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Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. Here we present optical dispersion engineering in a superlattice structure comprising alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate greater than 90% narrow band absorption in less than 4 nm of active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in square-centimetre samples. These superlattices show evidence of strong light–matter coupling and exciton–polariton formation with geometry-tuneable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically thin layers. Square-centimetre scale, multilayer superlattice structures based on atomically thin two-dimensional chalcogenide monolayers enable the realization of excitonic metamaterials.

Reproducible wafer-scale growth of two-dimensional (2D) materials using the Chemical Vapor Deposition (CVD) process with precise control over their properties is challenging due to a lack of understanding of the growth mechanisms spanning over several length scales and sensitivity of the synthesis to subtle changes in growth conditions. A multiscale computational framework coupling Computational Fluid Dynamics (CFD), Phase-Field (PF), and reactive Molecular Dynamics (MD) was developed – called the CPM model – and experimentally verified. Correlation between theoretical predictions and thorough experimental measurements for a Metal-Organic CVD (MOCVD)-grown WSe2 model material revealed the full power of this computational approach. Large-area uniform 2D materials are synthesized via MOCVD, guided by computational analyses. The developed computational framework provides the foundation for guiding the synthesis of wafer-scale 2D materials with precise control over the coverage, morphology, and properties, a critical capability for fabricating electronic, optoelectronic, and quantum computing devices.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ∼107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

The electronic and optical properties of two-dimensional materials can be strongly influenced by defects, some of which can find significant implementations, such as controllable doping, prolonged valley lifetime, and single-photon emissions. In this work, we demonstrate that defects created by remote N

X-ray and ultraviolet photoelectron spectroscopy are performed on transition metal dichalcogenides (TMDs) MoS2, MoSe2, and MoTe2 monolayers on Au surfaces, to identify charge transfer processes and...

Transition metal dichalcogenide (TMD) monolayers and heterostructures have emerged as a compelling class of materials with transformative properties that may be harnessed for novel device technologies. These materials are commonly fabricated by exfoliation of flakes from bulk crystals, but wafer-scale epitaxy of single-crystal films is required to advance the field. This article reviews the fundamental aspects of epitaxial growth of van der Waals–bonded crystals specific to TMD films. The structural and electronic properties of TMD crystals are initially described along with sources and methods used for vapor phase deposition. Issues specific to TMD epitaxy are critically reviewed, including substrate properties and film-substrate orientation and bonding. The current status of TMD epitaxy on different substrate types is discussed along with characterization techniques for large-areaepitaxial films. Future directions are proposed, including developments in substrates, in situ and full-wafer characterization techniques, and heterostructure growth.

Defect engineering is important for tailoring the electronic and optical properties of two-dimensional materials, and the capability of generating defects of certain types at specific locations is meaningful for potential applications such as optoelectronics and quantum photonics. In this work, atomic defects are created in single-layer WSe2 using focused ion beam (FIB) irradiation, with defect densities spanning many orders of magnitude. The influences of defects are systematically characterized. Raman spectroscopy can only discern defects in WSe2 for a FIB dose higher than 1 × 1013 cm-2, which causes blue shifts of both A'1 and E' modes. Photoluminescence (PL) of WSe2 is more sensitive to defects. At cryogenic temperature, the low-energy PL induced by defects can be revealed, which shows redshifts and broadenings with increased FIB doses. Similar Raman shifts and PL spectrum changes are observed for the WSe2 film grown by chemical vapor deposition (CVD). A four microsecond-long lifetime is observed in the PL dynamics and is three orders of magnitude longer than the often observed delocalized exciton lifetime and becomes more dominant for WSe2 with increasing FIB doses. The ultra-long lifetime of PL in single-layer WSe2 is consistent with first-principles calculation results considering the creation of both chalcogen and metal vacancies by FIB, and can be valuable for photo-catalytic reactions, valleytronics and quantum light emissions owing to the longer carrier separation/manipulation time.

Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and ReaxFF reactive force field-based molecular dynamics simulations, we provide insights into WS

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. Here we present optical dispersion engineering in a superlattice structure comprising alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate greater than 90% narrow band absorption in less than 4 nm of active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in square-centimetre samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tuneable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically thin layers.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS

In this article, we adopt a radical approach for next generation ultra-low-power sensor design by embracing the evolutionary success of animals with extraordinary sensory information processing capabilities that allow them to survive in extreme and resource constrained environments. Stochastic resonance (SR) is one of those astounding phenomena, where noise, which is considered detrimental for electronic circuits and communication systems, plays a constructive role in the detection of weak signals. Here, we show SR in a photodetector based on monolayer MoS2 for detecting ultra-low-intensity subthreshold optical signals from a distant light emitting diode (LED). We demonstrate that weak periodic LED signals, which are otherwise undetectable, can be detected by a MoS2 photodetector in the presence of a finite and optimum amount of white Gaussian noise at a frugal energy expenditure of few tens of nano-Joules. The concept of SR is generic in nature and can be extended beyond photodetector to any other sensors. Here, the authors take advantage of stochastic resonance in a photodetector based on monolayer MoS2 for measuring otherwise undetectable, ultra-low-intensity, subthreshold optical signals from a distant light emitting diode in the presence of a finite and optimum amount of white Gaussian noise.

Here we benchmark device-to-device variation in field-effect transistors (FETs) based on monolayer MoS2 and WS2 films grown using metal-organic chemical vapor deposition process. Our study involves 230 MoS2 FETs and 160 WS2 FETs with channel lengths ranging from 5 μm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of 2D FETs across 1 × 1 cm2 chips owing to high quality and uniform growth of these TMDs followed by clean transfer onto device substrates. We are able to demonstrate record high carrier mobility of 33 cm2 V−1 s−1 in WS2 FETs, which is a 1.5X improvement compared to the best reported in the literature. Our experimental demonstrations confirm the technological viability of 2D FETs in future integrated circuits., Here, the authors perform a benchmark study of field-effect transistors (FETs) based on 2D transition metal dichalcogenides, i.e., 230 MoS2 and 160 WS2 FETs, and track device-to-device variations to gauge the technological viability in future integrated circuits.

In this article, we adopt a radical approach for next generation ultra-low-power sensor design by embracing the evolutionary success of animals with extraordinary sensory information processing capabilities that allow them to survive in extreme and resource constrained environments. Stochastic resonance (SR) is one of those astounding phenomena, where noise, which is considered detrimental for electronic circuits and communication systems, plays a constructive role in the detection of weak signals. Here, we show SR in a photodetector based on monolayer MoS2 for detecting ultra-low-intensity subthreshold optical signals from a distant light emitting diode (LED). We demonstrate that weak periodic LED signals, which are otherwise undetectable, can be detected by a MoS2 photodetector in the presence of a finite and optimum amount of white Gaussian noise at a frugal energy expenditure of few tens of nano-Joules. The concept of SR is generic in nature and can be extended beyond photodetector to any other sensors., Here, the authors take advantage of stochastic resonance in a photodetector based on monolayer MoS2 for measuring otherwise undetectable, ultra-low-intensity, subthreshold optical signals from a distant light emitting diode in the presence of a finite and optimum amount of white Gaussian noise.

