Symposium S.NM09—Layered van der Waals Heterostructures—Synthesis, Physical Phenomena and Devices

Until recently, flat bands were achieved as Landau levels at high magnetic field. Then, Pablo Jarillo-Herrero of MIT and coworkers demonstrated flat minibands in graphene-based superlattices, discovering correlated insulators and superconductors at different fillings. We have now discovered dramatic magnetic states in such systems. Specifically, in magic-angle twisted bilayer graphene also aligned with a hexagonal boron nitride (hBN) cladding layer, we observe a giant anomalous Hall effect and signs of chiral edge states. This all occurs at zero magnetic field, near 3 electrons per moire cell in the conduction miniband [1]. Remarkably, the magnetization of the sample can be reversed by applying a small DC current. Although the anomalous Hall resistance is not quantized, and dissipation is significant, we suggest that the system is an incipient Chern insulator, similar to an integer quantum Hall state. In a different superlattice system, ABC-trilayer graphene aligned with hBN, again near 3 electrons per moire cell a Chern insulator emerges [2]. A magnetic field of order 100 mT is needed to quantize the anomalous hall signal. This trilayer system can be tuned in-situ to display superconductivity instead of magnetism [3]. We will discuss possible magnetic states, and complementary probes to examine which state actually emerges as the ground state in each system.

[1] A.L. Sharpe et al., “Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene”, Science (2019).
[2] G. Chen et al., “Tunable Correlated Chern Insulator and Ferromagnetism in Trilayer Graphene/Boron Nitride Moire Superlattice”, arXiv:1905.06535 (2019).
[3] G. Chen et al., “Signatures of tunable superconductivity in a trilayer graphene moiré superlattice”, Nature (2019).

This research was primarily supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515.

2D layered materials enable the construction of hybrid heterostructures with emergert electronic, optical, and quantum properties resulting from the interplay between the individual nanosheets. However, the synthesis of 2D heterostructures with controlled twist angles and stackings is still a great challenge which currently limits their development. In this talk, I will discuss our recent work on the synthesis and assembly of 2D heterostructures through non-equilibrium synthesis and processing approaches including isoelectronic doping, atomic implantation, and van der Waals epitaxy. First, by tailoring isoelectronic doping of chalcogens and metals in 2D TMDs (e.g., MoSe₂, WS₂) during CVD synthesis, the uniform alloys, gradient alloys, and lateral heterostructures are controlled grown on substrates which exhibit many novel properties including tunable bandgaps, enhanced photoluminescence, modulated charge carriers, etc. Second, with well controlled kinetic energy of Se clusters inpulsed laser deposition (PLD) plasmas, the CVD-synthesized 2D WS₂crystals are precisely converted to monolayer Janus heterostructure and bilayer heterostructures by laser implantation. Last, I will briefly discuss the vdW epitaxial growth of GaSe/MoSe₂heterostructures and corresponding gate-tunable photovoltaic properties. The interlayer stacking, interfacial structure and optical properties are characterized by atomic-resolution STEM, Raman and PL spectroscopy. The atomistic origins of the emerging properties of vdW heterostructures, including interlayer excitons, persistent photoconductivity, and gate-tunable photovoltaic response, will be discussed. Our results demonstrate that with nonequilibrium synthesis and processing approaches, 2D heterostructures can be precisely realized for future applications in atomically thin optoelectronics and quantum information science.

Research sponsored by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facilities Div. (characterization science). Throughout the presentation, facilities available for collaboration at the Center for Nanophase Materials Sciences (CNMS) user facility will be presented.

Co-integrating graphene with other two-dimensional (2D) materials represents an important step toward the development of novel 2D material-based electronic and optoelectronic devices. In order to fully exploit the unique physical properties of graphene and the semiconducting transition metal dichalcogenides (TMDs) in functional applications, it is essential to control the crystallinity, the material sequence, the stacking order or twisting angle, and the interface cleanliness.

In this study, we investigate the direct growth of WSe₂by gas source chemical vapor deposition (CVD) on top of CVD grown graphene. Isolated millimeter-size graphene crystals are first produced by CVD on Cu foils and then transferred onto fused silica or sapphire which are more compatible with the gas-source CVD process. This graphene template is used as the synthesis substrate for the high temperature growth of WSe₂using tungsten hexacarbonyl (W(CO)₆) and hydrogen selenide (H₂Se) as precursors.

Using scanning electron microscopy (SEM) and atomic force microscopy (AFM), we compare the nucleation site density, size and shape of WSe₂crystals grown on single layer graphene, bare C-plane sapphire and bare fused silica. WSe₂crystals typically exhibit a triangular shape with a lateral size exceeding 1 micrometer.

The nucleation site density of WSe₂crystals has also been studied as a function of the number of graphene layers and their relative twisting angle. The samples have been characterized by Kelvin probe force microscopy (KPFM), transmission electron microscopy (TEM), and Raman spectroscopy in order to better understand why single-layer graphene leads to a lower WSe₂seeding density compared to twisted bi-/tri-layer graphene and an even lower density compared to AB stacked bi-/tri-layer.

WSe₂crystals grown on graphene are found to follow 2 specific orientations rotated by a 60° angle. The alignment of the zigzag edges of the triangular WSe₂crystals with the zigzag edges of the underlying hexagonal graphene domain is indicative of van der Waals epitaxy rather than remote epitaxy with the underlying substrate.

Finally, given that the graphene transfer process inherently leads to impurity intercalation, we investigate the impact of the transfer protocol and post-transfer annealing on the cleanliness of graphene/substrate and WSe₂/graphene interfaces. The cleanliness of the interface is mainly studied using photoluminescence (PL) and Raman spectroscopy.

In this talk recent advances on the synthesis and application of large-scale highly-crystalline van der Waals heterostacks will be presented. The electronic performance of single-crystal graphene arrays obtained via patterned growth [1] will be discussed in terms of homogeneity and repeatability. Particular focus will be put in the heterostack obtained by directly synthetizing via chemical vapor deposition (CVD) graphene bilayers and few layers with controlled angle [2, 3]. Also, the synthesis of azimuthally aligned tungsten disulfide (WS₂) on graphene will be presented [4, 5]. It will be shown via microstructural and electronic characterization that monolayer WS₂ on graphene presents a remarkable spin-orbit splitting [5]. Nanomechanical properties such as superlubric sliding of WS₂ flakes on graphene triggered by scanning probe microscopy will be discussed [6]. Furthermore, the fabrication and performance of an entirely scalable hybrid WS₂/graphene photodetector will be presented [7]. Finally, the synthesis of transition metal dichalcogenide heterostructures with controlled angle via liquid phase CVD will be introduced.
[1] V. Miseikis, F. Bianco, J David, M. Gemmi, V. Pellegrini, M. Romagnoli, C. Coletti, 2D Materials 4 (2), 021004 (2017).
[2] S. Pezzini et al., in preparation
[3] C. Bouhafs et al., in preparation
[4] G. Piccinini, S. Forti, L. Martini, S. Pezzini, V. Miseikis, U. Starke, F. Fabbri, C. Coletti, 2D Materials, Volume 7 (1), (2019)
[5] S. Forti, A. Rossi, H. Buch, T. Cavallucci, F. Bisio, A. Sala, T.O. Mentes, A. Locatelli, M. Magnozzi, M. Canepa, K. Muller, S. Link, U. Starke, V. Tozzini, C. Coletti, Nanoscale 9 (42), 16412-16419 (2017).
[6] H. Büch, A. Rossi, S. Forti, D. Convertino, V. Tozzini, C. Coletti, Nano Research 11 (11), 5946-5956 (2018)
[7] A. Rossi, D. Spirito, F. Bianco, S. Forti, F. Fabbri, H. Buch, A. Tredicucci, R. Krahne, C. Coletti, Nanoscale,10, 4332 - 4338 (2018).

Acknowledgement
The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement 785219 – GrapheneCore2.

Twisted moiré pattern introduces new phenomena to Van der Waals layered materials by periodically changing the local registry of atoms, making it a new degree of freedom for 2D material engineering. Conventional twisted structures are fabricated by mechanically stacking layers, however, it still remains challenging to get clean interfaces. Direct vapor phase growth may yield small fractions of twisted bilayers as byproduct, there is no rational control over the twist angle. In this work, we demonstrate that continuously twisted structures beyond Eshelby twist can be realized starting with screw dislocation spirals of layered MX₂ materials. Super twisted spirals of MX₂ materials up to tens of microns in lateral size are grown, and the orientation of layers and moiré superlattices can be directly observed and measured using scanning transmission electron microscope. Our study provides a new strategy to the rational control of twist angle of 2D materials from direct growth, which could enable the study of twisted moiré superlattices.

Vertical stacking of monolayers via van der Waals (vdW) interaction opens promising routes toward engineering physical properties of two-dimensional (2D) materials and designing atomically thin devices. Increasingly, the bottleneck in this field is the controlled synthesis of these materials through methods such as chemical etching and chemical vapor deposition (CVD). In this talk, I will present insights into synthesis and growth of 2D materials developed from analytical, phase-field, and machine learning models. First, we adapt the state-of-the-art positive and unlabeled (PU) machine learning framework to predict which theoretically proposed 2D materials in the MXene family have the highest likelihood of being successfully synthesized. By considering both the MXenes and their precursors, we identify 18 MXene compounds that are highly promising candidates for synthesis. Next, we develop a general multiscale model for scalable CVD growth of layered materials and predict the necessary growth conditions for vertical (initial + subsequent layers) versus in-plane lateral (monolayer) growth. An analytic thermodynamic criterion is established for multilayer growth that depends on the sizes of both layers, the vdW interaction energies, and the edge energy of 2D layers. We connect the model to experimental controls and find that temperature and adatom flux from vapor are the primary criteria affecting the self-assembled growth. This model agrees with experimental observations of various monolayer and bilayer transition metal dichalcogenides grown by CVD. Finally, we consider CVD synthesizable transition metal dichalcogenide heterostructures as a robust platform for engineering quantum confinement of Dirac fermions using a multiscale model for electronic properties.

The rapid development of two-dimensional (2D) materials calls for the development of novel 2D semiconductors with large bandgap for applications in short-wavelength LEDs, type-I heterojunctions, etc. Gallium sulfide (GaS), a wide-bandgap semiconductor, fulfils these requirements but was not fully explored due to lack of controllable synthesis methods, unexplored layer-dependent properties and unanswered stability issues. Here, to answer these questions, we developed a CVD synthesis method of thickness-controlled (1–4L), uniform and continuous films of both defective GaS and defect-free GaS. A simple method for determination of thickness by Raman spectroscopy is developed. An enhanced stability of the defect-free GaS was demonstrated under laser and strong UV lights, and by controlling defects in GaS, the photoresponse range could be changed from vis-to-UV to UV-discriminating. The defect-free GaS is suitable for large-scale UV-sensitive high-performance photodetector arrays for information encoding under large vis-light noise, with short response time, excellent UV photoresponsivity and a 26-times increase of signal-to-noise ratio compared with small bandgap 2D semiconductors. By comprehensive characterizations from atomic-scale structures to large-scale device performances in 2D semiconductors, we provide insights into the role of defects, the importance of neglected material-quality control and how to enhance device performance. Finally, both layer-controlled defective and defect-free GaS prove to be promising platforms for study of novel phenomena and new applications in WS₂-GaS van der Waals heterojunctions.

