Wang, K. A. Ulman, D. J. Charboneau, J. B. Curley, D. J. Hynek, J. V. Pondick, S. Yazdani, N. Hazari, S. Y. Quek, H. Wang, J. J. Cha, Advanced Electronic Materials 2020, n/a, 2000873.

Extracting hot carriers as a photocurrent remains a challenge in materials science and quantum devices because the hot carrier lifetime is within the range of ultrafast picosecond. A more profound challenge is to understand the hot carrier drift dynamics. Here, we collect hot carriers as a photocurrent in-situ 2D WS2 single layer photoconductor, characterized by the ultrafast photocurrent spectroscopy (UPCS). The hot carrier is induced by the rich exciton species interaction, attributed to the trion Auger recombination under low exciton density and biexciton Auger recombination at high exciton density, respectively. We further identify the hot carrier transport properties, manifested by the minimum mobility of ~ 10 cm²/Vs and drift length of ~8 nm.

Advancement in flexible microelectronics based on 2D materials urgently needs reliable, high dielectric strength and chemically inert dielectric materials, which can be economically produced using large area and low-temperature synthesis techniques. Among possible candidate dielectric materials, ultra-thin amorphous boron oxynitride recently emerged as an attractive material, which can be synthesized at low temperatures using physical vapor deposition methods. In this study, amorphous boron oxynitride films were deposited at room temperature on rigid and flexible substrates using radio frequency magnetron sputtering of a hexagonal boron nitride target in various argon-nitrogen background gas ratios. The effect of nitrogen content in the sputtering gas, changes in target-to-substrate distance, and magnetron power density to the target on the film composition were investigated with X-ray photoelectron spectroscopy and correlated to the dielectric breakdown strength and bandgap obtained from current-voltage measurements and Tauc plots, respectively. Films grown in pure Ar atmosphere at a large target-to-substrate distance displayed compositions containing metallic boron, which led to a decrease in the dielectric breakdown strength. The use of Ar:N₂ mixtures and pure N₂ eliminated metallic boron bonding. Decreasing the target-to-substrate distance and target power density during reactive sputtering resulted in B:N stoichiometry ratio closer to one and about 25 – 30 at.% oxygen. Amorphous boron oxynitride films grown in pure N₂ at a small target-to-substrate distance exhibited dielectric breakdown strength of 2.1 – 2.4 MV cm^-1, smooth surface topography, and a bandgap of 5.4 eV. Charge transport was field-dependent, with field-enhanced Schottky emission, Frenkel–Poole emission, and space-charge-limited conduction being observed in various field ranges. Test MIS structures comprised of ~30 nm thick boron oxynitride insulator and transparent ZnO semiconductor also grown at room temperature revealed a low interface trap concentration N_it of 7.3 × 10¹⁰ cm^-2, and interface state density N_ss of 7.5 × 10¹² eV^-1 cm^-2. The results demonstrate the suitability of amorphous boron oxynitride for applications as an insulator or gate dielectric in flexible or transparent thin-film transistors and other electronics when a low processing temperature is required.

The large-scale growth of semiconducting thin films on insulating substrates enables batch fabrication of atomically-thin electronic and optoelectronic devices and circuits without film transfer. Here we report an efficient method to achieve rapid growth of large-area monolayer MoSe₂ films based on spin coating of Mo precursor and assisted by NaCl. Uniform monolayer MoSe₂ films up to a few inches in size were obtained within a short growth time of 5 min. The as-grown monolayer MoSe₂ films were of high quality with large grain size (up to 120 μm). Arrays of field-effect transistors were fabricated from the MoSe₂ films through a photolithographic process; the devices exhibited high carrier mobility of ~27.6 cm²V^-1s^-1 and on/off ratios of ~10⁵. The growth method was also applicable to other 2D transition metal dichalcogenides (TMDs), and rapid growth of large-area monolayer MoS₂, WS₂, and WSe₂ films were all successfully achieved.Our findings provide insight into the batch production of uniform thin TMD films and promote their large-scale applications.

Electrically conducting 2D metal–organic frameworks (MOFs) have attracted considerable interest, as their hexagonal 2D lattices mimic graphite and other 2D van der Waals stacked materials. However, understanding their intrinsic properties remains a challenge because their crystals are too small or of too poor quality for crystal structure determination. Here, we report atomically precise structures of a family of 2D π-conjugated MOFs derived from large single crystals of sizes up to 200um, allowing atomic-resolution analysis by a battery of high-resolution diffraction techniques. A designed ligand core rebalances the in-plane and out-of-plane interactions that define anisotropic crystal growth. We report two crystal structure types exhibiting analogous 2D honeycomb-like sheets but distinct packing modes and pore contents. Single-crystal electrical transport measurements distinctively demonstrate anisotropic transport normal and parallel to the π-conjugated sheets, revealing a clear correlation between absolute conductivity and the nature of the metal cation and 2D sheet packing motif.

