Chemical Vapor Deposition of a Single-Crystalline MoS Monolayer through Anisotropic 2D Crystal Growth on Stepped Sapphire Surface.

Recently, a step-flow growth mode has been proposed to break the inherent molybdenum disulfide (MoS) crystal domain bimodality and yield a single-crystalline MoS 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 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 crystal orientation. Strikingly, as the MoS crystals continue to grow laterally, the sapphire steps transform the MoS crystal geometry into diamond-shaped domains presumably by anisotropic diffusion of ad-species and facet development. Even though these MoS 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 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 monolayers peaks at 25 cm V s with the highest value reaching 28 cm V s. 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.