The firmly defined dimensionalities of two-dimensional (2D) layered materials including transition metal dichalcogenides (TMDCs) exposed numerous unusual properties that have been recently at the center of the quantum materials and information sciences research. In pursuit of the accelerated growth and discovery of 2D materials, many efforts have concentrated on developing new approaches including physical and chemical vapor deposition techniques. However, complex, uncontrolled gas-phase reactions, and flow dynamics have made the synthesis of these multi-component 2D crystals exceedingly challenging. This work demonstrates a novel laser-assisted synthesis technique (LAST), which significantly reduces the existing growth complexities and remarkably accelerates the growth of 2D materials. The uniqueness of this approach arises from the direct vaporization technique of stoichiometric powders by the laser heating process. We show that this directed laser heating permits pressure-independent decoupling of the growth and evaporation kinetics, enabling the use of stoichiometric powder as precursors for the growth of various high-quality 2D materials including MoS₂, MoSe₂, WSe₂, and WS₂.

Mr. Nurul Azam is currently a Ph.D. student in the Department of Electrical and Computer Engineering (ECE) at Auburn University (AU). His research in the Laser-Assisted Science and Engineering (LASE), Lab under the supervision of Prof. Mahjouri-Samani, focuses on investigating and developing novel approaches for the synthesis of 2D quantum materials and heterostructures.

In the past decade, there have been unprecedented advancements in research and development of materials for quantum applications such as ultrasensitive detection, quantum information processing, energy efficient ultrafast electronics, quantum light-emission, and many more that require a fundamental understanding of never-before observed physical and chemical phenomena in known materials. Of these new generation of materials, transition metal dichalcogenides (TMDs) have become attractive candidates for more extensive investigation due to their novel and non-classical behaviors observed at nanoscale as well as potential for their incorporation into devices and applications. For instance, nanoscale TMDs such as tungsten diselenide (WSe₂), tungsten disulfide (WS₂), and molybdenum disulfide (MoS₂) have been shown to exhibit highly localized, intense, quantum emission of single photons that have not been previously observed in their bulk counterparts. While understanding fundamental mechanism and origin, as well as the physical structure of these quantum materials and their properties are crucial, parallel efforts in developing large scale, controlled synthesis of these materials is just as imperative. Currently, much of the studies on TMD materials for quantum applications have primarily been on larger flakes and crystals, or exfoliated films, which limits scalability of the material production as well as the need for transfer of the exfoliated material onto a suitable substrate.

In this work, we present a systematic synthesis of 2-dimensional, single- to few-layers of transition metal chalcogenides thin film directly on silicon substrate via chemical vapor deposition (CVD). Our approach not only sheds light into understanding the growth mechanism of 2-dimensional TMDs, but also provides optimal growth parameter for consistent, large scale production of these materials. The results presented will provide basis for synthesis of next-generation of 2-dimensional materials as well as shed light into bottom-up building approach of van der Waals heterostructures for quantum applications.

*This work was partially supported by the Office of Naval Research through Naval Research Laboratory Base Program.

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent charge oscillations that exist in strongly optically anisotropic 2D materials (e.g., MoO₃). These polaritons propagate through the material’s volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffraction dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈ 2 ps) in single-crystal α-MoO₃ are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO₃ crystals deposited on periodic gratings showed longer HPhPs propagation lengths (≈ 2 ×) and lower optical compressions in suspended regions compared to regions in direct contact with the substrate. Additionally, PTIR data reveal polymeric contaminants, resulting from sample preparation, localized under parts of the MoO₃ crystals. This work enhances the ability to engineer nanophotonic devices by leveraging substrate morphology to control polariton propagation.

Two-dimensional (2D) heterostructure semiconductors are emerging as the potential candidates for broad applications, in which the transition-metal dichalcogenides (TMDs) have attracted the most attention due to their abundant advantages. TMDs have attracted tremendous interest due to the advantages of low-cost, earth-abundant, non-toxic, and environmentally friendly. Owing to the atomically thin structure, unique electronic and optical properties and rich diversity in chemical compositions, they have displayed considerable potential in the applications of integrated optoelectronic devices and systems such as p–n diodes, photodetectors, transistors, nanogenerators, and sensors as well as in catalysis. However, the investigations of these TMD heterostructure semiconductors usually only limit to the synthesis approach and opt-electronic properties, lacking the in-depth guidelines from the atomic exploration. Within the lateral heterojunction, the unique ripple effect has been identified as the consequence of the important interplay between different TMDs. Herein, the anion coupling induced charge disturbance within the WSe₂/WS₂ lateral heterojunction has been identified as the origin for the ripple structure induced by in-plane interfacial instability. Different scales of the first-principle simulations all verify the similar ripple phenomenon near the interfacial region of the WSe₂/WS₂ lateral heterojunction. Beyond the simple factor of lattice misfit, we confirm that the Se-S p-p coupling effect induced the uneven charge distribution is the intrinsic reason for such a unique structural phenomenon. By the introduction of the perturbation elastic entropy concept, we successfully predict the interfacial ripple scale in the WSe₂/WS₂ lateral heterojunction systems, supporting the simulation of experimental size. This work has compensated the knowledge gap between the theoretical investigation and experimental synthesis, which is essentially beneficial for the future fabrication of 2D heterostructure semiconductors with superior performance. The fundamentally general mechanism for the experimentally observed phenomenon is of pivotal significance in the further exploration in such 2D heterojunction semiconductor materials.

The method of mathematical modelling is a good tool for constructing new materials [1].
The present work considers the problem which concerns quantum sifting of isotopes of gas components at cryogenic temperatures. The authors also focus on the task of constructing the membrane itself. The calculations show that the membrane should have two identical ultrathin layers. By now, graphene nitride with regular pores has been synthesized using the double epitaxy method [2]. Mono-atomic layers of porous boron nitride [3] can also serve as the working layer of the membrane. Over the last decade, a group of new graphene-like mono-element 2D materials, such as germanene [4], silken [5, 6], phosphorus [7], and stanene [8] have also been synthesized. In principle, all porous analogues of these materials are suitable for application as the working layer of the membrane. This appears to be possible since the result of resonant passage of one of the mixture components is largely determined by the distance between the plates.
The considered problem of particles passing through a composite barrier is a generalization of the well-known Landau problem of particles with a certain energy passing through a unitary barrier. When constructing an analytical solution to the generalized problem of wave dynamics, the authors proceeded from the Schrödinger integral equation. The solution was built as a result of dividing the functions containing a shift in the independent variable into a nonlinear differential operator and separated exponential functions of the parameter and the argument. Next, algebraic operations with differential operators were performed, and the resulting operator was applied to separated functions. The resulting solution for the wave function is completely determined by the spectrum of the composite barrier. Studying the asymptotic behaviour of the solution allowed for finding the zeros of the reflection coefficient. As a result, resonance modes of transmission of one of the components were found. These modes were determined by the sequence of resonant distances between the plates. Choosing the distance between the layers from this sequence it is possible to achieve a selectivity effect in which a particle with a mass of m and energy of E passes through a potential barrier consisting of two identical parts without reflection. In other words, one of the components shows complete passage, while the other component is characterized by partial passage only. This ensures operation of the membrane as a gas separator. The calculations show that, at certain temperatures, it is possible to achieve hyperselectivity of mixture separation.

References:
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Synthesis and study of new two-dimensional (2D) materials nanosheets is very important for both fundamental study of physical phenomenon as well as for new applications in nanoelectronics and optoeletronics. Lead Iodide (PbI₂) has been reported to be an excellent candidate for applications in thin films, transistors and PbI₂ perovskites. However, not a lot of study has been carried out on easy and facile preparation of this material to produce scalable monolayer films. Liquid phase exfoliation (LPE) is considered to be an efficient method to obtain 2-dimensional nanosheets on a large scale. Based on previous findings on screening solvents for LPE, in this study, I will present results on obtaining several hundreds of >99% monolayer (PbI₂) using CHCl₃ as a solvent. We have then used atomic-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) imaging to study the monolayer flakes of PbI₂. Here, I will report on a new structure of PbI₂ which has not been studied before. PbI₂, when suspended on graphene, converts into more energetically stable H-phase crystal structure compared to T-phase structure which was found to be more stable on lacey carbon TEM grid. I will also present results on the defects and edge structure of the monolayer PbI₂ and the unusually mobile atoms on the surface. The structural change of the PbI₂ depending on substrate, i.e. graphene, opens up the possibility to control and further study its properties by using other different kinds of substrates.

*Sapna Sinha, Arthur France-Lanord, Yuewen Sheng, Jeffrey C. Grossman, Kyriakos Porfyrakis, Jamie H. Warner. Atomic Structure and Defect Dynamics of 2D Monolayer Lead Iodide Nanodisks with Epitaxial Alignment on Graphene, (accepted, in press, Nature Comms.)

Layered van der Waals Heterostructures such as MoS₂ monolayers have attracted great attentions for the application in optoelectronic devices owing to their outstanding physical, chemical, and mechanical properties. Chemical vapor deposition (CVD) is commonly used for scalable growth of layered materials. During CVD synthesis, reactions of MoO₃ and H₂S reactants are essential reaction steps where MoO₃ flakes are reduced and sulfurized, transferring to MoS₂ crystals. Recent studies suggested that addition of H₂ gas in this synthesis process could lead to synthesis of higher-quality MoS₂ monolayers. However, effects of H₂ partial pressure on synthesis of MoS₂ still remain elusive. Here, we present quantum molecular dynamics (QMD) simulations for CVD synthesis of MoS₂ layer using MoO₃ flakes and H₂S gas precursors with and without H₂ gas. Our QMD simulations revealed that H₂ molecules indeed act as effective reducing agents for the MoO₃ flake. As such, our work provides a valuable input for the scalable growth of layered van der Waals Heterostructures.
This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC00014607.

Emerging quantum materials, such as novel two-dimensional (2D) materials and topologically nontrivial materials, have gained increasing attention due to their unique electronic and photonic properties. The realization of the optoelectronic applications of these materials still faces several challenges. For example, it is critical to gain clear understandings of (1) the fundamental light-matter interactions, which govern many of the key material properties, and (2) the coupling with other nanostructures, which is a required structure for devices and systems. This talk introduces new discoveries and pioneering works using optical spectroscopic techniques on these critical challenges, and novel applications of 2D materials in chemical and biological sensing. The first part of this talk presents the essential material properties investigated using spectroscopy, including interlayer coupling of Moirè patterns of 2D materials, anisotropic light-matter interactions of 2D quantum materials, and quantum optical emission properties of nanoengineered 2D layers. The spectroscopic techniques, including low-frequency Raman spectroscopy and polarization-and time-resolved spectroscopy, can also be widely used in other material systems such as semiconductor superlattices. The second part of this talk focuses on the interaction of 2D materials with other nanostructures and related applications. The interactions of 2D materials and selected organic molecules revealed novel enhancement effect of Raman signals for molecules on 2D surface, which offers a new paradigm in biochemical sensing. However, there is a selection rule regarding the 2D material substrates for molecules, which is critical for the design of biochemical molecular sensing platform and has been revealed recently. Overall, the works presented in this talk are significant in fundamental nanoscience, and offer important guidelines for practical applications in optoelectronics, sensing, and quantum technologies. The methodologies used here also provide a framework for the future study of many new low-dimensional and quantum materials.