Defect engineering is important for tailoring the electronic and optical properties of two-dimensional materials, and the capability of generating defects of certain types at specific locations is meaningful for potential applications such as optoelectronics and quantum photonics. In this work, atomic defects are created in single-layer WSe2 using focused ion beam (FIB) irradiation, with defect densities spanning many orders of magnitude. The influences of defects are systematically characterized. Raman spectroscopy can only discern defects in WSe2 for a FIB dose higher than 1 × 1013 cm-2, which causes blue shifts of both A'1 and E' modes. Photoluminescence (PL) of WSe2 is more sensitive to defects. At cryogenic temperature, the low-energy PL induced by defects can be revealed, which shows redshifts and broadenings with increased FIB doses. Similar Raman shifts and PL spectrum changes are observed for the WSe2 film grown by chemical vapor deposition (CVD). A four microsecond-long lifetime is observed in the PL dynamics and is three orders of magnitude longer than the often observed delocalized exciton lifetime and becomes more dominant for WSe2 with increasing FIB doses. The ultra-long lifetime of PL in single-layer WSe2 is consistent with first-principles calculation results considering the creation of both chalcogen and metal vacancies by FIB, and can be valuable for photo-catalytic reactions, valleytronics and quantum light emissions owing to the longer carrier separation/manipulation time.

Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and atomistic simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and the microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Imaging reveals the films to have nearly a single orientation with imperfectly stitched domains. Through atomic-resolution imaging and ReaxFF reactive force field-based molecular dynamics simulations, we observe two types of translational mismatch and discuss their atomic structures and origin. Our results indicate that the mismatch results from relatively fast growth rates. Through statistical analysis of >1300 facets, we demonstrate that the macrostructural features are constructed from nanometer-scale building blocks, describing the system across sub-{\AA}ngstrom to multi-micrometer length scales., Comment: 20 pages, 4 figures

© 2018 IEEE. In 2-D van der Waals heterostructures, interactions between atomic layers dramatically change the vibrational properties of the hybrid system and demonstrate several interesting phenomena that are absent in individual materials. In this paper, we have investigated the vibrational properties of the heterostructure between transition metal dichalcogenide (TMD) and hexagonal boron nitride (hBN) on gold film at low- and high-frequency ranges by Raman spectroscopy. Nineteen Raman modes have been observed from the sample, including a new interlayer coupling mode at 28.8 cm-1. Compared to reported experimental results of tungsten disulfide (WS2) on SiO2/Si substrates, the Raman spectrum for WS2 on hBN/Au emerges a blue shift of about 8 cm-1. Furthermore, a remarkable enhancement of Raman intensity can be obtained when tuning hBN thickness in the heterostructure. Through systematic first-principles calculations, numerical simulations, and analytical calculations, we find that the 28.8 cm-1 mode originates from the shearing motion between monolayer TMD and hBN layers. In addition, the gold substrate and hBN layers form an optical cavity and the cavity interference effects enhance the obtained Raman intensity. This paper demonstrates the novel vibrational modes of 2-D van der Waals heterostructure as an effective tool to characterize a variety of such heterostructures and reveals a new method to enhance the Raman response of 2-D materials.

Tungsten disulfide (WS2) films were grown on c-plane sapphire in a cold-wall gas-source chemical vapor deposition system to ascertain the effect of the chalcogen precursor on the film growth and properties. Tungsten hexacarbonyl (W(CO)6) was used as the tungsten source, and hydrogen sulfide (H2S) and diethyl sulfide (DES-(C2H5)2S) were the chalcogen sources. The film deposition was studied at different temperatures and chalcogen-to-metal ratios to understand the effect of each chalcogen precursor on the film growth rate, thickness, coverage, photoluminescence, and stoichiometry. Larger lateral growth was observed in films grown with H2S than DES. The reduced lateral growth with DES can be attributed to carbon contamination, which also quenches the photoluminescence. Thermodynamic calculations agreed well with the experimental observations, suggesting formation of WS2 with both sulfur precursors and additional formation of carbon when deposition is done using DES.

The effect of substrate (c-plane sapphire versus epitaxial graphene/6H-SiC) on the CVD growth and properties of thin 3R NbS2 films was investigated using niobium (v) pentachloride (NbCl5) and hydrogen disulfide (H2S) as precursors in a hydrogen carrier gas. The growth temperature ranged from 400 °C to 800 °C and the sulfur to niobium ratio (S/Nb) ranged between 640 and 5100 under 100 Torr total pressure. Growth on sapphire resulted in fine grained nanocrystalline NbS2 films under all conditions studied whereas smooth NbS2 films with triangular-shaped domains, on the order of 1–2 μm in size, were obtained on epitaxial graphene under identical conditions. The differences in growth morphology were attributed to enhanced surface diffusion of Nb adatoms on the passivated graphene surface compared to that of c-plane sapphire. This work demonstrates that the substrate choice plays a very crucial role in the deposition of 3R NbS2.

