Symposium NM01—Beyond Graphene 2D Materials—Synthesis, Properties and Device Applications

The well-controlled synthesis of monolayer (ML) TMDC targeting large-scale fabrication is still technically challenging. Although metal-organic chemical vapor deposition (MOCVD) has stood out as a scalable and reproducible technique, details of growth are not completely understood, and some issues need to be addressed before realizing the “Lab-to-Fab” transition. One concern is C incorporation, which not only hinders lateral growth but also affects film quality. Furthermore, it is challenging to close a ML without premature bilayer (BL) formation. Coherent ML films with sparse BL nucleation are a prerequisite for growing planar 2D-2D heterostructures. In this study, we focus on WS₂ as a 2D TMDC well-suited e.g. for red light emission. In S-rich conditions (S/W molar ratio >4,000), W adatoms are considered to be the growth-limiting species. W accumulation and thus BL nucleation will inevitably start when the size of ML nuclei exceeds the on-WS₂ migration length. Analyzing the morphology after the onset of BL nucleation suggests this length on a scale <100 nm. Therefore, a two-stage process is proposed aiming at: (i) achieving a sufficient nucleation density in the nucleation stage and (ii) enhancing the migration of W adatoms (especially close to ML coalescence) in the lateral-growth stage.
All processes are performed on (0001)-oriented sapphire in AIXTRON CCS reactors in 7×2" and 1×4" configuration, equipped with LayTec in-situ temperature measurement and spectroscopic reflectance monitoring. Tungsten hexacarbonyl (WCO) and di-tert-butyl sulfide (DTBS) were chosen as precursors with H₂ as carrier gas. Processes were started with a 15 min H₂ sapphire desorption (150 hPa, 1050 °C on wafer surface). Scanning electron microscopy (SEM), Raman and photoluminescence (PL) spectroscopy were used for characterization.
To reduce C incorporation, different surface temperatures (from 700 to 840 °C) and chamber pressures (50 and 20 hPa) were investigated, revealing that both parameters affect C incorporation. This can be explained by complex multi-step precursor pyrolysis driven by collisions between precursor and H₂ molecules at high temperature. A pressure of 20 hPa leads to vanishing C-related Raman modes up to 840 °C and was fixed for further experiments.
Temperatures of 700 and 750 °C were investigated to optimize the initial nucleation density for later closing a ML. The temperature-dependent migration length of W adatoms on sapphire defines the maximum distance between neighboring nuclei. Nucleation at 700 °C for 15 min yields a high nucleation density of ~210 µm^-2 (50% higher than at 750 °C), with typical size of ML triangles ≤50 nm and a total ML coverage ~24%.
To suppress further ML nucleation, the lateral-growth stage is initiated by a temperature ramp of +10 K/min. By comparing the morphology and optical properties of the WS₂ films, the optimal temperature is determined to be 820 °C. In parallel, the WCO flux is ramped down (-75% over 144 min), aiming at reducing the W arrival rate and thus extending migration length. The impact of this ramp is clearly visible in the in-situ transient data, indicating a decelerated growth. This approach leads to a fully-coalesced (>99%) ML with very small BL coverage (~20%). The predominant ML nature of WS₂ was confirmed by Raman and PL studies. This MOCVD process can be used in both 7×2" and 1×4" configuration, showing excellent homogeneity and only small deviations in surface coverage for 2" and 4" substrates. Nevertheless, more advanced characterization methods like X-ray photoelectron spectroscopy (XPS) are necessary to study the stoichiometry, e.g. revealing the dependence of sulfur vacancy density on growth parameters.
In conclusion, a novel two-stage migration-enhanced MOCVD process is proposed to prepare fully-coalesced WS₂ ML films with suppressed BL formation. These results pave the way for the direct growth of 2D-2D heterostructures in a single process.

Transition metal dichalcogenides (TMDs) are a class of layered materials that have accumulated immense interest for integration in next-generation electronic, spintronic, and photonic devices. From the initial mechanical exfoliation experiments used to isolate thin TMD flakes, a considerable about of research effort has gone into realizing industrially-adaptive, scalable synthesis methods for large-area TMD films. While vapor-phase synthesis has enabled progress in improving grain size and orientation of TMD films on a wafer-scale, the lack of scalable doping and defect control methods impedes the implementation of TMDs in many application spaces. In this work we use a completely gas-phase metal organic chemical vapor deposition (MOCVD) system to grow high quality Re-doped MoS₂ (n-type) with excellent doping control. A combination of absorbance, Raman spectroscopy, and transmission electron microscopy (TEM) will evidence controllable and uniform Re-Mo substitution over large areas. Moreover, the effect of Re introduction on MoS₂ lattice and band structure will be investigated by time-resolved photoluminescence (TRPL), scanning tunneling microscopy/spectroscopy (STM/STS), and field effect transistor (FET) characteristics. These results along with density functional theory calculations elucidate the need to reduce defect ionization energy (IE) in doped 2D systems. The high IE of Re in ML Re-MoS₂ prevents dopant activation and results in deteriorating transport properties with higher doping levels. This work will show how dopant IE can be effectively lowered by growing few-layer (FL) doped films. In addition to investigating the transport properties of doped FL films, this work will present a novel interrupted MOCVD growth scheme for precisely controlling layer number over large areas. Employing this growth scheme, the synthesis of heterostructures made with layers of differing doping percentages enabling additional functionalities.

Semiconducting transition metal dichalcogenide (TMDs) monolayers have been of considerable interest for device applications. However, commonly used exfoliated flakes have size limitations and difficulties controlling the number of layers attained which limits their utility. Vapor phase techniques such as chemical vapor deposition (CVD) and metalorganic CVD (MOCVD) have been demonstrated as auspicious methods for the epitaxial growth of TMDs over wafer-scale areas. TMDs, however, exhibit a three-fold symmetry which typically results in two energetically equal domain orientation alignments at 0^o and 60^o when grown on common substrates such as sapphire, hBN and graphene. When these oppositely oriented domains coalesce during growth it produces inversion domain boundaries (IDBs) that exhibit metallic behavior which is undesired. Recent work has demonstrated that steps on the sapphire surface due to miscut can alter the energy landscape for nucleation and give rise to a preferred domain orientation for WS₂ [1] and MoS₂ [2] which enables a significant reduction in IDBs in wafer-scale TMD monolayers. In this work, we demonstrate that for WSe₂ grown by MOCVD on c-plane sapphire, growth conditions also play a significant role in domain orientation and can be used to control the crystallographic direction of domains on the sapphire substrate.
MOCVD growth was carried out in a stainless steel cold-wall reactor using W(CO)₆ and H₂Se as precursors in a H₂ carrier gas. The substrates used for growth were 2” diameter c-plane (0001) sapphire with nominal miscut of 0.2+/- 0.1° toward the m-plane. A three-step nucleation-ripening-lateral growth process was used to control the nucleation density and lateral growth rate to achieve epitaxial films [3]. The effects of growth temperature (850^oC to 1050^oC) and reactor pressure (50 to 700 Torr) on surface coverage and domain alignment were investigated. In all cases, the WSe₂ grows epitaxially on the (0001) sapphire surface with an epitaxial relationship of [110]// [110], as determined by in-plane x-ray diffraction. However, the preferred direction of the WSe₂ domains on sapphire was observed to change from 0^o to 60^o as the reactor pressure was raised from 200 Torr to 700 Torr holding all other parameters constant. The reactor pressure is believed to control the concentration of Se adsorbed on the sapphire which alters the surface chemistry and preferred nucleation site for the WSe₂. Further studies are underway to investigate this mechanism and determine the impact of preferred alignment on film properties.
1. M. Chubarov, et al., ACS Nano, 2021, 15, 2532-2541.
2. T. Li, et al. Nature Nanotechnol., 2021, https://doi.org/10.1038/s41565-021-00963-8.
3. X. Zhang, et al. Nano Lett, 2018, 18, 1049-1056.

Two-dimensional (2D) materials have garnered significant attention for the role they could play in advancing nanoelectronics. In particular, the transition metal dichalcogenides (TMDs), a class of 2D semiconductors, have shown good electron and hole mobilities at sub-nm channel thicknesses compared to silicon, owed in part to their layered structure which avoids increased surface roughness scattering [1]. Additionally, their large band gaps (> 2 eV) lead to transistor off-state leakage currents that are orders of magnitude lower than in silicon [2-4]. This makes them especially promising for memory applications such as dynamic random access memory (DRAM), where leakage current in access transistors limits performance, power consumption, and scaling [4]. Scaling of DRAM in recent decades has relied on the introduction of increasingly-complex 3D structures for both the capacitor and silicon access transistor to limit these leakage currents [5]. The demonstration of growth of monolayer TMDs on these kinds of non-planar 3D substrates, however, is limited. In addition, while layer-transfers can be achieved on planar substrates, they are significantly more difficult on 3D high-aspect ratio (high-AR) structures.

Here we demonstrate the direct growth of monolayer TMDs (MoS₂ and WSe₂) on high-AR trenches via chemical vapor deposition (CVD) at atmospheric pressure. This is achieved using solid source precursors in a quartz tube furnace in conjunction with substrates treated with perlyene-3,4,9,10 tetracarboxylic acid tetrapotassium salt (PTAS) as a seeding molecule [6,7]. We find that grains of monolayer TMDs nucleate and grow laterally, following the 3D contours of the SiO₂/Si substrate, including along sidewalls and into trenches. The grains are triangular under optical microscopy, which is seen in single crystals of TMDs grown on planar substrates, revealing the continuity of grains down into and over multiple trenches. Raman spectroscopy and photoluminescence (PL) measurements indicate monolayers for both TMDs.

Scanning electron microscopy (SEM) imaging shows that grains which nucleate on planar regions are able to continue growing along sidewalls. Conformal monolayer growth in trenches with AR ~ 2:1 is also observed for both TMDs examined here. Cross-sectional transmission electron microscopy (TEM) reveals that conformal growth of the 2D material is achievable even in trenches with high AR ~ 13:1, including at high radius of curvature corners and the bottoms of trenches (down to ~ 12 nm in width). This is further verified by electron energy-loss spectroscopy (EELS) analysis. The lack of bridging, clustering, or voids in the TMD films grown in these high-AR trenches is especially promising, and would be difficult or impossible to achieve with conventional sputtering, CVD, or layer transfers of other materials. Additionally, our grown films have grain sizes much larger (> 10 μm) than the height of a single sidewall or trench, which could allow for reduced device-to-device variability in large-scale and extremely dense 3D circuits.

In summary, we have demonstrated direct, conformal growth of monolayer TMDs onto trenches with ARs as high as 13:1. The ability for TMDs to be grown on arbitrary 3D structures and trenches could open up new design windows which, when combined with their superior performance and leakage at the monolayer limit, could propel the continued scaling and evolution of high-density 3D memory systems.

[1] C. English et al., Nano Lett. 16, 3824 (2016)
[2] C. Bailey et al., MRS-EMC 125 (2019)
[3] C. Bailey et al., MRS Spring Meeting (2020)
[4] C. Kshirsagar et al., ACS Nano 10, 8457 (2016)
[5] A. Spessot and H. Oh, IEEE Trans. Elec. Dev 67, 1382 (2020)
[6] K.K.H. Smithe et al., 2D Materials 4, 011009 (2017)
[7] J. Chen et al., ACS Photonics 6, 787 (2019)

Layered materials beyond omnipresent graphene such as chalcogenide-based materials that possess tunable and defined crystallographic structures and elemental compositions offer a broad portfolio of potential applications in sensors, detectors, and energy storage devices. The lacking control of large scale and homogeneous formation of 2D layered materials MX₂ (X = S, Se) in commercial formation processes motivated us to develop a unique synthetic approach to layered 2D materials. Complete characterization of synthesized complexes enabled their controlled decomposition to desired materials.
Herein we report a uniform synthesis of molecular building blocks to form (air)stable precursor classes [M{S(C₂H₄)₂NMe}_x] (M = Mo^IV, W^IV, Ti^IV, Zr^IV, Hf^IV, Nb^IV, Ta^IV, Sn^IV, x = 2; M = Ge^II, Sn^II, x = 1), which reliably delivered layered 2D van der Waals heterostructures.
Following a simple synthetic protocol, the reaction of tridentate SNS donor ligand with suitable metal compounds resulted in (air)stable molecular precursors, which have been characterized on an atomic scale. These precursors enabled the formation of homogeneous crystalline layered chalcogenide based heterostructures by thermal decomposition MoS₂, WS₂, TiS₂, SnS/Se₂, SnS/Se. These molecular building blocks provide an extraordinary synthetic pathway to a unique molecular precursor class, which delivered viable approach for large scale synthesis to chalcogenide-based van der Waalsmaterials for application in novel electronics, optics, and photonic devices.

Exfoliation of van der Waals materials has been the go-to approach to obtain 2D nanosheets. But recent efforts in expanding the 2D library has resulted in fabricating synthetic 2D crystals which have no layered bulk analogues. These efforts have been focused mainly on the surface growth of molecules in high vacuum or using template assisted growth techniques. Here, we report an alternate approach to making 2D crystals of covalent solids by chemical conversion of van der Waals layers. As an example, we used 2D indium selenide (InSe) obtained by exfoliation and converted it by direct fluorination into indium fluoride (InF₃), which cannot be obtained by direct exfoliation. The conversion of InSe into InF₃ is found to be feasible for thicknesses down to three layers of InSe, and the obtained stable InF₃ layers are doped with selenium. We study this new 2D material by optical, electron transport, and Raman measurements and show that it is a semiconductor with a direct bandgap of 2.2 eV, exhibiting high optical transparency across the visible and infrared spectral ranges. We also demonstrate the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation, into InF₃ films. The concept of chemical conversion of cleavable thin van der Waals crystals into covalently bonded non-cleavable ones opens exciting prospects for synthesizing a wide variety of novel atomically thin covalent crystals. The method can be extended to other materials. I will discuss our most recent results in this area of research.

Transition metal dichalcogenides are prominent materials for nanoelectronics and nanophotonics. Chemical vapor deposition remains the most effective way for the mass production of their 2D layers with high quality. The method has continuously been improved, from conventional growth using oxides as precursors, to recently developed salt-assisted growth,¹ where alkali metal halide salts are additionally included in the synthesis. Millimeter-sized 2D MoS₂ crystals have been achieved in experiments using the salt-assisted strategy, yet the details of the chemistry occurring during the synthesis are still ambiguous. Here, we employ first-principles calculations and ab initio molecular dynamics simulations to reveal the entire growth process,² including the sublimation of solid precursors, the gas sulfurization reactions, and the crystal growth at the flake edge, during the chemical vapor deposition synthesis of MoS₂ monolayers. The comparisons between the conventional and salt-assisted synthesis are made and the rules for choosing the best halide salts are discussed.³ Our study should promote the understanding of the chemical vapor deposition synthesis of transition metal dichalcogenides, and facilitate the process optimization to achieve their even faster growth in experiments.

1. J. Zhou et al., Nature 556, 355–359 (2018).
2. J. Lei et al., ACS Nano 15, 10525–10531 (2021).
3. J. Lei et al., In preparation.

Wafer-scale 2D single crystals are ideal for maximizing their performances in device applications. Here we demonstrate a few potential routes toward the synthesis of wafer-scale 2D single crystal:
Considering the weak van der Waals (VDW) interaction between 2D materials and various substrates, our theoretical analysis demonstrated that a single crystalline 2D material may across a grain boundary (GB) of a substrate without forming any defect and, therefore, a wafer-scale 2D single crystal might be synthesized on a polycrystalline substrate.[1] Such a strategy was realized by a “feedstock local feeding” approach.[2]
Through the analysis of the epitaxy of 2D materials growth, we concluded that the unidirectional alignments of a large number of 2D islands can be realized by using low-symmetry substrate. [3-4] Recently, under the guidance of the general approach and theoretical analysis, we have realized the growth of wafter-scale graphene, hBN and WS2 on designed substrates.[5-8]

Reference:
[1] X Zhang, et al., (2012), The Journal of Physical Chemistry Letters 3, 2822-2827
[2] T Wu, et al., (2016), Nature materials 15, 43
[3] Q Yuan, et al. (2014), J. Phys. Chem. Lett., 5, 3093
[4] J. Dong Nat. Comm. (2020) 11, 5862
[5] X Xu, et al., (2017), Sci. Bull. (2017) 62, 1074
[6] L Wang, et al., Nature (2019) 570, 91-95
[7] M. Wu, et al., Nature (2020) 581, 406-410
[8] J. Wang, et al., Nat. Nano., in press

Borophene, the two-dimensional (2D) monolayer of boron, has drawn tremendous attention due to its unique polymorphism and intriguing properties. The state-of-art syntheses employ the metal substrates that provide strong binding to boron atoms, stabilizing 2D borophene phases. However, the very same strong adhesion [1] that enables monolayer and recently even bilayer [1] growth, prevents subsequent exfoliation and hinders obtaining, characterization or use of free-standing layers. On the other hand, more inert weakly-binding substrates that would ease up exfoliation, do not provide sufficient areal attraction to enable 2D growth. Following our recent work on the lateral, edge-to-step 2D epitaxy [2] we investigate the possibility of borophene growth stabilization through dimensionality reduction on the nucleation stage. Using ab initio calculations of the epitaxial borophene growth on the h-BN surface we propose a novel growth methodology near naturally present 1D-defects. We demonstrate an order of magnitude reduction in the nucleation barrier due to covalent bonding to the exposed substrate while maintaining minimal areal adhesion [3]. This approach not only exemplifies the importance of the lateral epitaxy mechanism in the growth of 2D materials but also provides a practical approach to the synthesis of novel nanomaterials.
[1] Liu, X.; Wang, L.; Yakobson, B. I.; Hersam, M. C. Nano Lett. 2021, 21, 1174;
[2] Bets, K. V.; Gupta, N.; Yakobson, B. I. How the Complementarity at Vicinal Steps Enables Growth of 2D Monocrystals. Nano Lett. 2019, 19, 2027.
[3] Q. Ruan, L. Wang, K.V. Bets, B.I. Yakobson, Step-Edge Epitaxy for Borophene Growth on Insulators, ACS Nano, accepred (2021).

Novel confinement techniques facilitate the formation of non-layered two-dimensional (2D) materials, e.g. GaN, with thickness-dependent optoelectonic properties. However, a gap lies in the experimental demonstration of 2D oxides, such as gallium oxide (GaO_x), despite theoretical prediction. Here we demonstrate the synthesis of 2D oxides (GaO_x, InO_x, SnO_x) at the monolayer quasi-free-standing epitaxial graphene (QFEG)/SiC interface is highly dependent on buffer layer properties prior to metal intercalation. Using 2D Ga as the quintessential 2D metal, we identify that as-intercalated 2D Ga transforms to amorphous GaO_x via air oxidation at room temperature. High-resolution transmission electron microscopy verifies the formation of crystalline monolayer and bilayer GaO_x obtained via subsequent high-temperature O₂ annealing, verified via Density Functional Theory. Furthermore, QFEG/X/n-SiC (X = Ga, GaO_x) 2D/3D heterostructures exhibit Schottky and tunneling junction properties, respectively. Importantly, the bandgap of 2D gallium oxide is calculated as 6.6 ± 0.6 eV which is significantly larger than that of bulk β-Ga₂O₃ (~4.8 eV), suggesting strong quantum confinement effects for the gallium oxide in 2D limit.

The development of future quantum technologies relies largely on quantum materials, whose properties such as complex magnetism, topology, and superconductivity, do not follow the laws of classical physics. Two-dimensional (2D) materials are key members here because of their unique properties that their bulk counterparts do not have and their high degree of tunability. Mechanical exfoliation with scotch tape is a classical and popular way to obtain 2D materials. However, the application of mechanical exfoliation is restricted to van der Waals (vdW) layered materials as the mechanical force is not strong enough to break covalent and ionic bonds. In this work, we use soft chemical synthesis as a synthetic method to fine-tune the properties of layered materials and to access various metastable transition metal dichalcogenides (TMD) compounds that high-temperature solid-state methods cannot directly access. Starting from layered KMX₂ (M=transition metal, X=S, Se, or Te), we monitor the changes of structural and physical properties, such as magnetism or superconductivity, as we deintercalate potassium from the starting materials. In addition, we carefully examine fully deintercalated layered end compounds, MX₂, which are synthesized via different soft-chemical synthetic routes, finding that although lattice parameters can differ, some physical properties, such as superconductivity, remain robust. Some soft-chemical syntheses routes are shown to be more suitable for exfoliating samples to monolayers.

Until the early 1990’s the ZT of thermoelectric materials was believed to be restricted to unity. This meant that the thermodynamic efficiency was too low for mass production in competitive markets such as heating, ventilation and air conditioning and electronic cooling in computers. At that time seminal papers by L.D. Hicks and M.S. Dresselhaus made it clear that the dimensionality of materials can be used to increase boundary scattering of phonons reducing the lattice thermal conductivity. Reducing the lattice thermal conductivity is thought to be the critical factor in improving the ZT of materials. Besides dimensionality, alloying and structure disorder can also further reduce the lattice thermal conductivity of a material. Typically insulating amorphous solids and glasses have lower thermal conductivity than dense solids. However, in turbostratic structured layered crystals the thermal conductivity is shown to be as low as 0.05 W/mK. TEM analysis show that VLS grown MoS₂ nanowires have stacking faults starting at the metal catalyst and existing throughout the length of the wires. The lattice thermal conductivity and electrical conductivity of these materials were measured with the thermal bridge and four-point probe methods, respectively. MoS₂ nanowires show reduced thermal conductivity approaching what is reported in literature for the out of plane direction in bulk samples while the GaS_1-xTe_x nanowires demonstrate glass like thermal conductivity as low as 0.5W/mK. Additionally, MoS₂ nanowires demonstrate semiconducting behavior with high resistivity on the order of 10⁴ ohm-cm while GaS_1-xTe_x nanowires have resistivity on the order of 10^-2 ohm-cm. The reduction of thermal conductivity in MoS₂ nanowires is attributed to the stacking disorder and dimensionality, while the lattice thermal conductivity behavior in GaS_1-xTe_x nanowires can be attributed to alloying, dimensionality and possible stacking disorder. In conclusion, the VLS growth technique allows for the ability to introduce stacking disorder and grow nanowires with reduced lattice thermal conductivity. The influence of alloying and metal catalyst selection on stacking disorder will need to be considered in future studies of layered nanowire thermal conductivity.

Van der Waals integration, in which freestanding building blocks are physically assembled together through weak van der Waalsinteractions, offers a bond-free material integration strategy without lattice and processing limitations. Here I will discuss the fundamental forces involved in van der Waals integration and generalize this approach for flexible integration of highly disparate materials to produce artificial heterostructures with minimum interfacial disorder and to enable high-performing devices. I will start with a brief introduction of some early background on using this approach for damage-free dielectric integration to enable high speed transistors from atomically thin graphene and 2D semiconductors, and then focus my most of my discussion on our recent efforts in exploring this approach for low-energy metal integration to enable high performance contacts for delicate materials. In particular, I will highlight our recent advancement in exploiting the van der Waals integration approach for seamless integration of conventional metal thin film electrodes with the 2D semiconductors to create nearly ideal metal-semiconductor junctions essentially free of interface states and Fermi level pinning, for the first time the experimental achieving Schottky-Mott limit that has been difficult to achieve in Schottky diodes, and realizing the 2D diode approaching the intrinsic exciton physics limit. Next, we further extends this approach for damage-free integration of metal contacts on delicate halide perovskite thin films, realizing high performance van der Waals contacts with an atomically clean contacting interface and greatly reduced contact resistance, for the first time allowing systematic electrical transport studies of halide perovskites down to cryogenic temperatures and establishing technical foundation towards unraveling the complex transport behavior and exploring new physics in this unique class of “soft-lattice” materials. These advancements highlight the unique merit of the bond-free van der Waalsintegration approach in creating pinning-free heterostructure interfaces. I will conclude with a brief perspective on these strategies for creating diverse artificial heterostructures as a versatile material platform with electronic structure by design to unlock new physical limits and enable a new generation of devices with unprecedented performance or unique functions.

With the continuous scaling of silicon integrated circuits, identifying alternative materials for both the front-end-of-line (transistors) and back-end-of-line (BEOL, interconnects) technology is becoming increasingly important. Forthcoming sub-5 nm technology node requires a 20 nm interconnect half-pitch size, resulting in the conventional barrier and liner materials thicknesses of less than 4 nm. However, the traditional barriers like TaN/Ta occupy a significant portion of the interconnect cross-section, and this down-scaling reduces the Cu ion blocking efficiency. Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been recently explored as promising candidates for Cu diffusion barrier and liner applications. However, most 2D TMDs are grown at elevated temperatures (>800°C) using chemical vapor deposition, making them incompatible with BEOL processes. Plasma-assisted growth can reduce the growth temperature, but poor conformality and plasma-induced defects remain a challenge. Furthermore, some 2D TMDs growth approaches can only deliver micrometer-scale flakes, and BEOL requires films that are uniform over large areas for circuit integration.
We report a BEOL-compatible growth process of crystalline WS₂ thin films (~2 nm) on dielectric substrates via atomic layer deposition. This complete thermal growth technique offers precise thickness control and large-area conformality. We have successfully deposited high-quality WS₂ thin films on 2 cm x 2 cm SiO₂/Si substrates using an approach that is fully scalable to 8-inch wafers. The quality of the WS₂ thin film is confirmed by microscopic (atomic force microscopy and transmission electron microscopy) as well as spectroscopic (Raman spectroscopy, x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and energy dispersive x-ray spectroscopy) measurements. Electrical characterization of the WS₂ thin films was used to evaluate their properties for diffusion barrier and liner applications. We will discuss the potential of the thermal ALD growth of 2D TMDs for interconnect scaling beyond the 5 nm node.

MoS₂ has been an attractive material over the years, having comparable carrier mobility with oxide TFT and much greater photo-responsivity than conventional Si photodiode. Furthermore, its unique atomic structure enables to develop flexible devices, in typically flexible photodetector. Despite its various favorable properties, only few successes of practical level studies in MoS₂ photodetector have been reported yet might be due to the difficulty of large-area, high-quality synthesis and gate dielectric constructions on it.
Here, we show a flexible, active-matrix type photodetector implemented by MoS₂ circuitry that could operate for both visible light and X-ray. The MoS₂ photodetector exhibit about 10AW^-1 level of photoresponsivity that is almost 100 times larger than Si photodiode and integrated with the MoS₂ TFT having a large on/off ratio of ~ 10⁷ and mobility over 10 cm². 60 x 60 pixels in 3² cm² are enough to obtain spatial mapping through a target image. In addition, by bending it along the target feature it could effectively avoid vignetting effect in contrast to conventional rigid image sensors. Our study in flexible MoS₂ circuitry will motivate it to be achieved practical level development as well as flexible electronics.

Transition metal dichalcogenides (TMDs) exist in several polymorphs including 2H (usually semiconducting), and 1T/1T’ (usually semi-metallic). Our theoretical calculations and experimental measurements show that TMDs are optically-dense and can feature a difference in refractive index greater than unity between phases ([endif]-->) in the near-infrared (NIR). Fast, non-thermal martensitic phase-change behavior has been observed in TMDs, and transformation strains are expected to be low due to the layered crystal structure. For these reasons, we suggest that TMDs may be useful as active materials to control the phase of light in integrated photonic circuits, surpassing the performance of traditional phase-change materials [1].
Sulfide TMDs are attractive for application to photonics because they present lower optical loss and higher material stability than selenides or tellurides. However, sulfide TMDs also have higher transformation energy barriers. We address this problem by designing sulfide TMD alloys that are thermodynamically-adjacent to phase boundaries. We use density-functional theory (DFT) and the quasi-harmonic approximation to calculate free energy-composition diagrams at finite temperature for polymorphs in the MoS₂-TiS₂-ZrS₂ system. Our calculations predict that the free energy difference between phases can be reduced to near-zero through alloy design. We then synthesize sulfide TMD thin films through a two-step process of metal sputtering, followed by sulfurization in H₂S in a chemical vapor deposition (CVD) furnace. This two-step method, combined with combinatorial sputtering, enables rapid exploration of composition space and phase boundaries. By controlling the CVD furnace conditions, and in particular the trace oxygen concentration, we are able to lower the processing temperature for large-area alloy thin films to 500 °C and below; this is important for yielding good film morphology and for integration with photonic integrated circuit (PIC) manufacturing. We use transmission electron microscopy (TEM) and synchrotron nanodiffraction and nanoflourescence to characterize the competing processes of alloy stabilization and secondary phase formation. Finally, we demonstrate a test concept to study phase-change behavior with electrical stimulation and optical readout for a composition spread across a wafer, and we characterize the phase-change behavior of sulfide TMD alloys by Raman spectroscopy, spectroscopic ellipsometry, and infrared spectroscopy.
[1] A. Singh, S. S. Jo, Y. Li, C. Wu, M. Li, and R. Jaramillo, Refractive Uses of Layered and Two-Dimensional Materials for Integrated Photonics, ACS Photonics 7, 3270 (2020).

The current preparation of 2D materials for use in devices mostly focuses on Chemical Vapor Deposition (CVD) and micro-exfoliation from bulk crystals. While CVD offers good scalability and big coverage of the sample it induces many defects during growth. Micro-exfoliation, on the other hand, yields monolayers with excellent optical and electronic properties but lacks user-friendliness and scalability for application.
Molecular Beam Epitaxy (MBE) in ultra-high vacuum (UHV) offers unparalleled control of growth parameters and scalable growth of defect-free 2D materials with extremely good quality. [1] One of its current limitations is the reliance on metallic Ir(111) or Au(111) substrates which quench the optical response of grown materials, making them unsuitable for optoelectronic devices.
In this work, we demonstrate a method to transfer an MBE grown transition metal dichalcogenide (TMD) heterostructure (HS) to a SiO₂/Si wafer that is a more suitable substrate for the investigation of optical properties and device fabrication. The original sample was prepared by heteroepitaxial growth of TMDs under UHV conditions. Temperature programmed growth followed by a CVD step results in a full layer of graphene on Ir(111) substrate. Then we performed a one-step MBE growth to prepare homogeneous regular MoS₂ islands during which individual Mo atoms were intercalated below graphene. The resulting HS MoS₂/Gr/Mo/Ir(111) is then used as a heterogeneous vdW substrate for growing WS₂ with the two-step method. The resulting sample consists of lateral and vertical quasi-freestanding heterostructures WS₂/MoS₂/Graphene/Mo on Ir(111), with TMD islands of lateral size below 100nm. Scanning tunneling spectroscopy reveals type-II band alignment for the lateral and a symmetric upward bend bending for the vertical HS. [2]
The large, continuous graphene layer beneath the TMD HSs allows for the transfer from Ir(111) to a SiO₂/Si wafer. Electrochemical delamination from the Ir(111) crystal was used with PMMA as a supporting layer on top of the sample, which was then placed on the SiO₂/Si wafer. AFM of the transferred HS reveals mostly uniform height across the sample with some bi- and three-layer folding from the transfer process. Micro-Raman spectroscopy confirms the presence of the TMD HS on SiO₂/Si wafer and fluorescence spectroscopy reveals the characteristic exciton resonances which blueshift approaching the low temperature (4.2 K) limit, confirming its semiconducting nature [3].
Wide-field hyperspectral fluorescence microscopy yields spatially and spectrally resolved images of the sample revealing spectrally homogeneous areas over hundreds of microns. Some sample areas show a different fluorescence instead of the typical TMD HS signature. This emission may originate from chemical complexes that could result from a reaction with graphene dangling bonds during the electrochemical delamination process.
The combination of MBE growth and electrochemical delamination transfer technique provides a controllable and scalable new method of preparing 2D materials on any desirable surface suitable for application in electronic and optoelectronic devices.
References
[1] Joshua Hall et al 2018 2D Mater. 5 025005
[2] B. Pielić et al 2021 “Electronic structure of quasi-freestanding WS2/MoS2 heterostructures” ACS Appl. Mater. Interfaces 2021, XXXX, accepted, 10.1021/acsami.1c15412
[3] K. P. O’Donnel, X. Chen, Appl. Phys. Lett. 58, 2924 (1991);

IIntensive research activity on Transition Metal Dichalcogenides (TMDs) has been carried out in recent years because of the unique physicochemical properties of these materials [1]. TMDs exhibit a wide range of electronic properties, since they occur as semiconductors, metals, semimetals and superconductors [2]. TMDs are also used as sensors and in energy storage applications. MoTe₂ occurs in several crystalline phases (hexagonal 2H, octahedral 1T, distorted octahedral 1T′ and orthorhombic T_d) among which metallic 1T′, semimetallic T_d and semiconducting 2H are thermodynamically stable. The most interesting MoTe₂ phase seems to be T_d with theoretically predicted and experimentally verified type-II topological Weyl semimetal properties.
MoTe₂ epilayers are deposited on GaAs(111)B substrates using Molecular Beam Epitaxy [3]. The orientation, strain, the presence of defects, morphology and crystal phases of the layers as well as the smoothness of GaAs/MoTe₂ MnTe/MoTe₂ and NiTe₂/MoTe₂ interfaces were investigated with transmission electron microscopy methods. The thin cross-sections of the as-grown samples were prepared with Focused Ion Beam (FIB) technique. Depending on the growth conditions, the atomically flat, smooth or porous, moss-like layers are obtained. The epitaxial relations between the MoTe₂ layer and the substrate are determined as follows: [011] and [11-1] GaAs directions are parallel to [-2110] and [0001] directions of MoTe₂, respectively. In the cross-sections of smooth epilayers the 2H_c MoTe₂ phase is dominant. Additionally MoTe₂ layers were grown on MnTe and NiTe₂ buffers deposited on GaAs(111)B substrates [4] in the same process i.e., directly before the growth of MoTe₂.
In the case of NiTe₂ buffer the GaAs(111)B surface shares a trigonal symmetry with NiTe₂. Moreover, the surface lattice constant of GaAs(111), a = 3.997 Å matches very well that of NiTe₂, a = 3.855 Å. Several nanometers thick NiTe₂ epilayers do not show any structural defects in TEM cross-sections [3]. The lattice mismatch of 3.5% was accommodated by a network of periodic “pseudo misfit dislocations” at GaAs/NiTe₂ interface. In the case of thin NiTe₂ strain analysis, using HRSTEM images shows strain fields propagating to the epilayer. In the case of MnTe/GaAs(111)B buffer the hexagonal and cubic MnTe phases are observed. The large surface lattice mismatch (~12%) between MnTe and GaAs is accommodated by a dense network of misfit dislocations. No threading dislocations were observed, however the stacking faults, partial dislocation and nano-twins were evidenced especially in the case of cubic MnTe. In all the cases high quality several ML thick continuous MoTe₂ , layers were successfully deposited on NiTe₂ and MnTe. In general the c axis of MoTe₂ epilayer follows c axis of hexagonal MnTe and NiTe₂ and the [111] direction in the case of cubic MnTe. Despite of the Van der Waals epitaxy, the residual strain analysis performed by the GPA method [5] on cross-sectional HRSTEM images, shows inhomogeneous lattice deformation propagating from the interface region. It can be due to the inhomogeneous thermal expansion resulting in the post growth deformation. It seems that such residual strain can destabilize the dominating 2H MoTe₂ polytype and enable occurrence of 1T’ MoTe₂ phase, but also formation of Mo₆Te₆ phase seems to be related to the strain or vice versa strain is originating from the Mo₆Te₆ inclusions. The ability to fill the GaAs(111)B substrate cavities and unevenness by MoTe₂ layers due to high degree of elastic deformation of this VdW compound is also revealed in our cross-section TEM studies.
1. H. Huang , X. Fan, et al, Nanoscale 3 1247 (2020)
2. R. Roldan, et al., Annalen der Physik 526, 347-357 (2014)
3. Z. Ogorzalek, B. Seredynski, et al. , Nanoscale,12, 16535 (2020).
4. B Seredynnski, Z Ogorzalek,et al. , Crystal Growth & Design 21, 10, 5773-5779 (2021)
5. S. Kret, et. al., Phys. Status Solidi Basic Res., 227 (1), 247–295 (2001).

Controlling the interlayer twist angle in an artificial two-dimensional (2D) van der Waals (vdW) interface offers an experimental route to create a moire superlattice. In the small twist angle homojunction, vdW interlayer interaction can cause atomic-scale structural reconfiguration at the interface, creating the arrays of moire domain structures. The boundaries of these domain structures are structural solitons, forming 2D dislocation line arrays. The crossing points of these dislocation lines are topological objects, which cannot be removed by local deformation. Particularly, in the crystal symmetry engineered twisted bilayer polar crystals, we find unconventional domain anti-ferroelectricity can arise to exhibit the ferroelectric/antiferroelectric switching mechanism. We performed an in-situ transmission electron microscopy (TEM) investigation on dual gated twisted bilayer transition metal dichalcogenides (TMD) devices that enable real-time observation of polar domain dynamics in a 2D system. In combination with the theoretical investigation, we find the polarizability of the twisted bilayer TMD sensitively depends on the moire length and the domain shapes. As the size of domain approaches to the sample size, spontaneous polarization in the engineered TMD interface shows analogy as well as interesting distinction compared with the conventional ferroelectrics, exhibiting the electrically switchable bipolar structure in the engineered TMD. However, as the domain size becomes smaller than the size of the samples, the clear demonstration of alternating polar domains appears with an antiferroelectric configuration. Unlike atomic-scale antiferroelectric, these antiferroelectric domain structures cannot be transformed into a single ferroelectric domain due to the topological protection of the domain topology. We will discuss the implication of engineering (anti)-ferroelectric domain structures for the ferroelectric device applications based on 2D twisted homojunction.