Graphene is ideally suited for next generation communication systems [1], due to its unique optoelectronic properties [2] and CMOS compatibility [1]. Here, we present a graphene-based photodetector (GPD) for telecom wavelengths, combining an on-chip Si ring-cavity with high-mobility (〉10,000 cm²/Vs) single layer graphene (SLG) encapsulated in hBN. The constructive interference inside the ring results in a ∼100% absorption of the guided light. Dual-gate electrodes on the active layer generate a p-n-junction in SLG, enabling a response due to the photo-thermoelectric (PTE) effect [3]. At optical input power levels ~ 0.1mW, comparable to those required by receivers used in 100 GBs^-1 links [1], the GPD reaches a responsivity ∼ 90 V/W, one order of magnitude higher than the highest reported to date for integrated PTE based GPDs [4], an electrical 3-dB cut-off frequency ∼ 11 GHz, and a noise-equivalent power ∼ 40 pW/√Hz. For higher input powers (~ 1mW), the strong light-matter interaction inside the Si ring results in electronic temperature increases of the photo-excited carriers ∼ 100 K, and the transition to the ‘strong heating’ regime, characterized by sublinear scaling between photovoltage and input power [5]. Our devices can thus provide a route to studying strongly heated hot electrons in photo-excited SLG under continuous-wave illumination.

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[3] N. Gabor et al., Science, 334, 648, (2011).
[4] J. E. Muench et al., arxiv: 1905.04639, (2019).
[5] K. Tielrooij et al., Nat. Nanotechnol., 10, 437, (2015).

2D semiconductors based on TMDCs are highly promising materials for ultrathin, flexible and large-area photodetectors due to their thickness in the nm range and their effective absorption. The high exciton binding energy in these materials leads to a fast recombination and poor charge carrier separation, which represents an inherent drawback of such devices. To overcome this challenge, 2D heterostructures have been developed, separating optically generated charge carriers and thus increasing photoresponsivity. However, up to now, such heterostructure devices have been fabricated by stacking exfoliated flakes [1] or by transferring CVD-synthesized layers on top of each other [2]. These approaches suffer from process-induced defects and contaminations, and are hardly suited for industrially relevant fabrication routes.

We realized photodetectors based on TMDC heterostructures that are directly grown by MOCVD on a sapphire substrate [3], without any change between systems and without delicate transfer steps that bear the risk of uncontrolled contamination. The 1 mm² active area of the devices consists of a MoS₂/WS₂-heterobilayer, and interdigital contact patterns have been defined by electron beam lithography and metallization with Ti/Au. As a reference, devices based on MoS₂ and WS₂ monolayers, respectively, have been fabricated.

The photocurrent of the devices is analyzed under illumination with a defocussed laser (λ = 442 nm). Using either MoS₂ or WS₂ monolayers for light detection, only very low responsivities - far below 1 mA/W - are obtained. Obviously, low carrier mobilities and fast recombination rates are detrimental for the photosensitivity in monolayer devices. The situation is completely different for the directly grown MoS₂/WS₂-heterostructures: A several orders of magnitude enhanced responsivity of up to 16 A/W is achieved under an illumination of 45 mW/cm², corresponding to absolute photocurrents in the mA range. We attribute the large gain not only to an effective charge carrier separation, but also to an efficient hole trapping, leading to an enhanced electron flow in the heterostructure. This interpretation is consistent with the observation of a reduced responsivity for higher excitation densities, where these trap states become saturated.

[1] Long et al., ACS Nano 13, 2511 (2019)
[2] Tan et al., Adv. Mater. 29, 1702917 (2017)
[3] Andrzejewski et al., Nanotechnology 29, 295704 (2018)

Two-dimensional materials are inherently anisotropic – with strong bonding in-plane and weak bonding out-of-plane. Consequently, their optical properties are birefringent, particularly at frequencies of the optically active infrared phonons. At these frequencies, the strong optical anisotropy results in hyperbolicity, where the permittivity tensor along at least one axis is negative, while at least one is positive. Hexagonal boron nitride (hBN) is an exemplary material in this regard. Materials which exhibit hyperbolicity can support volume-confined polaritonic modes, which have a wavelength much shorter than that of light in free space. Such spatially compressed modes have important implications for IR technologies, as they may be utilized to create planar meta-optics that are much more compact than the current state of the art. In particular, 2D materials can be used to realize metasurfaces, capable of shaping both nearfield- and far field light waves for a broad range of applications.
Conventional metasurface designs use geometrically fixed structures, or materials with excessive propagation losses, thereby limiting their potential applications. Here demonstrate that this can be overcome through the realization of a reconfigurable hyperbolic metasurface comprising a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM), such as single-crystal vanadium dioxide (VO₂) or GST. Here, the metallic and dielectric domains in PCM provide spatially localized changes in the local dielectric environment that modify the polariton wavelength supported in hBN by a factor of 1.6. Using this platform, we demonstrate the first reduction to practice of in-plane HPhP refraction, and the means for launching, reflecting and transmitting of HPhPs at the PCM domain boundaries. Ultimately, this phenomenon can be used to create planar refractive optics, such as lenses or waveguides, but on length scales far below the diffraction limit. Further, my employing doped semiconductor materials with controllable plasma frequencies, such direct control of hyperbolic polaritons can also be obtained via modification of the free carrier density of the underlying semiconductor.

Heterostructuring of 2D materials, through the formation of vertical or lateral structures, may provide access to a multitude of novel photo-physical and electronic properties than can be tailored through a wide breath of available 2D materials. In this research, we will focus on chemical vapor deposition grown lateral heterostructures of MoS₂/WS₂with a goal of measuring and understanding the local carrier distribution and conductivity across the atomically precise material interfaces. We have used advanced atomic force microscopy methods, including scanning microwave microscopy to elucidate the local, spatially varying carrier distribution in 2D MoS₂/WS₂under dark conditions as well as under illumination with photon energy-resolved narrowband illumination. Our results show that strong spatial variations in the photoconductive response exists, even within individual flakes that were imaged. We corroborate our analysis with spatially resolved photoluminescence mapping and discuss our results in terms of the dynamics of long-lived photo generated carriers.

Two-dimensional (2D) transition metal dichalcogenides have been the subject of sustained research interest due to their extraordinary electronic and optical properties. At the same time, they are also a very rich system from the perspective of structural phases and phase transitions. Thus far transformation from 2H semiconducting to 1T metallic phase in MoS₂ has been well demonstrated. Likewise, thermally induced defect creation, etching and oxidation have also been well studied. However, one aspect of structural properties of semiconducting TMDCs that has been understudied is the structural transformation in a highly non-equilibrium or far out of equilibrium thermodynamic conditions. In this talk, we will discuss our recent progress in studying disintegration and phase evolution of atomically thin MoS₂ under varying temperatures (upto 700 C) and heating rates (25 C/sec). We probe these transformations at the atomic scale using aberration corrected scanning transmission electron microscope (AC-STEM) while heating the samples in-situ. We observe strong dependence of the resulting structures and phases on the heating rates and temperatures. In particular, for fast heating rates (~25 C/sec), we observe abrupt melting of the few layer regions into high ordered and crystalline, hexagonal islands of sizes < 20 nm. Importantly we also find, lateral heterostructures of 2H and 3R phases including presence of ordered, intermediate phases and sharp boundaries. For slower heating rates (~25 C/min) ex-situ in a furnace, this transformation occurs via sulphur sublimation resulting in formation of etch pits to nanocrystalline and amorphous regions that are sub-stoichiometric. We further use Electron Energy Loss Spectroscopy (EELS) to distinguish atomic level electronic structure information. The use of non-equilibrium heating rates to achieve highly crystalline and quantum confined features from 2D atomic layers opens new avenues for fundamental materials engineering and applications in confined dimensions.

In moire flat band systems, interlayer moire patterns can be used to engineer isolated superlattice bands in which the Coulomb interaction is comparable to the bandwidth. As in a Landau level, this leads to electron interaction dominated physics despite the low density of electrons, enabling full density control using electrostatic gates. Unlike Landau levels, however, moire flat bands occur under time reversal symmetric conditions. I will discuss experiments that observe the spontaneous breaking of time reversal symmetry, observed as magnetic hysteresis. Remarkably, hysteresis is accompanied by a quantized anomalous Hall effect, persisting to zero magnetic field and elevated temperatures. I will discuss the origins of this effect in the spontaneous orbital polarization of the electron system into a single, topological nontrivial superlattice band, and describe magnetic imaging data that confirms the orbital character of the magnetism.

For more than 50 years, magnetophonon resonance (MPR) has proved to be a powerful tool for investigating electron-phonon interactions in a wide range of elemental and compound semiconductors and heterostructures. Its experimental signature is a series of oscillations in the magnetoresistance, periodic in inverse magnetic field, given by the equation, p ω_c = ω_p , where p is an integer, ω_c is the Landau level splitting and ω_p is the phonon frequency. The successful observation of MPR in graphene has proved elusive due, in part, to the relative weak interaction of the massless Dirac Fermions with the lattice phonons.

Here we report the observation of large amplitude, temperature-dependent MPR oscillations in the magnetoresistance of large area, electrostatically gated Hall bars of exfoliated monolayer graphene encapsulated by hexagonal boron nitride (hBN) [1,2]. The oscillations are observed only in devices with a width which exceeds the ballistic carrier mean free path, 5 μm. We observe two distinct series of resonant peaks arising from inelastic scattering by transverse acoustic (TA) and longitudinal acoustic (LA) phonons. The LA phonon resonant amplitude is significantly weaker than that of the TA phonons. This is due, in part, to the carrier screening of the LA phonon modes. The magnetic field values of the resonant peaks are found to shift linearly with carrier sheet density from 10¹² to 10¹³ cm^-2. These experiments also allow us to measure the dispersion curves of the LA and TA phonons up to wavevectors of ~ 10⁹ m^-1. We model the experimental data to high accuracy using the Kubo formalism. We demonstrate that the LA and TA phonon speeds and the electron-phonon coupling strengths determined from the magnetophonon resonance measurements provide an excellent fit to the measured dependence of the resistivity at zero magnetic field over a temperature range of 4-150 K. Thus, MPR provides a spectroscopic tool for studying electron-phonon interaction in two-dimensional semiconductors.

