Two-dimensional (2D) layered semiconducting transition-metal dichalcogenides (TMDCs) are promising candidates for next-generation ultrathin, flexible, and transparent electronics. Chemical vapor deposition (CVD) is a promising method for their controllable, scalable synthesis but the growth mechanism is poorly understood. Herein, we present systematic studies to understand the CVD growth mechanism of monolayer MoSe2, showing reaction pathways for growth from solid and vapor precursors. Examination of metastable nanoparticles deposited on the substrate during growth shows intermediate growth stages and conversion of non-stoichiometric nanoparticles into stoichiometric 2D MoSe2 monolayers. The growth steps involve the evaporation and reduction of MoO3 solid precursors to sub-oxides and stepwise reactions with Se vapor to finally form MoSe2. The experimental results and proposed model were corroborated by ab initio Car–Parrinello molecular dynamics studies.
Solid–Vapor Reaction Growth of Transition-Metal Dichalcogenide Monolayers
In this work, we analyzed the CVD growth mechanism of 2D MoSe2. Samples were quenched halfway through a typical CVD growth process and compared to those that were slowly cooled to completion. The intermediate phases in the quenched samples were then analyzed. Based on the co-existence of MoSe2, aligned nanoparticles (APs) at the crystalline edges of MoSe2, and randomly distributed nanoparticles (RPs), as well as evidence of an in-plane “feedstock” and MoSe2 nucleation from the RPs, we developed a three-step growth model and proposed chemical reactions for each step, which were corroborated by molecular dynamics simulations. The first model of the CVD growth process of TMDCs has thus been developed. Furthermore, the growth of MoSe2 was shown to be a reversible reaction, where the MoSe2 edges recede during the slow cooling process.