Moiré superlattices, formed by stacking two-dimensional van der Waals layers with an interlayer rotation and/or lattice constant mismatch, have electronic band structures that are highly sensitive to structural modifications. For example, twisted bilayer graphene exhibits unconventional superconductivity and ferromagnetism at a ‘magic’ interlayer twist angle of 1.1°, associated with formation of flat electronic bands. At the same time, natural lattice deformations, strain, and disorder can also dramatically influence the behavior observed in these systems. Visualizing the structure and strain fields of moiré materials is therefore paramount to understanding and controlling their emergent electronic behavior. In this talk, I will present work on the development of a technique termed Bragg interferometry, based on four-dimensional scanning transmission electron microscopy (4D-STEM), for directly and quantitatively mapping interlayer atomic displacements and strain fields in moiré superlattices, including twisted bilayer graphene and twisted bilayer transition metal dichalcogenides. This work sheds light on the structural changes underpinning the twist angle dependent electronic properties of moiré systems and provides a new framework for directly visualizing lattice reconstruction mechanics, disorder, and strain in a wide array of moiré materials.

Heterostructure materials form the basis of much of modern electronics, from transistors to lasers and light-emitting diodes. Recent years have seen a renewed focus on creating heterostructures through the vertical integration of two-dimensional materials, including graphene, hexagonal boron nitride, and transition metal dichalcogenides (TMDCs). However, fundamental challenges associated with materials processing have limited material quality and impeded scalability. This talk discusses a method to convert sub-nanometer metal films deposited on silicon and sapphire into TMDC heterostructures and superlattices of MoSe₂/WSe₂ and MoS₂/WS₂ through vapor-phase processing. The resulting heterostructures exhibit novel properties compared with stand-alone TMDCs, including reduced bandgap, enhanced light-matter coupling, and improved catalytic performance. This robust and scalable synthetic method provides new opportunities to generate a wide range of artificially stacked 2D superlattices with controlled morphology and composition.

Layered Materials (LMs) have potential for quantum technologies, as scalable sources of single photon emitters (SPEs)[1,2]. LM heterostructures can be built with tuneable properties depending on the constituent materials and their relative crystallographic orientation[3,4]. Quantum emitters in LMs hold potential in terms of scalability, miniaturization, integration. Generation of quantum emission from the recombination of indirect excitons in heterostructures made of different LMs is a path with enormous potential. I will discuss how LM combinations can be used to generate SPEs and confinement of interlayer excitons[5].
1.C. Palacios-Berraquero et al., Nat. Commun. 8, 15093 (2017)
2.C. Palacios-Berraquero, et al., Nat. Commun. 7, 12978 (2016)
3.P. Rivera et al., Nat. Nanotech. 13,1004 (2018)
4.M. Barbone et al. Nat. Commun 9, 3721 (2018)
5. A. R. P. Montblanch et al. Commun Phys. 4, 119 (2021)

Atomic scale defects such as sulfur vacancies can influence the electronic structure of 2D transition metal dichalcogenides (TMDs). A relatively simple way to probe defects is by photoluminescence (PL) spectroscopy. In this study, we have probed the PL properties of 2D TMDs annealed up to 700○C in hydrogen, argon, and nitrogen atmospheres. Annealing of monolayer TMDs results in creation of defects that leads to changes in the PL spectra. We have studied monolayers of MoS₂ grown by chemical vapor deposition (CVD) and mechanical exfoliation. In the as-prepared samples, there is a red shift of the PL peak by ∼60meV in CVD samples compared to mechanically exfoliated samples. This is attributed to thermal strain that can be relaxed by annealing in inert environments. Annealing in argon and nitrogen also leads to narrowing of the exciton peak at ∼1.83eV up to 550○C. Above this temperature, the peak becomes broader with higher trion component. Annealing in hydrogen leads to appearance of an additional peak around 1.3 – 1.4eV. Our results suggest the creation of sulfur vacancies give rise to near infrared emission from low energy defect states.

With the success of twisted bilayer graphene and transition metal dichalcogenides (TMDs) in gaining unprecedented and tunable physical properties, vertical stacking of van der Waals (vdWs) candidates has raised extensive research interests. Effective engineering of 2D heterostructure properties can be approached through manipulation of defects or moiré superlattices. By proton irradiation of monolayer and bilayer TMD heterostructures, our research provides fundamental insights in understanding the correlation between structural defects and the tuning of optical properties. Specifically, we will present observations from the temperature dependent photoluminescence and Raman results by comparing pristine monolayers and irradiated monolayers with supporting substrates, as well as free-standing monolayers. Red-shifting of key photoluminescence peak without quenching upon irradiation will be discussed to explain possible mechanisms behind. The study provides enormous potentials and opportunities for enhancing performances of flexible optoelectronic devices using defect tuned vdW heterostructures.

Two dimensional (2D) transition metal dichalcogenides (TMDs) are widely studied due to their tunable properties and potential for ultrathin optoelectronic devices. In recent years, point defects in these materials have been intensively studied, with potential application as single photon emitters and as catalytically active centers for hydrogen evolution and CO₂ reduction. In molybdenum disulfide (MoS₂), native point defects appear in the form of monosulfur vacancies (V_S), disulfur vacancies (V_2S), or molybdenum vacancies (V_Mo). However, the main focus in defect studies of MoS₂ is on isolated V_S, as they are the most abundant defects found in both exfoliated and CVD-grown MoS_2. Thus, the control of V_S generation is a key factor for the fabrication of high performance devices, as well as for enhancing catalytic activity of this material. Thus far, the deterministic creation of defects at a controlled location and the formation of monosulfur vacancies (V_S) remains a challenge. In this contribution, we present the successful generation of V_S in MoS₂ monolayers using spatially focused (130 nm beam diameter) soft X-ray synchrotron radiation and monitored in situ using X-ray photoelectron spectroscopy (XPS). High-resolution XPS maps allow precise characterization of V_S rich regions within monocrystalline MoS₂ flakes and associated spectral analysis enables elucidation of the effect of V_S on the MoS₂ electronic and chemical properties. Thereby, additional chemical components arising approximately 1 eV towards lower binding energies (BE) of the molybdenum (Mo) 3d peaks are indicative of undercoordinated Mo and are assigned to V_S. Complimentary optical spectroscopic techniques confirm that the MoS₂ lattice is preserved throughout the vacancy generation process. In addition, we demonstrate that of X-ray induced V_S are metastable in air for at least five minutes, but targeted passivation of previously generated V_S can be achieved in an oxygen environment. These results provide further insight into the V_S passivation process and reveal its effect on the electronic and optical characteristics of MoS₂.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have attracted enormous attention because of exceptional optical properties such as large exciton binding energy, and strong light-matter interaction at the ultimate thickness limit. Such remarkable properties make them promising for high-performance light-emitting devices such as LEDs, LASERs, and single-photon emitters. However, high efficiency in the luminescence of those 2D semiconductors is inherently limited to monolayer regime due to indirect-to-direct bandgap transition. In addition, a quantum well (QW) structure for luminescence enhancement is hard to be implemented by using semiconductor TMDs as a light-emitting layer due to a lack of large bandgap materials for construction of type-I band alignment and facile fabrication methods compatible with these atomically thin materials. In this regard, a facile fabrication of the TMDs-based multiple quantum wells (MQWs) that enables for both confining excitons and enlarging active volume remains a considerable challenge.
Here, we demonstrate the novel approach to fabricate atomic-layer-confined MQWs via monolithic bandgap engineering of TMDs and artificial van der Waals stacking. A fundamental building block of QWs, the WO_X/WSe₂ hetero-bilayer, was prepared by monolithic oxidation of the WSe₂ bilayer, followed by stacking the blocks into the MQWs. By examining the band alignment of WO_X/WSe₂, we confirm that the hetero-bilayer WO_X/WSe₂ constructs the type-I quantum well for efficient exciton confinement and radiative recombination. Unlike the case of stacking monolayers only, the super-linear increases of photoluminescence with the number of QWs were achieved: about 5-fold enhancement for triple QWs. Furthermore, the quantum-confined radiative recombination in MQWs was verified by a large exciton binding energy of 193 meV and a short exciton lifetime of 170 ps. This work paves the way toward monolithic integration of 2D superlattices for novel quantum optoelectronics.

2D van der Waals (vdW) crystals provide a powerful materials platform for studying two dimensional (2D) electron transport. In this talk, I will start with the large-scale processes for generating 2D crystalline semiconductor films and superlattices that could be used to fabricate atomically thin integrated circuits. Then we will discuss new directions, where we use these 2D materials to realize non-electronic 2D transport phenomena, for example, observed from phonons, photons, and mass. These new approaches could empower the development of 2D phononics, 2D photonics, and 2D mechanics.

The successful isolation of two-dimensional (2D) crystals through mechanical exfoliation have led to numerous scientific breakthroughs and paved avenues for next-generation nanoscale devices, with the majority of these studies focused on inorganic van der Waals crystals. In this talk, I will present characterizations of the mechanically exfoliated Fe-based magnetic molecules down to the monolayer thickness. These molecular are bound by anisotropic van der Waals interactions into 2D assemblies, and can be exfoliated into single-crystalline flakes similar to inorganic atomic crystals. With reducing thickness, we observe a substantial increase in the spin-state transition thermal hysteresis, which suggests the important roles of substrate and interlayer coupling in molecular systems. We further demonstrate the capability of fabricating organic-inorganic heterostructures with a monolayer MoS₂, which exhibits an efficient interface coupling. Our results establish molecular solids with strong anisotropy of intermolecular interactions as precursors to a novel class of 2D materials, affording new possibilities for designing hybrid organic-inorganic heterostructures and devices.

Electric double layer transistors (EDLTs) are field-effect transistors where the gate oxide is replaced by an electrically insulating but ionically conducting electrolyte with mobile ions. Large capacitance densities (~10 μF/cm²) can be achieved using this approach; however, microns-thick ionic liquids or solid electrolytes after often used. While these electrolytes are useful for exploring the fundamental physics of semiconducting materials – especially two-dimensional (2D) crystals – the thicknesses are impractical and can interfere with optical measurements. The physics underlying EDL gating is such that the steady-state capacitance density should be independent of electrolyte thickness. Moreover, the RC time constant associated with EDL formation and dissipation should decrease with decreasing thickness. Indeed, both COMSOL modeling and a recent report by Gonzalez-Serrano et al. (J. Electronic Materials) support this picture by impedance measurements on metal-insulator-metal structure down to 8 nm thick electrolytes. Here, we extend this ultrathin film exploration to side-gated, 2D crystal FETs. The electrolyte is polyethylene oxide (PEO):CsClO₄. Hall measurements show that the graphene FET sheet carrier density remains constant throughout the measured thickness range (10 nm – 1 μm). Finite element modeling in which the ion transport is self-consistently couped to the carrier transport in the semiconductor, shows that the potential drop across the electrolyte is 50% smaller than the applied potential, suggesting that higher carrier densities can be achieved at larger gate voltages without concern for inducing electrochemical reactions. Indeed, Hall measurements extended to +/- 6V increase the electron and hole densities without inducing electrical signatures associated with electrochemistry. Both results are important for achieving ultrahigh carrier densities in 2D FETs, and demonstrate progress towards scaling to VLSI-relevant thicknesses and decreasing the EDL time constants to those relevant for electronics.
This work was funded in part by an Alfred P. Sloan Foundation fellowship and the NSF DMR-2011839 Penn State MRSEC Center for Nanoscale Science.

Direct bandgap and high charge mobility are useful properties of semiconducting transition metal dichalcogenides (TMDs) for electronic applications.¹ Metallic phases of TMDs have also shown promise for low resistance contacts in field effect transistors and novel photo-detectors.² Previously, local phase conversion of mechanically exfoliated 2H-MoS₂ flakes to metallic 1T-MoS₂ flakes was demonstrated via alkali metal intercalation.³ In this study, we show vertical and lateral partial 1T-2H phase transition of patterned mechanically exfoliated MoS₂ flakes . Raman spectroscopy of the lateral 1T-2H flakes shows clear boundaries between the two phases, additionally, it is observed that magnetic force microscopy (MFM) can also distinguish between the two phases. MFM reveals that the 1T phase is magnetic. Partial phase transformation in the vertical direction leads to 1T-2H strained heterostructures as characterized by Raman spectroscopy. The magnetic properties of the 1T phase contacted to the semiconducting 2H phase along with the presence of strain at the interface are important considerations for developing electronics and optoelectronics from multiphase TMDs.
¹Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N., & Strano, M. S. (2012). Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature nanotechnology, 7(11), 699-712.
²Wang, Y., Fullon, R., Acerce, M., Petoukhoff, C. E., Yang, J., Chen, C., ... & Chhowalla, M. (2017). Solution–processed MoS2/organolead trihalide perovskite photodetectors. Advanced Materials, 29(4), 1603995.
³Kappera, R., Voiry, D., Yalcin, S. E., Branch, B., Gupta, G., Mohite, A. D., & Chhowalla, M. (2014). Phase−engineered low-resistance contacts for ultrathin MoS2 transistors. Nature materials, 13(12), 1128-1134.

Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room-temperature with long-range ferromagnetic order is still debatable to date. Here, we report the room-temperature ferromagnetism with long-range order in semiconducting V-doped WSe₂ monolayer synthesized by chemical vapor deposition. Ferromagnetic hysteresis curves are clearly manifested with exclusive Al₂O₃ passivation via vibrating-sample-magnetometer, exhibiting coercivity remanent to 360K, while retaining high on/off current ratio of 10⁵ at 0.1% V-doping. This is also confirmed by the presence of magnetic domains from magnetic force microscopy. V-substituted W atoms keep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution transmission-electron-microscopy. We further demonstrate that the magnetic order and Curie temperature can be modulated by applying the back gate, implying the validation of the Zener model in establishing the long-range ferromagnetic order in V-doped WSe₂ monolayer, which is consistently supported by our density functional theory calculations. Interestingly, the competition between ferromagnetic and antiferromagnetic states is theoretically predicted. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.

Introduction
Thermodynamics approach is a simple but useful method to inspect reaction tendency systematically, which has successfully proposed HfO₂ and ZrO₂ in the selection of high-k gate dielectric materials for Si-based devices.[1] As candidate materials for next generation transistors, the stability of 2D materials is a critical issue that should be solved. In this research, thermodynamics approach was applied not only to verify the oxidation tendency of 2D materials but to predict the candidate materials that can solve oxidation issue by ameliorating the surface oxide.
Stability of 2D materials for oxidation
In the first part of this research, the Gibbs free energy change (ΔG) of the oxidation reaction of 75 different layered materials are calculated in consideration of following two reactions:(i) O₂(g) + M_xA_y → M_wO_z + A and (ii) O₂(g) + M_xA_y → M_wO_z + M_vA_z, where M and A stands for metal and anion of 2D materials, respectively. Equation (i) describes the reaction that 2D materials form metal oxide and isolated anion after oxidation, while equation (ii) covers the situation that other compounds with higher anion ratio form. By comparing all possible oxidation reactions, the most favorable oxidation route can therefore be determined by finding lowest ΔG among oxidation reactions of layered materials. Our calculation results reveal that most of 2D materials indeed show tendency to oxidize in room temperature as we have already encountered surface oxidation issue in many 2D devices.
Since most of 2D materials are thermodynamically unstable to oxygen, how to deal with the surface oxide becomes an indispensable issue in 2D devices. Based on thermodynamic calculation, two possible concepts that can remove surface oxide of 2D materials are proposed.
2.1 Conversion of surface oxide to high-k oxide
The first method is reducing surface oxide through metal deposition on channel region and/or contact region of 2D devices. By selecting metals that exhibit higher oxygen affinity than 2D materials, surface oxide can be reduced. By comparing ΔG of metal oxidation and 2D material oxidation, yttrium (Y), exhibiting highest oxidation ability, is found to be best candidate on all the 2D channels since its oxidation resultant, Y₂O₃, is also a high-κ oxide. For metal contact region, on the other hand, titanium (Ti) should be selected since partially oxidized TiO_x is still conductive and can serve as a good contact medium, which has been experimentally proved. [2]
2.2 Recovery of 2D materials from surface oxide
In second method, heterostucture is introduced to recovery oxidized 2D materials. Through heterostructure formation between oxidized 2D chalcogenide (at bottom) and another 2D material with the same chalcogen group and higher oxidation tendency (on top), oxygen of bottom 2D oxide is expected to be absorbed by top 2D material, while the chalcogen groups of top 2D material can be transferred to bottom. Thus, bottom 2D material is expected to be fully recovered, while top 2D material can be oxidized and serve as buffer layer for top gate insulator. Based on our systematic calculation, Zirconium and Hafnium chalcogenide could serve as appropriate choice for oxide reduction and chalcogen supply due to their higher reactivity than other typical 2D chalcogenides and high-κ oxide resulted.
3. Conclusion
The lingering questions regarding the oxidation of 2D materials was revealed by thermodynamic approaches in this research. Furthermore, the thermodynamic prediction of the conversion of the surface oxides and the recovery of original 2D materials proposed could contribute to the future development of 2D material-based devices from the perspective of gate and contact material selection, especially for 2D materials that are vulnerable to oxidation.
References: [1] D. Schlom et al., MRS Bulletin 2002, 27, 198. [2] T. Leichtweiss et al., Journal of Materials Chemistry A 2014, 2, 6631. [3] Y.-R. Chang et al., ACS Appl. Mater. Interfaces 2021, 13, 43282.

The chemical characteristics of the semiconductor surface can be altered by the molecular stimulation in the environment, resulting in a tunable electric performance of devices. Transition metal dichalcogenides (TMDs) have been studied as sensor platforms due to the properties of a large surface-to-volume ratio and ultrathin body thickness. The interaction between TMDs and molecular relies on van der Waals forces because the dangling bonds do not exist at the surface. As a result, the sensitivity and electric properties TMDs devices is greatly controlled by engineering surface states.
In this report, the surface morphology engineering of MoO_x/MoSe₂ heterojunction is demonstrated using applying O₂ and partial etching processes. The roughness of the MoSe₂ device is enhanced by partial etching of MoO_x/MoSe₂ surface with dipping in (NH₄)₂S for 30 min at 323K.
The Atomic force microscopy (AFM) is performed to analyze the topographical change of MoO_x/MoSe₂. After exposure to O₂ plasma, the particles with variable diameters (0.12 um~ 0.89 um) and height (1 nm ~ 60 nm) are observed on the surface of MoO_x/MoSe₂. However, the large-sized particles are entirely removed from the surface of MoO_x/MoSe₂ after dipping in (NH₄)₂S solution for 30 min at 323K, while the sub-5nm-diameter particles remain across the surface. As a result, the oxidation surface’s root-mean-square (RMS) roughness is about 3.3 nm, while the bare MoSe₂ surface is about 0.5 nm. After chemical treatment of MoO_x/MoSe₂, the RMS roughness significantly increases with partial etching of MoO_x/MoSe₂ layers.
The electric characteristics of the MoSe₂ device upon the two-step process are probed at room temperature in ambient conditions. After exposure of O₂ plasma, the linear transfer curve is observed consistent with the absence of gate dependence; the linear behavior of transfer characteristic is originated from the formation of defects induced conducting channel in MoO_x. Conversely, after dipping in (NH₄)₂S solution, the gate dependence of MoO_x/MoSe₂ can be observed with a similar maximum current value at n-branch with the value of value of bare MoSe₂. It is noted that the hysteresis in increased from 19.3 V_TH to 40.5 V_TH after chemical treatment, resulting from the existence of defects at the interface of MoO_x/MoSe₂.
Consequently, the present demonstration of a functionalization of chemical sensor surface can provide the potential method to enhance sensitivity to chemical stimulation.

Traditional photodiode materials such as germanium and III-V usually require stringent material synthesis processing like high synthesis temperature and crystal lattice matching to their host material and are therefore complex to integrate with other material platforms. On the other hand, two dimensional (2D) materials have naturally terminated surfaces, and do not require a crystal lattice matching to be integrated. However, the manufacturing process of 2D materials flakes based photodetectors are often manual labor intensive, and the high temperature required to synthesize large area 2D materials makes the direct integration of the photodetector into the existing photonic circuits difficult. It has recently been shown that tellurium can be evaporated and deposited at low temperature to form a crystalline with the high carrier mobility. Here, we show that tellurium can be lithographically defined and evaporated at low temperature directly onto photonic structures to form easy-to-fabricate functional photodetectors with platform independent manner. The fabricated photodetectors show high responsivity at low-frequencies at broad wavelength range, including visible, telecom and mid-infrared, and the detectors operate at high speed as well. Lastly, we demonstrate that these detectors can be fabricated on a variety of integrated platforms.

Metallic 2D materials have recently emerged as a class of potentially inexpensive catalysts for hydrogen evolution reaction (HER) with enhanced activity and increased stability. The overpotential and Tafel slope values of metallic 2D catalysts approach those of platinum. However, the amount of generated hydrogen of 2D catalysts remains lower than industrial Pt and Ir electrolyzers. This talk will discuss the synthesis of the metallic niobium disulfide with very high current densities (several Amps cm-2) in a proof of concept electrolyzer for hydrogen generation. Metallic 2D materials can also be good for the HER or other electrocatalytic reactions. Therefore, we investigate the properties of metallic 2D materials towards the HER and other reactions to identify the best catalysts for catalytic reactions.

Two-dimensional MoS₂ has attracted much attention for optoelectronic devices [1] due to its semiconducting properties and the layer dependent electrical [2] and optical properties [3], [4]. In order to utilize MoS₂ for applications in future technologies it is crucial to gain a proper understanding of its growth process so as to implement a method to actively control the number of layers from mono-layer (ML) to few-layer (FL). To achieve high-quality ML or FL MoS₂ an atomically flat and inert layered substrate is advantageous [5]. Muscovite mica fulfills these requirements, and it is mechanically flexible, which allows the investigation of strain-induced changes in MoS₂ . In particular, the growth mode of FL MoS₂ on Muscovite mica has not been investigated yet.
Applying controlled strain to MoS₂ and other TMDs is interesting since it allows the manipulation of the electronic structure, e.g., a transition from direct to indirect band gap can be observed [6]. In order to understand the observed electronic properties, the number of MoS₂ layers and the degree of strain must be known. The identification of the number of MoS₂ layers is often done by employing Raman spectroscopy, where the peak separation between the high-frequency optical phonon modes correlates with the number of layers [7]. However, in the case of strained layers, the interpretation of the Raman spectrum is more challenging because the modes also shift with strain [8], especially since the reference values for the peak separation mainly arise from mechanically exfoliated ML and FL MoS₂. Since the number of layers and the strain result in similar Raman shifts this could lead to a wrong determination of the number of MoS₂ layers on fully covering MoS₂ films solely based on Raman spectroscopy in CVD-grown material.
In this study, we investigate the growth mode of MoS₂ on Muscovite mica sheets in order to better understand the interplay between the film thickness, strain and growth mode. We corroborate how misleading the determination of the number of MoS₂ layers is solely based on Raman spectroscopy due to the occurrence of strain and changes in growth mode, which is corroborated by DFT simulations and atomic force microscopy (AFM). We show that layer-by-layer growth of CVD-grown ML to FL MoS₂ films on mica sensitively depends on the substrate temperature. This can be traced back to the thermal expansion of different amounts of encapsulated gases dissolved in the mica sheets, which changes the strain in the 2D layer during the sulfurization. Our analysis shows that mica substrates can be used to introduce and investigate strain in 2D materials without the need to apply external stress and that the growth mode needs to be studied with AFM in combination with Raman spectroscopy to disentangle the different contributions that impact the Raman peak separation.
[1] Choi et al. 20, 116-130, Materials Today, 2017.
[2] Splendani et al. 10, 1271-1275, Nano Letters, 2010.
[3] Lee et al. 4, 2695-2700, ACS Nano, 2010.
[4] Molina-Sánchez et al. 70, 554-586, Surf. Sci. Rep., 2015.
[5] Chae et al. 111, 143106 Appl. Phys. Lett., 2017
[6] Hiram J. et al. 13(8), Nano Letters, 2013
[7] Qingqing Ji et al. 13(8), 3870–3877, Nano Letters, 2013
[8] Hong Li et al. 6, 7381, Nature communications, 2015.

The fabrication of highly porous 3D foams and assemblies from 1D and 2D nanomaterials, such as CNT, Graphene and MXenes is studied intensively due to their unique set of properties. In general, these 3D assemblies are characterized by extremely low densities, high porosities, large surface areas, as well as unique mechanical and electrical properties. With respect to the latter, electrically conductive 3D nanomaterial foams have recently gained attention based on their 3D Joule-Heating capabilities, enabling new applications in the fields of gas sensing, through-flow heaters, loudspeakers, air filtration and bolometers.
Here, we demonstrate that by utilizing rapid Joule-heating of structural unique highly porous framework materials based on Graphene microtubes, electrically triggered, controllable and repeatable rapid gas explosions of cm³-sized gas volumes can be induced, enabling new high power actuation systems, tailorable microfluidic pumps, as well as propulsion systems. [1] The fabrication of the graphene foams is based on a wet-chemical approach in which a porous sacrificial template of tetrapodal ZnO is coated with 2D nanomaterial dispersions like graphene, CNT’s or MXenes. Removal of the ZnO results in a 3D network of interconnected hollow microtubes. [2, 3] Based on their high electrical conductivity (up to ~90 S m^-1) and extremely low volumetric heat capacity (1-5 kJ m^-3 K^-1) these 3D hollow microtube assemblies can be electrically heated by applying short power pulses (1-100 ms), thereby reaching heating and cooling rates of ~105 K s^-1 and ~103 K s^-1. [1] Their relatively large surface area combined with a highly accessible open porous structure (porosity >99.99%) enables a fast and efficient heat transfer from the framework structure to the gas phase. Thermodynamically, this results in a fast, explosion-like volume expansion. In contrast to explosions based on chemical reactions, no reactants are consumed, thus the here shown process of electrically powered gas explosions is repeatable with repetition rates of ~10 Hz. [1] By adjusting parameters like applied power, system pressure and material volume the process can be exactly tuned for the desired application scenarios, such as actuators and microfluidic pumps.
Furthermore, an outlook on recent experiments with other stimuli and different 2D materials like MXenes or H-BN [4] are presented and application possibilities are discussed.
References
[1] F. Schütt et al., “Electrically powered repeatable air explosions using microtubular graphene assemblies,” Materials Today, 48, 2021, doi: 10.1016/j.mattod.2021.03.010.
[2] F. Schütt et al., “Hierarchical self-entangled carbon nanotube tube networks,” Nat Commun, 8, 2017, doi: 10.1038/s41467-017-01324-7.
[3] F. Rasch et al., “Wet-Chemical Assembly of 2D Nanomaterials into Lightweight, Microtube-Shaped, and Macroscopic 3D Networks,” ACS applied materials & interfaces, 11, 2019, doi: 10.1021/acsami.9b16565.
[4] F. Schütt et al., “Conversionless efficient and broadband laser light diffusers for high brightness illumination applications,” Nat Commun, 11, 2020, doi: 10.1038/s41467-020-14875-z.

Triboelectric nanogenerators (TENGs), which convert mechanical energy into electrical energy via triboelectric effect, have been widely studied because of their potential applications. Recently, dynamic Schottky diode or p–n junction-based TENGs which generate direct current were reported. However, most of them exhibited low output voltage because their semiconducting friction layers were directly constructed on an electrode. In this study, we report a “depletion layer formation -driven” TENG, the working mechanism of which differs from previously reported ones. When n-type MoS₂ and p-type polypyrrole (PPy) are in contact, electrons and holes diffuse into PPy and MoS₂, respectively, contributing to generating output voltages and currents. To increase the number of charges diffusing across the p–n junction, Ag nanoparticles (NPs) and a piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) layer were placed below MoS₂. Additionally, PPy was p-doped using Pt NPs. These MoS₂-based TENGs exhibited an instantaneous power density of 14.4 mW cm⁻² with an external load resistor of 1 MΩ and a transferred charge density of 0.047 μC cm⁻² in a single cycle when charging a capacitor. These values are higher than the previously reported values, demonstrating that depletion layer formation -based TENGs can be regarded as another type of TENG.

Recently, the two-dimensional (2D) Ruddlesden-Popper class of organic-inorganic hybrid perovskites like (C₄H₉NH₃)₂PbBr₄ have attracted significant research attention because of their interesting photophysical properties and better stability compared to typical bulk perovskites [1]. Despite their direct bandgap, solar cell power conversion efficiencies and other device performance parameters have been drastically low [2]. This poor optoelectronic performance in visible and near-infrared range can be attributed to the significantly large exciton binding energies and low visible-range optical absorption which are material-specific limitations [3]. Further, for commercial applications, the overall air stability of 2D perovskites needs improvement, hence, development of 2D perovskites for the all-important visible-range optoelectronics and flexible sensors has been challenging.
Here, we investigate the interface of 2D perovskites with transition metal dichalcogenides (TMDs) using first-principles based density functional theory calculations. TMDs have been widely studied for fast and high responsivity photodetection, hence conjugating TMDs with 2D perovskites can open up greater possibilities in optical device applications. Specifically, we show that diverse 2D perovskite/TMD heterostructures can result in distinct band alignments at the interface (types-I, II or III) based on the choice of constituents and can result in new functionalities beyond the individual materials. For example, the type-II band alignment in BA₂PbBr₄/MoS₂ (BA is C₄H₉NH₃) heterostructure, with favourable bandgap of ~ 1.1 eV (HSE06 calculated), can lead to easier interlayer separation of tightly bound excitons under visible light, leading to higher photocurrents and improved solar cell efficiencies.
We also demonstrate a type-I band alignment for recombination-based devices in a BA₂PbBr₄/WSe₂ heterostructure with effective bandgap in the visible range, whereas, the BA₂PbI₄/MoS₂ system shows a type-III band alignment which can favour tunnelling of charge carriers from the valence of the 2D perovskite to the conduction band of TMD. The distinct heterostructures also enable modulation in transport properties like carrier effective masses, band gaps and band offsets. Additionally, we explore the effect of externally applied tensile strain on the intrinsic properties of the 2D perovskite/TMD heterostructure to evaluate its application in flexible substrates. We show significant strain tolerance under both uni- and bi-directional application of strain with only a minor change in band gap and effective mass. The TMD layer on top of the 2D perovskite can provide enhanced strain tolerance since it functions as a capping layer which restricts the interaction of the 2D perovskite layer with moisture and oxygen.
In summary, our study explores the possibility of engineering different band alignments at the interface of the 2D perovskite/TMD heterostructures to unravel new device applications and evaluates the application of these heterostructures in flexible devices.
References:
[1] K. Leng et al., Nat. Rev. Mater. (2020).
[2] L. Dou et al., Science (80-. ). 349, 1518 (2015).
[3] G. Grancini, M. K. Nazeeruddin, Nat. Rev. Mater. 2019, 4, 4.
[4] A. Varghese, Y,. Yin. M. Wang, S. Lodha, N. V. Medhekar, arXiv:2108.07488 (2021).

The optical and electronic properties of two-dimensional (2D) materials can be tuned through physical and chemical adsorption of gases. They are also ideal sensor platforms, where charge transfer from the adsorbate can induce a measurable change in the electrical resistance within a device configuration. While 2D materials-based gas sensors exhibit high sensitivity, questions exist regarding the direction of charge transfer and the role of lattice defects during sensing. Here we measured the dynamics of adsorption of NO₂ and NH₃ on monolayer WS₂ using in situ photoluminescence (PL) and resonance Raman spectroscopy. Experiments were conducted across a temperature range of 25–250 °C and gas concentrations between 5–650 ppm. The PL emission energies blue- and red-shifted when exposed to NO₂ and NH₃, respectively, and the magnitude of the shift depended on the gas concentration as well as the temperature down to the lowest concentration of 5 ppm. Analysis of the adsorption kinetics revealed an exponential increase in the intensities of the trion peaks with temperature, with apparent activation energies similar to barriers for migration of sulfur vacancies in the WS₂ lattice. The corresponding Resonance Raman spectra allowed the simultaneous measurement of the defect-induced LA mode. A positive correlation between the defect densities and the shifts in the PL emission energies establish lattice defects such as sulfur vacancies as the preferential sites for gas adsorption. Moreover, an increase in defect densities with temperature in the presence of NO₂ and NH₃ suggests that these gases may also play a role in the creation of lattice defects. Our study provides key mechanistic insights into gas adsorption on monolayer WS₂, and highlights the potential for future development of spectroscopy-based gas sensors based on 2D materials.

The isolation of graphene has led to a new field of research on two-dimensional (2D) materials which exhibit interesting electrical, optical and opto-electronic properties. In this study, few layer (3-4 L) 2D materials (MoS₂ and WS₂) and their heterojunction (vertical and lateral MoS₂ - WS₂) fabricated by chemical vapor deposition have been investigated. Raman spectroscopy has been employed to obtain the magnitude of difference between E_2g (~385 & 358 cm^-1) and A_1g (~404 & 419 cm^-1) peaks of MoS₂ and WS₂ which has been used as a signature of the number of layers. The two-dimensional material which comprises the channel material for the back-gated field effect transistor (FET) devices has been systematically characterized using Kelvin probe force microscopy (KPFM), photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS) in order to determine the band offsets and band alignment of the MoS₂ – WS₂ heterostructure. The successful operation of n-type and ambipolar conduction behavior of the fabricated FET devices was achieved. A high ON/OFF ratio (~ 10³), low subthreshold swing (668 mV/decade) of n-type MoS₂ device, high electron mobility (19 cm²/V.s) of MoS₂ – WS₂ heterostructure device was recorded. In the present study, a precise modulation of electronic conduction between n-type and ambipolar transport along with low threshold voltages, depending on chemically active species present during NaCl precursor - assisted growth has been emphasized. The optoelectronic capability of 2D semiconductors was exploited by fabricating optical sensors and studying their photocurrent generation mechanisms for light wavelengths in UV (300-450 nm), visible (500-670 nm) and near-infrared (700-1100 nm) range. The prototype of a 2D complementary metal−oxide−semiconductor (CMOS) logic inverter switch integrating high electronic and optical responses of the MoS₂−WS₂ heterostructure junction owing to ambipolar FET operation, capable of detecting light with gate tunability was achieved. The achieved results encompassing superior photoabsorption, atomically thin thickness, and high-performance indices demonstrates enormous potential that soft 2D heterostructure devices may open a new paradigm in artificial retinal implants and photoelectronics.

Transferring two-dimensional (2D) materials plays an essential role in 2D material studies. Recently, the moiré van der Waals heterostructures attract great interest since the superconductivity was demonstrated in twist bi-layer magic-angle graphene superlattice. To fabricate the 2D materials devices, the popular methods include the sacrificial layer assistant method, h-BN assistant method, and dry stamp method. However, the sacrificial layer, such as PMMA and PLLA, introduce organic residues from the transfer medium film itself. In addition, the h-BN assistant method has achieved great success thanks to the super-flatten interface of the h-BN. But it is hard to expose the top interface of transferred materials making the interface engineering, such as interfacial catalysis, keep challenging. Furthermore, the dry stamp method is well-developed as well, but the weak adhesion force between stamp and 2D materials limits its applications.
Here, we develop a capillary-force-assisted clean-stamp technique that uses a thin layer of evaporative liquid (e.g., water, ethanol, and acetone) as an instant glue to increase the adhesion energy between 2D crystals and polydimethylsiloxane (PDMS) for the pick-up step. After the liquid evaporates (takes tens of seconds to minutes), the adhesion energy decreases, and the 2D crystal can be released to an arbitrary surface. The thin liquid layer is condensed to the PDMS surface from its vapor phase, which ensures the low contamination level on the 2D materials and largely retains their chemical and electrical properties. To benchmark this method, we fabricated 2D materials photonic and electronic devices include a graphene-monolayer field-effect transistor (FET) and a graphene-MoS₂-graphene heterostructure photodetector. Furthermore, the moiré van der Waals heterostructures (h-BN stacks and transition metal dichalcogenide heterostructures) have been demonstrated with arbitrary numbers of layers and tunable twisted angles.

Novel synthesis of 2D transition metal dichalcogenides (TMDCs) and TMDC like structures are highly desired due to their extraordinary layer-dependant electronic and optoelectronic properties. Their semiconducting behaviour and huge intercalation potential make them suitable for applications in energy and conversion fields.
Herein, we expended the extraordinary molecular complex library of general formular [MIV(SEtN(R)EtS)₂] by the complexes containing titanium and tin as central atom (M=Ti, Sn; R= Me, Et). These complexes have been tested to be suitable single source precursors within chemical vapour depositions producing thin films of TiS₂ and SnS₂. The tridentate coordination of chelating ligand class (HSEt)₂NR (R = Me, Et) enable the isolation of (air-)stable monomeric metal complexes, whereas the metal center is distorted octahedral coordinated by four S and two N donor atoms, which have been confirmed by single crystal and NMR analysis. The clear decomposition at defined parameters has been confirmed by X-ray diffraction analysis, X-ray photoelectron analysis and atomic scale imaging. Control of phase purity and morphology of as-deposited thin films by single source precursor approach enable an adjustment for specific application, making this approach indicatory for providing materials with certain demands.