[1] P. Kumaravadivel et al. Strong magnetophonon oscillations in extra-large graphene, Nat. Commun. 10, 3334 (2019).
[2] M.T. Greenaway et al., Magnetophonon spectroscopy of Dirac fermion scattering by transverse and longitudinal acoustic phonons in graphene, Phys. Rev. B 100, 155120 (2019)

We demonstrate electronic constrictions on bilayer graphene that can be completely pinched off [1]. Split-gate devices in combination with a graphite backgates enable electronic tunability of the devices. The level spectrum of quantum point contacts in bilayer graphene has a peculiar magnetic field dependence quite different from what is known from semiconductors. We understand the details of this spectrum [2] based on the Berry curvature in bilayer graphene.
In few electron and hole quantum dots confined by p-n junctions [3] we investigate valley and spin splitting. Also many electron quantum dots with standard tunneling barriers as well as double and triple quantum dots [4] can be realized with an electronic quality comparable to the best dots fabricated in standard semiconductor environments. The spectrum of excited states in graphene dots is qualitatively different [5] compared to semiconductors because of the valley degree of freedom. These experiments open the door for using graphene quantum dots as spin qubits with potentially long coherence times.
We also discuss the properties of MoS₂ as a host material [6,7] for quantum dot-based qubits.
Work done in collaboration with A. Kurzmann, M. Eich, H. Overweg, M. Mangold, F. Herman, P. Rickhaus, R. Pisoni, Y. Lee, R. Garreis, C. Tong, K. Watanabe, T. Taniguchi, and T. Ihn

References
[1] H. Overweg, H. Eggimann, X. Chen, S. Slizovskiy, M. Eich, R. Pisoni, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, V. I. Fal'ko, T. Ihn, and K. Ensslin, Nano Lett. 18, 553 (2018)
[2] H. Overweg, A. Knothe, T. Fabian, L. Linhart, P. Rickhaus, L. Wernli, K. Watanabe, T. Taniguchi, D. Sanchez, J. Burgdörfer, F. Libisch, V. I. Fal’ko, K. Ensslin, and T. Ihn, Phys. Rev. Lett. 121, 257702 (2018)
[3] M. Eich, F. Herman, R. Pisoni, H. Overweg, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, M. Sigrist, T. Ihn, and K. Ensslin, Phys. Rev. X 8, 031023 (2018)
[4] M. Eich, R. Pisoni, A. Pally, H. Overweg, A. Kurzmann, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, K. Ensslin, and T. Ihn, Nano Lett. 18, 5042 (2018)
[5] A. Kurzmann, M. Eich, H. Overweg, M. Mangold, F. Herman, P. Rickhaus,R. Pisoni, Y. Lee, R. Garreis, C. Tong, K. Watanabe, T. Taniguchi, K. Ensslin, and T. Ihn, Phys. Rev. Lett 123, 026803 (2019)
[6] R. Pisoni, A. Kormányos, M. Brooks, Z. Lei, P. Back, M. Eich, H. Overweg, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, A.Imamoglu, G. Burkard, T. Ihn, and K. Ensslin, Phys. Rev. Lett. 121, 247701 (2018)
[7] R. Pisoni, T. Davatz, K. Watanabe, T. Taniguchi, T. Ihn, and K. Ensslin
Phys. Rev. Lett. 123, 117702 (2019),

Atomically thin crystals such as graphene, boron nitride and the transition metal dichalcogenides continue to attract enormous interest. Encompassing a wide range of properties, including single-particle, topological and correlated phenomenon, these 2D materials represent a rich class of materials in which to explore both novel physical phenomenon and new technological pursuits. By integrating these materials with one another, an exciting new opportunity has emerged in which entirely new layered heterostuctures can be fabricated with emergent properties beyond those of the constituent materials. In this talk I will discuss some of our recent efforts where, by tuning the geometry of these heterostructures at the nanoscale, we are able to realize yet a new level of control over their electronic properties. In particular I will discuss the significant role played by the rotational alignment between adjacent layers and the approach we are taking towards manipulating this degree of freedom to dynamically tune device properties in ways that are not possible with conventional materials.

Twisted bilayer graphene (TBLG) is a new and versatile platform to realize effects of strong electron-electron interaction as the mis-orientation angle (theta) between the graphene lattices profoundly affects the electronic structure of the combined system. While the layers behave independently at large theta (> 3 degree), new electronic bands emerge when theta is decreased, including nearly flat bands at the magic angle of 1.1 degree that has been shown the harbor superconductivity and magnetism. In addition of direct electrical transport, thermoeletric properties are also highly sensitive to electronic correlations, and often manifest in departure from the well-established Mott semiclassical framework. Here we present the results of measurement of thermoelectric power in TBLG over a wide range of theta. We have shown that thermoelectricity in TBL at large theta (> 2 - 3 degree) is expectedly determined by independent excitations in the two graphene layers, and can be described by the Mott relation [1,2]. At low theta (< 2 degree), however, we observed a strong departure from the semi-classical description, which is most pronounced at the half-filling of the underlying Moire lattice, and persists up to temperatures as high as 40 K [3]. In accordance with the strong enhancement in the electronic interactions at half filling, our experiments suggest the possiblity of a novel interaction-driven thermoelectricity in TBLG.

[1] P. B. S. Mahapatra et al. Nano Lett. Nano 17, 6822–6827 (2017).
[2] P. B. S. Mahapatra et al. arXiv:1910.02614
[3] B. Ghawri et al. (To be submitted, 2019)

The valley degree of freedom in two-dimensional (2D) honeycomb lattices has attracted much recent attention due to its non-trivial Berry curvature effects associated with two nonequivalent valleys. This feature gives rise to many interesting valley-dependent phenomena such as the valley optical selection rule and the valley Hall effect, each of which accessing the valley-dependent electron control using the optical and electrical means. The valley manipulation in these works, however, focus on the control of the electron occupation while the Berry curvature at the two valleys are remained constant. In this talk, we show the mechanical control of the valley degree of freedom which is enabled by the strain engineering of the Berry curvature. By fabricating flexible monolayer MoS₂ devices, we reversibly control strain on monolayer MoS₂ and demonstrate the generation of the current-induced valley magnetization due to the formation of the Berry curvature dipole. The measured valley magnetization is in excellent agreement with the calculated Berry curvature dipole, which depends on the magnitude and direction of strain. Our results show that the Berry curvature dipole acts as an effective magnetic field in current-carrying systems, providing a novel route to generate magnetization in 2D crystals.

Low-dimensional nanomaterials with properties distinctive from their bulk counterparts have been extensively investigated in recent years. Besides the widely explored graphene and transition metal chalcogenides (TMDCs), a new family of van der Waals (vdW) materials composed of group IV and VI elements, especially Ge/Sn monocalcogenides, have attracted great attention in recent years. They have shown strong in-plane anisotropy that is similar to phosphorene (black phosphorous layers), and a number of unique electrical, optical, thermal and mechanical properties. Here I would like to share some of our recent discoveries on this unique family of materials, including their valleytronic behaviors and Eshelby twist formation in crystal growth.

Unlike conventional valleytronic materials where cryogenic temperatures, strong electric or magnetic field are needed to differentiate the valleys, Tin sulfide (SnS) was expected to have non-degenerate valleys with selection rules that are fundamentally different. I will talk about our experimental demonstration of the valley effect in an ambient, and bias-free model system of SnS and its alloys with other IV-VI materials. We elucidate the direct access and identification of different sets of valleys, based primarily on the selectivity in absorption and emission of linearly polarized light by optical reflection/transmission and photoluminescence measurements, and demonstrate strong optical dichroic anisotropy of up to 600% and nominal polarization degrees of up to 96% for the two valleys with band-gap values 1.28 and 1.48 eV, respectively; the ease of valley selection further manifested in their non-degenerate nature. In addition, the band gap values can be tuned effectively through alloying of SnS with similar materials such as SnSe. Such discovery enables a new platform for better access and control of valley polarization.

The IV-VI material family is also enabling new opportunities from twisting topology generation. Twisting of vdW layers has been shown to provide a new degree of freedom to tailor the electrical and optical properties of vdW crystals. I will talk about the first observation of Eshelby twist in Ge monochalcogenides that results in continuous twisting of multiple vdW layers. We exploit the screw dislocation driven growth of GeS nanowires, with growth direction perpendicular to the vdW layers. Results from comprehensive characterization on the helical vdW crystals agree well with Eshelby’s theory. Moreover, the planar growth of the vdW layers co-exists with the screw dislocation driven growth and leads to radial expansion of the nanowires, which results in discretized Eshelby twists resembling a helical assembly of nanoplates. We attribute the discretization to the change of twist rates of the nanowires in conjunction with a substrate pinning effect. We further show that the twisting topology can be tailored by controlling the radial size of the structure. In these helical crystals, atomically sharp interfaces with various twist angles are created, providing an intriguing tunable platform to explore various interlayer coupling effects in vdW materials.

Van der Waals heterostructures present unique opportunities to synthesize artificial quantum materials. For example, a slight lattice mismatch or relative rotation between the constituent layers of a hetero-bilayer results in a long-range moiré superlattice which spatially modulates the electronic band-structure. Single particle wavepackets can be trapped in the periodic potential pockets with three-fold symmetry to form an intrinsic quantum dot lattice. Here I will discuss the properties of such quantum emitter arrays. In a heterostructure of bilayer 2H-MoSe₂ and monolayer WSe₂, we observe two interlayer exciton (IX) species trapped in moiré potentials with distinct spin-layer-valley configurations. Due to the phenomenon of locked electron spin and layer pseudospin in bilayer 2H-MoSe₂, the IX species exhibit opposite valley magnetic moments. Further, we find the 2H-MoSe₂ stacking intrinsically locks the atomic registries of the spin-layer locked IX species together. I will also present photon antibunching of moiré trapped excitons to unambiguously prove their quantum nature. Finally, I will discuss the properties of the moiré quantum emitters under applied electric fields and changing chemical potential.

An indirect exciton (IX), also known as an interlayer exciton, is a bound pair of an electron and a hole confined in spatially separated layers. Due to their long lifetimes, IXs can cool below the temperature of quantum degeneracy. This gives an opportunity to realize and study cold excitons.

We will overview studies of Bose-Einstein condensation of IXs. We will present direct measurements of spontaneous coherence and condensation of IXs in GaAs heterostructures. We will present phenomena observed in the IX condensate, including the exciton density wave, spin textures, Pancharatnam-Berry phase and long-range coherent spin currents, and interference dislocations.

We will present van der Waals transition-metal dichalcogenide (TMD) heterostructures where IX condensation can be realized at high temperatures. We will present IXs at room temperature and indirect charged excitons, i.e. indirect trions, in van der Waals heterostructures.

