From Molecular Precursors to MoS2 Monolayers: Nanoscale Mechanism of Organometallic Chemical Vapor Deposition

The development of a fully ab initio theory for the chemical vapor deposition (CVD) synthesis of two-dimensional (2D) materials is a prominent challenge in computational chemistry and materials science. Here, quantum-mechanical density functional theory calculations are used to discover the mechanisms underlying the nucleation and growth of monolayer 2H molybdenum disulfide (MoS2) during organometallic CVD. Starting with molybdenum hexacarbonyl (Mo(CO)(6)) and hydrogen sulfide (H2S) as molecular precursors, we elucidate processes such as the decomposition of Mo(CO)(6) to Mo(CO)(3), sulfidation of Mo(CO)(3), formation of metallic trigonal-phase (1T) Mo-S clusters, transition to semiconducting hexagonal-phase 2H MoS2, and the competition between the growth of Mo- and S-zigzag edges that lead to triangular and hexagonal flakes. We demonstrate thermodynamic and kinetic control over the formation of Mo- and S-zigzag edges. Additionally, we find the removal of hydrogen (H-2) to be the rate-determining step in the growth process. We further compute the free energy of formation of the investigated Mo-S clusters on amorphous SiO2, demonstrating the important role played by the SiO2 substrate in the initial stages of nucleation and growth. We also show the feasibility of forming Mo-S clusters with more than two Mo atoms on the SiO2 surface. Our work lays the foundation for developing fully ab initio models of 2D material synthesis.