Dual Laser Ablation (DLA) provides a novel method of creating high quality thin films with the ability to control the degree of ionization in the target plume. The spatially overlapped and temporally synchronized excimer (KrF) and carbon dioxide (CO2) laser pulse combination produces highly excited and ionized plasmas of composite materials leading to defect-free high-quality films. The low surface roughness of thin-films using the DLA method is ideal for growing heterostructures. In this work, the DLA technique has been utilized to grow tungsten disulfide (WS2 – a widely known optoelectronic semiconductor) and cobalt ferrite (CoFe2O4 – an exceptionally hard spinel ferrite) heterostructure on single crystal magnesium oxide (MgO) (100) substrates. Cobalt ferrite has attracted much attention due to many applications in spintronics, magnetocalorics, and magneto-optical devices. Recently, spin-Seebeck effect has been observed in cobalt ferrite-based materials. Here, we investigate structure, magnetic and spin-thermal-transport properties of these heterostructures by means of X-Ray Diffraction, Atomic Force Microscopy, Physical Property Measurement System, thermal transport, and ultrafast Magneto-optic Kerr Effect. Results show that the WS2/CFO heterostructures are very promising for applications in spincaloritronics and valleytronics.

Heterostructures composed of two-dimensional (2D) and three-dimensional (3D) materials have opened novel ways of advanced device manufacturing and research of emergenet phenomena. Advanced epitaxy techniques, such as van der Waals and remote epitaxy, enable to prepare high-quality architecture consisting of incommensurate materials. Nevertheless, remote epitaxy on non-graphene (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines in basic science and applied research.
Here, we present remote epitaxy of ZnO on monolayer MoS₂ and the realization of a whispering gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS₂. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the non-graphene 2D material interacted with overgrown and substrate layers, and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS₂ showed enhanced luminescence of MoS2 and multimodal emission. We will also discuss interfacial phenomena in 2D/3D heterostructures prepared by remote epitaxy.

In prospective applications such as flexible electrodes and supercapacitors[1], 2D-Metal organic frameworks (2D-MOFs) are subjected to various modes of mechanical stresses and strains, making their mechanical properties vital to their practical implementation[2]. The incorporation of metal ions makes 2D-MOFs susceptible to interaction with polar molecules, which can alter the bonding situation and strength of the metal coordination sites. Aqueous environments, which can contain ions as well as dissolved (macro)molecules, are suitable to study those interactions.
Here, we show how the Langmuir monolayer technique, in combination with interfacial rheology, can be used to measure the Young’s modulus of 2D–MOFs in the presence of water in a non-destructive and straightforward way. The results for the in-situ prepared 2D-MOF consisting of Palladium(II) meso-tetra(4-carboxyphenyl)porphine and Cu²⁺ are orders of magnitude lower than the ones determined with ex-situ AFM nano-indentation.
We further introduce a mathematical description for the chemical degradation of 2D MOFs, which is highly relevant given their complete exposure to the environment through their unequalled surface area. The model takes into account the framework topology together with defects and limited grain sizes, to predict both the loss of mass and mechanical stability. The model is verified for the aforementioned 2D-MOF by means of the Langmuir monolayer degradation technique[3], e.g. in the presence of acids as one of the highly relevant sources of damage for 2D-MOFs incorporating pH sensitive groups for metal coordination.
The presented methodology can be translated to all 2D materials synthesized or assembled at the air-water interface and provides a fast and straightforward assessment of the degradation and the mechanical properties of 2D materials.
Literature
1. J. Cheng, S. Chen, D. Chen, L. Dong, J. Wang, T. Zhang, T. Jiao, B. Liu, H. Wang, J.-J. Kai, D. Zhang, G. Zheng, L. Zhi, F. Kang and W. Zhang, Journal of Materials Chemistry A 6 (41), 20254-20266 (2018).
2. J. C. Tan and A. K. Cheetham, Chemical Society Reviews 40 (2), 1059-1080 (2011).
3. R. Machatschek, B. Schulz and A. Lendlein, Macromolecular Rapid Communications 40 (1), 1800611 (2019).

The large optical sensitivity of two-dimensional MoS₂ has been widely reported, spurring efforts to deposit this material over large areas. Indeed, large-area monolayer growth via chemical vapor deposition (CVD) has been successfully reported on SiO₂/Si and sapphire. Direct CVD of MoS₂ on Ga-polar GaN has also been investigated, while deposition on N-polar GaN has not yet been reported. In this presentation, we study how selection of each GaN face impacts CVD of MoS₂, and demonstrate how polishing of N-polar GaN permits monolayer MoS₂ to be obtained. This study serves as a critical steppingstone towards the future integration of MoS₂ with GaN-based lateral polar junctions (LPJs), which will be used for optical and quantum sensing.
A vertical, cold-wall, radio frequency (RF) heated, low-pressure metalorganic chemical vapor deposition (MOCVD) system was used to deposit the GaN thin films on sapphire substrates. Ga-polar GaN was obtained when a low-temperature AlN (LT-AlN) was used, whereas N-polar GaN was directly deposited on sapphire. The polarities were confirmed using KOH etch tests.
MoS₂ thin films were grown on each GaN surface using solid S (~200 mg) and MoO₃ (~ 1 mg) precursors, with 90 sccm of Ar as the carrier gas. 32 µL of perylene-3, 4, 9, 10 tetracarboxylic acid tetrapotassium salt (PTAS) was placed at the corners of the growth substrate using a pipette, which acts as seeds for growing MoS₂ thin films. Prior to the deposition of PTAS, the Ga-Polar GaN surface was treated with Piranha solution for 30 mins, whereas N-Polar GaN was cleaned with acetone-IPA due to the highly reactive surface of N-Polar GaN to acids. The growth temperature was set to 750 °C with the furnace pressure controlled at 760 Torr to ensure large epitaxial growth.
MoS₂ triangles are observed on Ga-Polar GaN templates with size up to 40 μm, which is significantly larger than the 1 μm triangles observed by other groups on GaN. The frequency separation of the E_2g and A_1g Raman mode is ~ 20 cm^-1 which confirms the monolayer nature of the films. The thickness and uniformity were further ascertained by AFM and SEM analysis which shows triangles with thickness of ~ 0.7 nm. Lastly, the A and B exciton peaks are observed at 1.85 eV and 2.01 eV respectively, which is typical for monolayer MoS₂ films. On the N-Polar GaN, however, multilayer MoS₂ islands with size ranging from 2 - 5 μm are observed, with a Raman peak separation of ~ 23 cm^-1, which points to film thickness of 3-4 layers. SEM shows the polycrystalline nature of the islands with grain size < 500 nm. The growth of thicker MoS₂ films can be attributed to the overall higher roughness of the N-Polar GaN templates (RMS ~ 6 nm) compared to the Ga-Polar GaN (RMS ~ 200 pm). The higher roughness can lead to higher nucleation rates, leading to smaller grain sizes that promote vertical growth. To test this hypothesis, N-Polar GaN was polished using a 0.02 μm colloidal solution for 60 mins, thereby reducing the RMS roughness to 0.9 nm. No degradation of the N-polar GaN template was observed via XRD. MoS₂ was grown on the polished N-polar GaN at 750 °C. Monolayer films with sizes from 2 - 4 μm are grown on the polished template with Raman separation of 20 cm^-1. The PL peak intensity, however, is ~3x less compared to monolayer on Ga-Polar templates. This is due to the polycrystalline nature of the films with very small grain sizes, as observed from SEM. SEM also shows that multilayer films are interspersed between the monolayer regions, which is also evident from Raman mapping.
Future work will focus on further reducing the N-polar GaN surface roughness to increase the size of MoS₂ crystals, followed by the deposition of MoS₂ on GaN LPJs.

The magnesium borohydride/magnesium boride system is one of the most promising systems for hydrogen storage. Magnesium borohydride, Mg(BH₄)₂, has theoretically high gravimetric (14.8 wt.% H₂) and volumetric (147 g/L) hydrogen capacities, and favorable enthalpy of hydrogen release (ΔH° = 38 kJ/mol H₂), which may allow for H₂ cycling between Mg(BH4)₂ and MgB₂ at moderate pressures and temperatures. To synthesize MgB₂ thin films for studies of adsorption, multiple synthesis strategies have been tested to first delaminate bulk MgB₂ and then deposit from suspension to target surfaces. Delamination approaches include heat treatment with anhydrous solvents, ball milling with and without solvents, and ultrasonication. Preliminary results showed that methanol, propylene glycol and dipropylene glycol are suitable solvents for synthesizing the MgB₂ nanoparticles. Deposition methods include drop casting the sample solution onto a hydrophobic surface (trichloro(octadecyl)silane on Silicon wafer) in an enclosed area to maintain low evaporation rate, and dip coating. Drop casting from methanol suspension experienced the unwanted coffee-ring effect, leading to non-uniform films with thicknesses varying to more than 100 µm. Dip coating enabled creation of arrays of suspended magnesium boride, MgB₂, nanoparticles with width and length ranging from 1 – 5 µm, forming an approximately 100 nm film.
A hydrophobic substrate was chosen to reduce the contact-pinning effect. A challenge of the initial attempt methanol and drop casting was the coffee-ring effect, which arises from the difference in evaporation rate at the edge and at the center of a droplet on a surface, inducing a capillary flow outward. We created a counter-Marangoni flow by using a co-solvent approach, in which one solvent has a higher boiling point but lower surface tension than the other, in order to suppress the coffee-ring effect. At the edge of a drop, the higher boiling point solvent would evaporate faster creating a surface tension gradient between the edge and the center, bringing more particles from the rear to the center, which is the reverse direction versus capillary flow. A common solvent used in the co-solvent method is water, because it has a low boiling point (100 °C at ambient condition) but very high surface tension ( γ= 72 mN/m at room temperature). However, MgB₂ is not compatible with water, which oxidizes the MgB₂. Instead, our effort uses mixtures of propylene glycol and dipropylene glycol as co-solvents for MgB₂ deposition. In this approach the substrate was dipped into the co-solvent containing suspended MgB₂ at 60 °C and withdrawn from the liquid at 0.012 mm/s. The sample was left dry in the 60 °C chamber and characterized by AFM (Atomic Force Microscopy) and SEM (Scanning Electron Microscopy).
Formation of such MgB₂ thin films and coatings is a crucial step for delivering the film onto the sensor surface of a quartz crystal microbalance (QCM), which is a gravimetric sensor that we are pursuing to characterize H₂ adsorption on MgB₂. One fundamental challenge is to understand the thermodynamics of the first stage of hydrogen sorption, which is the physisorption of molecular H₂ on MgB₂. Due to very slow kinetics, cycling between Mg(BH₄)₂ and MgB₂ has been accomplished only at high temperature (≥300 °C) and under high charging pressure (≥ 700 bar). The planar configuration of MgB₂ in this experiment enables interface studies, and the QCM, with ng/cm² sensitivity, allows data acquisition at very low partial pressures and reduced temperatures. The energetics of physisorption can be determined with a two-dimensional configuration, enabled by the thin film synthesis process reported here.

The high-contrast optical imaging of nearly transparent 2D materials such as clays and some oxide sheets has been an outstanding challenge; yet there is a critical research capability needed in the synthesis and processing of these materials, which show promise in a number of applications including barrier coatings, membranes, and composites. Using delaminated silicate clay and titania sheets as model systems, here it is shown that these weakly quenching, nearly transparent 2D sheets can be readily imaged by fluorescence quenching microscopy (FQM) based on a new mechanism. When these sheets are deposited on strongly quenching substrates, including doped silicon wafers and metals or indium tin oxide-coated glass slides, they can act as a “spacer” to reduce the degree of quenching of the dye layer by the substrate, appearing as highly visible bright sheets against a dark background. This new FQM strategy can visualize dielectric 2D materials with high contrast and layer resolutions comparable to scanning electron microscopy and atomic force microscopy. When different 2D materials are co-deposited, FQM not only differentiates them readily based on their quenching capabilities, but also can resolve their vertical stacking sequence based on the contrast of their overlapped areas.

Thermoelectric performance in 2D transition metal dichalcogenides (TMDs) are usually optimized through controlling carrier concentration, band engineering, and/or scattering mechanism. Applying uniform strain to TMDs has been demonstrated to increase electrical performance and carrier mobility owing to the alteration of electronic band structure and effective carrier mass. Here, we study the effects of non-uniform strain on the thermoelectric performance of MoS₂ using SiN_x substrates with a crested morphology. The non-uniform strain is demonstrated to suppress phonon-scattering which drastically enhances carrier mobility and thermoelectric power factor in crested MoS₂ by up to 2 orders of magnitude depending on carrier concentration as compared to conventional MoS₂ on flat SiO₂ surfaces. We investigate gate- and temperature-dependent carrier mobility and density to determine the mechanisms responsible for the drastic enhancement in thermoelectric performance. Distinctively, the ultra-high carrier mobility achieved at low carrier concentrations weakens the dependence of thermoelectric parameters on carrier concentration, therefore resulting in a substantial improvement in electrical conductivity without greatly compromising Seebeck coefficient. Our work provides a universal pathway towards engineering carrier mobility and superior thermoelectric performance for a wide range of room-temperature applications.

Developing a clean and sustainable energy system is a necessary challenge to meet our rising energy demands. Hydrogen, a clean renewable energy source, is a promising alternative to fossil fuels. Interest in hydrogen energy has soared since the first use of the term “hydrogen economy (a forthcoming energy system based on hydrogen fuel)” by John Bockris in 1970. Although 50 years past since the term was used, there are numerous obstacles to enabling a hydrogen economy due to the high cost of hydrogen production. Electrochemical water splitting through the hydrogen evolution reaction (HER) is a promising method to produce hydrogen due to its efficient and easy coupling with a renewable energy source. However, there is still a problem that platinum-based catalysts, the most active catalyst for HER found so far, are too costly to be used in the industry.

For these reasons, non-noble metal-based electrocatalysts are in the spotlight. Among them, two-dimensional van der Waals (vdW) materials are noteworthy electrocatalysts for the hydrogen evolution reaction (HER) because they have a high surface area and metallic conductivity. However, most pristine vdW materials do not show electrochemical activity in their basal planes; thus to improve the activity of the basal plane, various surface modifications such as defect engineering, interfacial engineering, and doping are required. If the parent layered material demonstrates activity in the basal plane before surface modification, its electrochemical activity can be tremendously improved after further modifications.

Among all the vdW layered materials, Fe₃GeTe₂ (FGT) is notable since it is a cleavable and stable 2D material down to the monolayer, preserving metallic character. FGT has a hexagonal crystal symmetry with space group P6₃/mmc, consisting of a “Fe₃Ge” substructure sandwiched between two layers of Te atoms, which are weakly bonded by vdW forces. Very recently, FGT was suggested as a potential electrocatalyst candidate for the oxygen-evolution reaction (OER) and other gas adsorption reactions (CO, NO, etc.). In this work, we report on the experimental and theoretical investigations of the HER activity of FGT for the first time.

Firstly, FGT was synthesized by mixing high-purity elements in a stoichiometric ratio in a glove box. The mixture was sealed under vacuum in a quartz tube before placement into a furnace. The sample was heated at 800 ^oC followed by slow cooling. The phase purity of the samples was examined by a powder X-ray diffractometer (PXRD) and Rietveld refinement. Then, three types of electrodes (bulk powder, sonicated powder, and densified pellet) were prepared to explore the HER activity. The results indicated that the HER activity of the sonicated FGT, which exposes more basal and edge planes, is higher than that of the bulk. Moreover, the SPS pellet electrode, which is the densest and whose PXRD showed not only single-crystal-like preferred orientation along the c-direction but also the presence of some edge planes, requires the smallest overpotential (η₁₀ = − 0.105 V) among all bulk TMDs reported so far.

To understand the excellent HER activity of FGT, DFT calculations were performed. The calculated ΔG_H values of the hexagonal basal plane Te layer were found to be nearly equal to that of the puckered Te layer in monoclinic MoTe₂ (1T′-MoTe₂), supporting the basal plane activity of FGT. Furthermore, the calculated ΔG_H values of another prevalent (106) layer (edge layer) are even closer to zero, thus supporting the increase in activity with particle size reduction, which favors increased edge sites.

This study introduces FGT as a highly HER active hexagonal vdW material with an active basal plane and edge sites, thus paving the way for further studies of related iron-based vdW materials, their composites, and their surface functionalization as high-performing electrocatalysts.

The fabrication of macroscopic and multi-scaled assemblies (such as sponges, foams, aerogels) from Graphene and related 1D and 2D nanomaterials is an extensively studied field due to their broad range of applications, such as in bioelectronics, energy storage, catalysis as well as environmental protection. However, transferring the nanoscopic functionality of these low dimensional materials into the macroscopic world is still challenging to achieve. In my talk I demonstrate new approaches for the synthesis of cm³-sized multi-scaled framework architectures, composed of 1D and 2D nanomaterials, such as Graphene, MXenes, Carbon Nanotubes [1] and hexagonal boron nitride (h-BN) [2]. With porosities in the order of 99.99% and densities as low as 0.2 mg/cm³ the properties of these so-called aeromaterials are determined by the nanoscopic functionality of the source material, resulting in a unique set of electrical, thermal, optical and mechanical properties. I will further highlight and discuss new applications that are enabled by different aero-materials, ranging from high-brightness and efficient laser-based light sources [2], electro-pneumatic micropumps and high-power actuators for soft-robotics [3], as well as bioinspired conductive composites for bioelectronics [4].
References
[1] F. Schütt, S. Signetti, H. Krüger, S. Röder, D. Smazna, S. Kaps, S. N. Gorb, Y. K. Mishra, N. M. Pugno & R. Adelung, Nature Communications, 8,1–10, (2018)
[2] F. Schütt, M. Zapf, S. Signetti, J. Strobel, H. Krüger, R. Röder, J. Carstensen, N. Wolff, J. Marx, T. Carey, M. Schweichel, M.-I. Terasa, L. Siebert, H.-K. Hong, S. Kaps, B. Fiedler, Y. K. Mishra, Z. Lee, N. M. Pugno, L. Kienle, A. C. Ferrari, F. Torrisi, C. Ronning & R. Adelung, Nature Communications, 11, 72107, (2020)
[3] F. Schütt, F. Rasch, N. Deka, A.Reimers, L. M. Saure, S. Kaps, J. Rank, J. Carstensen, Y. K. Mishra, D. Misseroni, A. R. Vázquez, M. R. Lohe, A. S. Nia, N. M. Pugno, X. Feng & R. Adelung, Materials Today, 48, 7-17 (2021)
[4] C. Arndt, M. Hauck, I. Wacker, B. Zeller-Plumhoff, F. Rasch, M. Taale, A. S. Nia, X. Feng, R., R. Schröder, F. Schütt & C. Selhuber-Unkel, Nano Letters, 8, 3690-3697, (2021)

Two dimensional (2D) materials with nontrivial band topology have attracted enormous attention after the discovery of graphene because of their dissipationless conduction. This led to the discovery of a family of 2D quantum materials; for example, to mention a few are stanene, alumenene, silicene, borophene and phosphorene. Among these stanene is particularly interesting because it was predicted to be a large gap (0.3 eV) quantum spin Hall insulator exhibiting Dirac cone-like linear energy bands in its buckled honeycomb structure. Gold is an interesting substrate for stanene growth that also exhibits a Rashba spin-orbit split surface state. However, both experimental as well as theoretical studies of Sn/Au(111) show formation of Au-Sn surface alloy. In this work, we report formation of ultra-thin compound layer of Au₂Sn by deposition of about 1 monolayer equivalent of Sn on Au(111) at elevated substrate temperature of 400 K using x-ray photoelectron spectroscopy, angle resolved photoemission spectroscopy, low energy electron diffraction and scanning tunneling microscopy. It exhibits a sharp (√3Χ√7)R19° low energy diffraction pattern indicating formation of an order layer a surface unit cell of oblique symmetry with three domains. Despite being a non-honeycomb unit cell, our angle resolved photoemission spectroscopy measurements demonstrate unexpected occurrence of a linear band resembling Dirac cone at the zone-center of this 2D layer. This band, recorded with 21.2 eV photon energy, disperses linearly over a wide binding energy range starting from 1.5 eV at kιι= 0.2A^-1 to the Fermi level, where the two branches of Λ meet at the Γ point. This band remains unchanged even when a different photon energy of 23 eV is used, indicating that it is surface related. Moreover, as expected for a Dirac cone, it is unaffected by the variation of the azimuthal angle e.g. from the Γ-M direction to the Γ-K direction. The magnitude of the slopes (dE/dk ) of the left and the right branches are same (6.9±0.1), and the calculated Fermi velocity turns out to be 1.05Χ10⁶ m/s, which is very similar to that of graphene (1Χ10⁶ m/s). These linear bands occur only for the (√3Χ√7)R19° surface, and vanishes for the (√3Χ√7)R19° surface at 493 K. Interestingly, we have also observed the linear band for the (√3Χ√7)R19° phase in nano-meter thick Au-Sn film of average composition Au₇Sn. The results of our density functional theory calculations for different possible atomic arrangements to simulate these bands will also be discussed.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been regarded attention as an emerging 2D-semiconductor class with a high surface to volume ratio and a wide range of band gaps. Among TMDs, atomically thin chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) has attracted much attention owing to its high carrier mobility, outstanding chemical stability, absence of dangling bond, and tunable electronic energy band.
To realize further practical applications with CVD-grown MoS₂, its modulated the electrical characteristics would be significantly important in building integrated logic circuits beyond a laboratory-scale single device. In this frame, for the last decade, there have been a lot of efforts to modulate or enhance the electrical performance of TMDs-based devices via chemical doping. Specifically, surface charge transfer doping (SCTD) has been intensively investigated which can induce electron donation or withdrawal between deposited chemical dopants and host materials. The carrier concentration of TMDs can be controlled by the SCTD without the lattice distortion of TMDs. Among the candidates of organic dopants, benzyl viologen (BV) is one of the most promising chemical dopants for n-type doping effect in TMDs which acts as an electron donor due to the difference in energy level between the reduction potential and the vicinity of the conduction band edge of TMDs. For conducting SCTD, organic dopants are typically deposited onto the TMDs via solution-immersion or dopant vaporization with controlling immersion time, dopant concentration in solution, and de-doping time. However, these doping methods have inherent limitation to introduce spatially selective doping on-demand which is of fundamental importance to realize the integrated circuit applications.
In this presentation, to the best of our knowledge, we will report spatially controlled SCTD on MoS₂ via using drop-on-demand selective inkjet printing with BV inks for the first time. To perform a reliable SCTD, we have optimized the solvent system of BV ink droplet to create well-defined pL-scale droplets considering a specific range of the inverse Ohnesorge number, which is a prominent parameter to determine inkjettability. By controlling the BV concentration in inks and the doping region, different SCTD effect could be delivered onto MoS₂ channels resulting in their elaborately modulated electrical characteristics. Raman spectroscopy, Photoluminescence, and X-ray photoelectron spectroscopy were conducted to support the modulated electrical properties of MoS2-FETs with controlled surface charge transfer doping via selective inkjet printing.

Ferromagnetism has been successfully observed in 2D layered crystals, which holds great promise for new quantum phases and spintronic phenomena. Although it is a huge breakthrough in condensed matter physics to achieve long-range ferromagnetic order, most 2D magnets still suffer from multiple challenges including Curie temperatures below room temperature, chemical instability, and so on. These significantly limit their potential applications in computing and storage. Recently, we successfully observed 2D ferromagnetism above room temperature, by doping Co atoms into a non-magnetic 2D layered system, graphitic ZnO. Robust ferromagnetism at room temperature has been uncovered in 2D Zn_1-xCo_xO (ZCO) via magneto-optic Kerr effect, superconducting quantum interference device, and X-ray magnetic circular dichroism techniques. ZCO also displays exceptional stability at ambient conditions as well as at elevated temperatures. Our work indicates that the one-atom-thick ZCO can serve as an exciting candidate to investigate fundamentally novel magnetic and topological behaviors with above-room-temperature T_C, environmental stability, and dopant-tunability.

GeI₂ is an interesting two-dimensional (2D) wide-band gap semiconductor because of diminished edge scattering due to an absence of dangling bonds. Angle-resolved x-ray photoemission spectroscopy (ARXPS) indicates a germanium rich surface, and a surface to bulk core-level shift of 1.8 eV in binding energy, between the surface and bulk components of the Ge 2p_3/2 core-level, making clear that the surface is different from the bulk. Temperature dependent studies indicate an effective Debye temperature (θ_D) of 186 ± 18 K for the germanium X-ray photoemission spectroscopy (XPS) feature associated with the surface. These measurements also suggest an unusually high effective Debye temperature for iodine (587 ± 31 K), implying that iodine is present in the bulk of the material, and not the surface. From optical absorbance, GeI₂ is seen to have an indirect (direct) optical band gap of 2.59 (2.83) ± 0.01 eV, consistent with the expectations. Temperature dependent magnetometry indicates that GeI₂ is moment paramagnetic at low temperatures (close to 4 K) and shows a diminishing saturation moment at high temperatures (close to 300 K and above).

Graphene nanoribbons (GNRs) with atomically precise width have opened new possibilities for next-generation electronic devices due to their tunable physical and electrical properties. While on-surface synthesis of GNRs allows convenient imaging and property study through scanning tunneling microscopy (STM), transferring synthesized GNRs onto non-metallic substrate is challenging, limiting the realization of GNR-based electronic devices. In this work, we present wet chemical synthesis, detailed STM imaging, and electrical characterization through scanning tunneling spectroscopy (STS) of a zigzag-shape GNR, which is expected to have a small energy band gap around 0.5 eV. Compared to large energy band gap GNRs, which may result in poor device performance due to large Schottky barriers and carrier effective masses, zigzag-shape GNRs have potential for high-performance electronic devices. The ribbons were synthesized from polymer precursor graphenized by the Scholl reaction with triflic acid, through 1,2-migrations of aryl groups on the naphthalene rings during the Scholl reaction, with projected short, metallic zigzag edges and long, semiconducting armchair edges. The GNR powders are deposited onto hydrogen passivated p-type silicon substrates by a dry contact transfer technique for STM characterization. High-resolution STM images and STS characterizations provide insights into the geometric structures, as well as electronic structures including energy band gap and spatial distribution of local density of states of zigzag-shape GNRs. STM images exhibit an alternating asymmetric dimerized internal structure, which only shows up on one side along the ribbon and flips to the other side when the ribbon is twisted or flipped. This asymmetric internal structure is uncommon in all forms of GNRs and may be the starting point of electronic applications. The experimental STS data along the dimerized internal structure side of the GNR shows an alternating energy band gap corresponding to internal structure; the STS data across the GNR shows a smaller energy band gap of about 0.5 eV on the side with internal structure and a larger energy band gap about 1.5eV on the other side. Several other aspects of the GNRs are also investigated and analyzed, including the manipulation of ribbon positions using STM tips, and alignment of ribbons with the Si substrate. The reduced band gap of zigzag-shape GNRs allows better transport characteristics and thus higher electrical conductivity, making them promising for device applications.

Three-atom-thick transition metal dichalcogenide (TMD) semiconductors such as MoS₂, WS₂, MoSe₂, WSe₂ are a class of layered 2D crystals with weak interlayer van der Waals bonding and strong intralayer covalent bonding. Owing to the van Hove singularity induced strong optical absorption and exciton generation in the visible part of the electromagnetic spectrum, TMDs are suitable for strong light-matter interactions in optoelectronic devices. Coupling the effect of temperature while a light-matter interaction is occurring in TMDs will enable to design 2D semiconductor-based optoelectronic and photonic circuits for extreme environment studies. Here thin WS₂ layer is exfoliated from a bulk crystal and transferred onto 300 nm SiO₂ coated Si substrate followed by Raman vibrational studies with a temperature variation of 80 to 400K. It is observed that both the in-plane (E’_2g) and out-of-plane (A_1g) vibrational mode frequencies vary non-linearly with the temperature. The optothermal phenomena occurring in nm-thick WS₂ crystals will be discussed. This fundamental study will lead to unveil the dependence of temperature and laser power on in-plane and out-of-plane Raman shifts of TMDs to study the thermal conductivity, hot carrier diffusion coefficient, and thermal expansion coefficient.

Tellurium thin films have been used in electronics, optoelectronics, energy devices and sensors due to their inherent structural anisotropy, high hole mobility and large photoconductivity (Liu, Wu, and Goddard 2018; Wu et al. 2018; Bianco et al. 2020). Crystalline Te is composed of an array of covalently bonded parallel atomic chains on a two-dimensional (2D) hexagonal lattice parallel to the [0001] direction. The bonding between the nearest-neighboring atoms between the chain is weak van der Waals (vdW) bonding. The appealing properties of tellurium have fueled a resurgence of interest in synthesizing ultrathin Te films. Te thin films were synthesized by the liquid-phase exfoliation approaches and physical vapor deposition (PVD). However, the yield and quality of the thin films are limited due to tellurium’s bonding nature. In addition, the in-plane orientation of the Te crystal on the substrate is random. In this respect, the van der Waals epitaxial growth of Te on the surface of transition metal dichalcogenides (TMD) was performed by PVD. It was shown that Te flakes with 5 nm thickness are oriented such that the c-axis of Te is aligned with the armchair direction of the substrates. In addition, from the interaction between Te and WSe₂ crystal structure, we observed a moiré superlattice. Heterostructures forming moiré lattice structures are the subject of intense study owing to their tunable electronic and excitonic spectra. For this reason, the characterization of moiré lattices is critical to pave the way towards mapping the electronic property modulation onto atomically resolved structure.
When 2D atomic crystals are brought into close proximity to form a hetero structure, the alignment between two crystal structures lead to the formation of moiré patterns(Wang et al. 2019). Various techniques have been used to observe moiré lattices, including convergent beam electron diffraction (CBED), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and dark field scanning transmission electron microscopy (STEM)(Reidy et al. 2021). Among those techniques STM and STEM give atomic resolution real space images. Out of these methods, four-dimensional scanning transmission electron microscopy (4DSTEM), which records a 2D image of the diffracted electron beam at each probe positions, gives unique information about the moiré lattice structure, orientation, and structure-dependent properties (Savitzky et al. 2020).
In this study we investigate the structure of moiré lattices and map moiré structure-property relations by combining scanning nanodiffraction techniques. Ultrathin Te will be grown by the PVD method by using a two-heating zone oven. Pure argon is used for the delivering the precursor vapor. The epitaxial growth of Te on WSe₂ is carried out at 450 ^oC for 10-15 minutes. The grown Te/WSe₂ heterostructure is transferred onto a carbon coated transmission electron microscopy (TEM) grid. By using the deflection of the probe in 4DSTEM scans, integrated differential phase contrast (iDPC) analysis will be performed to identify the moiré lattices. Electron microscope studies will be coupled with image simulations by py4DSTEM and abTEM image simulations packages will be used to develop an image simulation method for this system. Besides of the characterization of the structure of moiré lattices, strain analysis will be performed to understand the strain generated due to the misorientations in the interfaces by using scanning nanodiffraction analysis. With the help of the electron microscopy studies, corroborated with DFT calculations, the role of the WSe₂ substrate in modulating the structural and electronic properties of tellurium films is demonstrated. The strain mapping analysis extracted from scanning nanodiffraction analysis helps to enlighten the lattice mismatch between substrate and flakes affecting their mechanical, optical, and electronic properties.

2D semiconductors attracted significant attention of the research community in recent years due to combination of unique optoelectronic properties and great promises in diverse applications like optoelectronics, light harvesting and quantum computing. Relative ease of vertical stacking of these Van der Waals materials and possibility of the synthesis of their lateral heterosctructures open new horizons in the control of the optoelectronic properties of such hybrid materials and rational engineering of new unique states like vertical or lateral interlayer excitons, defect-bound excitons, quantum localization of the excitons due to extreme mechanical strain etc. In particular, the 1D interface in lateral heterostructures as well as the vertical multilayers comprised of different 2D semiconductors look specifically attractive in terms of rational design of optoelectronic properties of such hybrid materials. Characterization of the optical properties and chemical composition of the 1D/2D interfaces at proper length scale, which is often of the order of few nanometers or less, becomes a crucial necessity for further progress in understanding their properties and rational device engineering.
In presented work we report deep subdiffractional Raman imaging achieved by means of the tip enhanced Raman spectroscopy (TERS) of lateral and vertical heterostructures of WS₂ and WSe₂ transferred to gold.
Thanks to dramatically enhanced sensitivity and spatial resolution enabled by the gap mode TERS, we first observed that within 20-50 nm at the junction line in WS₂-WSe₂ lateral heterostructures that grew on top of the bilayer WS₂, TERS spectra were clearly different from the core WSe₂@2LWS₂ vertical heterotrilayer showing noticeable intensity of two new peaks at ~ 290 cm^-1 and ~ 330 cm^-1. The positions of these two junction-specific peaks were strikingly reminiscent of the A₁¹ and E² modes correspondingly of Janus WSSe structures¹. Formation of the Janus –type junction was a rather non-trivial, though not totally impossible hypothesis.
In order to probe it, we collected TERS maps of the junction in lateral WS₂-WSe₂ heteromonolayers and it turned out that the junction-specific spectra actually occurred on the WSe₂ side of the lateral heterostructure. With certain degree of confidence we can guess that the ~ 290 cm^-1 band was the second overtone of the mode at ~145 cm^-1 as the latter was also evident in the junction specific spectra and in some cases we even observed a weak peak at ~435 cm^-1 that would be the third overtone of the 145 cm^-1 band. TERS spectra on the WS₂ side of the lateral WS₂-WSe₂ heteromonolayer junction remained practically the same as the TERS spectra of the core WS₂.
Junction specific peaks in WSe₂ spectra look very similar to the longitudinal acoustic modes at M point which are associated with defects in this material, therefore we can make a conclusion that at the junction in lateral WS₂-WSe₂ heterostructures defects are preferentially concentrated on WSe₂ side. How such asymmetry affects the electronic and optoelectronic properties of the heterostructures remains the question for further investigation.
(1) Petrić, M. M.; Kremser, M.; Barbone, M.; Qin, Y.; Sayyad, Y.; Shen, Y.; Tongay, S.; Finley, J. J.; Botello-Méndez, A. R.; Müller, K. Raman Spectrum of Janus Transition Metal Dichalcogenide Monolayers WSSe and MoSSe. Physical Review B 2021, 103 (3). https://doi.org/10.1103/PhysRevB.103.035414.

Two dimensional (2D) nanomaterials have been intensively investigated as emerging materials for future devices, including electronics, photonics, and electrochemical energy storage devices. The mechanical stability of each 2D component is critical to the reliability of the fabricated devices. Currently, research on experimental mechanics of 2D materials has been focused on quantifying mechanical properties and understanding fracture behaviours using different techniques. Confined to 2D geometry, cracks in 2D materials generally favour a brittle behaviour with minimum plasticity at room temperature, which continues the dilemma of mutually exclusive fracture toughness and mechanical strength in bulk materials.
Considerable research has been devoted to improving fracture toughness of 2D materials. For example, carbon nanotubes (CNTs) were integrated into graphene as an extrinsic toughening strategy. The fabricated rebar graphene displays a zigzag fracture surface, guided and redirected by the embedded CNTs. Such toughening mechanism is similar to improving fracture resistance extrinsically by introducing fiber/lamella bridging, oxide wedging, transformation toughening, etc. In addition to rebar graphene, h-BN has been carefully investigated as it has the same structure of graphene but is composed of two elements. h-BN retains the high strength (~100 GPa) and elastic modulus (~0.8 TPa). The fracture behaviour of monolayer single crystalline h-BN has long been taken as an ideal brittle material subject to Griffith’s law. By combining computational analysis and in situ tensile test, the monolayer h-BN has an exceptionally high fracture toughness. The effective energy release rate is up to one order of magnitude higher than its Griffith energy release rate and that of graphene, yet their strength is very close. Stable crack propagation in monolayer h-BN is observed, and the corresponding crack resistance curve is obtained. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene. The novel lattice asymmetry induced intrinsic toughening mechanism discovered at the 2D limit shed important light into toughening of heterogeneous 2D materials.

Electron microscopy is a widespread and often essential tool for structural and chemical analysis at the atomic level. Image resolution is dominated by the energy (or wavelength) of the electron beam and the quality of the lens. Two-dimensional materials are imaged with low beam energies to avoid damaging the samples, limiting spatial resolution to ~1 Å. By combining the full 4D-phase space information collected by a new generation of direct electron detectors, which have the dynamic range to record the complete distribution of transmitted electrons at every beam position (4D-STEM), and a ptychographic phase retrieval algorithm to process the data, we have been able to increase the spatial resolution well beyond the traditional lens limitations reaching a 0.39 Å resolution for MoS₂, at the same dose and imaging conditions where conventional imaging modes reach only 0.98 Å [1]. The ultimate limit to spatial resolution in an electron microscope is set by the thermal vibrations of the atoms themselves, which are on the order of 10-20 pm [2]. Using multislice electron ptychography, we are now able to see the details of thermal vibrations of individual atom columns.
The improved resolution, dose efficiency and robustness to environmental noise enabled by ptychography make it easy to identify defects such as sulfur monovacancies, as well as subtle structural arrangements and tilts on the chalcogenide sublattice that are undetectable by conventional imaging modes. For twisted bilayers, we are able to resolve the shear distortions and interactions between the layers. Ptychography is also sensitive to the phase shift from magnetic fields, making it possible to map magnetic order in both 2D and stacked materials.
[1] Y. Jiang, Z. Chen, Y. Han, P. Deb, H. Gao, S. Xie, P. Purohit, M. W. Tate, J. Park, S. M. Gruner, V. Elser, and D. A. Muller. “Electron Ptychography of 2D Materials to Deep Sub-Ångström Resolution” Nature 559, (2018): 343–349.
[2] Z. Chen, Y. Jiang, Y.-T. Shao, M. E. Holtz, M. Odstrčil, M. Guizar-Sicairos, I. Hanke, S. Ganschow, D. G. Schlom, and D. A. Muller. “Electron Ptychography Achieves Atomic-Resolution Limits Set by Lattice Vibrations” Science 372, (2021), 826–831.
[3] Research supported by US National Science Foundation (grants DMR-1539918, DMR-1429155, DMR-1719875) & US AFOSR MURI program

Conventionally, luminescence processes in layered materials are probed using a focused laser for excitation, in which the ultimate spatial resolution is diffraction limited to ~1 μm. However, many submicron structures are present in luminescent layered material systems e.g. depletion regions [1], heterostructure interfaces [2] and localised defects [3]. Scanning electron microscopy cathodoluminescence (SEM-CL) can reach < 100 nm resolution [4], whilst still allowing one to probe complex device structures. Here, we use SEM-CL to probe the luminescence properties of a monolayer WSe₂ device, encapsulated in 20 and 100 nm hBN. We uncover a submicron region which exhibits a 3x enhancement in luminescence intensity, close to the grounded region of the WSe₂ flake. To understand the effect grounding the WSe₂ could have on luminescence, we employ Monte-Carlo simulations which give insight into the distribution of carriers in this device under electron bombardment, helping explain the enhancement. This lays foundational understanding of cathodoluminescence processes in layered materials heterostructures.