Spontaneous symmetry breaking lies at the heart of the description of interacting phases of matter. Here we argue that a driven interacting system subject to a linearly polarized (achiral) driving field can spontaneously magnetize (acquire chirality). In particular, we find when a metal is driven close to its plasmon resonance, it hosts strong internal ac fields that enable Berryogenesis: the spontaneous generation of a self-induced Bloch band Berry flux, which supports and is sustained by a circulating plasmonic motion, even for a linear polarized driving field. This non-equilibrium phase transition occurs above a critical driving amplitude, and depending on system parameters, can enter the spontaneously magnetized state in either a discontinuous or continuous fashion. Berryogenesis relies on nontrivial interband coherences for electronic states near the Fermi energy generated by ac fields readily found in a wide variety of multiband systems. We anticipate that graphene devices, in particular, which can host high quality plasmons, provide a natural and easily available platform to achieve Berryogenesis and spontaneous non-equilibrium (plasmon-mediated) magnetization in present-day devices, e.g., those based on graphene plasmonics. If we have time, we will also discuss other manifestations of non-trivial quantum geometry in Dirac systems.

Organic-inorganic (hybrid) perovskite have recently emerged as a new semiconductor platform for next generation optoelectronics devices. These perovskite solids feature weak bonds between their organic and inorganic building blocks, which results in an intrinsic softness and dynamics disorder of the lattice and an acute sensitivity to external stimuli. Here, we present comprehensive in-situ studies of light induced structural dynamics of in layered two-dimensional (2D) hybrid perovskites. We correlate the changes in the structure of the 2D perovskite to modification of both the physical properties and the figures of merit in solar cells and light emitting devices. We propose a new microscopic model to explain the evolution of the structure and optoelectronic properties under external stimuli. These results demonstrate the direct correlation between structural characterization of halide perovskites under external perturbation and intrinsic physical properties.

Bi-dimensional nano-materials and related heterostructures are establishing themselves as intriguing material systems for the development of a new class of electronic, photonic and plasmonic devices with ad hoc-properties, that can be engineered “from scratch”. Huge potential can be envisaged in a variety of application fields, ranging from saturable absorbers to optical modulators, from optical communication modules to spintronics, from near-field components to photodetectors. Their peculiar band-structure and electron transport characteristics, which can be easily manipulated via layer thickness control, suggest they could also form the basis for a new generation of high-performance devices operating in the Terahertz frequency range (1-10 THz) of the electromagnetic spectrum. This talk will review latest achievements in the developments of active and passive THz photonic and nano-electronic devices exploiting 2D nano-materials and combined heterostructures and will discuss future perspectives of this rapidly developing research field.

The incorporation of magnetic dopants is a way to realize magnetism in semiconductors. The 2D material WSe₂ is a semiconductor with a bandgap of 1.3 eV, and Fe-doped WSe₂ is predicted to have room temperature, long-range ferromagnetism. Moreover, WSe₂ has been extensively studied for applications in electronic devices, and can be prepared by deposition methods that are compatible with semiconductor fabrication processes. 2D magnetic films are particularly intriguing in technologies that rely on exchange coupling since the efficiency of the exchange with the magnet is inversely proportional to the magnet thickness 1/t_FM. Chalcogenide-based TMD magnets can also have ideal interfaces with chalcogenide-based topological insulators (like Bi₂Se₃), enabling efficient spin torque devices and potentially realizing the quantum Hall effect without an external magnetic field.
Focusing on materials growth and integration techniques that utilize non-equilibrium, kinetically restricted strategies, coupled with in-situ characterization, enables the realization of atomic configurations and materials that are challenging to make but once attained, display enhanced and unique properties. In this work, we will highlight the growth of Fe-doped WSe₂ and report on its up to room temperature ferromagnetic properties. The evolution of Fe-doped WSe₂ films under different doping levels indicates that strain from the impurity atom itself coupled with strain imparted by the substrate can drive phase separation at high Fe concentrations. We find that suppressing that film strain helps promote more Fe incorporation (and higher Curie temperatures).
Cr₂O₃ has only 0.2% lattice mismatch with WSe₂ and is a magnetoelectric that can couple with ferromagnetic films. By using seed-layer techniques, we successfully prepared layered Fe-WSe₂ on Cr₂O₃ and demonstrate magnetic coupling between the two, a potentially promising route toward realizing electric field control of 2D magnets. Magnetic measurements show a clear hysteresis loop at 100 K in a 2 Tesla cooling field, from which a large negative bias field of 600 Oe is resolved. The bias field vs temperature mesurements, where a negative bias field emerges below 250 K, indicates the existence of ferromagnetic exchange coupling in this heterostructure.
This work is supported in part by NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST through award number 70NANB17H041. This work is also supported by the National Science Foundation under awards 1917025 and 1921818.

Novel electronic and optical properties of monolayer transition metal dichalcogenides (MX₂, M=W, Mo and X=S, Se) have garnered a lot of attention in the recent years. A major challenge to integrating these materials with existing device fabrication processes is the availability of epitaxial single-crystal monolayers over a wafer scale. Our approach focusses on synthesis of these monolayers using gas source precursors, like those used in traditional III-V semiconductor industry. These precursors are placed in temperature and pressure-controlled bubblers placed outside the chamber and can be metered independently into the growth zone with precision. Uniform epitaxial binary TMD monolayers including MoS₂, WS₂, WSe₂ and MoSe₂ have been deposited on 2” sapphire wafers in a cold-wall CVD reactor using metal hexacarbonyl and hydride chalcogen precursors. A multi-step growth process comprising of precursor modulation was developed for WSe₂ to independently control nucleation density and the lateral growth rate of monolayer domains on the sapphire substrate [1]. For the sulfides, additional temperature modulation was introduced to increase crystalline quality. Using this approach, uniform, coalesced monolayer and few-layer TMD films were obtained on 2” sapphire substrates. Deposition of these materials in the same reactor provides insight into the factors controlling their growth, which in essential for the growth of heterostructures and/or alloys. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to sapphire with narrow X-ray full-width-at-half-maximum indicating minimal rotational misorientation of domains within the basal plane [2]. Transmission electron microscopy analysis of these materials revealed additional information about the coalesced film structure. For instance, ~95 % single oriented WS₂ domains were observed when the film growth rate was modulated. For WS₂, it was also confirmed that despite translational line defects present in the film, mirror boundaries were predominantly absent. WS₂ also showed no defect related photoluminescence peak at 80 K, indicating its high quality.
In this talk the factors affecting the growth of TMDs (MX₂, M=W, Mo and X=S, Se) will be presented. The role of the substrate surface in the nucleation and growth of these TMDs will be discussed. In addition, the structural and optoelectronic characteristics for these films will also be presented.

The authors acknowledge financial support of the U.S. National Science Foundation through the Penn State 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and EFRI 2-DARE Grant EFRI-1433378.
[1] Zhang X, Choudhury TH, Chubarov M et al., 2018. Nano Lett. 18(2):1049–56
[2] Chubarov M, Choudhury TH, Zhang X et al., 2018. Nanotechnology. 29(5):55706

MXenes (formula M_n+1X_n), are a class of two-dimensional (2D) materials, which are derived from a MAX crystal structure M_n+1AX_n where the M represents early transition metals (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta), the X is for C and/or N, and the A elements include Al, Si, P, and others. The MAX phase is structured with the A atoms sandwiched between the M and X layers from which MXene monolayer sheets can be derived by selectively etching out the A atoms from the bulk MAX crystal. While MXenes have been extensively explored theoretically, they are just starting to be made in experiment due to the difficulty in having to chemically etch to separate the layers, compared to van der Walls [1]. Hence, it is important to make progress in the chemical exfoliation in order to study them experimentally and understand how their behavior matches theory.
Extensive work has been done predicting intrinsic 2D ferromagnetic and antiferromagnetic materials with high Curie temperature T_C (or Neel temperature) [2] coupled with recent discoveries of low-dimensional materials with intrinsic long-range magnetic ordering [3]. But still few low-dimensional magnetic materials have been even synthesized. In addition to great electronic properties such as tunable bandgap with tensile strain or electric field, and mechanical properties [4], MXenes can have exciting magnetic properties [5]. For example, from theory pristine Ti₂C and Ti₂N are nearly half-metallic ferromagnets, Cr₂N is antiferromagnetic, and Cr₂C is a half-metallic ferromagnet which can undergo ferromagnetic metallic to anti-ferromagnetic insulating state transitions using appropriate surface functionalization with F, OH, H, or Cl groups [5]. These spintronic and manipulatable magnetic/electronic properties give MXenes an edge over the more popular graphene material [1].
Here we show our work on synthesizing chromium carbide (Cr₂C) nanosheets from the parent bulk chromium aluminum carbide (2:1:1 Cr₂:Al:C atomic %) MAX phase powder through wet etching. We show the processes that has been developed to successfully exfoliate the material to reduce it to low-dimensional flakes using HCL+LiF, and HCL+NaF chemical etching. We also show how we have been able to reduce the etch time from 96 hours to 48 hours. After the etch, the samples are dried. Using the scanning electron microscope, a clear accordion structure is seen as is expected for MXenes, showing sheets of Cr₂C after the Al is etched away. We find a large number of accordion structures, with sheet lengths 5-10 μm in size, similar to mechanically exfoliated sizes of commonly studied 2D materials. We will show our results separating the accordion structures into low-dimensional flakes using mechanical exfoliation following the chemical exfoliation and will show energy-dispersive x-ray spectroscopy results showing that the Al content has been significantly reduced by our etching procedure. Lastly, we will show characterization of the structural, electronic, and magnetic properties of the Cr₂C flakes.
References
[1] Khazaei, M., Ranjbar, A., Arai, M., Sasaki, T. & Yunoki, S. Electronic properties and applications of MXenes: a theoretical review. J. Mater. Chem. C 5, 2488–2503 (2017).
[2] Jiang, Z., Wang, P., Xing, J., Jiang, X. & Zhao, J. Screening and Design of Novel 2D Ferromagnetic Materials with High Curie Temperature above Room Temperature. ACS Appl. Mater. Interfaces 10, 39032–39039 (2018).
[3] Li, S., Kang, W., Huang, Y., Zhang, X. & Zhou, Y. Magnetic skyrmion-based arti fi cial neuron device. 01, (2017).
[4] Naguib, M. & Gogotsi, Y. Synthesis of two-dimensional materials by selective extraction. Acc. Chem. Res. 48, 128–135 (2015).
[5] Anasori, B., Xie, Y., Beidaghi, M., Lu, J., Hosler, B. C., Hultman, L., Kent, P. R. C., Gogotsi, Y. & Barsoum, M. W. Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes). ACS Nano 9, 9507–9516 (2015).

A high-speed, two-dimensional, solid-state, non-volatile memory has been demonstrated using electric double layer (EDL) gating of two-dimensional (2D) field effect transistors (FETs) with nanosecond scale switching speeds. The “monolayer electrolyte” FET (i.e., MEFET) consists of cobalt crown ether phthalocyanine (CoCrPc) and lithium ions deposited as a monolayer on the surface a 2D semiconductor by drop casting and annealing. CoCrPc is an electrically insulating (band gap ~ 1.34 eV) and the four crown ethers each solvate a Li+ ion. In response to an applied electric field, Li⁺ ions diffuse through the crown ether cavity and are positioned either near or away from the channel resulting in onand off states, respectively. We have previously shown switching behavior on 2D FETs using this material with potential application in non-volatile memory. Achieving well-ordered monolayers of CoCrPc and complete solvation of Li⁺ into the crown ethers are critical for device operation because the location and orientation of the molecules governs the device mechanism. Here we will show how the quality of the drop-casted CoCrPc/LiClO₄ depends on concentration and annealing temperature. When the well-ordered electrolyte layer deposited on a WSe₂ FET is capped with h-BN, the off-state is stabilized (as confirmed by density functional theory (DFT)) and 50 nanosecond switching speeds (limit of the instrument) are demonstrated with an on-off current ratio > 10³. The device is stable for at least 1000 cycles (max. measured) after programming with 50 ns pulse and the retention of each state persists for at least 28 hours (max. measured). Top-gated MEFETs are currently being fabricated to reduce the program/erase voltages.