Monolayer WSe2, a 2D transition metal dichalcogenide (TMDCs), has been demonstrated as a good candidate for potential applications in optoelectronics. It is imperative to know the crystalline quality of WSe2 over the wafer scale prior to its applications. Azimuthal reflection high-energy electron diffraction (ARHEED) is demonstrated to be a powerful technique to measure the symmetry, lattice constants, and in-plane orientation domain dispersion in wafer-scale, continuous monolayer WSe2 epitaxially grown by metal organic chemical vapor deposition on c-plane sapphire substrate. The constructed 2D reciprocal map from ARHEED reveals few degrees’ dispersion in WSe2 orientation domains due to the step meandering/bunching/mosaic of sapphire substrate. Minor 30° orientation domains are also observed. The methodology can be applied to study other TMDCs epitaxial monolayers, graphene, and confined atomically thin hetero-epitaxial metals.

We introduce the first reactive force field (ReaxFF) for Mo/Se/H interactions, which enables large-scale molecular dynamics simulations of the synthesis, processing, and characterization of 2D-MoSe2 and whose parameters are trained primarily on first-principles energetics data including both periodic and non-periodic calculations. This new potential elucidates the structural transition from metallic to semiconducting phases, the energetics of various defects, and the Se-vacancy migration barrier. A theoretical model developed based on this potential and the Wulff construction also describes an observed morphology evolution of 2D-MoSe2 domains during growth. Since controllable edge-mediated growth kinetics of 2D-MoSe2 are of great interest to the 2D community, we believe that this new ReaxFF potential trained against the edge formation energies of MoSe2 nanoribbons with different Se coverages will be a powerful complementary tool to experimental studies by simulating the edge-growth kinetics of 2D-MoSe2 at high speed and low cost.

Sulfidation of selected transition metal thin films (Mo, W, Re, Nb, Ta) was combined with thermodynamic calculations to study the synthesis of transition metal dichalcogenides (TMDCs) and understand variations among the metals as well as processing atmosphere. Metal seed layers were prepared by DC magnetron sputtering and sulfidized using sulfur vapor and H2S. Surface chemistry, structure, and morphology of the films were investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM), respectively. XPS analysis revealed that after treatment with sulfur vapor (p (S2) = 1–10 Torr), Mo, W, and Re films were transformed into MoS2, WS2, and ReS2, respectively. However, Nb and Ta films changed little, and Nb2O5 and Ta2O5 remained the predominant components. Alternatively, conversion of Nb and Ta films to NbS2 and TaS2 was feasible under H2S. Raman spectroscopy also revealed improved crystallinity for Mo, W, and Re sulfidized under H2S. Isobaric and isothermal stability diagrams were calculated to identify feasible processing conditions (sulfur partial pressure and temperatures) for the sulfidation of all of the metals, and our findings were in good agreement with the XPS and Raman results. It was found that for Mo, W, and Re a p (S2) = 10−5 bar is sufficient for the metals to be converted to sulfide phases at 750 °C. On the other hand, due to very high stability of Nb2O5 and Ta2O5, even at very low p (O2), a sulfur partial pressure of 103–104 bar is required to make NbS2 and TaS2, respectively. Nevertheless, thermodynamic calculations confirmed that Nb and Ta could be transformed to NbS2 and TaS2 under 760 Torr H2S. AFM analysis revealed very smooth films for MoS2, WS2, and NbS2 films, but dewetting of TaS2, and ribbons for ReS2. These results provide guidance for designing new processes for synthesizing 2D TMDCs.