[1] Frisenda, R., Molina-Mendoza, A. J., Mueller, T., Castellanos-Gomez, A. & Zant, H. S. J. van der. Atomically thin p–n junctions based on two-dimensional materials. Chem. Soc. Rev. 47, 3339–3358 (2018).
[2] Sahoo, P. K., Memaran, S., Xin, Y., Balicas, L. & Gutiérrez, H. R. One-pot growth of two-dimensional lateral heterostructures via sequential edge-epitaxy. Nature 553, 63–67 (2018).
[3] Palacios-Berraquero, C. et al. Large-scale quantum-emitter arrays in atomically thin semiconductors. Nat Commun 8, 1–6 (2017).
[4] Zheng, S. et al. Giant Enhancement of Cathodoluminescence of Monolayer Transitional Metal Dichalcogenides Semiconductors. Nano Lett. 17, 6475–6480 (2017).

The high-contrast optical imaging of nearly transparent 2D materials such as clays and some oxide sheets has been an outstanding challenge; yet there is a critical research capability needed in the synthesis and processing of these materials, which show promise in a number of applications including barrier coatings, membranes, and composites. Using delaminated silicate clay and titania sheets as model systems, here it is shown that these weakly quenching, nearly transparent 2D sheets can be readily imaged by fluorescence quenching microscopy (FQM) based on a new mechanism. When these sheets are deposited on strongly quenching substrates, including doped silicon wafers and metals or indium tin oxide-coated glass slides, they can act as a “spacer” to reduce the degree of quenching of the dye layer by the substrate, appearing as highly visible bright sheets against a dark background. This new FQM strategy can visualize dielectric 2D materials with high contrast and layer resolutions comparable to scanning electron microscopy and atomic force microscopy. When different 2D materials are co-deposited, FQM not only differentiates them readily based on their quenching capabilities, but also can resolve their vertical stacking sequence based on the contrast of their overlapped areas.

The nuanced interactions of a two-dimensional (2D) layered semiconductor/metal interface continue to be a central focus for study since device scaling demands the highest possible performance in the thinnest possible channel. Unlocking the intrinsically nanoscale nature of such a junction requires a detailed examination of the chemical and physical properties of the materials. Indeed, there is an intensive effort to understand and control the nature of attractive interactions between 2D layered semiconductors and metals and to examine its impact on the chemical bonding, electronic band structure, and contact resistance at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS₂ films and gold, with a focus on the occupied electronic states near the Brillouin zone center (i.e., the Γ point) and the work function, to illuminate the nature of the WS₂-Au interaction. To delineate the WS₂ spectra of a locally varying interaction strength with Au, we employ a microscopy approach and tailored sample structures, ranging from the WS₂/Au semi-epitaxial junctions to the suspended WS₂ regions. Spectroscopic analysis shows the shifts in the electron binding energy (E_B) of the occupied electronic states depending on the interaction strength. In the strong interaction limit, we have discovered a new source for contact heterogeneity – local stacking order. We hypothesize that these stacking perturbations will generally arise when the substrate-2D material interaction is stronger than the inter-2D layer interaction. Together, these findings show that optimizing 2D semiconductors’ electronic properties will require a careful control of the interface and conditioning steps for the underlying Au structure could directly impact the interface, in addition to the electron screening and defects that have been widely considered.
This research was conducted in collaboration with Alex M. Boehm, Catalin D. Spataru, Cherrelle J. Thomas, and R. Guild Copeland at Sandia National Laboratories, and Jose J. Fonseca and Jeremy T. Robinson at Naval Research Laboratory. We acknowledge insightful discussions with R. Tandon and N. Bartelt. The work at Sandia National Laboratories was supported by Sandia’s LDRD program and the Center for Integrated Nanotechnologies user program, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science (DE-AC04-94AL85000). The work at the US Naval Research Laboratory was funded by the Office of Naval Research. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly-owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Two-dimensional (2D) semiconducting monolayers such as transition metal dichalcogenides (TMDs) are promising channel materials to extend Moore's Law in advanced electronics. To make high-performance devices based on 2D semiconductors such as WS₂ monolayers, many issues need to be resolved. Here, we like to discuss two advancements we have achieved recently. (1) We discover that hydroxide vapor phase epitaxy enables the growth of WS₂ monolayers with a significantly lower density of structural defects, which make the electron mobility peaked at ~ 200 cm²/Vs. (2) Ultrahigh-k dielectrics can be applied onto short-channel (<30 nm) 2D monolayer transistors to greatly lower the subthreshold swing (down to 70 mV dec^-1) with an ON/OFF current ratio up to 10⁷.

Molybdenum disulfide (MoS₂) is a promising van der Waals (vdW) semiconductor material for building next-generation electronic devices. Monolayer and multilayer MoS₂ FETs are theoretically expected to possess high carrier mobility, high ON/OFF ratio, and steep subthreshold swing. However, most reports of MoS₂ FET show a relatively low ON-current, which makes the circuits based on MoS₂ FET suffer from serious RC delay and therefore low processing speed. Enormous efforts have been done on improving the contact interface between the MoS₂ channel and the electrode metal. Although the contact resistance has been largely reduced, the ON-current of the FET based on a single MoS₂ channel is still not sufficient. A device with multiple MoS₂ channels and dual-gate on each channel has the potential to further enhance the ON-current. To achieve this device structure, folding provides a convenient method to double the number of gates and channels. Here, we developed a deterministic folding technique of vdW materials and heterostructures using microfluidic forces. Electron beam lithography is utilized to define the folding edge, which allows precise control of the folding area and angle with a sub-100 nm and 1-degree spatial resolution. By folding a graphene/hBN/MoS₂ heterostructure, we converted a single-gate FET to a dual-gate one. Since the two gates are made from the same graphene and hBN flakes, the structure of the double-side gate FET is perfectly symmetrical. Electrical characterization shows the current density after folding is almost as twice high as before folding because the dual-gate creates an effective channel on both the top and bottom sides of MoS₂. Furthermore, we fabricated a two-channel dual-gate device by folding a metal/hBN/MoS₂/hBN/metal heterostructure to realize a four-time enhancement on current density. The folding technique is also applicable to arbitrary vdW heterostructures, and hence offers more possibility of nanofabrication for vdW material-based devices with novel structures and properties.

Strain engineering has been adopted as a powerful strategy to control the physical properties of materials enabling devices with enhanced functionality and improved performance. High strain tolerance of two-dimensional semiconductors makes them a fertile ground for manipulation of their electronic properties via application of the elastic strain fields. Here, we investigate the modulation of the perpendicular-to-plane transport behavior of the two-dimensional MoS₂ junctions under the mechanical stress induced by a probing tip of an atomic force microscope (AFM). We show that the junction resistance can be tuned by up to four orders of magnitude upon exerting a mechanical force by the AFM tip. Analysis of the stress-induced evolution of the I-V characteristics indicates a combined effect of the tip-induced strain and strain gradient. While the strain effect results in a gradual reduction of the MoS₂ barrier height with the tip loading force increase, the induced strain gradient, which generates a flexoelectric field, modify the barrier profile. In addition, we show that the tip-generated flexoelectric effect leads to a significant enhancement of the photovoltaic effect in the MoS₂ junctions. A combination of the optical and mechanical stimuli facilitates reversible photomechanical tuning of resistance of narrow band 2D semiconductors and the development of devices with the enhanced photovoltaic response.

Over the last decade, the discovery of graphene has triggered a new paradigm of two-dimensional (2D) crystal materials. They are characterized by a periodic network structure and topographical thickness at the atomic/molecular level, enabling the investigation of fundamental exotic physical and chemical properties down to a single-layer nanosheet. Thereby, robust technologies and industrial applications, ranging from electronics and optoelectronics to energy storage, energy conversion, membranes, sensors, and biomedicine, have been inspired by the discovery and exploration of such new materials.
In contrast to the tremendous efforts dedicated to the exploration of graphene and inorganic 2D crystals such as metal dichalcogenides, boron nitride, black phosphorus, metal oxides, and nitrides, there has been much less development in organic 2D crystalline materials, including the bottom-up organic/polymer synthesis of graphene nanoribbons, 2D metal-organic frameworks, 2D polymers/supramolecular polymers, as well as the supramolecular approach to 2D organic nanostructures. One of the central chemical challenges is to realize a controlled polymerization in two distinct dimensions under thermodynamic/kinetic control in solution and at the surface/interface. In this talk, we will present our recent efforts in bottom-up synthetic approaches towards novel organic 2D crystals with structural control at the atomic/molecular level and beyond. We will introduce a surfactant-monolayer assisted interfacial synthesis (SMAIS) method that is highly efficient to promote supramolecular assembly of precursor monomers on the water surface and subsequent 2D polymerization in a controlled manner. 2D conjugated polymers and coordination polymers belong to such materials classes. The unique structures with possible tailoring of conjugated building blocks and conjugation lengths, tunable pore sizes and thicknesses, as well as impressive electronic structures, make them highly promising for a range of applications in electronics and spintronics. Other application potential of organic 2D crystals, such as in membranes, will also be discussed.

Ferroelectric materials exhibit a net electric dipole moment whose orientation can be changed by an external electric field. This useful property has been used to create non-volatile ferroelectric field effect transistors (FeFETs), which may find application in neuromorphic computing due to their efficient emulation of synaptic plasticity. Integration of 2D ferroelectric semiconducting α-In₂Se₃ with non-ferrolectric gate dielectrics enables ferroelectric semiconductor-field effect transistors (FeS-FETs), The FeS-FET output depends on the history of the gate bias, which modulates of the free carrier distribution and the polarization of bound charges in the semiconductor. The combined effect on the potential profile and the metal-semiconductor contact barriers creates the possibility for optical neuromorphic devices based on, for example, tunable Schottky barrier photodiodes. While several groups have reported large hysteresis in α-In₂Se₃ based FeS-FETs, device characteristics vary widely, and the microscopic connections between polarization switching and resistance changes can remain obscured. We will describe the use of functional imaging to correlate polarization switching with modulation of the dark current and photocurrent regulated by the metal-semiconductor junction, laying a foundation for design of optoelectronic neuromorphic devices. Scanning photocurrent microscopy (SPCM) was used to map the spatially dependent photoresponse and isolate the contribution of the metal-semiconductor junctions, which dominate at the reverse biased (Schottky) junction. Photocurrent spectra were acquired at the contacts to directly measure the effective barrier height by internal photoemission, and the modulation of barrier height with external field poling of the In₂Se₃. Piezoresponse force microscopy was used to establish domain structure in exposed regions of the channel, and Kelvin probe force microscopy to establish a self-consistent explanation for local ferroelectric domain switching and changes in device output. The improved microscopic understanding of the origins and magnitude of hysteresis support the design and rational optimization of FeS-FET based devices.

Two-dimensional (2D) materials have been extensively explored as a promising candidate for future device technologies because of their unique layer-dependent physical, optical, mechanical and thermal properties. Among various 2D materials, WSe₂ is presented as one of the outstanding candidates for the realization of a single material 2D CMOS technology due to its tunability for both electron and hole transport, with relatively high mobility. In addition to, the idea of ferroelectric-based 2D devices with Negative capacitance (NC) features triggered explosive novelty in the device functionality and energy efficiency. In this talk, we will present some of our recent results in this regard. Specifically, we will discuss experimental data that show how the NC in doped high-k dielectrics like Si:HfO₂ could lead to supply voltage reduction and technology booster for WSe₂-based devices. A detailed study on possible gate stacks for vdW 2D/2D TFETs using Si:HfO₂ with/without internal metal and different linear dielectrics will be reported. Additionally, the energy losses of the gate stack per switching operation will be explored in the wide range of temperatures, from cryogenic (77 K) to the temperatures close to operational ones in modern microprocessors (100 ^oC). An analysis of energy-efficient metrics in the hysteretic ferroelectric stack, discussing charging, discharging, energy loss, and gate efficiency factor, will be proposed and supported by experimental results. We will show both DC and pulsed measurements trying to have further insights into recent controversies concerning the negative capacitance regime in ferroelectrics.

The electrical performance of semiconductor devices has a significant dependence on the surface states of the semiconductors because the channel between source and drain regions for charge carrier transports is mostly formed near the surface with applying gate bias. Therefore, it is critical to engineering the surface chemistry of the semiconductor material. Coupling an intrinsically atomically thin body with a finite bandgap, layered transition metal dichalcogenides (TMDCs) have been employed as semiconducting channel platforms with a large ON/OFF ratio and near theoretical subthreshold swing. Here, chemically reconfigurable synapse behaviors of TMDC via molecular semiconductor interaction are demonstrated with three terminal MoSe₂ synapse devices.
Molecular adsorption on the MoSe₂ surface is induced with dipping in (NH₄)₂S(aq) diluted in H2O with 25%, while the solution temperature is held at 323 K. H₂S or HS molecules can be adsorbed on the MoSe₂ surface during this chemical treatment and act as trapping centers for transported carriers. The adsorbed molecules can be desorbed from MoSe₂ surfaces with wash with isopropanol. Electric measurements are performed at room temperature in ambient conditions. In the transfer characteristic of MoSe₂ devices, molecular adsorption induces a significant increase of hysteresis from 38.3 V_TH to 59.0 V_TH, consistent with charge carrier trapping near the channel. In contrast, molecular desorption from MoSe₂ surfaces results in a decrease of value to 45.3 V_TH with the removal of trapping center from the surface of synapse devices.
Afterward, to mimic the biological synapse, the electrical synapse response of MoSe₂ transistors is modulated by a pulse generator wherein excitatory modes; thereby synapse plasticity is controlled by spike-like gate bias as a presynaptic input. After applying pulse gate bias with 50 ms pulse duration, for bare MoSe₂ devices, there is no synapse response, including changes in conductance, consistent with the absence of the plasticity. However, after chemical treatment, molecular adsorption increases conductance upon pre-synapse inputs consistent with a significant increase from 5.9 S to 68.6 S (a postsynaptic conductance), consistent with the manifest of plasticity. The observed plasticity can disappear with the desorption of molecules from the surface of synapse devices.
To emulate the visual sensory behavior of humans, the artificial synaptic plasticity of synaptic devices has been explored by applying light pulse. As light stimulation with pulse modulation, the channel’s conductance near-linearly increases as the number of light pulses increases, consistent with optical potentiation. Afterward, electric depression of molecularly functionalized synapse can be observed with the linear decrease of conductance. Consequently, since the present report demonstrates the chemically reconfigurable plasticity of TMDCs synapse devices with the optically sensory response, it can be a milestone to develop a brain-inspired computing system that integrates sensory function upon chemical and optical stimulation.

Transition metal dichalcogenides (TMDCs) are a member of the two-dimensional van der Waals semiconductor materials with a thickness-dependent energy bandgap. A few nanometer-thick TMDCs channel can be fabricated through the mechanical exfoliation method, and it shows a high level of carrier mobility and good chemical/thermal stability. Since TMDCs materials are free from short-channel effect, the fabrication of highly integrated circuits with atomic level scale and ultra-low power is possible that was almost not available in conventional Si devices.
WSe₂, one of an ambipolar material of TMDCs, have high carrier mobility (hole: ~250 cm²/Vs, electron: ~250 cm²/Vs) and thickness-dependent energy bandgaps with 1.7 eV (direct, monolayer) and 1.2 eV (indirect, multilayer). Electron or hole transport properties of WSe₂ can be selectively amplified by using the charge transport doping method. When under-stoichiometric oxidation was carried out to WSe₂, the exposed layer is converted to WO_X. WO_X is known as an effective hole injection layer and induces p-doping of the underlying WSe₂ lattice. In addition, Ar⁺ bombardment by plasma treatment generates the formation of Se vacancy of WSe₂ lattice. Se vacancy acts as an electron donor to induce effective n-doping.
Ambipolar WSe₂ is generally applied to normally-on p- or n-type field-effect transistors (FETs). Most of TMDCs materials are used to fabricate ultra-low-power devices because of their fast on-off switching speed and minimization of standby power. However, the normally-on characteristic of WSe₂ restricts the low-power device operation. In this study, an n-p-n lateral p-n homojunction FET with a normally-off characteristic is fabricated. Our normally-off WSe₂ FET has a p-type channel and n-type contact. Selective p- and n-type doping was formed through UV-assisted oxidation and Ar⁺ plasma treatment with electron-beam resist (ER) mask. The n-doping of the contact region and the patterning of source/drain electrodes used the same ER mask.
Forming of the dual p-n junction was confirmed by the photocurrent mapping using a 532 nm laser. Thickness change of WSe₂ caused by oxidation and Ar⁺ plasma treatment was measured in the TEM analysis. Normally-on n-doped WSe₂ FET (undoped channel) was used to compare the electrical properties with the normally-off device. Transfer characteristics show the change of ambipolar to the p-type dominant device. Normally-off characteristics are proved by the current level at V_GS = 0 V and V_DS = +1 V (n-i-n undoped channel: 17.8 nA → n-p-n heterojunction: 0.09 pA). Improved electrical parameters (on/off ratio: ~10⁸, carrier mobility: ~126 cm²/Vs, channel resistance ~ 1000 kΩ×μm) were observed. The results of this study will pave the way for the ultra-low-power device in nano-electronics based on TMDC materials. Details of our work will be discussed in the presentation.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been regarded attention as an emerging 2D-semiconductor class with a high surface to volume ratio and a wide range of band gaps. Among TMDs, atomically thin chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) has attracted much attention owing to its high carrier mobility, outstanding chemical stability, absence of dangling bond, and tunable electronic energy band.
To realize further practical applications with CVD-grown MoS₂, its modulated the electrical characteristics would be significantly important in building integrated logic circuits beyond a laboratory-scale single device. In this frame, for the last decade, there have been a lot of efforts to modulate or enhance the electrical performance of TMDs-based devices via chemical doping. Specifically, surface charge transfer doping (SCTD) has been intensively investigated which can induce electron donation or withdrawal between deposited chemical dopants and host materials. The carrier concentration of TMDs can be controlled by the SCTD without the lattice distortion of TMDs. Among the candidates of organic dopants, benzyl viologen (BV) is one of the most promising chemical dopants for n-type doping effect in TMDs which acts as an electron donor due to the difference in energy level between the reduction potential and the vicinity of the conduction band edge of TMDs. For conducting SCTD, organic dopants are typically deposited onto the TMDs via solution-immersion or dopant vaporization with controlling immersion time, dopant concentration in solution, and de-doping time. However, these doping methods have inherent limitation to introduce spatially selective doping on-demand which is of fundamental importance to realize the integrated circuit applications.
In this presentation, to the best of our knowledge, we will report spatially controlled SCTD on MoS₂ via using drop-on-demand selective inkjet printing with BV inks for the first time. To perform a reliable SCTD, we have optimized the solvent system of BV ink droplet to create well-defined pL-scale droplets considering a specific range of the inverse Ohnesorge number, which is a prominent parameter to determine inkjettability. By controlling the BV concentration in inks and the doping region, different SCTD effect could be delivered onto MoS₂ channels resulting in their elaborately modulated electrical characteristics. Raman spectroscopy, Photoluminescence, and X-ray photoelectron spectroscopy were conducted to support the modulated electrical properties of MoS2-FETs with controlled surface charge transfer doping via selective inkjet printing.

Recently, researches on high-density memory have been focused to develop AI implementation or high-performance memory. Accordingly, interest in multi-level memory began to surge. In the early days, these researches were conducted using the characteristics that change with the current level by controlling the range of the gate voltage for multi-level implementation. And recently, they have been conducted for improving the performance of existing devices using modification of device structure or additional processes such as light source or plasma. However, compared to the areas where these applied studies are actively conducted, basic research is insufficient. Therefore, we investigated the multi-level generation mechanism in the floating gate memory of the basic structure and conducted a study to control and predict the number of multi-level using this. We fabricated floating gate memory devices using two-dimensional materials. MoS₂ was used for the channel, hBN for the tunneling insulating layer, and graphene for the floating gate. SiO₂/Si substrates were used as the gate insulating layer and the gate electrode under two-dimensional materials. We took advantage of the characteristic that the current level changes from the reading voltage by controlling the range of the gate voltage mentioned above, and the characteristic that the slope of the transfer curve of the transistor changes according to the thickness of the insulating layer in that the floating gate memory is similar to the transistor structure. As a result of the experiment, multi-level was implemented in a floating gate memory device having a basic structure, and the number of multi-level was controlled by tuning the thickness of hBN.

Corrosion annually cost society trillions of U.S. dollars worldwide. Atomically thin layers of graphene have been proposed to protect surfaces through the direct blocking of corrosion reactants such as oxygen with low added weight. The long term efficacy of such an approach, however, is unclear due to the long-term desired protection of decades and the presence of defects in as-synthesized materials. Here, we demonstrate catalytic permeation of oxygen molecules through chemical vapor-deposited (CVD) graphene by imparting sub-eV kinetic energy to molecules. These molecules represent a small fraction of a thermal distribution thus this exposure serves as an accelerated stress test for understanding decades-long exposures. We used in situ synchrotron X-ray photoelectron spectroscopy, and the permeation rate of the energized molecules increased 2 orders of magnitude compared to their non-energized counterpart. Graphene maintained its relative impermeability to non-energized oxygen molecules even after the permeation of energized molecules indicating that the process is non-destructive and a fundamental property of the exposed material. Molecular dynamics-based simulation suggests kinetic energy-mediated chemical reactions catalyzed by common graphene defects as a responsible mechanism.

TlGaX₂ [X = Se, S] is a family of layered 2-D ternary chalcogenides. These p-type semiconductors have band gaps within the green to ultraviolet range, which makes them ideal candidates for optoelectronic applications.
Current examples of such applications, such as phototransistors and detectors, make use of multistep processes, such as mechanical exfoliation with a PMMA transfer or thin film synthesis via thermal evaporation, which makes potential future scalability of these devices cumbersome. Liquid phase exfoliation (LPE) is a far more facile process that has been shown to work for a large number of layered van der Waals materials. Herein, we demonstrate that these TlGaX₂ materials can be exfoliated with a facile one step sonication in IPA, leaving behind little to no residue.
Despite structure-property relationships in materials being highly important for their future device applications, this has to date not been fully addressed for this family of semiconductors. For example, while it was reported that Se vacancies in TlGaSe₂ can lead to a change in the nature and size of the bandgap, a quantitative relationship was not determined.
In this work, we use high-resolution scanning transmission electron microscopy (HRSTEM), in combination with EDX/EELS, to experimentally address this aspect of defects in TlGaSe₂. Our experiments clearly indicate the presence of stacking faults (as were seen previously in XRD studies with bulk TlGaSe₂, and with isostructural KInS₂) and surface relaxation in LPE-TlGaSe2. Using complimentary density functional theory (DFT) simulations, we explore the effect of defects on the electronic structure. Furthermore, density functional perturbation theory (DFPT) is used to quantify the effect of the stacking faults on phonon suppression, and how this relates to the thermoelectric properties of the materials.

Photocathodes are essential components for various applications requiring photon to free electron conversion, for example, high sensitivity photodetectors and electron injectors for free-electron lasers. Alkali antimonide thin films are widely used as photocathode materials owing to their high quantum efficiency in visible spectral range, however, their lifetime can be limited even in ultra-high vacuum due to their high reactivity to residual gases and sensitivity to ion back-bombardment in these applications. An ambitious technical challenge is to extend the lifetime of bialkali photocathodes by coating them with suitable materials that can isolate the photocathode films from residual gases while still maintaining their highly emissive properties. We propose the use of graphene, an atomically thin two-dimensional material with gas impermeability, as a promising candidate for this purpose. Here, we report that high-quality bialkali antimonide can be grown on a 2-layer (2L) suspended graphene substrate with a peak quantum efficiency of 15%. More importantly, by comparing the photoemission through varying layers of graphene we demonstrate that photoelectrons can transmit through few-layer graphene with a maximum QE of over 0.7% at 4.5 eV for 2L graphene, corresponding to a transmission efficiency of 5%. These results demonstrate important progress towards fully encapsulated bialkali photocathodes having both high quantum efficiencies and long lifetimes using atomically thin protection layers.

Since carbon nanotubes (CNT) and graphene, which was previously spotlighted materials, do not have a band gap, it is very difficult to apply them to devices. For this reason, many researchers have been interested in a new material, a transition metal chalcogenide. The transition metal chalcogenide, represented by the chemical formula of MX2, is composed of a transition metal (M) corresponding to groups 4 to 6 of the periodic table and a chalcogen element of group 7 of the periodic table defined as X. This compound has a layered structure, and each layer is bound to each other by very weak Van derr waals forces. Since the electrical and optoelectronic properties change depending on the number of stacked layers, this compound is a very good material for device applications.
Among the transition metal chalcogenide, molybdenum disulfide (MoS2) increases the bandgap as the stacked layers decrease, and when it finally becomes a single layer, the bandgap characteristics change from an indirect bandgap to a direct bandgap, and the direct bandgap is approximately 1.9 eV. For a wide application of MoS2, a stable deposition method and a functionalization strategy should be developed. Many studies have been conducted on the method of large-area growth of this material. However, there are no studies on a stable and simple functionalization method yet. In this study, we will introduce a stable and efficient functionalization method of MoS2.
MoS2 with three layers was stably nestled deposited on a silicon oxide substrate through physical exfoliation. After that, it was treated with Ar+ plasma to make sulfur defects in MoS2. Ar, a representative inert gas, does not chemically bond with the surface of MoS2, so Ar was selected, so that Ar+ ions accelerated by an electric field only physically damage the surface of MoS2. MoS2 with sulfur vacancies was exposed to mercaptopropionic acid (MPA). If sulfur vacancy is exposed to MPA with an organic thiols structure attached to the HS group, the HS group can act as a donor of the sulfur atom. Therefore, sulfur vacancy and HS groups enable dative bonds to share electron pairs to Molybdenum atoms by ligand conjugation. As HS atoms were filled in sulfur vacancies, carboxyl group were formed on the MoS2 surface.
The optimal MPA concentration was found by exposing different concentrations of MPA to MoS2 with sulfur defects. The concentration of the MPA aqueous solution was increased from 25 mM to 220 mM, and the atomic ratio of the COO group was confirmed through X-ray Photoelectron. Spectroscopy. It was confirmed that the quantification of the carboxyl functional group was maximized at the concentration of 55 mM, and it was confirmed that the concentration of 55mM was the optimal concentration for functionalizing MoS2 with sulfur defects. At a concentration higher than 55 mM, it was shown that the atomic ratio of the carboxyl group decreased, because the HS groups present at a high concentration exert a repulsive force from each other and interfere with sulfur defects and bonding.
In this paper, we introduced a method to efficiently functionalize MoS2 through selective destruction of sulfur atoms without completely etching the MoS2 layer. By controlling the minimized Ar plasma power and time, it led to partial destruction of sulfur atoms on the surface of MoS2. By exposing the MPA aqueous solution to the generated sulfur defects, carboxyl groups could be effectively formed on the surface of MoS2 through coordination bonds.
The proposed method can be applied to various 2D materials, and can be applied to gas sensors, humidity sensors, and biosensors. Typically, when MPA-functionalized MoS2 is reacted with a single cytosine solution, the carboxyl group and NH2 (amine group) of the single cytosine are amide-bonded, making it possible to manufacture a heavy metal measurement sensor.

Transition metal dichalcogenides (TMDs) have been studied intensely for their semiconducting properties that emerge at the atomically thin level. Van der Waals (vdW) heterostructure stacking of MoSe2 and WSe2 monolayers creates a type-II heterojunction which encourages the formation of strongly bound interlayer excitons due to the reduction in dielectric screening and increase in coulombic interaction strength [4, 5]. Interlayer excitons forming within the MoSe2/WSe2 heterobilayer have been shown to have increased lifetimes, large binding energies, and permanent dipole moments as opposed to “intralayer” excitons [5]. While TMD heterobilayers have undergone intense studies to better understand interlayer excitons, the effect of biaxial strain on these heterostructures are still unknown. Here we have designed and built a versatile biaxial straining platform for in situ Raman and Photoluminescence measurements of strained films. A thin polymer substrate (PET) is indented symmetrically by the platform to induce biaxial tensile strain via the change in the curvature of the substrate. The system is calibrated by mapping the location of features on the PET substrate as a function of indenter displacement. In this work, we will investigate strain induced PL and Raman peak shifting on biaxially strained MoSe2/WSe2 heterobilayers exfoliated from low point defect density bulk crystals which may correspond to excitonic emissions. From these observations, we may better understand the tunability of the interlayer exciton peak emission as a function of mechanical strain which may be useful for applications in enhancing properties like band gap and optical density in optoelectronic devices such as light emitters, photodetectors and photovoltaic cells.

References:
[1] Carrascoso et al., Nano Mater. Sci., in press (2021).
[2] Blundo et al., Appl. Phys. Rev. 8, 021318 (2021).
[3] Peng et al. Light: Science & Applications 9:190 (2020).
[4] Calman, NanoLett., 20, 1869−1875 (2020).
[5] Zheng et al., arXiv: 1911.00087 (2019).

High-performance electromagnetic shielding with strong absorption and lightweight are necessary for industry and military applications. To obtain the desired performance, two dimensional (2D) layered structure nano sheets, such as graphene and graphene-like, were adopted due to their unique structure and properties. Here, a hybrid nanofiller of reduced graphene oxide-molybdenum disulfide (rGO/MoS2) composite was prepared by a one-pot hydrothermal reaction. Further, nanocomposite foam of thermoplastic polyurethane was prepared by a unique phase separation technique. The hybrid nanostructure of rGO/MoS₂ prevents two-dimensional nano sheets re-stacked and can be applied in the electromagnetic wave absorption field. From the preliminary shielding tests conducted in the X band (8.2-12.4 GHz) frequency range, indicates the hybrid nanofiller and porous structure of foam helped in improving the conduction and polarization losses synergistically. Further, the individual effect of 2D rGO, MoS2 and synergistic effect of their hybrid nano filler on thermomechanical properties of the composite are also elucidated. We may reasonably conclude that rGO/MoS₂ reinforced composite foam can serve as excellent light-weight electromagnetic wave absorbers and can be widely used in practice.

WSe₂, one of the transition metal dichalcogenide (TMD) materials, has been intensively interested owing to its fascinating properties, such as layer-dependent bandgap (~1.7 eV for monolayer and ~1.2 eV for bulk). Harnessing of layer-dependence could enable to tune not only the bandgap but also electrical and optical properties, making TMDs attractive. Therefore, properly controlling the number of layers of TMDs is beneficial for the design and optimization of novel devices.
Recently, many studies have been reported to control the number of layers via chemical vapor deposition (CVD), which enables high-quality and large-area film growth. However, CVD essentially accompanies long-term or high-temperature processes to achieve fine control on layer count. These process conditions limit the metallization of device contacts and interconnections for the fabrication of electronic and optoelectronic devices. In addition, fabricated TMD films by metal-organic chemical vapor deposition (MOCVD) may contain unexpected carbon impurities due to residual organic precursors.
On the other hand, TMDs thin films could be fabricated at relatively low growth temperature and short deposition time via pulsed laser deposition (PLD). The limited source transfer by PLD is advantageous for precise control of the number of layers. It is also easy for the fabrication of heterojunction with other materials through in situ deposition without the additional transfer process. However, it remains a major challenge to synthesize high-quality stoichiometric thin films composed of micron-sized grains by PLD.
Our group successfully synthesized the high-quality WSe₂ thin films at low temperature via home-made PLD system combined with selenium gas flow system. Using our PLD system, selenium vacancies in WSe₂ thin films have been successfully compensated by control of the flow rate of selenium gas. Further, we obtained increased grain size of WSe₂ films by reducing the concentration of water and suppressing the unnecessary nucleation via the NaCl-assisted deposition process. By carrying out Raman spectral analysis and electrical measurements, we have confirmed our WSe₂ thin films show improved properties compared to those deposited in conventional PLD methods.
We expect that our study will pave the way for the development of efficient TMDs synthesis methods and be applied for the fabrication of next-generation electronic devices through follow-up studies.

1H-MoS₂ is a potential candidate material for next-generation electronic devices due to its intrinsic band gap. However, grain boundaries are frequently formed during the synthetic process, reducing the mobility considerably and thus being obstacles for the real application. Understanding the characteristics of this dislocation is essential not just for facilitating large-scale growth, but also in a way that it opens up a new possibility of utilizing a defect itself. The formation of localized states inside a band gap by dislocations at grain boundaries is well known, which are accompanied by the magnetism that is sensitive to the doping and tilt angle. Despite its importance, however, comprehensive investigation of the grain boundary’s electronic structure is limited.
Here, we show the properties of a localized defect state and an inherently induced in-plane dipole field in grain boundaries of 1H-MoS₂ using ab initio calculations. At the grain boundary of single-layer MoS₂, the dipole is observed. Although the field is most visible in the pentagon-heptagon 5|7 dislocation, its presence is not confined to a certain type of defect. We focus our discussion on 5|7 double boundary cell with periodic boundary conditions, which has three particular deep level defect states. These states are localized in a specific boundary dislocation, and two of them have an antibonding nodal line through the center of the defect line.
Furthermore, our ab initio based scanning Seebeck microscope (SSM) simulation demonstrates that the distinctive Seebeck domain of opposite sign is formed around the grain boundaries of 1H-MoS₂. Although it is commonly known that the intrinsic dipole field plays a crucial role in determining the sign of the domain, we found that the impact of the defect level is so massive that it could drastically change the Seebeck value, even the inversion of charge carrier type could locally happen. Another indication of the influence of defects state is the thin line following the nodal line of defect states. Through the antibonding line where the effect of defect states vanishes, a sharp line with an opposite Seebeck sign is easily observed. This result implies that in a polycrystalline system, one should carefully determine the influence of the deep level state before interpreting the meaning of the Seebeck coefficient. Moreover, the delicate engineering of grain boundary may allow the microscopic control of the Seebeck value. Our results suggest that the formation of nanoscale p-n junction or the application of a thin line of the inverted charge carrier type could also be done.

Monolayer molybdenum disulfide (MoS₂) possesses a desirable direct bandgap with moderate carrier mobility, whereas graphene (Gr) exhibits a zero bandgap and excellent carrier mobility. Numerous approaches have been suggested for concomitantly realizing high on/off current ratio and high carrier mobility in field-effect transistors, but little is known to date about the effect of two-dimensional layered materials. Herein, we propose a Gr/MoS₂ heterojunction platform, i.e., junction field-effect transistor (JFET), that enhances the carrier mobility by a foctor of ~10 (~100 cm²V^-1s^-1) compared to that of monolayer MoS₂, while retaining a high on/off current ratio of ~10⁸ at room temperature. The Fermi level of Gr can be tuned by the wide back-gate bias (V_BG) to modulate the effective Schottky barrier height (SBH) at the Gr/MoS₂ heterointerface from 528 meV (n-MoS₂/p-Gr) to 116 meV (n-MoS₂/n-Gr), consequently enhancing the carrier mobility. The double humps in the transconductance derivative profile clearly reveal the carrier transport mechanism of Gr/MoS₂, where the barrier height is controlled by electrostatic doping.

[N]-carbophenes (carbophenes) are a novel class of two-dimensional covalent organic framework, based on linear [N]-phenylenes, the simplest of which may have been recently synthesized.[1] Tight-binding density functional theory was used to determine the ground-state configurations of single layers, bilayers, and bulk carbophenes.[2] Carbophenes have formation energies per carbon atom similar to that of graphenylene. The similarity of formation energies between graphenylene and carbophenes suggests that when trying to synthesize one, the other may also be synthesized. The formation energies could explain why the first reported synthesis of graphenylene also indicated that they may have inadvertently synthesized 3-carbophene. The projected density of states demonstrates that the anti-aromatic nature of the cyclobutene units plays a direct role in creating bands around the Fermi level. Thus, carbophenes are promising candidates for investigations aiming to
reveal the nature of aromatic bonds.
This presentation focuses on replacing the hydrogen terminating agents of pristine carbophenes with the functional groups OH, NO₂, NH₂, COOH, and CO. At all levels of replacement, the functional groups studied are energetically favorable when compared to hydrogen. The band gaps of functionalized carbophenes are dependent on the type of carbophene, the type of functional group, and the degree of coverage, leading to the ability to tune the band gap over a range of ~2 eV. Due to the regular pore structures of single-layer carbophenes, the favorable AA stacking of multi-layers, and the ability to highly functionalize the pores, carbophenes are exciting materials for applications in nanofluidics, sensors, and gas (liquid) separation.
References
1. Du Q-S, Tang P-D, Huang H-L, Du F-L, Huang K, Xie N-Z, Long S-Y, Li Y-M, Qiu J-S, Huang R-B (2017) A new type of two-dimensional carbon crystal prepared from 1,3,5-trihydroxybenzene. Scientific Reports, 7:40796. https://doi.org/10.1038/srep40796
2. Junkermeier CE, Luben JP, Paupitz R (2019) N-Carbophenes: Two-dimensional covalent organic frameworks derived from linear N-phenylenes. Materials Research Express, 6(11):115103. https://doi.org/10.1088/2053-1591/ab4513

The rising global challenge in energy and environmental fields has prompted intensive research on renewable energy conversion and energy storage systems. With decades of endeavors, renewable energy-powered electrocatalytic generation of hydrogen from water has been considered a promising next-generation energy portfolio; nevertheless, the mass commercialization of water electrolyzers is severely hindered by relative sluggish kinetics on the anode and the shortened lifetime of the membrane due to reactive oxygen. To this end, biomass oxidation reaction (BOR), which converts biomass substrates into sustainable fuels and value-added chemical products, has been proposed as an effective alternative to replacing oxygen evolution reaction (OER) for increased efficiency of sustainable hydrogen production.

