Fully Ab Initio Mechanism of the Organometallic Chemical Vapor Deposition of MoS₂—Molecular Precursors to Transition Metal Dichcalcogenide Monolayers

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. In this talk, I will discuss the use of quantum-mechanical density functional theory calculations 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 leads to triangular and hexagonal flakes. We demonstrate thermodynamic and kinetic control, respectively, over the formation of Mo- and S-zigzag edges. Additionally, we find the removal of hydrogen (H₂) 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.