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS2 and WSe2, results in two energetically equivalent domain alignments, often referred to as 0o and 60o domains, when grown on substrates such as c-plane sapphire and graphene. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to break the surface symmetry giving rise to a preferred domain orientation. Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe2 and WS2 monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. The as-received sapphire substrates, which are miscut ~0.2o toward , consist of steps with sub-1 nm step height separated by 50-70 nm wide terraces. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850oC to 1000oC, was used to achieve epitaxial films using W(CO)6, H2Se and H2S as precursors in a H2 carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.

Metalorganic chemical vapor deposition (MOCVD) is a promising technique to form large-area, uniform films of monolayer or few-layer transition metal dichalcogenide (TMD) thin films; however, unintentional carbon incorporation is a concern. In this work, we report the presence of a defective graphene layer that forms simultaneously during MOCVD growth of tungsten diselenide (WSe2) on sapphire at high growth temperature and high Se:W ratio when using tungsten hexacarbonyl (W(CO)6) and dimethyl selenide ((CH3)2Se, DMSe) as precursors. The graphene layer alters the surface energy of the substrate reducing the lateral growth and coalescence of WSe2 domains. The use of hydrogen selenide (H2Se) instead of DMSe eliminates the defective graphene layer enabling coalesced monolayer and few-layer WSe2 films.

Bi2Se3 thin films were grown on c-plane sapphire substrates by hybrid physical-chemical vapor deposition (HPCVD) using trimethyl bismuth (TMBi) and Se pellets. A Se-rich environment is created by evaporating Se pellets in the vicinity of the substrate, which is used to suppress the formation of Se vacancies. The effects of pre-cracking temperature and substrate/Se temperature on the growth rate, structural and electrical properties of the Bi2Se3 films were investigated. C-axis oriented films were obtained which show a reduction in the carrier concentration as pre-cracking temperature was increased from 290 °C (1.6×1019 cm−3) to 350 °C (8.4×1018 cm−3). An additional reduction in carrier concentration (7.28×1018 cm−3) was observed on increasing the substrate temperature from 200 to 260 °C.

A detailed thermodynamic analysis of the solid and gas phases of the Mo-S-C-O-H system used for large area chemical vapor deposition (CVD) of MoS2 is presented and compared with experimental results. Given the multivariable nature of the problem, excellent agreement is observed. Deviations, observed from thermodynamic predictions, mainly at low temperatures and high flow rates have been highlighted and discussed. CVD phase diagrams which predict parameter windows in which pure MoS2 can be synthesized have been provided for important gas phase chemistries. Pure H2 as a carrier gas is shown to facilitate the largest contamination free process window. CO presence is shown to significantly reduce the nucleation rate and enable large island sizes but at the cost of carbon contamination. Oxygen leaks are shown to result in sulphur contamination. The absence of H2S during cooling is shown to yield Mo due to the reduction of MoS2 by hydrogen. Oxidation of Mo causes oxide contamination.

Molybdenum disulfide (MoS2) on the c-plane sapphire has been a very popular system to study in the two-dimensional (2D) materials community. Bottom-up synthesis of monolayer (ML) MoS2 with excellent electrical properties has been achieved on sapphire by various methods, making it a very promising candidate to be used in the next generation nano-electronic devices. However, large-area ML MoS2 with comparable quality as the relatively small size exfoliated ML remains quite a challenge. To overcome this bottle neck, a comprehensive understanding of the structure of the as-grown ML material is an essential first step. Here, we report a detailed structural characterization of wafer-scale continuous epitaxial ML MoS2 grown by metalorganic chemical vapor deposition on sapphire using an azimuthal reflection high-energy electron diffraction (ARHEED) technique. With ARHEED we can map not only 2D but also 3D reciprocal space structure of the ML statistically. From the oscillation in the ARHEED intensity profile along the vertical direction of the ML, we derived a real space distance of ~3 Å at the interface of ML and sapphire. Quantitative diffraction spot broadening analyses of the 3D reciprocal space map reveals low density defects and a small angular misalignment of orientation domains in ML MoS2. Based on atomic force microscopy height distribution analysis, cross-section scanning transmission electron microscopy, and density functional theory calculations, we suggest that there exists a passivation layer between MoS2 ML and sapphire substrate. This ARHEED methodology also has been applied to ML WS2 and is expected to be applicable to other ML transition metal dichalcogenides on arbitrary crystalline or non-crystalline substrates.