To inhibit the competing OER and realize the much-facilitated energy conversion, an exceedingly lower onset potential for BOR is strongly desired. Given the tremendous advances in material development, ultrathin two-dimensional (2D) layered double hydroxides (LDHs) are rapidly emerging as a prominent family of catalytic materials displaying lower onset potential for BOR. This is mainly ascribed to their easily regulated electronic and interfacial structures, abundant accessible active sites, large specific surface areas, and preferable electron transfer. On the other hand, the ultrathin 2D material systems generally offer sufficient active sites to collect strong operando spectroscopy signals, which are inherently beneficial to in-depth catalytic mechanism exploration and active site understanding. Consequently, a series of nickel-based ultrathin LDH nanosheets were innovatively constructed by a simple wet-chemistry synthesis strategy to efficiently convert 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA).

This poster will focus on work in the conversion of HMF to FDCA using ultrathin 2D NiFe LDHs and Jahn-Teller distorted ultrathin 2D NiMn LDHs. In view of abundant oxygen defects, a remarkably enlarged potential window and a high faradic efficiency (FE) of nearly 100% for HMF oxidation are accomplished. The judicious employment of X-ray techniques to build up a comprehensive and in-depth understanding of 2D material-based catalytic systems was highlighted in this poster. Specifically, ex-situ X-ray techniques were implemented to investigate the catalytic materials’ atomic-scale structures and electronic structural characteristics; meanwhile, in-situ X-ray techniques were adopted to track the dynamic structural evolution of catalysts in real-time under diverse operating conditions. This presented work is anticipated to offer new insights into the development and application of ultrathin 2D materials as high-performance electrocatalysts in hybrid water electrolysis systems for efficient sustainable-energy generation and large-scale production of fuels and industrially significant chemicals.

The development of two-dimensional (2D) materials based synaptic devices has been of considerable interest owing to their highly gate-tunable charge transport and efficient charge transfer across van der Waals (vdW) stacks. In this study, we demonstrate a vdW synapse transistor, exclusively built with 2D materials by i) controllably oxidizing the surface of environmentally sensitive HfS₂ to form the HfO_x/HfS₂ floating gate stack and then ii) assembling with other stable channel materials, few-layer MoS₂. For the flash memory device, electrons were injected through tunneling oxide HfO_x, which is uniform and ultrathin native oxide, and trapped in floating gate HfS₂. As a result, we can precisely modulate trapped charge density by controlling the gate voltage level and the device characteristics show large memory window (> 100 V), high on/off ratio (> 10⁶) and good retention performance. Furthermore, HfO_x/HfS₂ floating gate can be additively stacked to enhance the linearity of conductance update as a synaptic emulator. Our work will provide a facile device configuration for efficient neuromorphic computing as well as synaptic transistor.

A family of layered transition metal dichalcogenides (TMDs) is a promising candidate for nanoelectronics and neuromorphic computing owing to their excellent gate-controlled tunability in charge transport. However, despite an explosive amount of research activities, the interplay between intrinsic and extrinsic impurities as well as the effect of surrounding dielectric medium often makes it challenging to further understand the origin of their charge transport characteristics. Here, we report the ambipolar behavior in extrinsically doped TMD based field-effect transistors, abruptly appearing when being thinned down, and further demonstrate its modulation by inserting an interlayer between the substrate and semiconducting channel. When doped TMDs directly contacted with a disordered gate dielectric, ambipolar transport occurs, and it can be understood by the Poole-Frenkel emission and space charge limited current models. The present result indicates that the transport mechanism indicated as the bulk-limited current is another one to interpret and predict further functionalities of ambipolar behavior.

Molybdenum disulfide (MoS₂) is one of the most interesting 2D nanomaterials, due to its very promising catalytic properties, including hydrogen evolution reaction (HER). Addition of highly conductive carbon nanomaterials (CNMs) can enhance its catalytic properties, while addition of tungsten trioxide (WO₃) nanoparticles can influence its photocatalytic response. Combining these materials can significantly affect hydrogen production from water, based on a photoelectrocatalytic reaction.
In this study we propose decoration of MoS₂/CNMs hybrid materials with WO₃ nanoparticles to enhance their photocatalytic properties. MoS₂/CNMs hybrid materials were prepared by depositing MoS₂ nanoparticles on different carbon nanomaterials, such as graphene oxide (GO), reduced graphene oxide (rGO), or carbon nanotubes (CNTs) using an impinging jet reactor. The use of this method allows for continuous production of MoS₂ nanoparticles with repeatable properties and is easily scalable. WO₃ nanoparticles were obtained through microwave-assisted hydrothermal method. Decoration of MoS₂/CNMs hybrid materials with WO₃ nanoparticles was obtained through physical mixing. The physicochemical properties of the obtained materials were analyzed using different techniques, such as: scanning transmission electron microscopy (STEM), Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), particle size analysis by laser diffraction (PSD), and Fourier-transform infrared spectroscopy (FT-IR). The electrochemical measurements, including linear sweep voltammetry (LSV), cyclic voltammetry (CV), and chronoamperometry (CA) were performed in order to evaluate the photoelectrocatalytic and capacitive properties of the obtained materials.
The obtained results show that the catalytic behavior of the prepared materials is dependent on the type of used carbon nanomaterial and the addition of WO₃ nanoparticles is beneficial.
The proposed preparation method is facile and easily scalable, which can influence the commercialization of non-Pt catalysts for hydrogen evolution reaction leading to cheaper and cleaner energy.

Borophene refers to the family of synthetic two-dimensional polymorphs of boron, which has attracted significant attention due to anisotropic metallicity, correlated electron phenomena, and diverse superlattice structures. While borophene heterostructures have recently been realized, ordered chemical modification of borophene has not yet been reported. Here, we synthesize ‘borophane’ polymorphs by hydrogenating borophene with atomic hydrogen in ultrahigh vacuum [1]. Through atomic-scale imaging, spectroscopy, and first principles calculations, the most prevalent borophane polymorph is shown to possess a combination of two-center-two-electron boron-hydrogen and three-center-two-electron boron-hydrogen-boron bonds. Borophane polymorphs are metallic with modified local work functions, and can be reversibly returned to pristine borophene via thermal desorption of hydrogen. Hydrogenation also provides chemical passivation as borophane is shown to suppress oxidation for multiple days following ambient exposure.
Acknowledgement:
This work was supported by the Office of Naval Research (ONR N00014-17-1-2993) and the National Science Foundation Materials Research Science and Engineering Center (NSF DMR-1720139).
References:
[1] Qiucheng Li, Venkata Surya Chaitanya Kolluru, Matthew S. Rahn, Eric Schwenker, Shaowei Li, Richard G. Hennig, Pierre Darancet, Maria K. Y. Chan, Mark C. Hersam. Synthesis of borophane polymorphs through hydrogenation of borophene. Science, 2021, 371 (6534), 1143.

Transition metal dichalcogenides (TMDs) have attracted tremendous attention due to their unique optical and electronic properties. In particular, chemical vapor deposition (CVD) growth technique has provided an opportunity for TMDs to take a step forward for use in large device applications. In previous studies, an understanding of the exciton dynamics in TMDs has been considered important in fabricating and utilizing them in device structures, so several research groups have studied exciton behavior in TMDs such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) depending on laser irradiation. However, despite of the same compositional TMDs, they observed different aspects in the photoluminesence (PL) results, and the mechanism of this is still under debate. In this work, we investigated the exciton behavior of CVD-grown monolayer WS₂ under laser irradiation. The PL spectra of the as-grown and transferred WS₂ flakes were obtained using a confocal PL system with 532 nm excitation. As the laser irradiation time increased, both the blue-shifted PL peak and the red-shifted PL peak were observed at different positions on the same as-grown WS₂ flake. On the other hand, this complex behavior was not noticeable in the transferred WS₂ flake, and only the blue-shifted PL peak was clearly observed. Based on the laser power-dependent PL data, we will discuss how strain, laser heating, and vacancies affect the PL trend of TMDs before and after the transfer process.

A high-performance memristive device is a key component in neuromorphic computing. Conventional memristors based on metal oxides such as TiO_x can only mimic a single characteristic of neuromorphic layers and thus, it does not allow for the multi-terminal state-switching control which enables to simulate diverse neuron-synapse signal transfer processes more realistically. Recently, two-dimensional (2D) nanomaterials such as layered molybdenum disulfides (MoS₂) have drawn a great attention in simulating a dynamic process of neuromorphic computing with their superior and unique properties. Here, we demonstrate two 2D nanomaterial systems for developing memristive devices with various performances: h-BN/graphene/h-BN heterostructure system and MoS₂ system with a nano-gap. First, van der Waals h-BN/graphene/h-BN with interfacial charged impurities enabled the controllable carrier charge trap and release resulting in the gate bias-dependent hysteresis of electrical transfer curves. The trapped charge density of h-BN/graphene/h-BN field-effect transistors can be easily modulated by the magnitude and the polarity of the applied gate bias. The ‘set’ process (trap) can be achieved with applying a negative gate bias while the ‘reset’ process (release) can be done by applying a positive gate bias, showing the retention performance with the stable ratio of the currents for the set to the reset, and both the states were maintained more than few days. Secondly, to develop a high-performance memristive device, we demonstrate a MoS₂ nano-gap structure for mimicking actual human brain synapse structures. Completely separated MoS₂ flakes maintain high resistance state (HRS) but current tunneling occurred through the nano-gap by the field-emission of ions above a certain gate voltage. Continuous application of drain-source voltage formed the ion bridge at the nano-gap, as revealed by electrostatic force microscopy (EFM), and the bridge electrically connected each flake, significantly increasing the drain-source current, which can be a ‘set’ (low-resistance state, LRS) process. This study provides a fundamental understanding for how 2D nanomaterials can be utilized to design and develop various performance and characteristics of memristive devices for more realistically mimicking neuron-synapse signal transfer mechanisms.

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the charge transport across the interface is a critical factor to determine the device performance. Even though much effort has been recently made to improve the generation and dissociation of excitons in 2D semiconductors, effective strategies that can enhance charge extraction at the contact interface have rarely been explored. Here, we report an unexplored approach to boost the optoelectronic device performances of 2D heterojunctions via the phase-transition-induced modulation of the interface band alignment. In the proposed device, the atomically thin WO_x layer, which is monolithically formed by layer-by-layer oxidation of WSe₂, is used as a hole transport layer. When the WO_x interlayer was introduced at the semiconductor/electrode interface, the power conversion efficiency of the WSe₂-MoS₂ p-n junction devices increased by about an order of magnitudes, from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment resulting from the monolithic phase transition, as confirmed by band alignment analyses and supported by first-principle calculations. By further optimizing the electron transport of p-n junction devices, we achieved high V_OC of ~0.8 V approaching to the bandgap and high PCE of ~7 %. Our work suggests a new route to achieve band engineering in the heterostructures toward realizing high-performance 2D optoelectronics.

Two-dimensional transition metal dichalcogenides (2D TMDs) play a significant role in semiconductor industry to yield high performance, flexible, and light weight miniaturized devices. However, their ultra-thin thickness makes them susceptible to environmental changes, which might deteriorate the device performance. This may pose a problem in their use for space science applications which makes a study on the change in their properties due to environmental perturbations crucial. In this work, we have studied the effect of gamma radiation on the structural, morphological, chemical, electrical, and optical properties of large area monolayer WS₂ film synthesized by chemical vapor deposition on sapphire substrate. The film was irradiated with 1, 10, 50, 100, 200, and 400 kGy radiation dose. Structural and morphological changes in the irradiated film were observed from the optical microscopic images and atomic force microscopy. No significant change was observed up to 10 kGy, which indicate high radiation tolerance of our film, but from 50 kGy onwards, cracks were observed, and the film almost disappeared at 400 kGy. This might be due to the strain induced by radiation due to the difference in thermal expansion coefficient of sapphire and the film. AFM images showed increased surface roughness. Furthermore, gamma ray introduces sulfur vacancies in WS₂. Their presence was confirmed from the XPS spectra by the presence of W⁺⁶ peak which is the characteristic peak for WO₃. Due to missing S-W bond, tungsten atom may get oxidized and form WO₃. The peak was found to be intense at higher radiation fluence. The presence of sulfur vacancies was further confirmed from the Raman spectra by the occurrence of peak characteristic to WO₃. Raman spectra of WS₂ mainly exhibits two vibrational modes: E¹_2g (Γ) mode, which represents in-plane vibration of tungsten and sulfur atoms, and A_1g (Γ) mode, which represents out of plane vibration of sulfur atoms with respect to tungsten atoms. An increase in intensity ratio of A_1g /E¹_2g, and red shift in peak corresponding to A_1g mode was observed. This can be explained from the created sulfur vacancies. The vacancies create a missing S-W bond which reduce the restoring force and thereby increase intensity of A_1g peak and shifts it to a comparatively higher wave number. Also, PL spectra showed increased FWHM, intensity and reduced bandgap at high radiation dose. A red shift in excitonic peaks along with increased absorbance was also observed from ultraviolet-visible absorption spectra. Sulfur vacancies can form mid-gap/ interface states in WS₂ which can effectively reduce its band gap. The increased absorbance and broader FWHM can again be attributed to the surface defects and decreased film crystallinity.

In this study, synthesis of large area few layer two-dimensional (2D) SnS has been achieved on SiO₂/Si substrate using pulsed laser deposition technique. XRD and Raman spectra show the formation of pure SnS phase having a layered nature. Atomic force microscopy (AFM) and High resolution transmission electron microscope (HRTEM) reveal the morphological and structural characteristics of the film. XPS measurements gives the chemical state and composition of the as-grown film. Further, SnS film was uniformly decorated with palladium (Pd) nanoparticles (size 5-100 nm) using gas phase synthesis technique which indued n-type doping. Pristine SnS is highly sensitive and selective towards NO₂ gas at room temperature with limit of detection (LOD) of 100 ppb. Whereas, Pd-SnS film showed high response and selectivity towards H₂ gas with LOD of 10 ppm as compared to NO₂ at room temperature. The reason of high response might be ascribed to the large specific area (SA) in 2D SnS film, which is further enhanced with Pd nanoparticle decoration. Selectivity towards H₂ can be explained with n-type doping and surface reaction of Pd to form PdH.

Two-dimensional (2D) semiconductors have emerged as a promising material for low-power and high-performance electronics because of the intrinsic atomic thickness and the exceptional properties maintaining even with ultimate scaling. Besides, the competitive ability to electrostatically control the electrochemical potential allows us to design band-modulated 2D heterostructures for implementing a variety of gate-tunable electronic devices. Such a unique capability of 2D materials can also offer great potential for realizing an energy-efficient artificial synapse with high controllability. Nevertheless, the artificial synapse utilizing functionally unique properties has rarely been demonstrated, as appropriate materials and structures with robust memristive switching characteristics and an adequately integrated device architecture are not available.
Here, we report a functionally advanced artificial synaptic architecture, a three-terminal device consisting of a defect-controlled molybdenum disulfide (MoS₂) memristor on hexagonal boron nitride (h-BN), termed as a ‘weight tunable memristor’. Through the precise defect control of MoS₂ channel, the device exhibits low power switching phenomena even without applying gate voltages, which cannot be implemented in previously reported memtransistors utilizing gate dielectric as a pre-synaptic component. One more step, owing to the electrostatically controlled space charge limited current in the ultrathin channel, the device exhibits gate-controlled memristive switching characteristics. The device can implement essential synaptic characteristics, such as short-term plasticity and long-term plasticity. Notably, by electrostatic tuning with a gate terminal, we can additionally regulate the degree and tuning rate of the synaptic weight independent of the programming impulses from source and drain terminals, with sub-1 FJ pulse input. Moreover, the changed states are within stable region for 1500 consecutive pulses. These capabilities eventually enable the accelerated consolidation and conversion of synaptic plasticity, functionally analogous to the synapse with an additional neuromodulator in biological neural networks. Furthermore, such acceleration improves the recognizing accuracy and reduces learning step in MNIST pattern recognition, with considerable power-saving benefits. Our demonstration represents an important step toward highly networked and energy-efficient neuromorphic electronics.

Deterministic creation of quantum emitters in 2D materials is an important challenge in exploring their potential for quantum photonic devices. One of the promising approaches exploits ion beam irradiation for controlled defect engineering. However, the types of defects generated and their corresponding optical properties are not well understood. In this study, we investigate the electronic properties of defect states in monolayer MoSe₂ created by proton beam irradiation. Proton beam irradiation results in emergence of narrow subgap emission peaks, which are spectrally and thermally stable (up to 160 K). We probe the electronic origin of these peaks by studying the effect of electrostatic doping and their valley selectivity. We find that these peaks exhibit modest degree of circular dichroism, which suggests that the transitions involve valley-polarized holes, in accordance with the recent theoretical predictions for selenium-vacancy-induced optical transitions in MoSe₂. Our scanning transmission electron microscopy (STEM) imaging reveals that proton beam irradiation results in increase in selenium vacancies but no changes in metal vacancy concentration, further corroborating the role of unpassivated selenium vacancies in the observed emission.

Molybdenum disulfide (MoS₂) is an emerging 2D nanomaterial with unique properties. It has been found as a very promising catalyst for several reactions, including hydrogen evolution reaction (HER).
In this study, a novel, facile and low-cost method for preparing MoS₂-based materials is proposed. MoS₂ nanoparticles were obtained via wet chemical precipitation. The direct product of the synthesis was amorphous MoS₂. Further purification and annealing needed to be performed to get a high-quality product. This method allowed obtaining MoS₂ in an amorphous phase characterized by a high density of exposed sites ^1,2.
Moreover, to activate the basal plane, promote its charge transport and catalytic activity, carbon nanomaterials (CNMs) were proposed. For the synthesis of hybrid nanostructures MoS₂/CNMs, various carbon nanomaterials, such as graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotubes (CNTs) were used. The heterogeneous nucleation resulted in the production of carbon nanomaterials coated with heavily bonded MoS₂. The synthesis of hybrid nanostructures was conducted in the continuous flow reactor, i.e., impinging jet reactor. This type of reactor allows for continuous production and easy scalability. In addition, nanosized MoS2 is a p-type semiconductor with a bandgap in the range of 1.2–1.8 eV. To improve the light response-ability, an alternative semiconductor - titanium dioxide (TiO₂) was added to MoS₂/CNMs by mixing methods. TiO₂ nanoparticles were obtained via the hydrothermal method.
A thorough analysis of obtained materials was conducted, using analytical techniques, such as thermogravimetric analysis (TGA), X-ray diffraction (XRD), particle size analysis by laser diffraction (PSD), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, and scanning transmission electron microscopy (STEM). To check the catalytic potential towards HER, linear sweep voltammetry and cyclic voltammetry were conducted. The photocatalytic ability was measured using a photoelectrochemical cell.
The molybdenum disulfide nanoparticles were deposited on the carbon nanomaterials surface with success. Annealing at a lower temperature of 550 °C allowed for obtaining the mostly amorphous form, which was confirmed by XRD analysis. FT-IR analysis of MoS₂/CNMs revealed characteristic absorption peaks for carbon nanomaterials. However, their reduction may have occurred during annealing. The hybrid nanostructures exhibit smaller particle sizes and narrower particle size distributions, which was confirmed by STEM and PSD analysis. The above results may have a significant influence on the improvement of the catalytic properties of MoS₂. Furthermore, the proposed method of producing hybrid nanostructures will allow their production on a larger scale, thus facilitating their commercialization. The obtained materials exhibited superior catalytic activity due to the exposed active sites of amorphous MoS₂, the enhanced photocatalytic response by TiO₂, nanometric sizes, good particle dispersion, and enhanced charge transport.

1. Wojtalik, M., Bojarska, Z. & Makowski, L. Experimental studies on the chemical wet synthesis for obtaining high-quality MoS2 nanoparticles using impinging jet reactor. Journal of Solid State Chemistry (2020). doi:10.1016/j.jssc.2020.121254
2. Bojarska, Z., Mazurkiewicz-Pawlicka, M., Gierlotka, S. & Makowski, L. Production and properties of molybdenum disulfide/graphene oxide hybrid nanostructures for catalytic applications. Nanomaterials (2020). doi:10.3390/nano10091865

Doping in two-dimensional (2D) semiconductors while suppressing Coulomb scattering is essential to achieve high-performance electronic devices. Despite recent advances in the fabrication of artificial van der Waals heterostructures and their electronic device applications, modulation doping in 2D semiconductors has not been demonstrated so far. Typically, these atomically thin materials can be easily doped by simple molecular charge transfer methods. However, the ionized dopants generated on the surface are present in close proximity to the atomically thin channel and thereby can significantly hamper the charge transport by Coulomb interaction, leading to the charged impurity scattering. Here, we demonstrate a modulation doping in the WSe₂/h-BN/MoS₂ heterostructure that can diminish the doping-induced scattering, in which electrons in the underlying MoS₂ channel doped by remote charge transfer are spatially separated from molecular dopants on the WSe₂ surface. Remarkably, as decreasing the temperature, μ of the modulation-doped (MD) device monotonically increases and begins to saturate below 100 K, whereas, for the direct-doped (DD) device, μ peaks around 200 K and gradually decreases as further decreasing the temperature. Such a decreasing behavior of μ at low temperature is not observed in the case of the undoped samples. In particular, the MD device exhibits over an order of magnitude enhancement in μ at 10 K from 63 to 1,100 cm²V^-1s^-1 compared with the DD counterpart. These results could be explained by the reduction of the dopant-induced charge impurity scattering.

Hydrogen evolution reaction (HER) by electrocatalytic water splitting is an environmentally friendly method to produce hydrogen. Layered transition metal dichalcogenides (TMDCs) have received considerable attention as a promising HER catalyst because of high specific surface area and catalytically active edge sites. However, their catalytically inactive basal planes hinder HER performance. To overcome this problem, an efficient strategy is using Pt/TMDC hybrid nanocatalyst in which TMDC acts not only as the support of dispersed Pt but also as HER catalyst itself. Although There have been the efforts to synthesize better Pt/TMDC (Pt/MoS₂, Pt/WS₂) based HER catalyst, Pt/ReS₂ has not been studied yet. In this study, we developed edge enriched ReS₂ supported Pt nanocluster catalysts. ReS₂ nanoparticles are synthesized by same facile quick microwave assisted solvothermal method. Pt nanoclusters are dispersed on TMDCs by simple wet impregnation method. The optimized Pt/ReS₂ (400 °C Ar, 10 wt%) exhibits an excellent electrocatalytic activity with low overpotential (20 mV at 10 mA cm^-2) and small Tafel slope (38 mV dec^-1). This activity is better to that of commercial Pt-C catalyst for less Pt loading. Also, we demonstrate the Pt-ReS₂ interaction and its effect on electrochemical HER activity.

2D magnetic materials and their heterostructures are promising for next-generation spintronic and magneto-optical devices. These technological applications require the large-area synthesis/exfoliation and characterization of single crystalline ultra-thin materials. Recently the family of Fe-based 2D magnetic materials has been fabricated in wafer-scale by molecular beam epitaxy. These 2D materials exhibit Curie temperatures at or above room temperature along with various magnetic effects. To investigate the nuclear and magnetic structure of these large-area ultra-thin materials, neutron reflectivity measurement can be useful because of its large beam size, high penetration depth, and magnetic sensitivity. This can be achieved by transferring 2D van der Waals materials on top of the substrate with additional layers, which amplify the reflectivity from ultra-thin materials. In this work, both molecular-beam epitaxy and neutron reflectometry for large-area 2D magnetic materials will be presented.

For practical applications with industry-compatible complementary metal oxide semiconductor (CMOS) technology, one of the main challenges is the development of a technique with carrier type modulation. Although a variety of material candidates have emerged, oxide materials and carbon nanotube have a limitation with having p-type and n-type properties, respectively. Herein, we propose a facile route to achieve wafer-scaled p-type MoS₂ and n-type MoS₂ films that can be universally applied to produce 2D materials. The use of pre-deposited films on substrate and gas-phase precursors of H₂S allow us to obtain layer-controlled and uniformly thin films. Remarkably, our method requires a significantly shorter time to achieve a full coverage of 2-inched 1L MoS₂ compared with previous studies. Niobium (Nb) is selected as a dopant for electronic state modulated MoS_2. Various experiments have verified qualities of grown samples including thickness-dependent electrical behavior of doped TMD, and especially an innovative TEM analysis has confirmed an existence of dopants regardless of an atomic size.

Low resistance metal contact is critical for the development of electronics based on transition metal dichalconide (TMD) materials. ReS₂ is a semiconducting TMD with an atomic structure similar to that of MoS₂. While most TMDs grow parallel to the substrate when synthesized using chemical vapor deposition (CVD), ReS₂ has the tendency to orient itself vertically during growth. Such films drastically increases the surface area and exposes chemically active edges, making vertically-oriented ReS₂ an attractive material for energy and sensor applications. However, the contact resistances of vertically grown materials are known to be relatively high, compared to those of common flat TMDs, such as MoS₂. Most reported methods for lowering the contact resistance have been focused on exfoliated TMDs with only a few devices tested, and few works on flower-like T-phase materials exist. Moreover, nearly all reported studies have been conducted on only a few devices with exfoliated flake samples.
In this work, we investigated the effects of the metal-insulator-semiconductor (MIS) tunneling contact for ReS₂, comparing different fabrication approaches and materials. Most of the previous contact studies used mechanically exfoliated samples with small sample sizes (a few μm²). With such a small area, there would be only a few devices and an appropriate statistical study on the relationship between the dielectric thickness to the Schottky barrier height would not be attainable. In our analysis, over a hundred devices were fabricated and measured for a large scale statistical study. An etching-free transfer method was also employed to preserve the original T-phase structure of the high-quality film. Moreover, the Schottky barrier height of the CVD-grown ReS₂ contacts was modified by conformally coating MIS layer between the metal and the dendritic ReS₂ film using atomic layer deposition (ALD). Importantly, we compare various techniques and materials for lowering metal electrode contact resistance and found an optimized technique. We found that the reductions in barrier height were directly correlated with exponential reductions in contact resistance and increases in on-current by almost two orders of magnitude. This research was supported by National Research Foundation of Korea (NRF) grant no. 2018R1A6A1A03025708 and grant no. 2017R1C1B2012227.

Two-dimensional (2D) materials have recently attracted significant attention in electronic devices applications due to their unique electrical and optical properties. Among 2D materials, graphene, black phosphorus (BP), and transition metal dichalcogenides (TMDs) have been actively studied in various ways. Along with such attention, germanium sulfide (GeS) has also received attention as it has a similar structure to BP. Furthermore, it is worth noting that defects in 2D materials play a fundamental role in the variation of their electronic properties because they can be donors or acceptors.
This study investigates the effects of vacancies and substitutional atoms in monolayer GeS using density functional theory calculations. We choose group IV or chalcogen atoms as substitutional ones which substitute for Ge or S in GeS. It is found that the band gap of GeS with substitutional atoms is close to that of pristine GeS, while the band gap of GeS with Ge or S vacancies is much smaller than that of pristine GeS. In terms of formation energy, monolayer GeS with Ge vacancies is more stable than that with S vacancies, and notably GeS with Ge substituted with Sn is most favorable within the range of chemical potential considered. Defects affect the piezoelectric properties depending on vacancies or substitutional atoms. As a result, it is concluded Sn can effectively heal Ge vacancy in GeS, keeping high piezoelectric strain coefficients.

Water electrolysis and photoelectrochemical water splitting provide a means to generate energy in the form of H₂. This well-studied electrolysis reaction is comprised of two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Though the promise of converting water into energy is of obvious technological benefit, water electrolysis as its pitfalls. For optimal conversion, chemically pure alkaline or acidic water is desired to mitigate unwanted side reactions, limiting use in areas where clean water is readily abundant. Even more detrimental is the required overpotential needed for the OER reaction, which increases the power needed solely for the purpose of generation ubiquitous O₂ to complete total water electrolysis.

To circumvent these issues with sustainable H₂ generation from water, our research group has recently began focusing on using biomass and wastewater sources for water electrolysis. By creating catalysts that are stabile under wastewater-like conditions, were further maximize H₂ generation by replacing OER with electrocatalytic oxidation of the organic compounds that occur at much lower overpotentials. Thus, H₂ is generated via HER with the H⁺ obtained from the oxidized organic, providing a means to also generated value added chemicals instead of O₂. For this construct to successful work, innovated catalysts design approaches must be performed in tandem with fundamental structural studies at the atomic scale to develop a methodology to create catalyst for a wide variety of wastewater sources.

This talk will focus on our groups recent work in implementing pilot scale electrolysis cells for wastewater electrolysis coupled with fundamental studies using synchrotron radiation capabilities to better understand structure/function relationships in our catalysts. An emphasis will be made on our efforts using in-situ X-ray techniques to monitor structural changes of catalytic materials under reaction conditions in this talk to showcase the needed to correlate changes at the atomic scale with catalytic properties. The presented work is expected to find applications in clean energy generation and customized commodity chemical reactivity.

Two-dimensional hybrid perovskites, building on the merits of hybrid perovskites and 2D materials, should offer exciting opportunities for designing materials properties and devices functionalities. Rich engineering of 2D RP perovskites has been demonstrated by many different routes. It creates a way to study the ion migrations between different layers by building proper vertical heterostructures. However, 2D RP perovskites are not the proper platform to study the interlayer charge transfer when incorporated with other 2D materials, because of the separation of ligand spacers. Herein, we successfully apply a solution conversion method to obtain ultrathin 2D ABX₃ (MAPbI₃, FAPbI_3, and MAPbBr₃) perovskites by changing the reaction precursors and composition of the solution. In this method, XRD results prove the new perovskites belong to the tetragonal phase and have high quality. The samples' thickness is usually lower than 15 nm, and the thinnest one is only about 2 nm. Last but not least, the new perovskite nanolayers can be incorporated in a van der Waals stacking junction with other 2D materials and themselves. For instance, we have finished covering a single-layer WS₂ on MAPbI₃ or FAPbI₃ nanocrystal, and they both give a type-II semiconductor heterojunction. Even, homojunction, including MAPbBr₃/MAPbBr₃ and MAPbI₃/MAPbI₃, with different twist angles are successfully realized, and moire patterns could be observed using TEM. The ability of our strategy to obtain bare optically efficient 2D ABX₃ perovskites as building blocks for vertical junction creates much more possibilities for new designs and functionalities.

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.

Chemical and biological sensing using 2D nanomaterials has been an area of intense investigation including inorganic materials that exhibit high sensitivity, however it has been limited as a field due to the lack of selectivity among analytes. While 2D inorganic materials, such as transition metal dichalcogenides (TMD), can readily decipher between donor/acceptor groups, strategies to incorporate selectivity in these devices is often challenging. Organic 2D materials such as covalent organic frameworks (COFs), known for their highly crystalline repeated organic moieties, have been shown to act as selectivity agents due to their functionalization and porosity. This research focuses on the association of COFs with 2D nanomaterials using top-down microwave-assisted activation of TMDs and deposition of COF nanoparticles creating coupled heterostructures. Several known TMD and COF species have been examined to understand the fundamental surface interactions between 2D nanomaterials and few layer COFs, this unique methodology allows the study of mono- to few layer properties of both the organic and inorganic materials simultaneously. Kinetics and mechanistic studies were used to determine the manner of association of the inorganic and organic substrates. Photoluminescence and electronic measurement studies demonstrate a microwave power dependent electronic coupling of the inorganic and organic materials in these highly tunable hybrid materials.

Seok Joon Yun, Jin Bao Jiang, Dinh Loc Duong, and Young Hee Lee^1
1 IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 16419, Korea
Email: leeyoung@skku.edu
The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI₃ and CrGeTe₃ semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe₂ is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~10⁵ at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron-microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. We also observe a ferromagnetic hysteresis and circularly polarized light with spin-up and spin-down excitations at room temperature in diluted V-doped WSe₂, while maintaining the semiconducting characteristic of WSe₂ with a high on/off current ratio of five orders of magnitude. We further demonstrate that magnetic order in V-doped WSe₂ is enhanced by increasing Se vacancy concentration. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.

Ferromagnetic two-dimensional materials such as Fe₃GeTe₂ have attracted great interest due to their potential applications in ultra-compact spintronic devices. Recently, we have demonstrated the synthesis of large-area Fe₃GeTe₂/graphene heterostructures with high structural and interface quality by employing molecular beam epitaxy (MBE). In order to increase the Curie temperature from about 220 K towards room temperature, one promising approach reported in the literature is the synthesis of metal-rich Fe_3+xGeTe₂ (FGT) films. However, in these studies, micrometer-sized flakes exfoliated from bulk crystals have been studied. In this work, we report on electrical and magnetic properties of large-area FGT films with different chemical compositions.
The van der Waals epitaxy of FGT films with 10 to 20 nm thickness was accomplished via MBE at a substrate temperature around 300 °C. Epitaxial graphene on 4H-SiC (0001) was used as a substrate. Morphological and structural characterization performed with different methods including synchrotron-based, grazing-incidence X-ray diffraction confirmed the formation of continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality.
From temperature-dependent resistivity measurements, metallic transport behavior with positive temperature coefficient has been revealed for all FGT films with various chemical compositions. The magnetic characteristics of the films was investigated by the analysis of magnetotransport measurements, for which a large-area van der Pauw geometry was used. At low temperatures, the transverse resistance measured during subsequent downward and upward sweeps of an out-of-plane external magnetic field exhibits a clear square-shape hysteresis loop for all samples. This observation provides evidence for the occurrence of the anomalous Hall effect (AHE) and, consequently, the ferromagnetic order in the FGT films with a strong perpendicular anisotropy. As expected, the AHE of each individual film vanishes with increasing temperature. At the same time, the coercive field (H_C) extracted from the hysteresis loops decreases with increasing temperature, which can be explained by a uniform, long-range ferromagnetic order with uniaxial anisotropy described by a single magnetic domain. Most importantly, the Curie temperature of the FGT films extracted from the temperature dependence of H_C is found to increase with increasing Fe content. In fact, the FGT films with the highest Fe content remains ferromagnetic above room temperature. Information on the dominant scattering mechanism responsible for the AHE is gained from the relationship between the AHE conductivity (σ_xy) and the longitudinal conductivity (σ_xx). At low temperatures, σ_xy does not depend on σ_xx, which indicates that the AHE is dominated by an intrinsic mechanism, whereas extrinsic mechanisms are found to become more important at elevated temperatures. However, from the viewpoint of spintronic applications, the metallic transport behavior, the robust perpendicular magnetic anisotropy, and the room temperature ferromagnetism make metal-rich, large-area FGT films grown by MBE extremely promising for device fabrication.

Chromium triiodide (CrI₃) was the first layered van der Waals material to demonstrate the persistence of a sizable ferromagnetic hysteresis down to the monolayer [1]. This observation was accompanied by a striking transition from a layered ferromagnetic to a layered antiferromagnetic order as the number of layers of the crystal approached the two-dimensional limit [2]. The discovery of layered-dependent magnetism in CrI₃ motivated a wealth of fundamental studies seeking the origin of this phenomenon. Although this effect can be underpinned by the differences of the spin superexchange coupling in the distinct layer stacking alignments found in bulk and few-layer cystals [3, 4], CrI₃ still hosts deep conundrums. First and foremost, a clear explanation of why bulk CrI₃ exhibits a layer-stacking structural transition to a rhombohedral low-temperature phase whereas few-layer samples do not is still missing. Secondly, the exact crossover point in terms of the number of layers appears to be, surprisingly, in the mesoscopic scale, but is currently unknown. In addition, there exists other controversies regarding the presence of a peak at 50 K in the in-plane magnetization curve [5] and the coexistence of different structural phases in bulk single crystals of this material [6]. In this talk, I will provide experimental insights regarding the magnetic states and crystalline phases of CrI₃ beyond the reported monoclinic/rhombohedral stacking dichotomy, showcasing a far more intricate scenario than the one currently understood.
References
[1] B. Huang et al. Nature, 546, 270, 2017.
[2] D. R. Klein et al. Science, 360, 1218, 2018.
[3] N. Sivadas et al. Nano Lett. 18, 7658, 2018.
[4] Z. Sun et al. Nature, 572, 497, 2019.
[5] Z. Wang et al. Nat. Comm. 9, 2516, 2018.
[6] X. Guo et al. ACS Nano, 15, 10444, 2021.