Metal oxides, which is one of the most abundant substances in nature, are rarely found to be layered crystal. This hinders their potential to form into two-dimension (2D) since the cleavage of weak, interlayer van der Waals bonds in layered bulk crystal serves as the benchmark to obtain the atomically thin high-quality 2D layers such as ZnO¹. 2D-like non-layered metal oxides are usually created via a space-confined-growth^2-4 or soft-chemically exfoliation of unilamellar bulk crystal⁵, in which external ionic groups may be induced to stabilize the crystals. In this talk, I will discuss our recent discovery of a series of the layered planar hexagonal phase of oxide from elements across the transition metals, post-transition metals, lanthanides and metalloids⁶. These oxides can be naturally formed from metallic surfaces under a controlled oxidation environment without involving sophisticated equipment or other chemicals. Subsequently, the hexagonal 2D metal oxide monolayers can be facilely exfoliated on a substrate in a mechanical manner, similar to the way of obtaining graphene and other layered metal chalcogenides. We have selected the hexagonal TiO₂ as a representative to reveal the distinct properties of such hexagon crystal coordinated oxides apart from their bulk counterparts. The monolayered and few-layered hexagonal TiO₂ shows p-typed semiconductor behavior with the hole mobility up to 950 cm²V^-1s^-1 at room temperature. This research topic can possibly expand the exploration of metal oxides in the 2D quantum regime and initiate numerous applications in the future.
Reference
1 Tusche, C., Meyerheim, H. & Kirschner, J. Observation of depolarized ZnO (0001) monolayers: formation of unreconstructed planar sheets. Physical review letters 99, 026102 (2007).
2 Xiao, X. et al. Scalable salt-templated synthesis of two-dimensional transition metal oxides. Nature communications 7, 1-8 (2016).
3 Yang, J. et al. Formation of two-dimensional transition metal oxide nanosheets with nanoparticles as intermediates. Nature materials 18, 970-976 (2019).
4 Son, J. et al. Epitaxial SrTiO₃ films with electron mobilities exceeding 30,000 cm² V⁻¹s^{− 1}. Nature materials 9, 482-484 (2010).
5 Ma, R. & Sasaki, T. Nanosheets of oxides and hydroxides: ultimate 2D charge bearing functional crystallites. Advanced materials 22, 5082-5104 (2010).
6 Zhang, B. Y. et al. Hexagonal metal oxide monolayers derived from the metal-gas interface. Nature Materials, doi:10.1038/s41563-020-00899-9 (2021).

Layered van der Waals (vdW) materials, consisting of atomically thin layers, are of paramount importance in physics, chemistry, and materials science owing to their unique properties and various promising applications. However, their fast and large-scale growth via a general approach is still a big challenge, severely limiting their practical implementations. Here, we report a universal method for rapid (~60 min) and large-scale (gram scale) growth of phase-pure, high-crystalline layered vdW materials from their elementary powders via microwave plasma heating in sealed ampoules. This method can be used for growth of 30 compounds with different components (binary, ternary, and quaternary) and properties. The ferroelectric and transport properties of mechanically exfoliated flakes validate the high crystal quality of the grown materials. Our study provides a general strategy for the fast and large-scale growth of layered vdW materials with appealing physiochemical properties, which could be used for various promising applications.

Anisotropic nanomaterials composed of two halves with different structure, chemistry, or polarity, known as Janus nanomaterials, have distinct properties when compared to symmetrically functionalized analogues. These materials have recently gained significant attention in many applications such as electronic thin films, drug delivery, sensors, optics, oil/water separation membranes, photoactivated micromotors, photocatalysts, and interfacial modification. Two dimensional (2D) Janus nanosheets are especially intriguing considering their interfacial properties as well as their ability to assemble into higher order and hybrid structures. In this research, we describe an approach to mass synthesis of Janus nanosheets consist of graphene, metal and ceramic materials in highly controllable ways via a novel “Interfacial Nanoreactor” system. We have characterized and proven the asymmetric structures of the synthesized Janus graphene nanosheets by Langmuir-Blodgett (LB) surface pressure-area (π-A) curves, fluorescence spectroscopy, contact angle measurement and scanning electron microscope (SEM) characterizations. As an example of applications in oil and gas industry, we have demonstrated that the Janus nanosheets with proper hydrophilic/hydrophobic functionalization can be directed onto interface of aqueous-organic bi-phase system and significantly lower the interfacial tension (IFT) between aqueous and organic phases, which promotes the oil recovery process in oil reservoir. With loading of proper imaging contrast agents (dyes or quantum dots) on one side and controlled hydrophilicity or hydrophobicity on the other side, the Janus nanosheets can be directed to confined microporous area, which enables to probe structural microheterogeneity of geological samples such as shale rocks by optical or microscopic imaging techniques.

The suitability of solvents for the efficient liquid phase exfoliation (LPE) to produce 2D materials such as hexagonal Boron Nitride (hBN) from their bulk starting materials is typically assessed by UV-Vis concentration measurements. However, UV-Vis concentration characterisation of liquid phase exfoliated 2D materials relies on good materials dispersibility within a given solvent for the data to reflect the actual solvent exfoliation efficiency.

