M.Tech(Res.) Thesis Colloquium: Madhavan Nampoothiri D K

Understanding the Effect of Molecular Oxygen and Gold Substrate in the Chemistry of 2D MoS2 Growth During Chemical Vapour Deposition.

Two-dimensional (2D) transition metal dichalcogenides, such as molybdenum disulfide (MoS₂), are emerging candidates for varied applications, including neuromorphic computing, catalysis, gas separations, and optoelectronic devices. Powder-based chemical vapour deposition (CVD) techniques with molybdenum trioxide (MoO₃) and sulphur (S) as solid molecular precursors are commonly employed for the growth of large-area MoS₂ monolayers. Experimental studies suggest that passing O₂ along with the typical carrier gas can enhance the control on obtaining high quality, large single crystal MoS₂ domains. In this study, we track the detailed gas-phase reaction pathway towards the formation of MoS₂ with and without O₂ assistance to understand the mechanism by which O₂ affects the CVD process. We propose a reaction mechanism from first principles by observing the key intermediates obtained by running prolonged ab initio molecular dynamics (AIMD) simulations for both pathways. The reaction energies and activation barriers for these reaction steps were calculated using quantum-mechanical density functional theory (DFT). The ensuing results indicate that the formation of sulphur oxides such as SO₂ and S₂O drives the chemistry in the O₂ assisted pathway while S₂ drives the process in the standard pathway. Both reaction pathways proceed similarly with SO₂/S₂ insertion into (MoO₃)₃ ring trimer, followed by ring opening and chain breaking. The smaller MoO_xS_y chains then continuously sulfurize by S₂ and S₂O removal to form MoS₆ in the gas phase. We also study the effect of the Au(111) surface on the reaction mechanism starting from the MoO_xS_y chains. A possible mechanism towards the growth of a triangular MoS₂ nucleus is also explored. Our work is expected to shed light on the precise mechanism of oxygen-assisted and substrate supported CVD growth of 2D MoS₂ using first-principles calculations.