In the last years 2D materials have attracted a huge attention thanks to the amazing properties that arise when thickness approaches the single layer level and thanks to the large number of functionalities that they offer. Recently, they started to appear as high potential candidates for spintronics devices since, thanks to their intrinsic two-dimensional nature, they create thin, tuneable and free of defects barriers with sharp interfaces. Despite this potential, investigation of 2D materials for spintronics is still at the beginning. Only very recently few works started to appear using other 2D materials beyond graphene [1,2,3]. In particular, concerning the family of transition metal dichalcogenides (TMDCs), promising theoretical calculations were first reported predicting an extremely large magnetoresistance (MR) signal up to 300% in Fe/MoS₂/Fe MTJs [4]. However, experimental results are still far from these expectations. Indeed, most of experimental works have to face the main technological issue of avoiding bottom FM electrode oxidation during fabrication process, limiting in turn MR signals.
Here we will show an in-situ fabrication approach to avoid oxidation of the FM electrodes during the device fabrication process that tackles this fundamental problem. We will start reporting the successful fabrication of MoS₂-based MTJs, investigated as prototypical 2D semiconductive material, then showing perspective on the successful incorporation of more complex, air instable, 2D semiconductors as Black Phosphorous (BP).
The NiFe/MoS₂/Co MTJs with multilayer MoS₂ mechanically exfoliated flakes we report show the highest results obtained so far for MoS₂-based spin valve devices [5], thus confirming that our in-situ fabrication protocol allows maintaining high quality non-oxidized interfaces between the ferromagnetic electrodes and the 2D layer.
Beyond interfaces and material quality, we also suggest that an overlooked more fundamental physics issue related to spin-current depolarization could explain the limited MR observed so far in these magnetic tunnel junctions [5,6]. This points to a path towards the observation of larger spin signals in line with theoretical predictions above 100%. We show the impact of our work to be beyond MoS₂ and its broader transition-metal dichalcogenides family by opening the way towards observation of higher magnetoresistance signals and an accelerated screening of other 2D materials that are yet to be explored for spintronics.
[1] W. Wang, A. Narayan, L. Tang et al, Nano Lett., Vol. 15, p. 5261–5267, (2015).
[2] A. Dankert, P. Pashaei, M. Venkata Kamalakar et al., ACS Nano, Vol. 11, p. 6389–6395, (2017).
[3] W.C. Wong, S. M. Ng, H. F. Wong et al., IEEE Trans. Magn., Vol. 53, p.1600205 (2017).
[4] K. Dolui et al., Phys. Rev. B, Vol. 90, p. 041401(R), (2014).
[5] M. Galbiati et al., Phys. Rev. Appl. Vol. 12, p. 044022, (2019).
[6] M. Galbiati et al., ACS Appl. Mater. Interfaces, 10, 30017 (2018).

Research into new methods to control magnetic spin textures has been largely focused on
technological applications. However, complex magnets, particularly those characterized by
magnetic frustration, offer opportunities for ultra-low power applications. This is due to two
main characteristics of such systems: 1) the nearly degenerate phase space of magnetic states
available to such systems, and 2) novel magneto-electric coupling available to these materials.
In this work, we illustrate unusual mechanisms for spin-transfer are available to such strongly
correlated systems, revealing potential opportunities for applications of quantum materials.

Defects are inevitable in materials. Often they are undesired entities that reduce performance. Here, we describe defects in 2D materials and how they are essential for emerging applications. The discovery of memory effect in 2D atomically-thin nanomaterials is based on local defects such as vacancies or extended defects such as grain boundaries. These defects are in general reconfigurable by the provision of electrical or thermal energy. Non-volatile memory effect based on these defects has now been observed in over a dozen 2D materials including monolayers, multilayers and heterostructures. It is a rapidly advancing subject with rich physics and several applications. On this occasion, we will describe the basic mechanism of reconfigurable switching of localized vacancy defects in monolayer 2D materials. Furthermore, we will highlight examples of diverse applications including zero-power devices, non-volatile radio-frequency switches, and memristors for neuromorphic computing.

Ferroelectric Field Effect Transistors (Fe-FETs) are used as Nonvolatile memory due to the spontaneous polarization of ferroelectric properties, maintaining two stable states even without an external electric field. However, conventional Fe-FETs have several limitations, such as relatively short retention time, small memory window, and scalability. Here, we report all ferroelectric field-effect transistor (Fe²-FET) using 2-dimensional Van der Waals Ferroelectric materials as a gate dielectric (CuInP₂S₆) and a channel (α-In₂Se₃) to overcome the limitation of conventional Fe-FETs, along with improved device performances.
Fabricated Fe²-FET exhibits 14.47 V of wide memory window in sweep range of -10 V to 10 V (72% ratio of memory window ratio to sweep range), demonstrating excellent data-storage capability compared to previously reported Fe-FET devices. Moreover, our Fe²-FET exhibits a high on/off current ratio over 10⁶, long data retention, and stable cyclic endurance. Furthermore, we extend the Landau-Khalatnikov theory, which only considered the relationship between ferroelectric and dielectric, to the relationship between two ferroelectrics. Our theoretical analysis explains a mechanism of much enhanced ferroelectric properties of the integrated structure of Ferroelectric dielectric/semiconductor, resulting in wider hysteresis characteristics. Experimental characterizations, including PFM (Piezoelectric Force Microscopy), are provided. Finally, conductance modulation and short-/long-term plasticity characteristics are demonstrated by synaptic pulse measurement. It reveals that our Fe²-FET device can mimic the dynamics of a biological synapse. In addition, we offer its applicability to Neural-network through training and inference simulation using the CIFAR-10 dataset.

2D materials have emerged as promising materials due to their attractive properties, such as mechanical, electronic, optical, magnetic, and even topological properties[ref]. In addition, the vast variety of 2D material families, including graphene, h-BN, TMCs, MAXenes, and nanosheet family, and the ability of realizing heterostructures with a high degree of freedom provides us opportunities freeing up the structural design of a device and their material combinations. These attractive properties offer tremendous possibilities to expand the functionalities of 2D material-based devices for various future applications.[1]
Especially, in-memory computing has been gaining attention to overcome current issues in digital technology such as a tremendous increase in computing tasks requiring high energy and area efficiency, since in-memory computing allows us to avoid energy- and time-consuming data movement by using the memory device for computing tasks at the same time. The aforementioned advantages of using 2D materials could make them become a strong contender for in-memory computing and their future applications have been suggested as well.[2]
2D all-inorganic Sr2Nb3O10 (SNO) is considered as a promising candidate for future electronics since they are stable, non-toxic, wide-bandgap layered-nanosheet perovskite, enabling to exploit the advantages of 2D vdW material platform. Additionally, unlikely TMDs, substitutional doping can be easily available for tailoring the material properties. In this presentation, we will report Gr/SNO/Gr vertical heterostructured memristors device with different Ag dopant contents in SNO. We have systematically observed resistive switching behaviors of the 2D heterostructured memristors with different Ag contents exhibiting 4 distinctive states. During the operation of 2D memristors, we found that a high on/off ratio of 10^10, and the low resistance state was found to be very stable. In order to understand their transition dynamics between each state, we conducted voltage pulse measurements with various voltage pulse conditions. In addition, we observed that applied voltage pulses could potentiate and depress the conductance of 2D memristors. In addition, atomic probe tomography was conducted to investigate the origin of resistance switching phenomena.
We believe that our results pave the way for emerging 2D neuromorphic applications with 2D perovskites and will help to understand the micromechanism of the resistive switching phenomena in 2D perovskite-based heterostructured memristor devices.
Ref
[1] K. Cho et al. ACS Nano, 2019, 13, 9, 9713-9734
[2] C. Liu et al. Nat. Nanotechnol. 2020, 15, 545-557

Two-dimensional (2D) materials, such as Graphene, hBN and MoS₂, are promising candidates in a number of advanced functional and structural applications, owing to their exceptional electrical, thermal and mechanical properties. Understanding mechanical properties of 2D materials is critically important for their reliable integration into future electronic, composite and energy storage applications. However, it has been a significant challenge to quantitatively measure mechanical responses of 2D materials, due to technical difficulties in the nanomechanical testing of atomically thin membranes.
In this talk, we will report our recent effort to study fracture behaviours of 2D materials. Our combined experiment and modelling efforts verify the applicability of the classic Griffith theory of brittle fracture to graphene. Strategies on how to improve the fracture resistance in graphene, and the implications of the effects of defects on mechanical properties of other 2D atomic layers will be discussed. More interestingly, stable crack propagation in monolayer 2D h-BN is observed and the corresponding crack resistance curve is obtained for the first time in 2D crystals. Inspired by the asymmetric lattice structure of h-BN, an intrinsic toughening mechanism without loss of high strength is validated based on theoretical efforts. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene.

In this talk, I will present our work on deterministic straining and confinement of excitons in atomically-thin, two-dimensional (2D) semiconductors. More specifically, I will present predictable and reconfigurable strain engineering in atomically-thin WSe2 via three-dimensional (3D) wrinkle architectures. Strain exerted on WSe2 was periodically modulated to tensile and compressive strain at peaks and valleys of the wrinkles, respectively via 3D wrinkle architecture. Furthermore, owing to the deformable nature of the wrinkle architecture, the applied strain can be tuned reconfigurably by post stretching/releasing processes, with PL shift dynamically modulated. We also observed exciton transport across apex and valley of WSe2 as well as exciton localization at the apex due to strain gradient induced energy modulation of wrinkled WSe2. Finally, I will present extension of our strain modulation approach to interlayer excitons in MoS2/WSe2 vertical heterostructures as well as applications towards strained, deterministic single photon emitters.

Classic electro to mechanical gearboxes and actuators are bulky, difficult to miniaturize, and expensive.^1,2 Recent developments have shown that it is possible to utilise two-dimensional materials for lightweight and cheap electrochemical systems.³ Electrochemical actuators based on restacked nanosheets of metallic 1T phase MoS₂ have demonstrated good properties due to the higher electrical conductivity, elastic modulus and fast proton diffusion.⁴ In this study, the effect of different ion intercalation species in metallic MoS₂ restacked nanosheets on the electrochemical actuator performance is investigated. The specific capacity, X-ray diffraction and strain of the actuators with varying group 1 alkali species have been measured. We show the intercalation of hydrogen, sodium and potassium increase the interlayer spacing, as previously confirmed from XRD studies.⁵ However, the actuator displays more strain with sodium than potassium, contrary to the higher specific capacity of potassium compared to sodium. This is due to the solvency numbers of the alkali species which shows that the electrochemical actuation is directly related to the volume expansion of the nanosheets. Further, findings from XRD and Raman spectroscopy confirm that the actuation mechanism of bending is related to the expansion of both interlayer and intralayer spacing from the intercalation of these species. It has been shown that by changing the intercalating species within metallic MoS₂ actuators, it is possible to drastically tune actuator performance for high-strain and high-frequency applications.
¹Liu, A., Liu, H., Yao, B., Xu, W., & Yang, M. (2018). Energy consumption modeling of industrial robot based on simulated power data and parameter identification. Advances in Mechanical Engineering, 10(5), 1687814018773852.
²Jin, H. J., Wang, X. L., Parida, S., Wang, K., Seo, M., & Weissmüller, J. (2010). Nanoporous Au− Pt alloys as large strain electrochemical actuators. Nano letters, 10(1), 187-194.
³Bonaccorso, F., Colombo, L., Yu, G., Stoller, M., Tozzini, V., Ferrari, A. C., ... & Pellegrini, V. (2015). Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science, 347(6217).
⁴Acerce, M., Akdoğan, E. K., & Chhowalla, M. (2017). Metallic molybdenum disulfide nanosheet-based electrochemical actuators. Nature, 549(7672), 370-373.
⁵Acerce, M., Voiry, D., & Chhowalla, M. (2015). Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials. Nature nanotechnology, 10(4), 313-318

Converse flexoelectricity generates a mechanical response under non-zero electric field gradient. Flexoelectric actuators operating based on this effect are promising candidates for many applications owing to their fast response, capability to control the displacement precisely without hysteresis, suitability for vacuum/extreme temperature conditions, and a wide range of material choices. In spite of these advantages, there has been limited studies on flexoelectric actuators, largely because flexoelectric response is negligible in bulk materials. For nanoscale actuators, however, converse-flexoelectric effect is a promising mechanism, as electric field gradient is increased quadratically as the material’s thickness decreases. For this reason, atomically thin two-dimensional (2D) materials, such as molybdenum disulfide (MoS₂), are promising candidates for flexoelectric actuators. Here, we report a flexoelectric actuator based on 2D MoS₂. A monolayer MoS₂ was used as an active layer, while a parylene-C layer with a thickness of 500 nm was used as a supporting material. We also employed a comb-shaped electrode to generate a strong electric field gradient. In response to 40 V AC excitation at ~20 kHz, the actuator shows resonant displacements up to 50 nm. A finite element analysis (FEA) simulation with COMSOL Multiphysics shows that this response is mainly induced by flexoelectric effect. Our 2D MoS₂ actuator outperforms other existing flexoelectric actuators by more than two orders in terms of relative displacement to active layer thickness. Moreover, displacement amplitude varies linearly with applied voltage, allowing precise actuation control. Most importantly, flexoelectric responses are preserved in vacuum/cryogenic environments with an amplitude of actuation (~35 nm) reduced only 30 % at 10 K. This greatly outperforms commercial piezoelectric actuators which experienced a 60 % reduction in actuation at 10 K. We also observed that our flexoelectric actuator can survive 10¹⁰ actuation cycles with less than 12 % of performance fluctuation, regardless of operating temperature. These results demonstrate the superiority of 2D materials based flexoelectric actuators in low temperature/vacuum conditions which will be useful in various extreme conditions such as actuators for space applications.

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 method 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.
In this study, a deionized (DI) water-based solution process was developed for MoS₂ thin film by bottom-up thermolysis method for the first time. This method was relatively easy to handle, safe, and environmentally friendly process. The MoS₂ atomic layers were completed without sulfurization and with a single step of annealing. Thin-film transistors (TFTs) were prepared from the solution-processed MoS₂ on Al₂O₃ and SiO₂ dielectric with thermal evaporated Al source and drain electrodes. Electrohydrodynamic jet printing process for Ag electrodes was also applied to fabricated TFTs. The best result shows an improved mobility of 8.5 cm² V^-1s^-1 and a reasonable on-off ratio of about 1.0×10⁵ with solid output saturation. This new methodology can be applied to multifarious devices and has the potential for scalability in 2D materials.

Thermal switches have gained intense interest recently for enabling dynamic thermal management of electronic devices and batteries that need to function at dramatically varied ambient or operating conditions. However, current approaches have limitations such as the lack of continuous tunability, low switching ratio, low speed, and not being scalable. In this work, a continuously tunable, wide-range, and fast thermal switching approach is proposed and demonstrated using compressible graphene composite foams. Large (~8x) continuous tuning of the thermal resistance is achieved from the uncompressed to the fully compressed state. Environmental chamber experiments show that our variable thermal resistor can precisely stabilize the operating temperature of a heat generating device while the ambient temperature varies continuously by ~10°C or the heat generation rate varies by a factor of 2.7. This thermal device is promising for dynamic control of operating temperatures in battery thermal management, space conditioning, vehicle thermal comfort, and thermal energy storage. In addition, we study the microscopic mechanism behind the dynamic thermal transport using molecular dynamics simulation. Our simulation results also reveal the anisotropy in thermal conduction in compressed foams.

Subthreshold swing (SS) of conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) have been limited to the 60 mV/dec by the fundamental thermionic limitation, so called Boltzmann tyranny. To overcome the limitation, The negative capacitance (NC) field effect transistor using thin film of doped HfO₂-based ferroelectrics has been attracting attentions due to the compatibility of CMOS process, maintaining ferroelectricity under 10 nm thickness, high retention and endurance.^[1-3] In addition, Molybdenum disulfide (MoS₂) of transition metal dichalcogenides has been extensively studied for low-power and high performance transistors as channel layer due to high carrier mobility, thickness dependence of band structure and no surface roughness.^[4-6]
Here, we fabricated a ferroelectric field effect transistor (Fe-FET) using a HfZrO₂ ferroelectric as a gate dielectric and a CVD-grown monolayer MoS₂ as a channel layer. It was directly observed that the drain current changed rapidly in the region where the polarization is switched from I_g-V_g, and the I_on/I_off ratio and the subthreshold swing were observed to change according to the amount of polarization switching. In addition, we fabricated the negative capacitance field effect transistor using the series of HfZrO₂ and Al₂O₃ as a gate dielectric. It showed over 10⁷ I_on/I_off ratio and less than 60 mV/dec subthreshold swing without hysteresis behavior at the low voltage driving (1 V_g).
Reference
1. Min Hyuk Park et al. MRS communications. 8, 3, (2018)
2. Sayeef Salahuddin and Supriyo Datta. Nano Lett. 8, 2, 405-410 (2008)
3. Mengwei Si, et al. Nature Nanotechnology 13, 24-28 (2018)
4. Radisavljevic, B et al, Nature Nanotech. 6, 147-150 (2011)
5. Mak, K et al. Phys. Rev. Lett. 105, 136805 (2010)
6. Li, S.-L et al. Nano Lett. 13 (8), 3546-3552 (2013)
Acknowledgments
This research was supported by the Industrial Strategic Technology Development Program (20006492) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).

With increasing interest in amorphous/mixed-phase (A/MP) materials and their unique applications in energy storage applications, there is a large scope to design nanomaterials with homogeneously disordered structures in terms of anisotropy and defects. These defects act as additional active charge storage channels thereby increasing the capacitance of supercapacitors (SCs), lithium-ion batteries (LiBs), etc. A/MP materials accelerate ionic diffusion by opening more active charge transport channels, hence they exhibit a high rate performance. Additionally, they retain better structural integrity during vigorous charge/discharge cycles because they release the volume strain effectively during the redox reactions.
In this work, we report mixed-phase MoS₂/VS₂ nano-urchins homogeneously grown onto reduced graphene oxide (rGO) matrix (denoted as G-MVS) via a facile hydrothermal approach and its electrochemical performance in an aqueous electrolyte system. G-MVS has a high specific capacitance of 460 F g^-1 (287.5 F cm^-2) at 0.5 A g^-1 and 310 F g^-1 (193.75 F cm^-2) at 10 A g^-1. In the symmetric configuration G-MVS//G-MVS, a high energy density of 49 Wh kg^-1 (67.86 mWh cm^-2) at a power density of 700.25 W kg^-1 was achieved.

Tunneling field effect transistors, or TFETs for short, are designed to overcome an inverse subthreshold slope of 60 mV/dec making them superior to conventional MOS transistors. The drain current does not rely on injecting carriers from the thermically broadened Fermi distribution. Instead, carrier transport only depends on band-to-band tunneling and energy filtering in the Boltzmann regime. By applying individual potentials to buried gate electrodes in close proximity different band configurations along a semiconductor channel region can be adjusted to control tunneling probability. However, to achieve a homogenous field distribution along the channel, fabrication of buried and mutually insolated electrodes is a challenging process. Here, we report on a novel and simple design of buried gate electrodes based on structured Graphene. By exploiting the atomically thin thickness of two-dimensional Graphene, complex planarization processes are avoided. Additionally, we use ozone-enhanced atomic layer deposition for ultra-thin gate dielectrics allowing band transitions to become very narrow. This in turn, increases the band-to-band probability leading to very steep subthreshold slopes as confirmed by finite element simulations. In a proof-of-concept study we demonstrate the feasibility of our design using carbon nanotubes as TFET-channel material while maintaining channel lengths of 1.2 μm. However, our design is versatile and allows integration of any semiconducting 1D- or 2D-material and can be progressively scaled down to sub-100 nm dimensions.

Graphene nanoribbons (GNRs) with atomically precise width have opened new possibilities for next-generation electronic devices due to their tunable physical and electrical properties. While on-surface synthesis of GNRs allows convenient imaging and property study through scanning tunneling microscopy (STM), transferring synthesized GNRs onto non-metallic substrate is challenging, limiting the realization of GNR-based electronic devices. In this work, we present wet chemical synthesis, detailed STM imaging, and electrical characterization through scanning tunneling spectroscopy (STS) of a zigzag-shape GNR, which is expected to have a small energy band gap around 0.5 eV. Compared to large energy band gap GNRs, which may result in poor device performance due to large Schottky barriers and carrier effective masses, zigzag-shape GNRs have potential for high-performance electronic devices. The ribbons were synthesized from polymer precursor graphenized by the Scholl reaction with triflic acid, through 1,2-migrations of aryl groups on the naphthalene rings during the Scholl reaction, with projected short, metallic zigzag edges and long, semiconducting armchair edges. The GNR powders are deposited onto hydrogen passivated p-type silicon substrates by a dry contact transfer technique for STM characterization. High-resolution STM images and STS characterizations provide insights into the geometric structures, as well as electronic structures including energy band gap and spatial distribution of local density of states of zigzag-shape GNRs. STM images exhibit an alternating asymmetric dimerized internal structure, which only shows up on one side along the ribbon and flips to the other side when the ribbon is twisted or flipped. This asymmetric internal structure is uncommon in all forms of GNRs and may be the starting point of electronic applications. The experimental STS data along the dimerized internal structure side of the GNR shows an alternating energy band gap corresponding to internal structure; the STS data across the GNR shows a smaller energy band gap of about 0.5 eV on the side with internal structure and a larger energy band gap about 1.5eV on the other side. Several other aspects of the GNRs are also investigated and analyzed, including the manipulation of ribbon positions using STM tips, and alignment of ribbons with the Si substrate. The reduced band gap of zigzag-shape GNRs allows better transport characteristics and thus higher electrical conductivity, making them promising for device applications.

We demonstrate multilevel long- and short-term memory behavior operating at low voltages, based on van der Waals heterostack n-MoSe₂/n-MoS₂ channel field effect transistors (FETs). Our heterostack memory FET exploited the gate-voltage (V_GS)-induced trapping/detrapping phenomena for Program/Erase functioning, which was maintained for long retention times due to the existence of heterojunction energy barrier between MoS₂ and MoSe₂. More interestingly, trapped electron density was incrementally modulated by the magnitude of a pulsed V_GS, enabling the heterostack device to achieve multilevel long-term memory. For a practical demonstration, five different levels of drain current were visualized with modulated light emissions after our memory FET was integrated to an organic light emitting diode pixel circuit. In addition, our device was applied to a synapse-imitating neuromorphic memory in artificial neural network. We regard our unique heterostack channel FET to be a novel and promising electron device undertaking multifunctional operations related to upcoming fourth industrial revolution era.

Molybdenum disulfide (MoS₂) is most promising n-type two-dimensional (2D) transition metal dichalcogenide semiconductors for nanoscale electron devices. But, issues still remains in the 2D device and relatively high contact resistance is one of them. Although many scientific approaches have been made to obtain a record-low R_C, a practical and reliable technique should also be developed. Here thermal-evaporated ultrathin LiF is inserted between MoS₂ and source/drain metal to achieve minimum R_C in 2D-like MoS₂ field effect transistors (FETs). The R_C from the MoS₂ FET with LiF/Au contact is less than ∼600 Ω μm as measured by 4-bar method. The mobility measured from 2-bar FET scheme reaches that from 4-bar FET scheme which exclude mobility degradation from contact resistance. Furthermore, ultrathin LiF is compatible with ITO conductive oxide so that transparent MoS₂ FETs can be fabricated.

Van der Waals (vdW) heterostructures are quasi-two-dimensional (2D) materials made by stacking various layered materials such as transition metal dichalcogenides (TMDs), graphene, hBN and MXenes, that are promising electrodes for solid-state battery applications. The ability to maximize charge-storage capacity by stacking various 2D systems and achieve precise control over their thickness and strain, are some of their key features for use in energy storage devices. To realize these systems, it is essential to understand the mutual influence of the 2D materials on the thermodynamic and kinetic processes occurring within the resulting vdW heterostructures. For example, while it is known that Li intercalation in TMDs like MoS₂ is accompanied by a semiconducting (2H) to metallic (1T) phase transition, the influence of substrates such as hBN and graphene on the stability and the kinetics of this process remains largely unexplored. Using ab initio density functional theory (DFT) we calculate the kinetic barriers of Li intercalation in MoS₂ with and without these substrates. We then investigate the role of Li doping on the modified electronic structure of MoS₂ addressing the effect of screening resulting from hBN and graphene using GW calculations. Finally, we discuss the quasiparticle bandstructure of MoS₂ in the context of energy storage applications.

Xenes are new mono-elemental two-dimensional (2D) materials (e.g., silicene and germanene) that are “2D materials beyond graphene”. They represent a revolutionary new development in the science and technology of materials. Compared to normal 3D solids, the electronic and phononic characteristics of these materials are highly unusual.
Some Xenes have a strong spin-orbit coupling that gives rise to quantum spin Hall effect. They also have an electrically tunable band gap. These properties make Xenes potentially very useful in the design of exciting new solid-state devices such as heat management, thermoelectric energy conversion, quantum computing, field effect transistors and spintronics. Hence, there is strong contemporary interest in exploring silicene and other Xenes for diverse industrial and defense applications. Silicene is also of special interest because silicon-based device technology is already highly developed and may be transferred to silicene.
In general, the performance of a device, is sensitive to the phonon spectrum of its material. For example, thermal conductivity of materials is an important design parameter for the devices. It is sensitive to the DOS (density of states) of low-frequency phonons. For energy conversion devices, the thermal conductivity of the material should be low but the electrical conductivity high. Hence, for designing thermal devices, it is desirable to be able to vary the electrical and thermal conductivities independently. However, in natural materials there is usually a strong positive correlation between the two conductivities.
One way of satisfying this contradictory requirement is to introduce isotopic impurities in otherwise pure crystals. Isotopes of an atom have different masses but same electronic structure. A heavier isotopic impurity results into a lower phonon frequency with no effect on its electronic properties. Thus, in principle, isotopic impurities can tune the thermal conductivity of the host without affecting its electrical conductivity. For design purposes, therefore, it is important to study the effect of isotopic impurities on phonons in silicene.
Modeling the effect of isotopic impurities is also needed for characterization of silicene samples. The common isotope of silicon is ²⁸Si. Its abundance is about 92.2 %. The abundance of its two heavier isotopes is 4.7% for ²⁹Si and 3.1% for ³⁰Si. Hence a normal sample of silicene is likely to contain almost 8% isotopic impurities. This explains the strong topical interest in mathematical modeling of the effect of isotopic impurities on phonon spectra of Xenes, which is the objective of our research.
The modeling challenge in such calculations arises because the 2D materials have a strong size effect. Hence, a large crystallite needs to be simulated, which makes the conventional techniques based upon molecular dynamics or density functional theory computationally very expensive. Moreover, low frequency phonons are needed to model the large time behavior of the material system, for which the conventional techniques are poorly convergent.
In our calculations, we write phonon Green’s functions (GF) in complex frequency space. This representation yields an analytical expression for the perturbed phonon modes of the lattice. A major advantage of this technique is that the imaginary part of the GF directly gives the full phonon DOS. Further, our calculations show a very interesting fact that even isotopic impurities in a lattice can interact. Intuitively, isotopic impurities should be non-interacting, because they do not induce a strain field in the solid. We show that they have a thermodynamic interaction at non-zero temperatures.
In this talk, we will briefly describe our phonon GF method and its application to calculation of phonon modes in Xenes containing an isotopic impurity. Numerical results will be presented for silicene.

Surface alloy is a surface-confined mixture of different metals that are not miscible in bulk phase. Since it is a new type of alloy formed at the surface, it offers a new way to promote surface-confined reactions in catalysis and synthesis. This presentation covers the synthesis of two-dimensional MoS₂ using surface alloy as a virtual substrate by chemical vapor deposition (CVD). The role of surface alloy during the synthesis is confirmed by comparing the reaction mechanism with metal-organic chemical vapor deposition (MOCVD). X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) studies revealed the presence of surface alloy during the growth.

Due to its unique properties, graphene has a big potential to be a solution for various industries’ demands. Although there has been a substantial progress in terms of scientific manner, the desired situation couldn’t be achieved in engineering. The main reason for this is that graphene can be produced in “sample” scales with some control and reproducibility; however, increasing scale creates serious problems in terms of quality, reproducibility, or cost. In this context, a controlled, cost efficient, and repeatable large-scale synthesis of graphene is necessary for the industry to benefit from graphene’s superior characteristics. Recently, graphene encapsulated powders or particles have been attracting attention to be able to benefit from graphene’s properties at large scale; however, existing processing techniques does not satisfy all the requirements such as high-quality graphene, homogenous distribution, convenience for large scale production and low cost. In this study, a cost effective and novel method is proposed to synthesize high quality graphene encapsulated metal powders via plasma enhanced rotating low temperature chemical vapor deposition (CVD). Characterization studies show that by using the system developed within the scope of this study in our laboratory, individual graphene capsulated copper powders are successfully formed. We have systematically studied the effect of processing parameters (duration, plasma power, gas mixture and flow) by using Taguchi method, and optimize the processing parameters for minimum I(D)/I(G) and maximum I(2D)/I(G) ratios, and maximum graphene coverage as the responses of our interest. The mechanical and thermal test samples are prepared by pressing and sintering only copper powders and graphene encapsulated copper powders to understand the effect of graphene on the mechanical and thermal properties. This study is supported by TUBITAK (grant no 118F491).

Photoelectrochemical water splitting is one of the promising ways to generate sustainable energy in the form of hydrogen. However, the challenge remains in designing affordable, efficient, and stable photoelectrodes. Among 2D materials, MoS₂ has been reported as an effective hydrogen evolution reaction (HER) photocatalyst, however, requires the activation of the basal pane to utilize its full potential. Several strategies like exposing the maximum edge site of MoS₂ in HER through defect engineering via sulfur vacancy, strain engineering, and hetero-atom doping have been attempted to enhance the catalytic performance of MoS₂. However, these are not suitable for photoelectrochemical HER since defects in terms of sulfur vacancy create huge charge carrier recombination loss due to defect site mediated trapping of photo-generated carriers in MoS₂. Herein, we will demonstrate our recent findings on the MoS₂ basal plane activation using its heterostructuring with MoSe₂. The MoS₂, MoSe₂ and MoS₂/MoSe₂ heterostructure as photocathodes was hydrothermally grown on chemically etched silicon nanowires (SiNWs). MoS₂/MoSe₂/SiNW PC exhibited enhanced PEC HER activity with a photocurrent density of 19.35 mA/cm², 1.2 times better than pristine MoS₂ grown on SiNW (MoS₂/SiNW). The IPCE for the MoS₂/MoSe₂/SiNW PC is 38.4% which is 1.36 times better than MoS₂/SiNW PC. We attribute this performance enhancement to the activation of the basal plane of MoS₂ by MoSe₂ and also by the band edge alignment for efficient charge separation and transport. This futuristic strategy of basal plane activation by 2D-heterostructure will revamp PEC HER with efficient charge separation and high catalytic sites.

Customising material properties is always desirable. Early on, it was noted [1] that heterostructures and superlattices made from 2D materials, such as the transition metal dichalogenides [2], present a wealth of opportunities to explore and customise properties. These layered systems could prove pivotal in solid-state devices in the fields of sensing, renewable energy [3], information processing and communications technology [4]. We show here that the fundamental properties, band gap and thermal conductivity, for both optoelectronics and thermal management can be controlled by the choice of the two constituent layers.
We have performed a combination of both large scale first principles study of many transition metal dichalcogenides and other 2D semiconductors using density functional theory combined with analytical transport theory and tight binding. As the band structures of individual layers of a heterostructure have a high fidelity to their isolated counterparts, we show how one can predict the band gap of these structures, based on their components [5]. We explain what factors change in the electronic band structure of the superlattice/heterostructure and also discuss how it changes the phononic structure. From this, we go on to show how the thermal conductivity of these systems is reduced compared to their counterparts and how these two factors present an opportunity for improved thermoelectric performance.
[1] A. K. Geim and i. V. Grigorievam, Nature 499, 419 (2013).
[2] B. Miller, A. Steinhoff, B. Pano, J. Klein, F. Jahnke, A. Holleitner, and U. Wurstbauer, Nano Lett. 17, 5229 (2017).
[3] B. Amin, N. Singh, and U. Schwingenschlögl, Physical Review B 92, 075439 (2015).
[4] A. Raja, A. Chaves, J. Yu, G. Arefe, H. M. Hill, A. F. Rigosi, T. C. Berkelbach, P. Nagler, C. Schüller, T. Korn, C. Nuckolls, J. Hone, L. E. Brus, T. F. Heinz, D. R. Reichman and A. Chernikov, Nature Communications 8, 15251 (2017)
[5] F. H. Davies, C. J. Price, N. T. Taylor, S. G. Davies, and S. P. Hepplestone, Physical Review B, 103, 045417 (2021)

Paradigm shifts in batteries, ion separation, electronics, and myriad other technologies will require isolation and identification of the fundamental electrochemical characteristics of 2D heterointerfaces to enable rational materials design. To this end, we prepare individual vdW intercalation heterostructures of graphene and transition metal dichalcogenide (TMDC) layers that are encapsulated between layers of inert hexagonal boron nitride (hBN). We establish several operando experimental tools such as Hall voltammetry and intercalation frontier imaging to monitor the course of the electro-intercalation reaction at the mesoscopic 2D heterointerfaces. The high quality of our 2D intercalated heterostructures permits us to use low-temperature quantum transport methods in conjunction with spectroscopy to identify the fundamental layer capacities and equilibrium potentials of individual 2D. Particularly, we demonstrate the intercalation of AlCl₄^– ions into van der Waals heterostructure layers consisting of deterministically stacked hexagonal boron nitride (hBN) and few layer graphene. We use magnetoresistance charge transport and operando optical microscopy to evaluate the dynamics of intercalation processes in atomically thin samples. We develop a colorimetric method to estimate the diffusion coefficient. The diffusion coefficient for AlCl₄^–, measured in 2D graphitic hosts approaches 10^–5 cm² s^–1 at 320 K, establishing the intrinsic upper limit. These atomically thin single crystal measurements are compared to the cycling response of a high-performance rechargeable Al battery consisting of few-layer graphene–multiwall carbon nanotube composite cathode. Our results thus establish the distinction between intrinsic and ensemble electrochemical behavior in Al-based batteries and show that engineering ion transport to enhance diffusivity in these devices can yet lead to vast improvements in battery performance.

Van der Waals heterostructures have attracted great research attention due to their novel electronic structures and their potential in device development and applications. Especially, graphene has aroused significant interest in the study of two-dimensional layered materials with tunable electronic and optical properties. [1] In addition, metallic chalcogenides have shown great potential in optoelectronics and thermoelectric devices. [2] Therefore, epitaxy of graphene/metal chalcogenide layered materials with high quality is an important step in understanding underlying science and application based on two-dimensional materials. [3] In this study, we report high-quality, crystalline indium telluride grown on graphene by molecular beam epitaxy (MBE). This study systematically explores the growth mechanism and structural analysis of graphene/indium telluride mainly using transmission electron microscopy (TEM).
Electron diffraction pattern and atomic resolution STEM images were used to identify the phases of indium telluride and local atomic configurations. Additionally, in order to inspect the epitaxial relation of indium telluride on graphene, both in-plane and cross-sectional HR-TEM images of indium telluride clusters were obtained. Various phases of indium telluride were observed depending on the growth temperature and In/Te flux ratio, which was compared with thermodynamic calculations. The growth behavior of these heterostructures may offer great potential for science and engineering based on atomically thin two-dimensional materials.

References
[1] A. H. Castro Neto et al. “The electronic properties of graphene” Rev. Mod. Phys. 81 109 (2009)
[2] M. Alsalama et al. “Enhancement of thermoelectric properties of layered chalcogenide materials” Rev. Adv. Mater. Sci. 59 371 (2020)
[3] W. Huang et al. “2D layered group IIIA metal chalcogenides: synthesis, properties and applications in electronics and optoelectronics” CrystEngComm 18 3968 (2016)

Materials with superior electrocatalytic performance have attracted the attention of researchers through the past few years, aiming to find a convenient material that works at low overpotentials and possess high stability. Herein, we report the fabrication of 2D-BNC heterostructure nanosheets composed of boron carbon nitride nanosheets (BCN NSs) with different loads of copper and iron. The fabricated composites showed unique electrocatalytic properties for the hydrogen evolution reaction (HER). The morphology and structure of the electrocatalysts were characterized using FESEM, XRD and XPS. The electrocatalytic performance was investigated through ECSA, impedance and stability measurements. The electrocatalytic properties were studied with varying Cu/Fe:BCN weight ratios. For the fabricated nanohybrid the overpotential was reduced by 90%, with the value of -0.05 V at current density of 10 mA/cm2 vs RHE.
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Nanocarbon allotropes and their composites with transition metal oxides are a promising class of materials possessing vast surface area, high electrical conductivity, and corrosion resistance. Here we report a novel, liquid-phase synthesis of graphene-coated metal oxide nanocrystals which yields from one to ten graphene layers (T ≤390 °C) in nonpolar organic solvents. The process begins with the thermal decomposition of a metal carboxylate to generate CO and/or CH₄ which then become the carbon feedstock for the growth of high quality graphene templated onto iron oxide nanoparticles. By varying the reaction temperature, the carboxylate decomposition rates can be controlled to form nanoscale graphene films with tunable thickness up to 3 nm. These films can be extracted from the metal oxide substrate by acid etching to obtain graphene nanoflakes or porous graphene. Alternatively the graphene-iron oxide composites can be assembled into conductive and magnetic films useful for electrocatalysis and electrochemical sensing.

Three-atom-thick transition metal dichalcogenide (TMD) semiconductors such as MoS₂, WS₂, MoSe₂, WSe₂ are a class of layered 2D crystals with weak interlayer van der Waals bonding and strong intralayer covalent bonding. Owing to the van Hove singularity induced strong optical absorption and exciton generation in the visible part of the electromagnetic spectrum, TMDs are suitable for strong light-matter interactions in optoelectronic devices. Coupling the effect of temperature while a light-matter interaction is occurring in TMDs will enable to design 2D semiconductor-based optoelectronic and photonic circuits for extreme environment studies. Here thin WS₂ layer is exfoliated from a bulk crystal and transferred onto 300 nm SiO₂ coated Si substrate followed by Raman vibrational studies with a temperature variation of 80 to 400K. It is observed that both the in-plane (E’_2g) and out-of-plane (A_1g) vibrational mode frequencies vary non-linearly with the temperature. The optothermal phenomena occurring in nm-thick WS₂ crystals will be discussed. This fundamental study will lead to unveil the dependence of temperature and laser power on in-plane and out-of-plane Raman shifts of TMDs to study the thermal conductivity, hot carrier diffusion coefficient, and thermal expansion coefficient.

