Thermodynamics and kinetics in van der Waals epitaxial growth of Te

Abstract

Chemical vapour deposition (CVD) in a tube furnace and molecular beam epitaxy (MBE) in a vacuum chamber represent the most effective methods for the production of low-dimensional nanomaterials. However, the as-synthesized products always exhibit diverse morphologies and phases due to the varying thermodynamic and kinetic factors. A comprehensive investigation into these factors is thus imperative. Here, we employ tellurium (Te), a p-type semiconductor characterized by anisotropic properties, as a model system for van der Waals (vdW) epitaxy to elucidate the difference of kinetic and thermodynamic influences in CVD and MBE processes. From a thermodynamic perspective, the inherent structural anisotropy of Te crystals favors the growth of 1D nanowires. In the CVD process, Te predominantly forms 1D structures at low substrate temperatures (T_sub < 473 K) due to substantial thermal mass and high deposition rates. At higher T_sub (>633 K), diffusion becomes predominant, resulting in the formation of kinetically controlled 2D Te nanoflakes. In MBE, the formation of 1D Te nanowires is impeded by kinetic limitations stemming from a limited deposition flux, yielding 2D Te films at low T_sub (120–300 K). Only at higher T_sub (400 K), when the MBE system reaches a thermodynamic equilibrium, can 1D nanowires be synthesized. Our study reveals the distinct roles of thermodynamic and kinetic parameters in guiding the morphological evolution of Te nanostructures, and the findings provide a general framework for understanding the growth mechanism of other vdW epitaxial low-dimensional nanomaterials.