Defects have a profound impact on the electronic and physical properties of crystals. For two-dimensional (2D) materials, many intrinsic point defects have been reported, but much remains to be understood about their origin. Using scanning transmission electron microscopy imaging, this study discovers various linear arrays of W-vacancy defects that are explained in the context of the crystal growth of coalesced, monolayer WS2. Atomistic-scale simulations show that vacancy arrays can result from steric hindrance of bulky gas-phase precursors at narrowly separated growth edges, and that increasing the edge separation leads to various intact and defective growth modes, which are driven by competition between the catalytic effects of the sapphire substrate and neighboring growth edge. Therefore, we hypothesize that the arrays result from combined growth modes, which directly result from film coalescence. The connections drawn here will guide future synthetic and processing strategies to harness the engineering potential of defects in 2D monolayers.

Monolayer semiconducting transition metal dichalcogenides (MX2, M=W, Mo and X=S, Se) exhibit novel electronic and optical properties. Availability of epitaxial single-crystal monolayers over a wafer scale is a major hurdle in integrating these materials with existing device fabrication processes. Our approach focusses on synthesis of these monolayers using gas source precursors, like those used in traditional III-V semiconductor industry. The precursors are kept in temperature and pressure-controlled bubblers outside the growth 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. 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]. 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. 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

Lasing with a narrow linewidth is achieved from large area monolayer tungsten disulfide film synthesized by chemical vapor deposition after integrating onto silicon nitride heterostructure photonic crystal cavities.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal–organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1–10 μA·μm–1), and good field-effect transistor mobility (∼30 cm2·V–1·s–1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D inte...

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe

A simple microstructural rationale for successful anodization of metallic films into ordered oxide nanostructures has been identified. It applies to three of the most commonly studied systems, Zr, Ti and Al films and can be extended to other such oxides. A dense Zone T or II microstructure, in sputtered films, is the most critical ingredient. While T-substrate > 0.3T(melting) Ching is the simplest route, pressure and plasma heating can also be exploited. Such microstructures are also associated with a unique growth stress signature. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Layered transition metal dichalcogenides (TMDs), such as MoS2, are candidate materials for next generation 2-D electronic and optoelectronic devices. The ability to grow uniform, crystalline, atomic layers over large areas is the key to developing such technology. We report a chemical vapor deposition (CVD) technique which yields n-layered MoS2 on a variety of substrates. A generic approach suitable to all TMDs, involving thermodynamic modeling to identify the appropriate CVD process window, and quantitative control of the vapor phase supersaturation, is demonstrated. All reactant sources in our method are outside the growth chamber, a significant improvement over vapor-based methods for atomic layers reported to date. The as-deposited layers are p-type, due to Mo deficiency, with field effect and Hall hole mobilities of up to 2.4 cm(2) V(-1) s(-1) and 44 cm(2) V(-1) s(-1) respectively. These are among the best reported yet for CVD MoS2.