Van der Waals heterostructures have been shown to exhibit a multitude of quantum electronic transport phenomena, such as superconductivity and integer quantum Hall effects, which entail great promise for new nanoscale devices.¹ Alongside exceptional electronic properties, two-dimensional van der Waals materials present intriguing heat transport properties, such as ultra-high thermal conductivity.² Here, we use elastic scattering lattice dynamics³ calculations to investigate cross-plane thermal conductance across twisted and untwisted heterojunctions composed of graphene, molybdenum disulfide, and hexagonal boron nitride sheets. We found large degrees of cross-plane thermal conductance modulation through phonon filtering and localization based on heterojunction composition and twisting angle.

1. Novoselov K, Mishchenko A, Carvalho A, and Catro-Neto A 2016 2D Materials and van der Waals heterostructures Science 29, 353
2. Perez N M R 2010 Colloquium: The transport properties of graphene: An introduction Rev. Mod. Phys. 82, 3
3. Duchemin I and Donadio D 2011 Atomisitic Calculation of the thermal conductance of large scale bulk-nanowire junctions Phys. Rev. B 84, 11

Infrared photodetectors based on traditional thin-film semiconductors such as InGaAs, InSb, HgCdTe, Si BIB, and QWIP as well as novel type-II superlattice exhibit highly sensitive detection capability. However, these devices always need to work at low temperature, resulting in an additional large and expensive cooling system. Recently, two dimensional (2D) materials and their van der Waals heterostructures have attracted tremendous attention owing to their optoelectronic tunability and potential optoelectronic applications. Nevertheless, as a photoconductive detector, the signal-to-noise ratio could be very low without the suppression of dark current. Meanwhile, the performance of the photodetectors, which are based on 2D materials and layered van der Waals Heterostructures, including phototransistors, photoconductors with photogating, and photodiodes, is strongly affected by surface/interface and in-plane trap states resulting in the restricted electron-hole separation efficiency and low speed, and intrinsic ultrathin absorption thickness for 2D photodetectors suffers the low quantum efficiency.
Here we report the progress on novel uncooled infrared photodetectors based on 2D materials and their van der Waals heterostructures manipulated by localized fields. We fully exploit the detection ability of 2D materials and their van der Waals heterostructures by introducing localized-field, including ferroelectric filed, vertical heterojunction field, p-n junction photovoltaic field and so forth. With a strong induced localized-field, high performance infrared photodetectors based on Graphene, TMDs, Pd (Pt) Se, Black phosphorus, Black arsenic-phosphorus etc. may lead to a disruptive revolution in prospective low dimensional electronic/optoelectronic devices. Our study opens a new avenue for the controllable fabrication of built-in localized-field in 2D devices, which is a prominent challenge in low dimensional material researches.
Feng Wu, Qing Li, Peng Wang*, Hui Xia, Zhen Wang, Yang Wang, Man Luo, Long Chen, Fansheng Chen, Jinshui Miao, Xiaoshuang Chen, Wei Lu, Chongxin Shan, Anlian Pan, Xing Wu*, Wencai Ren, Deep Jariwala and Weida Hu*, High efficiency and fast van der Waals hetero-photodiodes with a unilateral depletion region, Nature Communications, 10, 4664 (2019).
Invited Review: Mingsheng Long, Peng Wang, Hehai Fang, and Weida Hu*, Progress, Challenges, and Opportunities for 2D Material Based Photodetectors, Advanced Functional Materials 29 (19), 1803807 (2019).
Invited Review: Peng Wang, Hui Xia, Qing Li, Fang Wang*, Lili Zhang, Tianxin Li*, Piotr Martyniuk, Antoni Rogalski, and Weida Hu*, Sensing infrared photons at room temperature: from bulk materials to atomic layers Small, 1904396 (2019) DOI: 10.1002/smll.201904396
Mingsheng Long, Anyuan Gao, Peng Wang, Hui Xia, Claudia Ott, Chen Pan, Yajun Fu, Erfu Liu, Xiaoshuang Chen, Wei Lu, Tom Nilges, Jianbin Xu, Xiaomu Wang*, Weida Hu*, Feng Miao*, Room-temperature high detectivity mid-infrared photodetectors based on black arsenic phosphorus Science Advances, 3, e1700589 (2017)
Mingsheng Long, Yang Wang, Peng Wang, Xiaohao Zhou, Hui Xia, Chen Luo, Shenyang Huang, Guowei Zhang, Hugen Yan, Zhiyong Fan, Xing Wu*, Xiaoshuang Chen*, Wei Lu, Weida Hu*, Palladium Diselenide Long-Wavelength Infrared Photodetector with High Sensitivity and Stability ACS Nano, 13, 2511–2519 (2019).
Peng Wang, Shanshan Liu, Wenjin Luo, Hehai Fang, Fan Gong, Nan Guo, Zhi-Gang Chen, Jin Zou, Yan Huang, Xiaohao Zhou, Jianlu Wang, Xiaoshuang Chen*, Wei Lu, Faxian Xiu*, and Weida Hu*, Arrayed van der Waals Broadband detectors for Dual band detectio, Advanced Materials, 29, 1604439 (2017).
Xudong Wang, Peng Wang, Jianlu Wang*, Weida Hu*, Xiaohao Zhou, Nan Guo, Hai Huang, Shuo Sun, Hong Shen, Tie Lin, Minghua Tang, Lei Liao, Anquan Jiang, Jinglan Sun, Xiangjian Meng, Xiaoshuang Chen, Wei Lu, and Junhao Chu, Ultrasensitive and broadband MoS2 photodetector driven by ferroelectrics Advanced Materials, 27, 6575–6581 (2015).

The prevalent von Neumann architecture in today’s processors involves memory and processing units to reside in physically separate locations. With memory speeds lagging behind the processor speeds, latency in accessing data from memory has resulted in the “von Neumann bottleneck”. To alleviate this issue, several alternative non-von Neumann architectures have been explored. Neuromorphic computing is one such non-von Neumann approach, inspired by the human brain’s ability of cognitive recognition. The brain operates through a network of neurons that are connected to each other by synapses. In 2008, after the discovery of memristors, the fourth circuit element, researchers have explored the idea of mimicking synaptic behavior with a single memristive device. Albeit significant advances, synaptic devices using phase change materials-based memristors and metal oxide-based resistive switching devices have some limitations. These devices exhibit high programming current in the range of µA to mA. Additionally, the synaptic weight update, i.e. the increase (decrease) of the synapse’s conductance with the application of a continuous stream of identical positive (negative) input voltage pulses, is non-linear. Non-linearity in weight update increases the complexity of using these devices for real-time unsupervised learning. So, it becomes necessary to employ a materials system which exhibits low programming current as well as a linear weight update.
Recently, two-dimensional (2D) materials are being largely explored to demonstrate their viability as electronic synapses and neurons. In this talk, we shall discuss the realization of a synaptic device using graphene/MoS₂ heterostructures. In these devices, CVD-grown monolayer graphene acts as an electrode to CVD MoS₂. These memristive devices exhibit low programming currents and a high dynamic range from 1 nA to 1 mA. In contrast with oxide-based or PCM-based synapses, these devices exhibit a gradual set and reset process when symmetric input voltage pulses are applied, resulting in a near-linear weight update. Linearity and symmetry in weight update is much desired for real-time learning applications. We shall also present the demonstration of an integrate-and-fire neuron using Ag/MoS₂/Au vertical structures. These devices possess the four crucial features of neuron – all-or-nothing spiking, threshold-driven firing, post-firing refractory period and stimulus strength based frequency response. Realizing neurons and synapses using the same materials system allows the monolithic integration of the essential building blocks of neuromorphic hardware, and bears potential for a highly scalable spiking neural networks suitable for unsupervised learning applications.

Two-dimensional (2D) van der Waals materials and related heterostructures have shown a wide variety of novel electronic and opto-electronic properties. However, a key challenge in fully realizing their potential is a general lack of manufacturing techniques capable of producing desired heterostructures at a large scale. Here, we demonstrate a highly scalable direct-laser-writing approach to fabricating in-plane heterostructures in two-dimensional In₂Se₃ layers. This approach derives from an optically activated solid-solid phase transition that leads to significant changes in local properties (semiconducting vs. metal-like), while preserving the single crystallinity of the local lattice, leading to well-defined heterointerfaces and demonstrating a scalable path to large-area device manufacturing. Carrier transport across in-plane heterojunction nanoscale devices fabricated by this technique exhibits asymmetric diode behaviors supported by the presence of interface energy barriers as revealed by Kelvin probe force microscopy. Scanning photocurrent microscopy shows short-circuit photocurrent in these devices, demonstrating their potential applications in efficient photodetection and photovoltaics. Our numerical modeling of the device characteristics reveals space-charge-limited and injection-limited conduction as the carrier transport mechanisms, in contrast to the standard diode model.

One of the most pressing questions in condensed matter physics is how electron correlations play out on two-dimension. For this question, the newly emerged magnetic van der Waals material can be helpful. In particular, TMPS3 with TM=transition metal elements have attracted significant attention as it exhibits all three fundamental magnetic models of magnetism; Ising (FePS3), XY (NiPS3) and Heisenberg (MnPS3) Hamiltonian. Using these materials, we have studied some of the fundamental theorems of modern magnetism: Onsager solution for the Ising model, The Berezinskii–Kosterlitz–Thouless transition for the XY model, and Mermin-Wagner theorem for the Heisenberg model. Besides, we have also investigated how correlation physics plays a role in optical spectroscopy data. In this talk, I will demonstrate how we can use this unique magnetic property of these materials to learn of the old physics.

For more than 50 years, magnetophonon resonance (MPR) has proved to be a powerful tool for investigating electron-phonon interactions in a wide range of elemental and compound semiconductors and heterostructures. Its experimental signature is a series of oscillations in the magnetoresistance, periodic in inverse magnetic field, given by the equation, p ω_c = ω_p , where p is an integer, ω_c is the Landau level splitting and ω_p is the phonon frequency. The successful observation of MPR in graphene has proved elusive due, in part, to the relative weak interaction of the massless Dirac Fermions with the lattice phonons.