Hexagonal boron nitride (h-BN) is the insulator of the family of two dimensional (2D) materials. It is a very important material as it find various applications in optoelectronics devices, sensors, tunneling junctions, graphene and other 2D FETs owing to its large electronic as well as optical band gap, atomically flat surface, high surface polar phonon density and suitable dielectric characteristics. Growth of h-BN has been reported on various metallic substrates such as Cu, Ni, Ru, Fe, etc. but owing to its inherent low solubilities of B and N species it is considered as a good candidate for large area growth of h-BN. Also, different facets of Cu surface have been reported to have different degree of van der Waals interactions with h-BN layer as a result of plane to plane registry. In 2D materials, epitaxial growth has been characterized as van der Waals stacking due to presence of heterointerfaces which are primarily not affected by the lattice mismatch strain between different crystallographic planes due to insignificant out of plane interaction energies. Thus, the concepts of three modes of growth namely Volmer-Weber (VW), Frank van der Merwe (FdM) and Stranski Krastanov (SK) solely based on thermodynamic factors such as surface energies are nullified. Even then, evidence of such modes of growth can be seen which is due to a combination of thermodynamic and kinetic factors which occur during the growth of such layered materials. We report growth of h-BN on Cu via chemical vapor deposition resulting in islands (I), layers (L) and islands plus layers (I+L) in a controlled manner. By controlling the supersaturation and by operating in the near equilibrium limit we observe growth to occur in layered mode resulting in large single crystal grains which coalesce to form flat layers of h-BN. With varying supersaturation at a value far away from the equilibrium limit outcomes to island growth. This is attributed to the higher local supersaturation at a nucleation site which results in growth happening in reaction limited regime thus favoring island growth. By altering the growth kinetics at the surface by increasing the precursor flux and thus increasing local supersaturation at elevated growth temperature we observe large single crystal grains forming which form the first few h-BN layers. With increase in layer thickness the dependence on the binding energy of Cu substrate which acts as the catalyst and the borazane precursor is insignificant, this results in subsequent nucleation on the few layer hBN surface resulting in formation of small islands. This is attributed to decreased surface diffusion as the growth happens in reaction limited regime due to high precursor flux. This suggests growth mode similar to Stranski-Krastanov growth owing to kinetic factors, where island growth becomes favorable to layer-by-layer growth after the first few layers due to a decrease in film substrate binding energy with increasing thickness. We observe such growth over large areas resulting in layer control from monolayer to few layers and island sizes as big as 50 um. This work also establishes that the reactivity of borazane can be modified by altering the physico-chemical chemistry. Thus, we report mode dependent controlled large area deposition of h-BN which would help exploit its properties in various application and help advance large scale manufacturing of 2D materials.

Ion Beam Synthesis of Layer-Tunable and Transfer-Free Graphene on Arbitrary Substrates Towards Versatile Applications

G. Wang¹, X.Q. Feng¹, J.K. Baldwin², Z.F. Di³, and Y.Q. Wang^2,4
1Department of Microelectronic Science and Engineering, Ningbo University, Ningbo, China
²Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
³State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Shanghai, China.
⁴Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.

Direct synthesis of transfer-free and layer-tunable graphene on an arbitrary substrate is highly valued for multiple daily life applications. Here, through employing ion implantation into a Cu/Ni dual metal smart Janus substrate, a well-controlled number of layers of graphene, primarily monolayer and bilayer, can be precisely synthesized via the equivalent fluence of the implanted carbon (C) ions. More importantly, the top Cu film, behaving like an excellent C-diffusion barrier (due to its limited solubility of C), gradually inter-diffuses into the bottom Ni layer during the post-implantation thermal annealing and leads to the implanted C atoms being regularly expelled toward the moving-in interface of the newly formed Cu-Ni alloy and eventually transformed into graphene structure on the substrate. The elemental diffusion during the graphene synthesis is confirmed by secondary ion mass spectrometry (SIMS). It is discovered that all of the implanted C atoms were expelled towards the substrate surface to form transfer-free graphene on arbitrary substrates as characterized by Raman spectroscopy, atomic force microscopy and select area electron diffractions. We demonstrated that the graphene films synthesized on such objective substrates as SiO₂, glass, and Si exhibited excellent properties and device performance as field-effect transistors, heating devices and near-infrared photodetectors without needing any post-transfer process. Our study affords a multifaceted plan for graphene on arbitrary substrates, with well-controlled layer and transfer-free versatile device manufacturing, expediting their applications in many fields.

Graphene is an two-dimensional (2D) material with a thickness equal to one atom. Its structure is made up of carbon atoms bonded together in a hexagonal pattern, similar to a honeycomb [1]. Graphene can be obtained by various methods differing in price, ease of production and quality of the resulting product. The most common fabrication techniques include chemical vapor deposition (CVD), mechanical exfoliation, graphene oxide reduction, liquid phase exfoliation, and epitaxial growth on silicon carbide [2]. This work is focused on the methods where for graphene flakes were produced from mineral graphite. They were mainly based on the electrochemical and ultrasound assisted chemical techniques. One of the method was based on the mineral graphite electrodes, which were exfoliated in a electrolyte under the specific conditions. The solution prepared in this way was subjected to ultrasounds and freeze drying. Another approach was the chemical exfoliation of expanded graphite using sulfuric acid and hydrogen peroxide in specific proportions and method of introduction, in the process assisted by ultrasounds. The quality of the obtained materials was examined using X-ray diffraction, Raman and IR spectroscopy and SEM/TEM microscopy. In order to optimize the flakes production method, a series of experiments was performed in which the pH of the solution, the applied voltage and current, the duration of electrolysis, the temperature in the electrolytic system, as well as the time and type of ultrasound interaction were changed. The systematic research allowed to develop scalable and effective methods of obtaining graphene flakes of various quality, which, depending on their physical properties, can be used in various applications. The best quality graphene flakes have been used as a filler for polymers, paints and varnishes. In order to obtain an appropriate dispersion of the flakes in the composites, the functionalisation of the graphene surface was performed. The composites obtained in this way were subjected to mechanical and biological studies.
Acknowledgements
The project was supported by the National Center for Research and Development, Poland under grant no. LIDER/25/0132/L-10/18/NCBR/2019
References
E.P. Randviir, D.A.C. Brownson, C.E. Banks, Materials Today, 2014, 17, 426–432,
X.-Y. Wang, A. Narita, K. Müllen, Nature Reviews Chemistry, 2018, 2, 0100.

A new method of hydrogen generation from solvents (water, methanol, ethanol) by irradiation with CW infrared laser diode of graphene scaffold immersed in solution is reported. The efficiency of hydrogen production increased exponentially with laser power. It was shown that the process was extremely efficient when the intense white light emission induced by laser irradiation of graphene foam was used. The mechanism of laser induced dissociation of solvents molecules is discussed. It was found that the production of hydrogen was extremely high about 80% and assisted by a small emission of O₂, CO and CO₂ gases.

The main application sectors of graphene composites are: aerospace and automotive industry, coatings, electronics, energy, medicine, lighting, sensors and 3D printing. The aforementioned applications indicate a huge market potential and demand for products in the form of pure graphene flakes and composites with them. A technology for the production of graphene flakes from mineral graphite was developed. The proposed technology allow to develop a less expensive way of obtaining graphene, which can be used as a filler in composites applicable in many industry fields. The obtained material is characterized by high stability, mono or multilayer structure and allow for a significant acceleration of its production process.
Graphene, which was then used to produce composites (paints, varnishes, filaments for 3D printing) was obtained from pure graphite. Experiments with the functionalization of the surface of graphene were carried out, thanks to which it is be possible to introduce it into solutions that were subjected to polymerization processes at later stages. The possibility of using graphene flakes as a filler in varnishes and polymers to improve their mechanical properties was investigated. Mechanical and microbiological tests were performed for all the produced composites and the results are presented.
Acknowledgements:
The project was supported by the National Center for Research and Development, Poland under grant no. LIDER/25/0132/L-10/18/NCBR/2019

Graphitic carbon fiber microelectrodes (CFMEs) have been used as biosensors for the detection of biomolecules such as neurotransmitters using fast scan cyclic voltammetry (FSCV). The electrodes are relatively small (7 microns in diameter), biocompatible, have high spatiotemporal resolution, and do not generally illicit an immune response. CFMEs were characterized with SEM imaging for surface features and EDS/EDX for chemical surface functionalization. Traditionally, CFMEs have been used to detect neurotransmitters such as dopamine, serotonin, norepinephrine, and others. The shape and position of the cyclic voltammogram (CV) is a chemical fingerprint for neurotransmitter detection with the peak oxidative current being proportional to concentration. The method is ideal for measuring fast, subsecond changes of neurotransmitters such as the phasic firing of dopaminergic neurons. Recently, the method has been expanded to measure other molecules such as metals, amino acids, peptides (including neuropeptides), hormones, proteins, neuromodulators, and other biomolecules.

In this study, we demonstrate the use of CFMEs as both FSCV sensors and field-effect transistor (FET) transducers for dynamic pH measurements. The electrochemical oxidation and reduction of functional groups on the surface of CFMEs affect their response over a physiologically relevant pH range. When measured with FET transducers, the sensitivity of the measurements over the measured pH range was found to be (101 ± 18) mV, which exceeded the Nernst value at room temperature of 59 mV by approximately 70 %. Finally, we validated the functionality of CFMEs as pH sensors with FSCV ex vivo in rat brain coronal slices with exogenously applied solutions of varying pH indicating that potential in vivo study is feasible. The sensitive, fast, biocompatible, and selective detection of pH fluctuations provides for a multitude of potential future applications such as the optimization of biomolecule measurement and measurement of pH in specific extracellular tumor microenvironments for cancer studies in addition to many others.

The thermal transport properties of biphenylene network (BPN), a novel sp²-hybridized two-dimensional allotrope of carbon atoms recently realized in experiments [Fan et al., Science, 372 852–856 (2021)], are studied using the density functional theory-driven solution of the Boltzmann transport equation. The thermal transport in BPN is anisotropic and the obtained thermal conductivities are more than an order of magnitude lower than that in graphene, despite similar sp²-hybridized planar-structure of both allotropes. The lower thermal conductivity in BPN is found to originate from enhanced anharmonicity which in turn is a result of reduced crystal symmetry of BPN.

One of the main challenges for the pressure-induced multilayer graphene to atomically thin diamond scale-up synthesis process is the requirement of high compressive pressures. Our previous studies revealed a reduction in graphene to diamane transformation stress upon shearing and fully hydrogenation separately, motivating current investigation on their combined effect on improving the synthesis conditions. We revealed that the transformation stress reduces by an order of magnitude upon the combined implementation of shear and hydrogenation. Our findings showed that a higher percentage of hydrogenation hinders the sp² to sp³ hybridization, which results in higher transformation stress due to the steric effect in the presence of shear. The comparative study of transformation stress between shear and no shear conditions proves the effect of shear on this particular phase transformation. Using machine learning models, we found that although shear and layer number significantly affect the graphene to diamond transformation stress, transformation stress is a strong function of hydrogenation amount and temperature. The presented results guide the future experimental synthesis of multilayer diamane structures.

Molybdenum disulfide (MoS₂) is a well-studied material that sparks enthusiasm in the research community on the account of its desirable properties and wide range of applications.^1,2 Due to its graphene-like layered structure, it can be easily transformed to a Two-Dimensional (2D-MoS₂) counterpart. 2D-MoS₂ exists in two polymorphs: a 1T-phase of metallic properties and a 2H-phase which is semiconducting. 2H-MoS₂ is a naturally occurring form, but metastable 1T-MoS₂ can be acquired in the process of chemical or electrochemical reduction, resulting in formation of a negative charge in the material. The negative charge allows to stabilise the dispersion of 1T-MoS₂ in water and is used as a driving force in a covalent functionalisation by various electrophiles.^3,4 To date, functionalisation of 2H-MoS₂ has proven to be challenging, on the account of stability and inertness on the material.⁵
In order to probe the reactivity of 2H-MoS₂, liquid phase exfoliated material was reacted with sodium borohydride (NaBH₄). It resulted in formation of a partial negative charge in the material (n-2H-MoS₂), identified by zetapotential measurement. This is the first time that accumulation of a negative charge has been observed on the 2H-MoS₂ surface, without a phase transition to metallic 1T-MoS₂ polymorph. The negative charge in n-2H-MoS2 stabilised the dispersion of the semiconducting material in water. It was demonstrated that the negative charge doping was not associated with functionalisation, chemical or structural changes of pristine 2H-MoS₂, which was demonstrated by various characterisation techniques. Most notably, X-ray Photoelectron Spectroscopy (XPS) did not indicate presence of NaBH₄ decomposition products, nor a change in oxidation state or coordination of molybdenum and sulfur atoms. Powder X-ray Diffraction (pXRD) did not show any signs of ion intercalation, or any other change in the unit cell of the material. Finally, Thermogravimetric Analysis (TGA), Raman Spectroscopy and Infrared Spectroscopy (IR) unambiguously demonstrated that there are no new moieties present at the surface of the material. n-2H-MoS₂ preserved its semiconducting properties as probed by Absorption and Emission Spectroscopies. The negative charge accumulated in the material allowed for a successful functionalisation by electrophilic organohalide reagents (including iodoethan-2-ol, iodomethane and iodoacetamide). Functionalised materials were characterised with Raman Spectroscopy, TGA and IR, which provided indirect evidence of covalent functionalisation.
1. S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V Yazyev and A. Kis, Nat. Rev. Mater., 2017, 2, 17033.
2. D. Gupta, V. Chauhan and R. Kumar, Inorg. Chem. Commun., 2020, 121, 108200.
3. D. Voiry, A. Goswami, R. Kappera, C. de C. C. e Silva, D. Kaplan, T. Fujita, M. Chen, T. Asefa and M. Chhowalla, Nat. Chem., 2015, 7, 45–49.
4. K. C. Knirsch, N. C. Berner, H. C. Nerl, C. S. Cucinotta, Z. Gholamvand, N. McEvoy, Z. Wang, I. Abramovic, P. Vecera, M. Halik, S. Sanvito, G. S. Duesberg, V. Nicolosi, F. Hauke, A. Hirsch, J. N. Coleman and C. Backes, ACS Nano, 2015, 9, 6018–6030.
5. X. Chen and A. R. McDonald, Adv. Mater., 2016, 28, 5738–5746.

Here, we have demonstrated the high-quality thin-film growth of MoS_2-xN_x on single-crystal c-Al₂O₃ substrates under ammonia ambient by the pulsed laser deposition (PLD) technique rendering highly stable and tunable 1T/2H biphasic constitution. Raman characterization reveals structural characteristics of a systematic enhancement of the 1T phase component due to the incorporation of covalently bonded N doping in the MoS₂ lattice inducing compressive strain. Interestingly, the film deposited at 300 mTorr NH₃ shows ~80% 1T phase. The transport measurements performed on MoS_2-xN_x films deposited at 300 mTorr NH₃ show very low room temperature resistivity of 0.03 mΩ cm which is 100 times enhanced over the undoped MoS₂ grown under comparable conditions. A triboelectric nanogenerator (TENG) device containing biphasic MoS_2-xN_x film as an electron acceptor exhibits threefold enhancement in the output voltage as compared to the pristine MoS₂. Device architecture, p-type N doping in MoS₂ lattice, favorably increased work-function, multiphasic component of MoS₂, and increased surface roughness synergistically contribute to superior TENG performance.

Borophene (two-dimensional boron) is unusual among synthesized two-dimensional (2D) materials in its lack of layered 3D bulk counterparts and the richness of 2D allotropes/polymorph. Borophene has also been found to play roles in several important environmental and energy applications, such as CO₂ reduction, hydrogen and oxygen evolution reactions, nitrogen fixation, etc [1]. Up to now, borophene synthesis has been realized on metal substrates. Due to the strong interactions with the substrates, however, the removal of borophene from metals have been difficult. Very recently, it was reported that boride is formed through the reconstruction of the top metal layer mixed with boron during borophene synthesis on Al(111) [2]. Metal borides have the potential to be superior substrates for borophene synthesis and separation. However, the interactions between borophene and borides, and the properties of borophene-boride hetero-structures have been rarely explored. Here, we investigate the properties of borophene-boride hetero-structures. Density functional theory (DFT) calculations are performed to explore their structures, binding energies, charge transfer, and work functions. This work allows us to explore alternative routes of borophene synthesis.

Acknowledgements
We acknowledge funding from the National Science Foundation MRSEC program under grant number DMR-1720139. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

References:
[1] Chand, H., Kumar, A., Krishnan, V., “Borophene and Boron-Based Nanosheets: Recent Advances in Synthesis Strategies and Applications in the Field of Environment and Energy.” Adv. Mater. Interfaces 2021, 8, 2100045.
[2] Alexei B. Preobrajenski, Andrey Lyalin, Tetsuya Taketsugu, Nikolay A. Vinogradov, and Alexander S. Vinogradov., “Honeycomb Boron on Al(111): From the Concept of Borophene to the Two-Dimensional Boride.” ACS Nano 2021 15 (9), 15153-15165

For optically active impurities in 2D semiconductors to exhibit quantum nature, their concentration must be sufficiently low such that individual defects can be selectively addressed. While this requires impurity concentration to be lower than ~0.1 ppm, the lowest concentration reported to date for intentionally introduced impurities in a 2D semiconductor is ~1000 ppm, far greater than the desired limit. In this work, we demonstrate ultralow Nb doping of monolayer WS₂ by liquid-phase precursor dilution and systematically study the impact of doping concentration on impurity emission characteristics over three orders of magnitude of dilution down to 0.5 ppm. The Nb-induced emission, which is observed ~130 meV below the A exciton emission, exhibits reduction of inhomogeneous broadening with decreasing Nb concentration. Scanning transmission electron microscopy (STEM) reveals that majority of Nb impurities are isolated, and decoupled to other defects, suggesting that negatively charged Nb atoms are responsible for the observed emission. We further conduct density functional theory (DFT) and GW-Bethe−Salpeter equation (GW-BSE) calculations to gain insight into the physical nature of the localized states.

The class of two-dimensional (2D) graphene-like lattices made of atoms out of carbon, collectively known as Xenes, today includes elements from the lightest boron to the heaviest tellurium [1]. In addition to their fascinating electronic properties, today their optical properties are attracting increasing attention aiming at the integration of 2D materials in ultra-scaled photonics devices [2]. In this framework, the widely studied silicene on Ag(111) looks promising for applications in electronics, conversely on such a substrate the optical properties can be hardly accessed because of the hybridization issues [3]. Here, this scenario will be revisited in light of the recently reported Xene heterostructures, where silicene and stanene are sequentially stacked on Ag(111) by molecular beam epitaxy (MBE) [4]. Concomitantly, the synthesis of silicene and silicon nanosheets on a transparent substrate like Al₂O₃(0001) allowed for the survey of the thickness-dependent behavior of the optical conductivity obtained from transmittance measurements via Kramers-Kronig constrained fit [5]. At the 2D limit, the optical conductivity is characterized by two main features at 1.4 and 4.5 eV that closely resemble those arising from π-π* and σ-σ* interband transition in freestanding silicene. Two distinct behaviors can be recognized: at the 2D limit, the optical conductivity is consistent with a Dirac-like energy bandstructure, whereas, conversely, for thicker silicon layers an anomalous optical behavior shows up suggesting a different energy bandstructure with respect to that of conventional silicon. On the other hand, limited to the group-14 (i.e. column-IVA) of the periodic table, it turns out that increasing the mass of the X element from carbon to tin, the spin-orbit coupling (SOC), a relativistic effect that scales as Z⁴ in elements of atomic number Z, converts a honeycomb lattice from an ideal 2D semimetallic state to a quantum spin Hall insulator (as predicted first for graphene) characterized by large bandgap opening and conductive dissipationless edge channels [6]. Therefore, the choice of a heavier element than silicon, like tin, would intriguingly afford to unravel the topological properties of the Xenes giving rise to the emergence of non-trivial topological properties even at room temperature. Interestingly, the Al₂O₃(0001) substrate turns out to be also well-suited even for stanene as predicted by theoretical modeling [7]. In close analogy with silicon [5], we investigated the optical properties of tin deposited by MBE on Al₂O₃(0001) [8]. The absorption coefficients from THz to ultraviolet (6 meV - 5 eV) photon range measured on ultra-thin tin nanosheets show two spectral features centered at ~1.25 and 4 eV that can be related to π-π* and σ-σ* interband transition in freestanding stanene albeit broadened and shifted towards lower frequency. Remarkably, as also confirmed by means of optical conductivity, ultra-thin tin nanosheets show hints of a bandgap opening being consistent with SOC induced predicted values. Finally, the insertion of a graphene layer in between Al₂O₃(0001) and tin is studied aiming at the creation of Xene heterostructures. By and large, the Xenes made of silicon and tin atoms might potentially pave the way to a new era in high-speed and low power nanophotonics based on 2D materials. The work is within the ERC-COG 2017 Grant N0. 772261 “XFab”.

References
[1] C. Grazianetti et al., Materials 14, 4170 (2021)
[2] C. Grazianetti et al., Optical Mater. X 12, 100088 (2021)
[3] E. Cinquanta, CG, et al., Phys. Rev. B 92, 165427 (2015)
[4] D. S. Dhungana, CG, et al. , Adv. Funct. Mater. 31, 2102797 (2021)
[5] C. Grazianetti et al., Nano Lett. 18, 7124 (2018)
[6] A. Molle et al., Nature Mater. 16, 163 (2017)
[7] H. Wang et al., Phys. Rev. B 94, 035112 (2016)
[8] C. Grazianetti et al., ACS Appl. Nano Mater. 4, 2351 (2021)

Monolayers of transition-metal dichalcogenides (TMDC), such as MoS₂, WSe₂, and MoTe₂, have opened the door to the theoretical and experimental studies of multiparticle excitations, such as trions and biexcitons, with relatively large binding energies. Upon carrier doping or strong optical pump fluence, these excitation complexes dominate the optical properties of these materials owing to the strong Coulomb interactions and reduced electronic screening in such systems. In this work, we perform first-principles calculations to study multiparticle excitations in monolayer MoTe₂. We employ interacting Green’s-function-based method, including the Bethe-Salpeter equation approach and beyond, to compute the interaction kernel for these multiparticle excitations, and utilize interpolation techniques to accurately resolve the spatially correlated features of such excitations. Our calculations allow us to resolve in fine detail the valley- and momentum-distribution of such multiparticle excitations for the lowest- and higher-energy trions, which differ significantly from simplified hydrogenic models. We also incorporate the dependence of the carrier doping in our calculations from first principles, and comment on the applicability of the traditional trion picture of one exciton plus an electron compared to the hybrid exciton plus Fermi-sea description to capture such excitations from first principles.

Quantum confinement and reduced dielectric screening in two-dimensional (2D) materials can result in enhanced many-body Coulomb interactions which in turn lead to large exciton binding energies of the order of several hundreds meV. A modification in the dielectric environment of 2D systems, such as the presence of charged free carriers, can screen the many-body interactions and modify the quasiparticle (QP) band gap as well as the excited state properties (energies and oscillator strengths). In general, the optical properties of bulk systems including electron-hole interactions are computed with ab initio many-body perturbation theory, using the GW plus Bethe-Salpeter equation (BSE) approach using the static random phase approximation. However, the static approximation is valid only if the exciton binding energy is negligible in comparison to the plasmon energy, which is no longer the case in 2D materials such as transition metal dichalcogenides (TMDs), and even less true for the carrier plasmon energy which is of the same order as the exciton binding energy. Here, we perform ab initio GW+BSE calculations on a MoTe₂ monolayer. We introduce a modified plasmon pole model (PPM) that explicitly accounts for both dynamical excitonic effects and local field effects, to accurately capture QP band gap renormalization, as well as modifications of the excitonic and optical properties, in doped MoTe₂ monolayers. Using this model, we find that, in agreement with photoluminescence measurements, neutral excitation energies remain almost unchanged upon doping, a consequence of a similar reduction in both the QP band gap energy and exciton binding energies. Both the dynamical screening and local field effects captured by our PPM lead to good quantitative agreement with experiments and enable studies of more complex charged excitations, e.g., trions, in these and other doped 2D systems.

This work is supported by the Center for Computational Study of Excited-state Phenomena in Energy Materials (C2SEPEM), funded by the US Department of Energy (DOE) under contract No. DE-AC02-05CH11231. Computational resources are provided by the National Energy Research Scientific Computing Center (NERSC), under contract No. DE-AC02-05CH11231. AC acknowledges the support from Wallonie Bruxelles International (WBI).

The stacking of two materials to control and tailor their opto-electronic properties has opened a new paradigm in materials research. For the monolayer Mo- and W- based transition metal dichalcogenides, stacking allows for control of many fundamental optical properties, such as the bandgap and nonlinear susceptibility. Charge transfer between the TMD layers leads to the formation of interlayer excitons that have received a great amount of scientific interest owing to their unique properties such as their permanent dipole moment and their interaction with the moire lattice. Many studies have investigated the optical properties of these fascinating excitations, but most have relied on diffraction limited laser spots which are many times larger than the largest moire periods. Local real-space optical studies of interlayer excitons remain a challenge. Here I will report on our recent efforts to study interlayer excitons in MoSe₂/WSe₂ heterostructures at the nanoscale. Combining tip-enhanced optical studies with nano-photocurrent measurements we see optical electronic effects on order the moire lattice period, as well as red-shifting of the ILE emission energy due to formation of nanobubbles. Our results show the promise and feasibility for optical studies of moire phenomena on their native length scales.

Atomically sharp edges of monolayer van der Waals materials form abrupt junctions that provide a novel platform to study symmetry-broken physical phenomenon and modification of the local electronic structure. Here, we report strong modification of the excitonic emission spectrum at the edges of monolayer black phosphorus (BP) samples. We find that while the quasi-1D exciton in the interior of monolayer BP exhibits a single Lorentzian-like photoluminescence (~40meV linewidth) at low temperatures (~5K), we observe additional fine-structure emission lines with very sharp linewidths of approximately 1-5 meV at the edges of monolayer BP flakes. We interpret the origin of such emission to be atomic reconstruction of the lattice at the edges of BP which modifies the quantum confinement. We observe the photoluminescence to be highly linearly polarized, with significant temperature and pump fluence dependence. Furthermore, we find that the fine-structure emission lines are highly tunable with electrostatic doping and can be turned on and off, on demand. Our results help facilitate the understanding of fundamental optical excitations in monolayer BP under local structural modification and may enable further engineering of photoluminescence in BP nanostructures such as quantum dots or nanowires for bright, polarized, and reconfigurable light emitters.

In monolayer transition-metal dichalcogenide semiconductors, many-body correlations can manifest in optical spectra when photoexcited electron-hole pairs (excitons) are introduced into a 2D Fermi sea of mobile carriers. At low background electron and hole densities, the formation of negatively and positively charged excitons (X^-, X⁺) is well documented. However, in WSe₂ monolayers an additional strong absorption resonance, often called X^-', emerges at higher electron doping. Its origin is not understood. Here we investigate the nature of the X^-' state, via polarized absorption spectroscopy of electrostatically-gated WSe₂ monolayers in high magnetic fields to 60 T. Field-induced filling and emptying of the lowest optically-active Landau level in the K' valley leads to repeated quenching of the corresponding optical absorption. Surprisingly, however, these quenchings are accompanied by changes in the absorption to higher-lying Landau levels in both the K' and K valleys, which are unoccupied. These results cannot be reconciled within a single particle picture, and demonstrate the many-body nature and inter-valley correlations of the X^-' quasiparticle state.

Recent advances in the fabrication and characterization of van der Waals heterostructures have demonstrated large tunability of their optoelectronic properties through the control of the microscopic parameters of the interface, such as the local atomic registry, strain and rotation. Beyond the quantum mechanical coupling between adjacent layers, 2D materials can impact and be impacted by the local electrostatic potential. At typical 2D-2D and 0D-2D materials distance (~3-5Å), near-field effects–resulting from the high multipoles of the electronic density–can be dominant.
In this work, we develop a theory of the near-field electrostatic effects of self-assembled organic layers and 2D materials. We show using density functional theory that the electrostatic potential of a vast number of experimentally achievable organic layers and 2D materials is well approximated by a discretized charge density model. This model can be derived from the atomic positions and multipoles of the electronic density. We derive simple analytical expressions of the electrostatic potential V(x,y,z) in the case of organic layers and h-BN, mono- and dichalcogenides, and show their applicability in tuning the electronic structure of 2D materials and van der Waals heterostructures.

In low-dimensional and nanostructured materials, the optical response is dominated by correlated electron-hole pairs---or excitons---bound together by the Coulomb interaction. Understanding the energetics and dynamics of these excitons is essential for diverse applications across optoelectronics, quantum information and sensing, as well as energy harvesting and conversion. By now, it is well-established that these large excitonic effects in low dimensional materials are a combined consequence of quantum confinement and inhomogeneous screening. However, many challenges remain in understanding their dynamical processes, especially when it comes to correlating complex experimental signatures with underlying physical phenomena through the use of quantitatively predictive theories. In this talk, I will discuss how excitons in low-dimensional and nanostructured materials, such as monolayer transition metal dichalcogenides, few-layer black phosphorus, carbon nanotubes, and molecular crystals, differ from typical bulk materials. In particular, we will look at how symmetry and dimensionality manifest in the exciton bandstructure and how those features of exciton dynamics and transport. I will also discuss new methods we have developed to simulate exciton dynamics, including exciton-phonon interactions, and core-level excitations associated with x-ray absorption spectra.

Atomically thin two-dimensional (2D) materials provide a versatile platform for atom-scale engineering. In addition to their superlative electronic characteristics, their discrete layered structures and van der Waals (vdW) interlayer bonding enable them to be grown, patterned, and stacked to generate heterostructured solids with atomically-precise vertical composition. The band structure can be further tailored by twist angle, creating moiré superlattices to realize novel and unprecedented tunability in quantum states of matter. However, current approaches to stacking 2D layers into high-quality vdW heterostructures are slow, stochastic, and artisanal. Further engineering and application of vdW solids requires a scalable and rapid production method to precisely design and control composition and structure over all three spatial dimensions (x, y, z) and interlayer rotation (θ). In this talk, I will discuss the development such an approach, Robotic 4D Pixel Assembly, for rapidly manufacturing designer vdW solids with unprecedented speed, area, patternability, and angle control. We utilize the robotic assembly of prepatterned pixels made from atomically-thin 2D components. Wafer-scale 2D films are grown, patterned through a clean, contact-free process, and assembled with engineered adhesive stamps actuated by a high vacuum robot. Our technique led to the fabrication of vdW solids of up to 80 individual layers, consisting of (10 to 1000 μm)² areas with pre-designed patterned shapes, laterally/vertically programmed composition, and controlled interlayer angle. This enabled efficient optical spectroscopy assays of vdW solids. Furthermore, our approach allows for the fabrication of twisted n-layer assemblies, where we observe atomic reconstruction of twisted 4-layer WS2 at unexpectedly high interlayer twist angles of ≥4°. To conclude, I will outline ongoing efforts to manufacture van der Waals heterostructures with enhanced control using high-quality 2D materials grown at the wafer scale.

High-performance printable electronic inks with 2D materials have the potential to enable the next generation of high performance low-cost printed digital electronics. [1] The recent advances in flexible inkjet printed transistor heterostructures [2, 3] and complementary inverters achieving a voltage gain |A_v| ≈ 0.1 [2] using liquid phase exfoliation of 2D materials will be shown and their limitations discussed. We will show that by utilising electrochemical exfoliation of semiconducting 2D materials [4] such as molybdenum disulfide (MoS₂) flake aspect ratios >100 and morphologically uniform inkjet printed transistor channels can be achieved. [5] We will demonstrate air-stable, low voltage (<5 V) operation of inkjet-printed n-type MoS₂, and p-type indacenodithiophene-co-benzothiadiazole (IDT-BT) field-effect transistors (FETs), estimating an average switching time of τ_MoS2 ≈ 4.1 μs for the MoS₂ FETs. We then integrate MoS₂ and IDT-BT FETs to realize inkjet-printed complementary logic inverters with a voltage gain |A_v| ≈ 4 when in resistive load configuration and |A_v| ≈ 1.4 in complementary configuration. The results presented represent a key enabling step towards ubiquitous long-term stable, low-cost printed digital ICs.[5]

[1] Torrisi, F. & Carey, T. Graphene, related two-dimensional crystals and hybrid systems for printed and wearable electronics. Nano Today 23, 73-96 (2018).
[2] Carey, T. et al. Fully inkjet-printed two-dimensional material field-effect heterojunctions for wearable and textile electronics. Nature Communications 8, 1202 (2017).
[3] Kelly, A.G. et al. All-printed thin-film transistors from networks of liquid-exfoliated nanosheets. Science 356, 69-73 (2017).
[4] Lin, Z. et al. Solution-processable 2D semiconductors for high-performance large-area electronics. Nature 562, 254-258 (2018).
[5] Carey, T. et al. Inkjet Printed Circuits with 2D Semiconductor Inks for High-Performance Electronics. Advanced Electronic Materials 7, 2100112 (2021).

The remarkable semiconducting properties of 2D transition metal dicalcogenides (TMDs) are of great interest to researchers hoping to harness such traits for optoelectronic applications. Nano-TMDs not only provide a new range of tunable optoelectronic properties, but also create a direct path towards ultrathin, flexible devices. However, large-scale production remains a major challenge in commercializing TMD-based devices¹.
Our group has recently addressed this issue by developing methods for the solution-based fabrication of nanoflake TMD thin films for large-area optoelectronic applications². In this presentation we highlight our results advancing these methods at each stage of the film fabrication process from improved nanosheet dispersions to roll-to-roll continuous deposition. In particular we present a novel electrochemical intercalation method to produce high-quality TMDs in a scalable fashion. We demonstrate that this scalable technique yields predominantly monolayered nanosheets with large lateral dimensions and that they possess superior optoelectronic properties compared to other solution-based methods such as ultrasonication. Thanks to the adaptability of this method, we show how it can be used to exfoliate a range of layered materials ( MoS₂, WS₂, WSe₂) as well as to tune the material's properties via controlled doping (Nb_1%MoS₂) and alloying (Mo_xW_1-xS_ySe_2-y). The ability to produce controllably doped and alloyed TMD nanosheets in large quantities is important for the inexpensive production of next-generation optoelectronic devices.
To process our exfoliated materials into large-area thin films we report the further advancement of our preparation methods toward a continuous roll-to-roll deposition of TMD-based thin films from nanoflake dispersions using a liquid-liquid self-assembly technique³. We demonstrate reproducible printing of 100mm wide TMD flake films on plastic substrates^{<span style="font-size:10.8333px">3</span>}. Thus, we design a pathway for fully solution-processable 2D-TMD devices beginning with as-received bulk TMD powder and finishing with large-area nanosheet thin films.
(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) Yu, X.; Prévot, M. S.; Guijarro, N.; Sivula, K. Self-Assembled 2D WSe2 Thin Films for Photoelectrochemical Hydrogen Production. Nat. Commun. 2015, 6, 7596. https://doi.org/10.1038/ncomms8596.
(3) Wells, R. A.; Johnson, H.; Lhermitte, C. R.; Kinge, S.; Sivula, K. Roll-to-Roll Deposition of Semiconducting 2D Nanoflake Films of Transition Metal Dichalcogenides for Optoelectronic Applications. ACS Appl. Nano Mater. 2019, 2 (12), 7705–7712. https://doi.org/10.1021/acsanm.9b01774.

The 2D layered materials are the potential components in next-generation semiconductor devices and quantum devices. At the beginning of 2D semiconducting material research, the binary metal chalcogenides have been popularly studied, such as MoS₂, WS₂, MoSe₂, WSe₂, etc. Currently, it has been found that approximately 100 types of 2D materials can be grown. However, according to recent high-throughput calculations, more than 1,000 types of two-dimensional materials have yet to be revealed. Here, we will introduce the recent advances for the growth of new 2D materials beyond binary metal chalcogenides, including 2D ternary metal chalcogenides, 2D oxides, and 2D metastable materials. For exploring the new 2D materials, we used the gas-phase growth, especially metal-organic chemical vapor deposition, to take advantage of precursor controllability. Furthermore, we investigated the electrical, optical, and mechanical properties of the newly discovered materials. Through the 2D material discovery effort, we expect to add new 2D materials in a library of 2D materials and improve the performance and functionality of 2D material devices.