There is significant interest in the growth of single crystal monolayer and few-layer films of transition metal dichalcogenides (TMD) and other 2D materials for scientific exploration and potential applications in optics, electronics, sensing, catalysis and others. The characterization of these materials is crucial in determining the properties and hence the applications. The ultra-thin nature of 2D layers presents a challenge to the use of x-ray diffraction (XRD) analysis with conventional Bragg-Brentano geometry in analyzing the crystallinity and epitaxial orientation of 2D films. To circumvent this problem, we demonstrate the use of in-plane XRD employing lab scale equipment which uses a standard Cu x-ray tube for the analysis of the crystallinity of TMD monolayer and few-layer films. The applicability of this technique is demonstrated in several examples for WSe2 and WS2 films grown by chemical vapor deposition on single crystal substrates. In-plane XRD was used to determine the epitaxial relation of WSe2 grown on c-plane sapphire and on SiC with an epitaxial graphene interlayer. The evolution of the crystal structure orientation of WS2 films on sapphire as a function of growth temperature was also examined. Finally, the epitaxial relation of a WS2/WSe2 vertical heterostructure deposited on sapphire substrate was determined. We observed that WSe2 grows epitaxially on both substrates employed in this work under all conditions studied while WS2 exhibits various preferred orientations on sapphire substrate which are temperature dependent. In contrast to the sapphire substrate, WS2 deposited on WSe2 exhibits only one preferred orientation which may provide a route to better control the orientation and crystal quality of WS2. In the case of epitaxial graphene on SiC, no graphene-related peaks were observed in in-plane XRD while its presence was confirmed using Raman spectroscopy. This demonstrates the limitation of the in-plane XRD technique for characterizing low electron density materials.

We present a multi-scale computational approach to model the gas-phase chemical kinetics for Metal-Organic Chemical Vapor Deposition (MOCVD) of WSe2 using W(CO)6 and H2Se as gas-phase precursors. This framework combines Quantum Mechanical (QM) methods based on Density Functional Theory (DFT), ReaxFF-based reactive molecular dynamics, and Computational Fluid Dynamics (CFD) to efficiently model the gas-phase physiochemical processes leading to WSe2 growth in a cold-wall horizontal MOCVD chamber. A detailed gas-phase chemical kinetic reaction model is developed to describe all major chemical reaction pathways from the precursors W(CO)6 and H2Se to the most thermodynamically stable molecules, with quantified kinetic rate constants. First QM calculations are performed to suggest key reaction types and to provide the necessary training set to determine ReaxFF reactive force-field parameters for the W/H/C/O/Se system. Using the developed force field, ReaxFF simulations are performed to identify all major chemical reaction pathways and determine their associated activation energies. Other kinetic parameters, together with the thermal and transport properties of all species involved, are estimated using well-established theories or correlations. This chemical kinetic model with thermal and transport parameters is then integrated into a reacting flow solver for full-scale CFD simulations of the MOCVD chamber under realistic operating conditions, to demonstrate its capabilities in predicting the major processes in the gas phase and qualitatively estimating thin film growth behavior. The predicted gas-phase concentrations of tungsten chalcogenides at the growth substrate correlate well with experimental measurements of average film thickness across the substrate, which suggests that crystal growth may result from surface deposition reactions of these species. This computational framework for the gas-phase chemical kinetics in MOCVD (prior to surface deposition and subsequent crystal growth) can thus test experimental MOCVD conditions, generate insights into more effective growth protocols, shed light on the significance of reactor geometry, and improve the reproducibility of MOCVD results across different experimental growth chambers. The computational framework is also transferable to other CVD chemistries.