Here we report the observation of large amplitude, temperature-dependent MPR oscillations in the magnetoresistance of large area, electrostatically gated Hall bars of exfoliated monolayer graphene encapsulated by hexagonal boron nitride (hBN) [1,2]. The oscillations are observed only in devices with a width which exceeds the ballistic carrier mean free path, 5 μm. We observe two distinct series of resonant peaks arising from inelastic scattering by transverse acoustic (TA) and longitudinal acoustic (LA) phonons. The LA phonon resonant amplitude is significantly weaker than that of the TA phonons. This is due, in part, to the carrier screening of the LA phonon modes. The magnetic field values of the resonant peaks are found to shift linearly with carrier sheet density from 10¹² to 10¹³ cm^-2. These experiments also allow us to measure the dispersion curves of the LA and TA phonons up to wavevectors of ~ 10⁹ m^-1. We model the experimental data to high accuracy using the Kubo formalism. We demonstrate that the LA and TA phonon speeds and the electron-phonon coupling strengths determined from the magnetophonon resonance measurements provide an excellent fit to the measured dependence of the resistivity at zero magnetic field over a temperature range of 4-150 K. Thus, MPR provides a spectroscopic tool for studying electron-phonon interaction in two-dimensional semiconductors.

[1] P. Kumaravadivel et al. Strong magnetophonon oscillations in extra-large graphene, Nat. Commun. 10, 3334 (2019).
[2] M.T. Greenaway et al., Magnetophonon spectroscopy of Dirac fermion scattering by transverse and longitudinal acoustic phonons in graphene, Phys. Rev. B 100, 155120 (2019)

Two-dimensional materials are inherently anisotropic – with strong bonding in-plane and weak bonding out-of-plane. Consequently, their optical properties are birefringent, particularly at frequencies of the optically active infrared phonons. At these frequencies, the strong optical anisotropy results in hyperbolicity, where the permittivity tensor along at least one axis is negative, while at least one is positive. Hexagonal boron nitride (hBN) is an exemplary material in this regard. Materials which exhibit hyperbolicity can support volume-confined polaritonic modes, which have a wavelength much shorter than that of light in free space. Such spatially compressed modes have important implications for IR technologies, as they may be utilized to create planar meta-optics that are much more compact than the current state of the art. In particular, 2D materials can be used to realize metasurfaces, capable of shaping both nearfield- and far field light waves for a broad range of applications.
Conventional metasurface designs use geometrically fixed structures, or materials with excessive propagation losses, thereby limiting their potential applications. Here demonstrate that this can be overcome through the realization of a reconfigurable hyperbolic metasurface comprising a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM), such as single-crystal vanadium dioxide (VO₂) or GST. Here, the metallic and dielectric domains in PCM provide spatially localized changes in the local dielectric environment that modify the polariton wavelength supported in hBN by a factor of 1.6. Using this platform, we demonstrate the first reduction to practice of in-plane HPhP refraction, and the means for launching, reflecting and transmitting of HPhPs at the PCM domain boundaries. Ultimately, this phenomenon can be used to create planar refractive optics, such as lenses or waveguides, but on length scales far below the diffraction limit. Further, my employing doped semiconductor materials with controllable plasma frequencies, such direct control of hyperbolic polaritons can also be obtained via modification of the free carrier density of the underlying semiconductor.

Organic-inorganic (hybrid) perovskite have recently emerged as a new semiconductor platform for next generation optoelectronics devices. These perovskite solids feature weak bonds between their organic and inorganic building blocks, which results in an intrinsic softness and dynamics disorder of the lattice and an acute sensitivity to external stimuli. Here, we present comprehensive in-situ studies of light induced structural dynamics of in layered two-dimensional (2D) hybrid perovskites. We correlate the changes in the structure of the 2D perovskite to modification of both the physical properties and the figures of merit in solar cells and light emitting devices. We propose a new microscopic model to explain the evolution of the structure and optoelectronic properties under external stimuli. These results demonstrate the direct correlation between structural characterization of halide perovskites under external perturbation and intrinsic physical properties.

Novel electronic and optical properties of monolayer transition metal dichalcogenides (MX₂, M=W, Mo and X=S, Se) have garnered a lot of attention in the recent years. A major challenge to integrating these materials with existing device fabrication processes is the availability of epitaxial single-crystal monolayers over a wafer scale. Our approach focusses on synthesis of these monolayers using gas source precursors, like those used in traditional III-V semiconductor industry. These precursors are placed in temperature and pressure-controlled bubblers placed outside the chamber and can be metered independently into the growth zone with precision. Uniform epitaxial binary TMD monolayers including MoS₂, WS₂, WSe₂ and MoSe₂ have been deposited on 2” sapphire wafers in a cold-wall CVD reactor using metal hexacarbonyl and hydride chalcogen precursors. A multi-step growth process comprising of precursor modulation was developed for WSe₂ to independently control nucleation density and the lateral growth rate of monolayer domains on the sapphire substrate [1]. For the sulfides, additional temperature modulation was introduced to increase crystalline quality. Using this approach, uniform, coalesced monolayer and few-layer TMD films were obtained on 2” sapphire substrates. Deposition of these materials in the same reactor provides insight into the factors controlling their growth, which in essential for the growth of heterostructures and/or alloys. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to sapphire with narrow X-ray full-width-at-half-maximum indicating minimal rotational misorientation of domains within the basal plane [2]. Transmission electron microscopy analysis of these materials revealed additional information about the coalesced film structure. For instance, ~95 % single oriented WS₂ domains were observed when the film growth rate was modulated. For WS₂, it was also confirmed that despite translational line defects present in the film, mirror boundaries were predominantly absent. WS₂ also showed no defect related photoluminescence peak at 80 K, indicating its high quality.
In this talk the factors affecting the growth of TMDs (MX₂, M=W, Mo and X=S, Se) will be presented. The role of the substrate surface in the nucleation and growth of these TMDs will be discussed. In addition, the structural and optoelectronic characteristics for these films will also be presented.

The authors acknowledge financial support of the U.S. National Science Foundation through the Penn State 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and EFRI 2-DARE Grant EFRI-1433378.
[1] Zhang X, Choudhury TH, Chubarov M et al., 2018. Nano Lett. 18(2):1049–56
[2] Chubarov M, Choudhury TH, Zhang X et al., 2018. Nanotechnology. 29(5):55706

The incorporation of magnetic dopants is a way to realize magnetism in semiconductors. The 2D material WSe₂ is a semiconductor with a bandgap of 1.3 eV, and Fe-doped WSe₂ is predicted to have room temperature, long-range ferromagnetism. Moreover, WSe₂ has been extensively studied for applications in electronic devices, and can be prepared by deposition methods that are compatible with semiconductor fabrication processes. 2D magnetic films are particularly intriguing in technologies that rely on exchange coupling since the efficiency of the exchange with the magnet is inversely proportional to the magnet thickness 1/t_FM. Chalcogenide-based TMD magnets can also have ideal interfaces with chalcogenide-based topological insulators (like Bi₂Se₃), enabling efficient spin torque devices and potentially realizing the quantum Hall effect without an external magnetic field.
Focusing on materials growth and integration techniques that utilize non-equilibrium, kinetically restricted strategies, coupled with in-situ characterization, enables the realization of atomic configurations and materials that are challenging to make but once attained, display enhanced and unique properties. In this work, we will highlight the growth of Fe-doped WSe₂ and report on its up to room temperature ferromagnetic properties. The evolution of Fe-doped WSe₂ films under different doping levels indicates that strain from the impurity atom itself coupled with strain imparted by the substrate can drive phase separation at high Fe concentrations. We find that suppressing that film strain helps promote more Fe incorporation (and higher Curie temperatures).
Cr₂O₃ has only 0.2% lattice mismatch with WSe₂ and is a magnetoelectric that can couple with ferromagnetic films. By using seed-layer techniques, we successfully prepared layered Fe-WSe₂ on Cr₂O₃ and demonstrate magnetic coupling between the two, a potentially promising route toward realizing electric field control of 2D magnets. Magnetic measurements show a clear hysteresis loop at 100 K in a 2 Tesla cooling field, from which a large negative bias field of 600 Oe is resolved. The bias field vs temperature mesurements, where a negative bias field emerges below 250 K, indicates the existence of ferromagnetic exchange coupling in this heterostructure.
This work is supported in part by NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST through award number 70NANB17H041. This work is also supported by the National Science Foundation under awards 1917025 and 1921818.

Van der Waals heterostructures present unique opportunities to synthesize artificial quantum materials. For example, a slight lattice mismatch or relative rotation between the constituent layers of a hetero-bilayer results in a long-range moiré superlattice which spatially modulates the electronic band-structure. Single particle wavepackets can be trapped in the periodic potential pockets with three-fold symmetry to form an intrinsic quantum dot lattice. Here I will discuss the properties of such quantum emitter arrays. In a heterostructure of bilayer 2H-MoSe₂ and monolayer WSe₂, we observe two interlayer exciton (IX) species trapped in moiré potentials with distinct spin-layer-valley configurations. Due to the phenomenon of locked electron spin and layer pseudospin in bilayer 2H-MoSe₂, the IX species exhibit opposite valley magnetic moments. Further, we find the 2H-MoSe₂ stacking intrinsically locks the atomic registries of the spin-layer locked IX species together. I will also present photon antibunching of moiré trapped excitons to unambiguously prove their quantum nature. Finally, I will discuss the properties of the moiré quantum emitters under applied electric fields and changing chemical potential.

We demonstrate electronic constrictions on bilayer graphene that can be completely pinched off [1]. Split-gate devices in combination with a graphite backgates enable electronic tunability of the devices. The level spectrum of quantum point contacts in bilayer graphene has a peculiar magnetic field dependence quite different from what is known from semiconductors. We understand the details of this spectrum [2] based on the Berry curvature in bilayer graphene.
In few electron and hole quantum dots confined by p-n junctions [3] we investigate valley and spin splitting. Also many electron quantum dots with standard tunneling barriers as well as double and triple quantum dots [4] can be realized with an electronic quality comparable to the best dots fabricated in standard semiconductor environments. The spectrum of excited states in graphene dots is qualitatively different [5] compared to semiconductors because of the valley degree of freedom. These experiments open the door for using graphene quantum dots as spin qubits with potentially long coherence times.
We also discuss the properties of MoS₂ as a host material [6,7] for quantum dot-based qubits.
Work done in collaboration with A. Kurzmann, M. Eich, H. Overweg, M. Mangold, F. Herman, P. Rickhaus, R. Pisoni, Y. Lee, R. Garreis, C. Tong, K. Watanabe, T. Taniguchi, and T. Ihn