The pursuit of artificial micro/nanoscale intelligent machines that can carry out multi-tasks demands the fabrication and integration of innovative functional materials. In the past decades, 2D transition metal dichalcogenide (TMD) materials, such as molybdenum disulfide (MoS₂), have received immense attention owing to their unique electrical, catalytic, biological, and mechanical properties for myriad applications. Nevertheless, 2D materials have been rarely explored in the research of micro/nanomachines, the success of which could bring unparallel opportunities to basic research and applications in micro/nanorobotics. Herein, we report the design, fabrication, and performance of an original one-side-open MoS₂/TiO₂ hollow Janus micromotor (OHJM) that autonomously propels in an aqueous solution with light simulation. Owing to the unique type-II bandgap alignment of the MoS₂/TiO₂ heterojunction, the OHJMs exhibit enhanced propulsion due to an extended light absorption to the visible region. They also demonstrate anomalous speed acceleration in response to an ionic environment, observed for the first time among various chemical-driven micromotors. Finally, the MoS₂/TiO₂ micromotors are applied for water treatment and disinfect 99.999% of E. coli in one hour. This research highlights the potentials of 2D materials for both the fundamental investigation and applications of micro/nanomachines.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are serious contenders for aggressively scaled field-effect transistors (FETs) due to their excellent scaling behavior. While most efforts concentrate on channel miniaturization, in future technology nodes, the contact length (L_C) is expected to dominate the total device footprint. One solution that enables maximum scalability is edge contacts; in this scheme, the channel is contacted through its sides along the same plane as the metal contacts. Because this structure does not suffer from current crowding, the performance of edge contacts is expected to be L_C-independent [1]. In addition, edge contacts are compatible with back-end-of-line (BEOL) processing. Current edge-contacted TMD transistor demonstrations have utilized different fabrication approaches to create the edge connection. Explored techniques included in-situ physical etching with Ar⁺ ions [1,2], ex-situ reactive ion etching (RIE) [3], and in-situ post-RIE plasma cleaning [4]. Optimizing the fabrication approach is central to the successful adoption of edge contacts, yet the influence of ex-situ versus in-situ processing on interfacial morphology and charge carrier transport is unclear. In this work, we fabricate multiple edge-contacted TMD transistors on the same channel using several fabrication approaches to provide crucial data for determining the best method. We extensively analyze the transistors using electrical characterization, cross-sectional transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) to uncover the implications of employing each technique. We discover intriguing distinctions between the different etching techniques in terms of device performance and edge contact structure formation. One key observation is that the ion beam etching of tungsten-based TMDs can result in residual tungsten in the contact region that modifies the metal and interface. This, and other, key insights pave the way for the realization of high-performing, extremely miniaturized metal edge contacts on the promising platform of 2D TMDs.
References
[1] Z. Cheng et al. "Immunity to Contact Scaling in MoS₂ Transistors Using in Situ Edge Contacts." Nano letters 19.8 (2019): 5077-5085.
[2] H. Abuzaid et al. "Unanticipated Polarity Shift in Edge-Contacted Tungsten-Based 2D Transition Metal Dichalcogenide Transistors," IEEE Electron Device Letters, vol. 42, no. 10, pp. 1563-1566, (2021).
[3] Z. Yang et al. "A Fermi-level-pinning-free 1D electrical contact at the intrinsic 2D MoS₂–metal junction." Advanced Materials 31.25 (2019): 1808231.
[4] A. Jain et al. "One-dimensional edge contacts to a monolayer semiconductor." Nano letters 19.10 (2019): 6914-6923.

Through the implementation of real-time impedance spectroscopy and development of a circuit model with physical meaning, redox exfoliated MoS2 nanoflakes demonstrate an ultrasensitive empirical detection limit of NO2 gas at 1 ppb, with an extrapolated ultimate detection limit approaching 63 ppt. This sensor construct reveals a more than three orders of magnitude improvement from conventional DC sensing approaches as the traditionally dominant interflake interactions are bypassed in favor of selectively extracting intraflake doping effects. This same approach allowed for an all solution-processed, flexible 2D sensor to be fabricated on a polyimide substrate using a combination of graphene contacts and drop-casted MoS2 nanoflakes, exhibiting similar sensitivity limits. Finally, material changes via annealing, selective coatings, and practical considerations for a mass-producible sensor will be discussed.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as one of the most promising channel materials for developing high-performance field-effect transistor (FET) based electronic nanobiosensors. However, the difficulty in limited chemical stability and the complicate chemistry in biomolecule functionalization are major obstacles in the route towards commercialization of 2D TMD-based biosensor. Here, we developed a universal functionalization method based on a single chemistry using hexaganol boron nitride (hBN) intermediate layer and pyrene-NHS (Pyrenebutyric acid N-hydroxysuccinimide ester) linker to facilitate the fabrication of large-area and highly sensitive TMD-based FET DNA nanosensors. This method can be applied to functionalize all kinds of TMDs and any other 2D materials with most DNA, RNA, and protein-based biomarkers. In addition, the encapsulation of hBN can also improve the stability of the TMDs beneath and, thus, the performance stability of as-fabricated nanobiosensors. This work contributes to the development of 2D materials based nanosensors for early-stage point-of-care diagnosis of many diseases, such as cancers, HIV, and COVID-19, etc.

Demonstration of magnetism in two-dimensional materials (2DMs) has enabled the production of magnetic tunnel junctions (MTJ) in van der Waals heterostructures. Since then, the search for MTJs with improved properties (e.g. enhanced tunnel magneto-resistance and spin injection) has spawned considerable efforts. However, unlike bulk magnetic tunnel junctions, the absence of epitaxial restrictions within the growing family of 2DMs yields an extremely large number of possible heterostructures and pose a daunting challenge for both experimental and computational efforts. Hence, guiding principles for the creation and optimization of the properties of these layered heterostructures is highly desirable.
Here we show that optimal spin injection in these systems can be realized through band engineering. First, we demonstrate that quantum transport calculations within the Landauer’s formalism quantitatively describe spin injection in these heterostructures, as exemplified in graphite/CrI₃ junctions [1]. More importantly, we find that, owing to the weak van der Waals interactions between layers, the emerging properties of the MTJs formed with 2DMs can be rationalized in terms of the bulk properties of its constituents [2]. This feature enables spin injection control using descriptors obtained from first principles that include complex band structure, band alignment, density of states, circumventing computationally demanding quantum transport calculations. Through this approach, we demonstrate that the choice of electrodes within the family of transition metal dichalcogenides may increase the spin injection in CrI₃ junctions by various orders of magnitude and improve tunneling magnetoresistance. We also apply the methodology to magnetic heterostructures combining transition metal dichalcogenides and halides and find other suitable MTJ candidate heterostructures. By screening possible 2DM constituents based on their bulk properties, our work may prove useful to the accelerated rational design of magnetic heterostructures based on van der Waals materials.
The authors gratefully acknowledge the support from the National Science Foundation (NSF) through Grant No. NSF-1848344. A.M.P. is thankful for the support from the Alabama EPSCoR GRSP.
[1] Heath et al., "Role of quantum confinement and interlayer coupling in CrI₃-graphene magnetic tunnel junctions", Physical Review B 101, 195439 (2020).
[2] Heath and Kuroda, "Spin-Injection Enhancements in van der Waals Magnetic Tunnel Junctions through Barrier Engineering", Phys. Rev. Applied 16, L041001 (2021).

Two-dimensional transition metal dichalcogenides (TMDs) have recently garnered much attention owing to their extraordinary physical, chemical, electrical, and optical properties. Initial applications in functional electronic and optoelectronic devices have revealed that these properties are particularly sensitive to extrinsic factors like substrate interactions, strain, and charge transfer which have limited performance. Thus, unlocking the intrinsic nature of TMD-metal interfaces is critical for studies of native TMD properties and optimal device development. Here, we find that the electronic structure of mechanically exfoliated TMD-Au interfaces exhibit pronounced heterogeneity arising from local variations in the strength of the interface interaction and provide a means to engineer these interfaces. To examine these nuanced interactions, we employ a direct probe of the electronic structures of WS₂ and WSe₂ with the spatial resolution to delineate the influences of nanoscale heterogeneities at TMD-Au interfaces. Spectroscopic analysis reveals key differences in work function, electron binding energy of the occupied states, and intensity near the Fermi level owing to distinct μm sized regions of strong and weak TMD-Au interface interactions. Regions of stronger TMD-Au interactions are found to experience a larger degree of charge transfer resulting in greater hole doping as opposed to the more weakly interacting regions. We then fabricate uniformly strong and weak TMD-Au interfaces via Au processing thereby selectively dictating the electronic properties of the TMDs. Our findings illustrate that the electronic properties of TMDs are greatly impacted by metal interface interaction strength and provide a means to engineer these important junctions via simple processing methods.

We acknowledge insightful discussions with G. Copeland, N. Bartelt, C. D. Spataru, K. Thürmer, C. Smyth, T. M. Lu, and S. Chou at Sandia National Laboratories. The work at Sandia National Laboratories was supported by Sandia’s LDRD program. The work at the US Naval Research Laboratory was funded by the Office of Naval Research. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly-owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Commonly studied transition metal dichalcogenide (TMD) crystals exhibit polymorphism, where the electronic structure of the material changes significantly with a change in the crystal structure. MoTe₂ has gained particular interest because the potential energy difference between the semiconducting 2H (trigonal prismatic coordination) and semi-metallic 1T' phase (distorted octahedral coordination) is the lowest (40 meV) among TMD polymorphs, making it promising for low voltage switching. Although the 2H phase is the most stable form, it can be transformed to 1T' by 0.3 - 3% by tensile strain thereby causing a large change in the electronic conductivity. Recent studies have experimentally shown this phase transition by mechanically stretching the entire substrate or applying local mechanical strain using atomic force microscopy (AFM) tip, but both strain methods would be difficult to implement in CMOS. What is needed is a straining mechanism driven by field-effect, where a single device can be controlled electrically by a nearby gate.
Here, we employ an ‘ionomer’ or single-ion conductor to impart strain, which has been used by the polymer physics community for artificial muscles and actuators. An ionomer contains mobile cations but has anions that are covalently bonded to a polymer backbone. Under an applied electric field, the cations accumulate at the MoTe₂ surface, effectively controlling electron transport in the material, while anions maintain their position in the polymer backbone creating a charge imbalance. The imbalance causes the ionomer to bend, which then induces strain in the MoTe₂.
In this work, the electrical and structural properties of a suspended MoTe₂ FET are measured simultaneously using a home-built set-up combining electrical measurements with Raman spectroscopy. With no gate voltage (V_G) applied, the insulating 2H phase is confirmed. For V_G > 2.5 V, the 1T’ phase is detected by a significant decrease in the electrical resistance accompanied by a characteristic 1T’ peak in the Raman spectra. To our knowledge, this is the first demonstration of using a gate to induce the phase transition; moreover, the transition occurs at a voltage that is ~ 1-2 V lower than phase transitions induced by electrostatic and electrochemical mechanisms. However, because a large portion of the flake is supported and not suspended, a contribution from the 2H phase persists. Mapping the 2H and 1T’ peaks across the entire flake reveals that the phase transition is reversible (i.e., the flake reverts to the semiconducting 2H phase when the voltage is removed), which is an essential feature of a switch. These proof-of-concept results are encouraging because there is plenty of opportunities to optimize the device structure and the ionomer to engineer a complete transition and enable a true low-voltage electronic switch.

Here, we report over 200% modulation in both the real and imaginary part of the complex refractive index in monolayer molybdenum diselenide (MoSe₂) by tuning the exciton resonances. By studying the dependence of the A and B exciton resonances for carrier densities up to 9x10¹²/cm² and temperatures between 4 K to 150 K, we observe both temperature and carrier dependent epsilon-near-zero response and transition from metallic to dielectric response. Our findings of large electrical tunability of the refractive index is accompanied by large changes in reflectance amplitude and phase. We directly measure the phase shift of the reflected light under applied bias by using an interferometer measurement and observe a monotonic modulation of the phase at the A exciton wavelength of 754 nm. The gate-tunable phase thus allows us to dynamically steer the reflected beam by applying a phase gradient across antenna arrays comprised of MoSe₂ monolayers. Our results suggest the potential of monolayer MoSe₂ as an active material for emerging photonics applications.

Transition metal dichalcogenides (TMDs) and other two-dimensional (2D) semiconductors oxidize readily in a variety of conditions.¹ A thorough understanding of this oxide formation is necessary for effective integration of 2D semiconductors into microelectronic and photonic devices. The reasons for this are twofold: (1) as oxides of emerging 2D semiconductors such as Zr- and Hf-based TMDs can exhibit high-k dielectric properties useful in device design,² and (2) as oxidation is inevitable for many TMDs and can cause material degradation that affects device performance.¹
Here, we investigate the processing and properties of oxides of a variety of 2D semiconductors (MoS₂, HfS₂, HfSe₂, ZrS₂, ZrSe₂). We combine scanning transmission electron microscopy (STEM) with spectroscopic ellipsometry (SE) measurements to provide a comprehensive characterization of the oxide composition, structure, and dielectric properties. We also focus on advancing 2D semiconductor oxide metrology, calibrating SE data through multi-sample correlation between SE and cross-sectional STEM for oxide thickness. Atomic resolution cross-sectional STEM allows us to study the interface structure and crystallinity of the oxide, while spectroscopic techniques such as energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) allow us to determine the chemical composition and bonding with nanometer resolution. We find that the native MoS₂ oxide after aggressive oxidation (500 ^oC, 1hr, 0.2 atm O₂) exhibits an unexpected morphology of plate-like MoO₃ crystals. Although the interface between the MoO₃ and underlying MoS₂ lattice is sharp and uniform, thick MoO₃ is non-uniform and exhibits void formation. We study the nucleation of MoO₃ crystals and their growth during the oxidation process. We use information gleaned to discuss kinetic mechanisms of MoS₂ oxidation and compare with other 2D semiconductors, considering the roles of mass transport, defects, interdiffusion, and sublimation. We discuss the complementary information available from in-situ environmental TEM (E-TEM) experiments. Our results provide quantitative guidance for accounting for native oxides in the design and processing 2D semiconductor devices.
References
1. Soon Jo, S. et al. Growth Kinetics and Atomistic Mechanisms of Native Oxidation of ZrSxSe2−x and MoS2 Crystals. Nano Lett. 120, 8592–8599 (2020).
2. Mleczko, M. et al. HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides. Sci. Adv. 3, 1700481 (2017).

The gallium monochalcogenide family of van der Waal (vdW) semiconductors is composed of the group III-VI binary compounds gallium sulfide (GaS), gallium selenide (GaSe), and gallium telluride (GaTe). Each layer has the atomic sequence X-M-M-X, where M is the metal gallium and X is the chalcogen sulfur, selenium, or tellurium. Alloying these binary compounds creates tunable optoelectronic property-composition relationships that depend on the electronegativity and atomic radius of the constituent chalcogens. The band gaps of GaSe_1−xS_x and GaSe_1−xTe_x ternary alloys have exhibited a linear and slightly bowed relationship with composition, respectively^1,2. Because of the greater mismatch between S and Te, we predict that band gap bowing is more extreme, allowing for a widely variable bandgap within a small composition range. Sulfur rich GaS_1-xTe_x nanowires have been synthesized in our laboratory using a gold-catalyzed VLS method. Room-temperature photoluminescence measurements and DFT calculations show that the GaS_1-xTe_x system undergoes a drastic decrease in the fundamental band gap, typical of a highly mismatched alloy. These wires grow tens of microns long in the c-axis, such that the limiting dimension of the nanowire is the cross-sectional vdW plane. This easy access to vdW interfaces and dangling bonds opens a host of possibilities for material manipulation such as intercalation of solutes.
While initial work has successfully synthesized GaS_1-xTe_x nanowires, the nanowires had no preferred orientation to the silicon surface. Synthesizing clean arrays of nanowires is desirable for observing anisotropic effects of experimental parameters and for scaling to bulk production for device implementation. We have grown GaS_1-xTe_x nanowires on Si and sapphire to study epitaxial effects, morphology, crystal structure, and growth direction. Sapphire-grown nanowires have a strong preference to grow perpendicular to the surface, compared to nanowires grown on Si substrates, which are more randomly oriented. This vertical growth may arise from the initial growth of a GaS_1-xTe_x layer from which nanowires may be growing epitaxially. Control over the nanowire growth area and density is studied by growing nanowires from a gold patterned surface. We explore the vertical growth of nanowires directly on a substrate surface by the use of gold patterned GaSe crystals.
References:
[1]Jung et al, ACS Nano 2015 9 (10), 9585-9593
[2]Fonseca et al., Chemistry of Materials 2018 30 (13), 4226-4232

Two-dimensional (2D) semiconductors are promising candidates for next-generation electronics because they lack dangling bonds and do not suffer the same mobility degradation from surface scattering as silicon does in ultrathin films [1]. Among 2D semiconductors, InSe is one of the few showing electron mobility over 1000 cm²V^-1s^-1 in few-layer exfoliated films at room temperature [2]. However, studies on monolayer InSe have been limited to bulk exfoliation [2,3] or synthesis methods that are difficult to integrate into silicon-compatible manufacturing (e.g. molecular beam epitaxy [4] or growth on mica that still requires mechanically transferring the InSe [5]). These demonstrations are useful as prototypes, but it is necessary to develop a direct growth process for InSe to be considered a candidate for beyond-silicon electronics.
In this work, we present the first solid-source chemical vapor deposition (CVD) growth of monolayer and multilayer InSe onto a manufacturing-friendly substrate, SiO₂ on highly doped silicon. Parameters such as pressure, temperature, and precursor mass are tuned in order to achieve an appropriate ratio of selenium and indium atomic fluxes to the substrate. We use the seeding promoter perylene-3,4,9,10-tetracarboxylic acid tetrapotassium acid salt (PTAS) to facilitate nucleation [6]. Post-growth characterization reveals monolayer and few-layer InSe grains of 5-10 µm in size. Photoluminescence with a 325 nm laser indicates a peak of 2.8 eV for the monolayers and 2.64 eV for few-layers, which is consistent with the emission of InSe previously demonstrated with exfoliated flakes [2]. Additionally, the material is characterized through Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), revealing the mono and few-layer nature grains of the growth.
We also perform electrical characterization on the as-grown InSe grains on SiO₂ (90 nm) on highly doped Si substrates, where the Si also serves as the back-gate. These devices are capped post-growth for subsequent processing with an alumina protective layer to prevent exposure to the ambient [7]. Then, using electron-beam lithography, we pattern field-effect transistors with gold source and drain metal contacts [1], and define channel dimensions using a XeF₂ dry etch. Few-layer back-gated transistors of 300 nm channel length show drain current on/off ratios greater than 10⁴ and a field-effect mobility of 6 cm²V^-1s^-1 at room temperature, for a drain-to-source bias of 0.1 V.
Overall, this work presents the first direct growth of monolayer InSe on amorphous SiO₂ using solid source CVD, with subsequent characterization, utilizing a transfer-free process. Further effort in tuning processing parameters points to the potential for InSe to advance towards the demonstrated exfoliated mobility and the material’s utilization in nanoelectronics.
[1] C. English, E. Pop et al., Nano Lett., 16, 3824 (2016)
[2] D. Bandurin et al., Nature Nanotech., 12, 223 (2017)
[3] P.-H. Ho et al., ACS Nano, 11, 7362 (2017)
[4] P.S. Avdienko et al., J. Opt. Soc. Am. B, 38, 2579 (2021)
[5] H.-C. Chang et al., Small, 14, 1802351 (2018)
[6] K. K. H. Smithe, E. Pop et al., 2D Mater., 11, 8456 (2017).
[7] Y. Y. Illarionov, E. Pop et al., IEEE Electron Dev. Lett., 38, 1763 (2017)

The study of 2-D transition metal dichacogenides(TMDCs) and their corresponding stable oxides is a growing area of interest due to their promising optoelectronic and thermal properties. In a recent experiment, we have observed the plasma assisted conversion of 2-D TMDCs into 2-D oxides and their subsequent rolling into nanoscrolls. With the goal of assigning a mechanism to this process, we use van der Waals corrected density functional theory simulations to determine the individual driving forces that play a role in nanoscroll formation. We then use the parameters derived from these computations into existing physio-mechanical models that have been developed to study CNTs in the past to find the equilibrium core radii of the nanoscrolls and compare them to our experiments. We finally conclude by presenting DFT computed properties of the 2-D oxide nanoscrolls in order to unearth their novel properties and potential applications.
AG and AS are supported by NSF grant number DMR-1906030

Janus materials possess a unique array of properties such as finite out-of-plane dipole moments, Rashba effect, strongly bound excitons, and strong interaction with light making these 2D materials ideal for a wide range of applications from piezoelectric devices to multi-layer 2D heterostructures. Janus MXY materials are 2D materials where a metal atomic layer M is sandwiched between layers X and Y of two different chalcogen, halogen, or pnictogen atoms. The properties of Janus materials are prone to alter due to interfacial interactions in a heterostructure. Furthermore, the properties of 2D materials can be dramatically altered by placing them on substrates. For example, placing 2D-MoS₂ on a sapphire substrate reduces the mobility of the carriers by more than an order of magnitude. Using our workflow package 2dSynth, we compute the energetic stability, electronic properties, and charge transfer for ~50 MXY Janus materials on 50 elemental, cubic phase, metallic substrate materials using van der Waals-corrected density functional theory. Furthermore, we use machine learning models to identify structure-property correlations at the 2D Janus-substrate heterostructure interface.

2D transition metal dichalcogenides (TMDs), with bandgap energies corresponding to near-infrared and visible light ranges, are promising materials for photodetectors, light emitting diodes, and solar cells. Organic semiconductors have distinct physical properties compared with inorganic counterparts: large binding energy of excitons, flexibility, and low dielectric constant. In this work, newly-developed p-conjugated organic molecules, named DY1, were integrated with MoS₂ monolayers grown on quartz substrates. We investigated photoluminescence (PL) characteristics of the DY1/MoS₂ hybrid heterostructures. The micro-PL measurements and wide-field PL image showed that the MoS₂ monolayers could change the spectra and reduce the intensity of the PL emission from DY1. The emission lifetime at the DY1/MoS₂ was larger than that of bare DY1. Kelvin probe force microscopy measurements under illumination helped us to visualize the light-induced contact potential difference at the DY1/MoS₂ interface. In the presentation, we will discuss the energy and charge transfer processes at the DY1/MoS₂ organic/inorganic heterointerfaces, based on the experimental results including energy band diagrams.

Next-generation of batteries beyond the Li-ion chemistry is needed for cost-efficient and large-scale systems. Na-ion batteries (SIB) have emerged as an alternative to Li-batteries. However, challenges relating to intrinsic properties of sodium ion intercalation remain and performance of SIBs must be improved. In this work, we have investigated anodes based on restacked metallic MoS₂ 2D transition metal dichalcogenide (TMDs) nanosheets for SIBs. We have found that electrodes from restacked metallic 1T phase MoS₂ nanosheets in SIBs offer distinct advantages. These include: (1) larger cations can be easily intercalated and de-intercalated into the electrodes; (2) enhanced electroconductivity that facilitates the diffusion of the ions; and (3) improved electrocatalytic activity that can catalyze desired reactions. Based on these principles, we have achieved SIBs with the first cycle capacity of 520 mAh g^-1 with a reversible efficiency of ~ 50 % and cycle stability of 80 % at 50 cycles. We employ Raman, XPS, and XRD measurements to figure out the intercalation mechanism of these materials before and after cycling.

A global shift to clean, low-carbon technologies requires development of low-cost, highly efficient energy storage. In particular, the electrode-electrolyte interface is key in development of stable, high-efficiency batteries [1], with high surface-area electrodes in the nano-dimensions being the pinnacle [2].
The usage of two-dimensional (2D) materials in energy storage is continuously expanding [3] due to efficient ion transport between the single-atomic layers, and superior, atomically smooth, surface-areas leading to increased ion adsorption and surface reactions [4]. Further surface-area enhancement of the 2D materials, using scalable industrial compatible processes, could revolutionise the energy storage sector.
In this work, ultra-high surface area “2D-MoS₂/black-Si” heterostructures were developed by merging two large-area scalable processes. Nanoscale grass-like black-Si acts as a scaffold for direct 2D growth, avoiding costly and complex transfer processes, significantly enhancing the surface-area of 2D-MoS₂. These hybrids are expected to out-perform current state-of-the-art, when used for applications such as battery anodes [5], but also lend themselves to gas sensing, water splitting and solar cells [6,7].
Our two large-area processes are top-down vertical silicon nanowires (SiNWs) and a 2-step MoS₂ growth. The former uses AgNO₃ and hydrofluoric (HF) acid solution, with a cyclic process on the Si surface, caused by nucleation from Ag nanoparticles and etching from HF. This results in grass-like nanowires, whose height is controlled by etch time. The Si surfaces boast ultra-low broadband reflectivity (<1%), and are typically used in photovoltaics [8]. The 2-step MoS₂ technique firstly deposits MoO₃ one atomic layer at a time via atomic layer deposition (ALD), followed by an anneal in hydrogen disulphide, converting the layers to MoS₂. This offers significant advantage over other MoS₂/SiNW heterostructure work published to date, which use electrodeposition, hydrothermal, or conventional chemical vapour depositions techniques [9-12]. Not only is our technique large-area compatible, but we are able to fine-tune the number of MoS₂ layers via ALD cycles, giving us greater control and quality, whilst also using the MoO₃ layer to protect the SiNW from the sulphur exposure in the anneal step. The result is a high-quality MoS₂, with layer number optimisation, conformally coating large-areas of black-Si.
The MoS₂/black-Si stacks were characterised using Raman Spectroscopy and photoluminescence (PL) for measuring MoS₂ layer number and quality, scanning electron microscopy (SEM) for assessing potential nanowire damage and transmission electron microscopy (TEM) for conformality and in-depth MoS₂ analysis. The use of an alumina interfacial layer, via ALD, was also investigated.
We successfully grew large-area monolayer and multilayer MoS₂ directly onto black-Si, with no nanowire degradation. By integrating an alumina interfacial layer, we further increased the MoS₂ quality, with fewer defects. This demonstrates the compatibility of our technique for fabricating scalable high-quality 2D-MoS₂/black-Si heterostructures in a tuneable and highly controllable way. Our next step is to directly assess the impact of our 2D material on silicon nanowire electrodes for next generation batteries, by using Electrochemical Impedance Spectroscopy.
References:
[1] 10.3389/fchem.2020.00821 [2] 10.1016/j.ssi.2016.11.028 [3] 10.1016/j.cclet.2019.10.028 [4] 10.1002/aenm.201600025 [5] 10.1002/adfm.200601186 [6] 10.1021/acsami.8b08114 [7] 10.1039/C4CS00455H [8] 10.1016/j.solmat.2016.10.044 [9] 10.1039/C7RA13484C [10] 10.1007/s12274-014-0673-y [11] 10.1007/s12633-018-0014-y [12] 10.1016/j.matdes.2016.07.098
Acknowledgements: This work was funded by University of Southampton Zepler Institute Stimulus Fund 2020/21, EPSRC (EP/R005303/1) and (EP/N00762X/1).

Two-dimensional (2D) transition metal borides (MBenes) were theoretically predicted to have high electric conductivity and high Li-ion capacity. It was predicted that Mo₂B₂ 2D nanosheets are stable and possible through exfoliation of Mo₂AlB₂. However, Mo₂AlB₂ is a metastable phase that cannot be directly synthesized. In addition, MoAlB is the only thermodynamically stable phase in the Mo−Al−B system. Partial deintercalation of Al from MoAlB was reported before using different topochemical strategies, such as NaOH, HF, and HCl-LiF. However, their electrochemical performance has not been studied before. In this study, we report on the successful etching of MoAlB to Mo₂AlB₂ and their electrochemical performance as electrode materials for Li-ion batteries for the first time. The preliminary results show an initial significant enhancement in Li storage from MoAlB to Mo₂AlB₂ where the specific capacity increased from 50 to 200 mAh/g when tested at a specific current of 20 mA/g. The Mo₂AlB₂ sample was milled after etching and the electrochemical performance showed further increase to 300 mAh/g at same specific current. Results from ex situ X-ray diffraction and photoelectron spectroscopy to shed light on the electrochemical lithiation mechanisms will be discussed.

Modern electronics require pairing of N- and P-type field effect transistors (FETs) via complementary metal oxide semiconductor (CMOS) technology. In three dimensional (3D or bulk) semiconductors, substitutional doping is used to achieve N- and P-type FETs. However, controllable incorporation of dopants into atomically thin transitional metal dichalcogenides (TMDs) is not possible without destroying the material. FETs based on 2D TMDs must rely on tuning the work function of metal contact for hole injection into the valence band (P-type) or electron injection into the conduction band (N-type). Recent studies have made progress in realizing high-performance N-type FETs with 2D TMDs^1,2. It has been proven to be exceptionally challenging to achieving P-type contacts on 2D TMDs with high work function contacts due to the defects at the metal/semiconductor interface caused by metal deposition. While mechanical transfer of Pt and Pd on multi-layered WSe₂ and MoS₂ has been reported^3,4, P-type devices on single layered WSe₂ and MoS₂ have yet to be realised. Here we demonstrate the realization of P-type devices on single and few-layered MoS₂ and WSe₂ using industry compatible electron beam evaporation of high work function metals such as Pd and Pt. Using atomic resolution imaging and spectroscopy and X-ray photoemission spectroscopy, we demonstrate near ideal van der Waals interfaces without chemical interactions between the 2D TMDs and 3D metals. The demonstration of high-performance P-type contacts on 2D TMDs solves a fundamental problem in the field and opens up pathways for realising the full potential of atomically thin body semiconductors for novel electronic devices.
1. Wang, Y. et al. Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors. Nature vol. 568 70–74 (2019).
2. Shen, P.-C. et al. Ultralow contact resistance between semimetal and monolayer semiconductors. Nature 593, 211–217 (2021).
3. Liu, Y. et al. Approaching the Schottky-Mott limit in van der Waals metal-semiconductor junctions. Nature 557, 696–700 (2018).
4. Jung, Y. et al. Transferred via contacts as a platform for ideal two-dimensional transistors. Nat. Electron. 2, 187–194 (2019).

Emerging layered ferroelectrics such as CuInP₂S₆ (CIPS) have anisotropic effective mass of carriers and can enable giant modulation of fermi level in van der Waals (vdW) heterostructures.¹ However, defects at the interface of metal contacts and ferroelectrics lead to trap states that affect the depolarizing fields and overall ferroelectric response. Unlike oxide-based ferroelectrics such as doped Hafnia, 2D ferroelectrics are more sensitive to surface adsorbates that leads to a higher density of interface trap states. In this work, through piezoresponse force microscopy (PFM) and electrical transport measurements we investigate how the local environment at the ferroelectric-electrode interface can influence polarization. We compare the case and for a strongly interacting metal such as titanium, that can lead to a dead interface layer with indium based ultraclean vdW contacts² with no chemical interaction at the interface. Our results unravel the fundamental polarization mechanisms in 2D ferroelectrics which would be useful for nonvolatile memories and tunnel junctions. Further, our approach could be extended to build efficient metal contacts on emerging Moiré ferroelectrics.
[1] Wu, J., Chen, H. Y., Yang, N., Cao, J., Yan, X., Liu, F., ... & Wang, H. (2020). High tunnelling electroresistance in a ferroelectric van der Waals heterojunction via giant barrier height modulation. Nature Electronics, 3(8), 466-472.
[2] Wang Y., Kim J. C., Wu R. J., Martinez J., Song X., Yang J., Zhao F., Mkhoyan A., Jeong H. Y., Chhowalla M. (2019) Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors Nature, 568 (7750), 70–74.

The integration of p- and n-type devices is the key to complementary metal-oxide-semiconductor technologies. If two-dimensional (2D) semiconductors based on transition metal dichalcogenides (TMDs) are to realize their potential for future electronics, rational integration of p- and n-type devices must be demonstrated.[1] In this work, we report our recent efforts on making p- and n-type field-effect transistors (FETs) using van der Waals (vdW) contacts and vertical integration of the p and n channel 2D TMDs.[2] We have analyzed the few-layer devices as well as the vertically integrated devices. The vertical heterostructures are characterized by a variety of techniques. The challenges associated with achieving a vertical p-n junction are discussed. Interesting effects such as negative differential resistance are also investigated. Our results suggest that while theoretical calculations show that it is possible to achieve novel devices such as tunnel FETs with vertical p-n heterostructures, it is challenging to achieve working devices even when very clean contacts are achieved.[3] Despite these challenges, we observe interesting phenomena in vdW contacted p-n vertical heterostructures.

References
[1] Lee, CH., Lee, GH., van der Zande, A. et al. Atomically thin p–n junctions with van der Waals heterointerfaces. Nature Nanotech 9, 676–681 (2014)
[2] Wang, Y., Kim, J.C., Wu, R.J. et al. Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors. Nature 568, 70–74 (2019)
[3] Chhowalla, M., Jena, D. & Zhang, H. Two-dimensional semiconductors for transistors. Nat Rev Mater 1, 16052 (2016)

Two-dimensional (2D) transition metal dichalcogenides (TMDs) like MoS₂ are promising candidates for the channel material in nanoscale transistors because they outperform Si at the ultra-thin monolayer limit [1]. However, the electrical performance of monolayer (1L) MoS₂ is limited by large contact resistance which is caused by Fermi level pinning [2] and reactions between the metal and MoS₂ [3]. To date, the most widely used contact metals to MoS₂ are Au [1] and Ni [4], both of which have failed to consistently yield contact resistance below 1 kΩ.µm. Such high melting point metals can cause damage when deposited directly on MoS₂ [5]. Recently, Shen et al. [6] demonstrated significant improvement of contact resistance to 1L MoS₂ using contacts made of Bi capped with Au. However, the low melting point (~271 °C) and the chemical instability of Bi, along with the incompatibility of Au with Si processing, prevent adoption into manufacturing for heterogeneous integration with Si technology.
In this study, we find that low melting point metals like In and Sn minimize the damage caused by metal deposition on 1L MoS₂ and yield some of the best contacts in the world today, while maintaining high temperature tolerance when properly alloyed. We studied contacts made of 10 nm In or Sn capped with 25 nm Au by fabricating and testing transfer length method (TLM) structures consisting of MoS₂ field-effect transistors (FETs). The best TLMs showed a contact resistance of 190 Ω.µm for In/Au contacts and 270 Ω.µm for Sn/Au contacts, which are among the lowest reported contact resistance values to date with 1L MoS₂. We carried out X-ray diffraction (XRD) and depth-profiled X-ray photoelectron spectroscopy (XPS) and found that, after a 2-hour 250 °C vacuum anneal, the low melting point metal alloyed completely with Au. The resultant alloys have a melting point greater than 480 °C, enabling In/Au and Sn/Au contacts that can withstand back-end-of-line (BEOL) processing temperatures. These devices survived encapsulation by atomic-layer deposition (ALD) at 250 °C, which is above the melting point of In and Sn, and maintained their good on-off ratio and high on-current.
We performed a similar study on Bi/Au contacts to 1L MoS₂ and found that the best TLM showed contact resistance of 700 Ω.µm after a 150 °C vacuum anneal. However, annealing at 250 °C caused the formation of voids in the Bi/Au metal lines, as confirmed by scanning electron microscopy (SEM), making the lines highly resistive. XRD and XPS showed that Bi and Au do not alloy together, even after the anneals, leading to melting of Bi. To increase the temperature tolerance of Bi-based contacts and to improve the compatibility of our novel alloyed contacts, we investigated the alloying of these lower melting point metals (In, Sn, and Bi) with Si-compatible high melting point metals (Pd and Pt). After a 250 °C anneal, XRD and XPS measurements show that In/Pd, Sn/Pt, and Bi/Pd alloy into compounds which have melting points above 480 °C.
In summary, we have demonstrated ultralow contact resistance using alloys between low and high melting point metals and laid out a path towards CMOS compatibility of this contact scheme. These findings could pave way for the adoption of In, Sn, or Bi contacts to 2D semiconductors with Si-compatible manufacturing.
References:
[1] C. D. English, K. Saraswat, E. Pop, et al., Nano Letters, vol. 16, 3824, 2016.
[2] S. Das, et al., Nano Letters, vol. 13, 100, 2013.
[3] S. McDonnell, et al., ACS Applied Materials and Interfaces, vol. 8, 8289, 2016.
[4] Q. Smets et al., International Electron Devices Meeting, 2019.
[5] K. Schauble, E. Pop, et al., ACS Nano, vol. 14, 14798, 2020.
[6] P.-C. Shen et al., Nature, vol. 593, 7858, 2021.

High contact resistance is a key limiter to the electronic performance of two-dimensional (2D) semiconductors like MoS₂. Some of the best contacts to monolayer (1L) MoS₂ to date are made with non-oxidizing metals like Au [1] and Ag [2]. However, these metals follow a Volmer-Weber (VW) growth mode and have low diffusive energy on MoS₂ [3], which causes them to deposit as islands [4], limiting their ability to form good contacts due to incomplete coverage. Depositing these metals can also damage the fragile 1L 2D semiconductor [4] and Ag can react with MoS₂ when annealed [5], which leaves room for improvement when using these and similar metals as contacts.
Here we study, for the first time, the role of an ultrathin Ge interlayer for Ag contacts to 1L MoS₂. Ge also follows a VW growth mode but with much smaller and denser nuclei, such that a thin layer of Ge is known to improve the uniformity of Ag deposition on SiO₂ [6]. We find the Ge interlayer improves the uniformity of the interface between Ag and MoS₂ and reduces the MoS₂ damage caused by otherwise promising Ag contacts.
We prepare 1L MoS₂ transistors on 90 nm SiO₂ on a highly-doped Si back-gate, and electrically test four different contact stacks: 0, 1, 2, and 3 nm Ge followed by ~35 nm Ag, deposited by e-beam evaporation. All 1L MoS₂ in this work is directly grown by chemical vapor deposition [2]. All transistors displayed on/off current ratios > 10⁶, but those with 1 nm Ge interlayer contacts demonstrate the best electrical performance, i.e. maximum on-state current of 220 μA/μm at V_DS = 1 V with carrier density n = 1.2×10¹³ cm^-2 for a 300 nm channel device after a 250 °C vacuum anneal. With this contact scheme, we measured contact resistance as low as 340 Ω.μm using transfer length method structures with channel lengths from 300 nm to 1 μm. These results show >10x improvement over pure Ag contacts