‘Green’ solvents possess low boiling points and toxicity, which are the attractive solvent choice for LPE of 2D materials but the yield is low due to the limited materials dispersibility in these solvents. Therefore, the efficient liquid phase exfoliation to produce 2D materials is still largely relying on e.g., 1-Methyl-2-pyrrolidinone (NMP) which unfortunately is harmful to the environment and therefore hinders the large scale production. Here, to assess the exfoliation efficiency of 2D materials in ‘green’ solvents independently of their dispersibility, we performed dedicated exfoliation efficiency studies to enhance the yield of liquid phase exfoliated hBN and compared yields to the most studied 2D materials system to date, e.g. graphene.

We found that isopropanol (IPA) has the highest exfoliation efficiency (comparable to that of NMP), despite the poor dispersibility of graphene in IPA. We could also show that the dispersibility and concentration of layered materials in IPA is higher after a brief post-exfoliation sonication treatment without damaging the exfoliated 2D materials.

By studying shear-mixing processes and understanding the actual exfoliation efficiency of the solvents in detail, it is possible to double the hBN yield (and increase the graphene yield 10-fold) using IPA as the exfoliation medium. Our studies prove that the yield of hBN (and graphene) in green solvents is not limited by the exfoliation efficiency but it is due to its poor dispersibility in green solvents. Our findings pave the way for other post-exfoliation methods to optimise the yield through sustainable solvents exfoliation for industrial-scale, especially for hBN production.

In the past decade, research in the field of two-dimensional materials has exponentially transitioned from fundamental studies to the development of complex 2D devices. While an increasing number of 2D materials, including graphene, and hexagonal-boron nitride, have been synthesized successfully in the lab, others have only been theoretically predicted. One very important example is two-dimensional silicon carbide (2D SiC). Theoretical studies have predicted that monolayer SiC has a stable planar graphene-like structure, a direct bandgap of about 2.58 eV, strong excitonic effects, and highly tunable electronic, optical, and magnetic properties. These characteristics are of great importance for many applications and make 2D SiC a potential game-changer for future energy-related technologies. Experimentally, however, the growth of 2D SiC has challenged scientists for decades, mainly because unlike graphite or other layered materials, bulk SiC is not a van der Waals layered structure. Bulk SiC is a covalently bonded material, and it is one of the strongest materials. Further still, bulk SiC exists in more than 250 polytypes, further complicating the synthesis efforts. Importantly though, our group has made groundbreaking progress on the synthesis of 2D SiC. We successfully grow the first monolayer SiC by adopting a unique wet exfoliation method, in which 6H-SiC was used as a precursor.<font size="1"> </font>The exfoliated 2D SiC nanosheets, are 100% stable in air and show visible light emission at around 480 nm (2.58 eV), which is in good agreement with theoretical predications.
As a stable atomic thick semiconducting material, 2D SiC has the potential to bring revolutionary advances across various technological fields ranging from power electronics, and optoelectronics to quantum information processing and beyond.2D SiC can address countless drawbacks associated with the gapless nature of graphene, silicon semiconductors (narrow band gap and low voltage breakdown), environmental instability of silicene and few-layer black phosphorous and the indirect bandgap of bulk SiC.

Laser induced graphene (LIG) is an emerging technique that enables directly patterning conductive carbon electrodes for a plethora of flexible devices, including supercapacitors and sensors. While the morphology and chemistry of the porous three-dimensional graphene produced by this method were previously shown to be tunable, the ability to create functional gradation wherein different parts of the patterned graphene exhibit different morphology and/or chemistry on the same flexible substrate has not been previously achieved. Here, we present a new methods for creating functionally graded LIG based on utilizing controlled gradients of optical energy flux to create spatial gradients of LIG morphologies having different electrical conductivities. Importantly, we demonstrate the seamless transition between neighboring regions having different porosity and conductivity. To demonstrate the morphology transitions in the LIG, we developed a method to continuously sweep laser fluence values by controlling the defocus level of the sample surface. This sweep enabled identifying the precise fluence thresholds that correspond to morphological transitions: first, from isotropic porous morphology to anisotropic networks at 12 J/cm²; second, from anisotropic networks to aligned nanofibers at 17 J/cm². Hence, our results provide new insights into the fluence-dependence of the physicochemical processes underlying LIG formation. Moreover, our approach enables generating a morphology diagram for LIG, which facilitates precise tunability of both the morphology, electrical and electrochemical properties of LIG patterns, based on easy-to-control processing parameters, such as laser power and degree of beam defocusing. In addition, this unprecedented ability to create spatially varying morphologies opens the door for one-step fabrication process of complex flexible devices with high surface area electrodes that are integrated with relatively denser and more electrically conducting traces.
Here, we present two methods for creating functionally graded laser induced graphene materials: The first method utilizes controlled gradients of optical energy flux to create spatial gradients of LIG morphologies having different electrical conductivities. The second approach creates spatially controlled surface functionalization through relasing the LIG in vacuum, nitrogen or oxygen environment in a controlled chamber. Through the second lase, patterned LIG surfaces can be created, leading to patterned surface properties and electrochemical properties.