References
[1] H. Overweg, H. Eggimann, X. Chen, S. Slizovskiy, M. Eich, R. Pisoni, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, V. I. Fal'ko, T. Ihn, and K. Ensslin, Nano Lett. 18, 553 (2018)
[2] H. Overweg, A. Knothe, T. Fabian, L. Linhart, P. Rickhaus, L. Wernli, K. Watanabe, T. Taniguchi, D. Sanchez, J. Burgdörfer, F. Libisch, V. I. Fal’ko, K. Ensslin, and T. Ihn, Phys. Rev. Lett. 121, 257702 (2018)
[3] M. Eich, F. Herman, R. Pisoni, H. Overweg, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, M. Sigrist, T. Ihn, and K. Ensslin, Phys. Rev. X 8, 031023 (2018)
[4] M. Eich, R. Pisoni, A. Pally, H. Overweg, A. Kurzmann, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, K. Ensslin, and T. Ihn, Nano Lett. 18, 5042 (2018)
[5] A. Kurzmann, M. Eich, H. Overweg, M. Mangold, F. Herman, P. Rickhaus,R. Pisoni, Y. Lee, R. Garreis, C. Tong, K. Watanabe, T. Taniguchi, K. Ensslin, and T. Ihn, Phys. Rev. Lett 123, 026803 (2019)
[6] R. Pisoni, A. Kormányos, M. Brooks, Z. Lei, P. Back, M. Eich, H. Overweg, Y. Lee, P. Rickhaus, K. Watanabe, T. Taniguchi, A.Imamoglu, G. Burkard, T. Ihn, and K. Ensslin, Phys. Rev. Lett. 121, 247701 (2018)
[7] R. Pisoni, T. Davatz, K. Watanabe, T. Taniguchi, T. Ihn, and K. Ensslin
Phys. Rev. Lett. 123, 117702 (2019),

The valley degree of freedom in two-dimensional (2D) honeycomb lattices has attracted much recent attention due to its non-trivial Berry curvature effects associated with two nonequivalent valleys. This feature gives rise to many interesting valley-dependent phenomena such as the valley optical selection rule and the valley Hall effect, each of which accessing the valley-dependent electron control using the optical and electrical means. The valley manipulation in these works, however, focus on the control of the electron occupation while the Berry curvature at the two valleys are remained constant. In this talk, we show the mechanical control of the valley degree of freedom which is enabled by the strain engineering of the Berry curvature. By fabricating flexible monolayer MoS₂ devices, we reversibly control strain on monolayer MoS₂ and demonstrate the generation of the current-induced valley magnetization due to the formation of the Berry curvature dipole. The measured valley magnetization is in excellent agreement with the calculated Berry curvature dipole, which depends on the magnitude and direction of strain. Our results show that the Berry curvature dipole acts as an effective magnetic field in current-carrying systems, providing a novel route to generate magnetization in 2D crystals.

Low-dimensional nanomaterials with properties distinctive from their bulk counterparts have been extensively investigated in recent years. Besides the widely explored graphene and transition metal chalcogenides (TMDCs), a new family of van der Waals (vdW) materials composed of group IV and VI elements, especially Ge/Sn monocalcogenides, have attracted great attention in recent years. They have shown strong in-plane anisotropy that is similar to phosphorene (black phosphorous layers), and a number of unique electrical, optical, thermal and mechanical properties. Here I would like to share some of our recent discoveries on this unique family of materials, including their valleytronic behaviors and Eshelby twist formation in crystal growth.

Unlike conventional valleytronic materials where cryogenic temperatures, strong electric or magnetic field are needed to differentiate the valleys, Tin sulfide (SnS) was expected to have non-degenerate valleys with selection rules that are fundamentally different. I will talk about our experimental demonstration of the valley effect in an ambient, and bias-free model system of SnS and its alloys with other IV-VI materials. We elucidate the direct access and identification of different sets of valleys, based primarily on the selectivity in absorption and emission of linearly polarized light by optical reflection/transmission and photoluminescence measurements, and demonstrate strong optical dichroic anisotropy of up to 600% and nominal polarization degrees of up to 96% for the two valleys with band-gap values 1.28 and 1.48 eV, respectively; the ease of valley selection further manifested in their non-degenerate nature. In addition, the band gap values can be tuned effectively through alloying of SnS with similar materials such as SnSe. Such discovery enables a new platform for better access and control of valley polarization.

The IV-VI material family is also enabling new opportunities from twisting topology generation. Twisting of vdW layers has been shown to provide a new degree of freedom to tailor the electrical and optical properties of vdW crystals. I will talk about the first observation of Eshelby twist in Ge monochalcogenides that results in continuous twisting of multiple vdW layers. We exploit the screw dislocation driven growth of GeS nanowires, with growth direction perpendicular to the vdW layers. Results from comprehensive characterization on the helical vdW crystals agree well with Eshelby’s theory. Moreover, the planar growth of the vdW layers co-exists with the screw dislocation driven growth and leads to radial expansion of the nanowires, which results in discretized Eshelby twists resembling a helical assembly of nanoplates. We attribute the discretization to the change of twist rates of the nanowires in conjunction with a substrate pinning effect. We further show that the twisting topology can be tailored by controlling the radial size of the structure. In these helical crystals, atomically sharp interfaces with various twist angles are created, providing an intriguing tunable platform to explore various interlayer coupling effects in vdW materials.

In moire flat band systems, interlayer moire patterns can be used to engineer isolated superlattice bands in which the Coulomb interaction is comparable to the bandwidth. As in a Landau level, this leads to electron interaction dominated physics despite the low density of electrons, enabling full density control using electrostatic gates. Unlike Landau levels, however, moire flat bands occur under time reversal symmetric conditions. I will discuss experiments that observe the spontaneous breaking of time reversal symmetry, observed as magnetic hysteresis. Remarkably, hysteresis is accompanied by a quantized anomalous Hall effect, persisting to zero magnetic field and elevated temperatures. I will discuss the origins of this effect in the spontaneous orbital polarization of the electron system into a single, topological nontrivial superlattice band, and describe magnetic imaging data that confirms the orbital character of the magnetism.

Until recently, flat bands were achieved as Landau levels at high magnetic field. Then, Pablo Jarillo-Herrero of MIT and coworkers demonstrated flat minibands in graphene-based superlattices, discovering correlated insulators and superconductors at different fillings. We have now discovered dramatic magnetic states in such systems. Specifically, in magic-angle twisted bilayer graphene also aligned with a hexagonal boron nitride (hBN) cladding layer, we observe a giant anomalous Hall effect and signs of chiral edge states. This all occurs at zero magnetic field, near 3 electrons per moire cell in the conduction miniband [1]. Remarkably, the magnetization of the sample can be reversed by applying a small DC current. Although the anomalous Hall resistance is not quantized, and dissipation is significant, we suggest that the system is an incipient Chern insulator, similar to an integer quantum Hall state. In a different superlattice system, ABC-trilayer graphene aligned with hBN, again near 3 electrons per moire cell a Chern insulator emerges [2]. A magnetic field of order 100 mT is needed to quantize the anomalous hall signal. This trilayer system can be tuned in-situ to display superconductivity instead of magnetism [3]. We will discuss possible magnetic states, and complementary probes to examine which state actually emerges as the ground state in each system.

[1] A.L. Sharpe et al., “Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene”, Science (2019).
[2] G. Chen et al., “Tunable Correlated Chern Insulator and Ferromagnetism in Trilayer Graphene/Boron Nitride Moire Superlattice”, arXiv:1905.06535 (2019).
[3] G. Chen et al., “Signatures of tunable superconductivity in a trilayer graphene moiré superlattice”, Nature (2019).

This research was primarily supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515.

Spontaneous symmetry breaking lies at the heart of the description of interacting phases of matter. Here we argue that a driven interacting system subject to a linearly polarized (achiral) driving field can spontaneously magnetize (acquire chirality). In particular, we find when a metal is driven close to its plasmon resonance, it hosts strong internal ac fields that enable Berryogenesis: the spontaneous generation of a self-induced Bloch band Berry flux, which supports and is sustained by a circulating plasmonic motion, even for a linear polarized driving field. This non-equilibrium phase transition occurs above a critical driving amplitude, and depending on system parameters, can enter the spontaneously magnetized state in either a discontinuous or continuous fashion. Berryogenesis relies on nontrivial interband coherences for electronic states near the Fermi energy generated by ac fields readily found in a wide variety of multiband systems. We anticipate that graphene devices, in particular, which can host high quality plasmons, provide a natural and easily available platform to achieve Berryogenesis and spontaneous non-equilibrium (plasmon-mediated) magnetization in present-day devices, e.g., those based on graphene plasmonics. If we have time, we will also discuss other manifestations of non-trivial quantum geometry in Dirac systems.

The prevalent von Neumann architecture in today’s processors involves memory and processing units to reside in physically separate locations. With memory speeds lagging behind the processor speeds, latency in accessing data from memory has resulted in the “von Neumann bottleneck”. To alleviate this issue, several alternative non-von Neumann architectures have been explored. Neuromorphic computing is one such non-von Neumann approach, inspired by the human brain’s ability of cognitive recognition. The brain operates through a network of neurons that are connected to each other by synapses. In 2008, after the discovery of memristors, the fourth circuit element, researchers have explored the idea of mimicking synaptic behavior with a single memristive device. Albeit significant advances, synaptic devices using phase change materials-based memristors and metal oxide-based resistive switching devices have some limitations. These devices exhibit high programming current in the range of µA to mA. Additionally, the synaptic weight update, i.e. the increase (decrease) of the synapse’s conductance with the application of a continuous stream of identical positive (negative) input voltage pulses, is non-linear. Non-linearity in weight update increases the complexity of using these devices for real-time unsupervised learning. So, it becomes necessary to employ a materials system which exhibits low programming current as well as a linear weight update.
Recently, two-dimensional (2D) materials are being largely explored to demonstrate their viability as electronic synapses and neurons. In this talk, we shall discuss the realization of a synaptic device using graphene/MoS₂ heterostructures. In these devices, CVD-grown monolayer graphene acts as an electrode to CVD MoS₂. These memristive devices exhibit low programming currents and a high dynamic range from 1 nA to 1 mA. In contrast with oxide-based or PCM-based synapses, these devices exhibit a gradual set and reset process when symmetric input voltage pulses are applied, resulting in a near-linear weight update. Linearity and symmetry in weight update is much desired for real-time learning applications. We shall also present the demonstration of an integrate-and-fire neuron using Ag/MoS₂/Au vertical structures. These devices possess the four crucial features of neuron – all-or-nothing spiking, threshold-driven firing, post-firing refractory period and stimulus strength based frequency response. Realizing neurons and synapses using the same materials system allows the monolithic integration of the essential building blocks of neuromorphic hardware, and bears potential for a highly scalable spiking neural networks suitable for unsupervised learning applications.

An indirect exciton (IX), also known as an interlayer exciton, is a bound pair of an electron and a hole confined in spatially separated layers. Due to their long lifetimes, IXs can cool below the temperature of quantum degeneracy. This gives an opportunity to realize and study cold excitons.

We will overview studies of Bose-Einstein condensation of IXs. We will present direct measurements of spontaneous coherence and condensation of IXs in GaAs heterostructures. We will present phenomena observed in the IX condensate, including the exciton density wave, spin textures, Pancharatnam-Berry phase and long-range coherent spin currents, and interference dislocations.

We will present van der Waals transition-metal dichalcogenide (TMD) heterostructures where IX condensation can be realized at high temperatures. We will present IXs at room temperature and indirect charged excitons, i.e. indirect trions, in van der Waals heterostructures.