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Creators/Authors contains: "Bachu, Saiphaneendra"

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  5. Atomistic simulation techniques have become an indispensable tool to acquire a fundamental understanding of growth and structural characteristics of two-dimensional (2D) materials of interest, thereby accelerating experimental research in the same field. A new ReaxFF reactive force field presented here is the first comprehensive empirical potential that is explicitly designed to capture the most prominent features of 2D WSe2 solid-phase chemistry, such as defect formation as a function of local geometry and chalcogen chemical potential, vacancy migration and phase transition, thus enabling cost-effective and reliable characterization of 2D WSe2 at large length scales and time scales much longer than what is accessible by first-principles theory. This potential, validated using extensive first-principles energetics data on both periodic and nonperiodic systems and experimental measurements, can accurately describe the mechanochemical coupling between monolayer deformations and vacancy energetics, providing valuable atomistic insights into the morphological evolution of a monolayer in different environments in terms of loading conditions and various concentrations and distributions of defects. Since understanding how growth is affected by the local chemical environment is vital to fabricating efficient and functional atomically thin 2D WSe2, the new ReaxFF description enables investigations of edge-controlled growth of single crystals of 2D WSe2 using reactive environmentsmore »closely matching experimental conditions at a low computational cost.« less
  6. 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 ∼107more »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.« less