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2D transition metal dichalcogenides (TMDs) for advanced logic transistor technologies are deposited by various modifications of the chemical vapor deposition (CVD) method using a wide variety of precursors. Being a major electrical performance limiter, the TMD crystal grain size strongly differs between the various CVD precursor chemistries from nano‐ to millimeter‐sized crystals. However, it remains unclear how the CVD precursor chemistry affects the nucleation density and resulting TMD crystal grain size. This work postulates guiding principles to design a CVD process for highly crystalline TMD deposition using a quantitative analytical model benchmarked against the literature. The TMD nucleation density reduces favorably under low supersaturation conditions where the metal precursor sorption on the starting surface is reversible and the corresponding metal precursor desorption rate exceeds the overall deposition rate. Such reversible precursor adsorption guarantees efficient long‐range gas‐phase lateral diffusion of precursor species in addition to short‐range surface diffusion, which vitally increases crystal grain size. The proposed model explains the large spread in experimentally observed TMD nucleation densities and crystal grain sizes for state‐of‐the‐art CVD chemistries. Ultimately, it empowers the reader to interpret and modulate precursor adsorption and diffusion reactions through designing CVD precursor chemistries. [ABSTRACT FROM AUTHOR]

Atomically thin, 2D semiconductors, such as transition metal dichalcogenides, complement silicon in ultra‐scaled nano‐electronic devices. However, the semiconductor and its interfaces become increasingly more difficult to characterize chemically and electrically. Conventional methodologies, including scanning probe microscopies, fail to capture insight into the chemical and electronic nature of the semiconductor, albeit vital to understand its impact on the semiconductor performance. Therefore, this work presents a unique and universal in situ approach combining time‐of‐flight secondary ion mass spectrometry and atomic force microscopy to map chemical differences between regions of different electrical conductivity in epitaxially deposited tungsten disulfide (WS2) on sapphire substrates. Surprisingly, WS2 regions of lower electrical conductivity possess a larger amount of sulfur compared to regions with higher conductivity, for which oxygen is also detected. Such difference in chemical composition likely roots from the non‐homogeneously terminated sapphire starting surface, altering the WS2 nucleation behavior and associated defect formation between neighboring sapphire terraces. These resulting sapphire terrace‐dependent doping effects in the WS2 hamper its electrical conductivity. Thus, accurate chemical assignment at a sub‐micrometer lateral resolution of atomically thin 2D semiconductors is vital to achieve a more detailed understanding on how the growth behavior affects the electrical properties. [ABSTRACT FROM AUTHOR]

Recently, a step-flow growth mode has been proposed to break the inherent molybdenum disulfide (MoS 2 ) crystal domain bimodality and yield a single-crystalline MoS 2 monolayer on commonly employed sapphire substrates. This work reveals an alternative growth mechanism during the metal-organic chemical vapor deposition (MOCVD) of a single-crystalline MoS 2 monolayer through anisotropic 2D crystal growth. During early growth stages, the epitaxial symmetry and commensurability of sapphire terraces rather than the sapphire step inclination ultimately govern the MoS 2 crystal orientation. Strikingly, as the MoS 2 crystals continue to grow laterally, the sapphire steps transform the MoS 2 crystal geometry into diamond-shaped domains presumably by anisotropic diffusion of ad-species and facet development. Even though these MoS 2 domains nucleate on sapphire with predominantly bimodal 0 and 60° azimuthal rotation, the individual domains reach lateral dimensions of up to 200 nm before merging seamlessly into a single-crystalline MoS 2 monolayer upon coalescence. Plan-view transmission electron microscopy reveals the single-crystalline nature across 50 μm by 50 μm inspection areas. As a result, the median carrier mobility of MoS 2 monolayers peaks at 25 cm 2 V -1 s -1 with the highest value reaching 28 cm 2 V -1 s -1 . This work details synthesis-structure correlations and the possibilities to tune the structure and material properties through substrate topography toward various applications in nanoelectronics, catalysis, and nanotechnology. Moreover, shape modulation through anisotropic growth phenomena on stepped surfaces can provide opportunities for nanopatterning for a wide range of materials.