Recently, single photons have been observed emanating from point defects in two-dimensional (2D) materials including WSe2, WS2, hexagonal-BN, and GaSe, with their energy residing in the direct electronic bandgap. Here, we report single photon emission from a nominal weakly emitting indirect bandgap 2D material through deterministic strain induced localization. A method is demonstrated to create highly spatially localized and spectrally well-separated defect emission sites in the 750–800 nm regime in a continuous epitaxial film of few-layer WSe2 synthesized by a multistep diffusion-mediated gas source chemical vapor deposition technique. To separate the effects of mechanical strain from the substrate or dielectric-environment induced changes in the electronic structure, we created arrays of large isotropically etched ultrasharp silicon dioxide tips with spatial dimensions on the order of 10 μm. We use bending based on the small radius of these tips—on the order of 4 nm—to impart electronic localization effects through morphology alone, as the WSe2 film experiences a uniform SiO2 dielectric environment in the device geometry chosen for this investigation. When the continuous WSe2 film was transferred onto an array of SiO2 tips, an ∼87% yield of localized emission sites on the tips was observed. The outcomes of this report provide fundamental guidelines for the integration of beyond-lab-scale quantum materials into photonic device architectures for all-optical quantum information applications.Recently, single photons have been observed emanating from point defects in two-dimensional (2D) materials including WSe2, WS2, hexagonal-BN, and GaSe, with their energy residing in the direct electronic bandgap. Here, we report single photon emission from a nominal weakly emitting indirect bandgap 2D material through deterministic strain induced localization. A method is demonstrated to create highly spatially localized and spectrally well-separated defect emission sites in the 750–800 nm regime in a continuous epitaxial film of few-layer WSe2 synthesized by a multistep diffusion-mediated gas source chemical vapor deposition technique. To separate the effects of mechanical strain from the substrate or dielectric-environment induced changes in the electronic structure, we created arrays of large isotropically etched ultrasharp silicon dioxide tips with spatial dimensions on the order of 10 μm. We use bending based on the small radius of these tips—on the order of 4 nm—to impart electronic localization eff...

The effect of reactive ion etching using chlorine or fluorine-based plasmas on aluminum-induced crystallization (AIC) of silicon on fused silica glass substrates was investigated with the goal of chemically modifying the substrate surface and thereby influencing the crystallization behavior. Chlorine etching of the glass prior to AIC resulted in six times faster silicon crystallization times and smaller grain sizes than films formed on untreated substrates while fluorine etching resulted in crystallization times double than those on untreated surfaces. The differences in crystallization behavior were attributed to changes in surface chemistry and surface energy of the glass as a result of the plasma treatment as supported by X-ray photoelectron spectroscopy and contact angle measurements. The different surface treatments were then combined with optical lithography to control the location of crystallization on the substrate surface to realize the production of patterned polycrystalline silicon films from i...

Nanostructured TiO2 is one of the most commonly used materials in photocatalytic applications and photochemical solar cells. This article describes a method to synthesize nanoporous anatase TiO2 membranes directly on stainless steel (SS), an easily available substrate by anodization to form amorphous TiO2 and a subsequent heat treatment to convert it into anatase, the photoactive phase. To obtain adherent membranes with interfaces that are resistant to peeling, both anodization and heat treatment parameters need to be optimized to obtain a heterostructure that contains a Ti film between the TiO2 membrane and the substrate.

Anodized nanotubular and nanoporous zirconia membranes are of interest for applications involving elevated temperatures in excess of 400 °C, such as templates for the synthesis of nanostructures, catalyst supports, fuel cells and sensors. Thermal stability is thus an important attribute. The study described in this paper shows that the as-anodized nanoporous membranes can withstand more adverse temperature–time combinations than nanotubular membranes. Chemical treatment of the nanoporous membranes was found to further enhance their thermal stability. The net result is an enhancement in the limiting temperature from 500 °C for nanotubular membranes to 1000 °C for the chemically treated nanoporous membranes. The reasons for membrane degradation on thermal exposure and the mechanism responsible for retarding the same are discussed within the framework of the theory of thermal grooving.

Anodized titania, synthesized on oxidized silicon substrate, has been used as oxygen gas sensor. The as-anodized films resulted in a sensitivity of 5756% at 125°C, when exposed to 100% oxygen. The gas-sensing performance of anodized films has been evaluated with post-anodization treatment in de-ionized water and aqueous ammonia solution. The sensitivity increases to 8646% and 16599%, with post-treatment in aqueous ammonia solution and de-ionized water, respectively. This is attributed to increase surface area and activation during the post- anodization treatment. The repeatability of sensor performance has also been evaluated, and it is observed that the de-ionized water treated film is unstable for repeated sensing, possibly due to some structural modifications.