Chemical Vapor Deposition

The USC group has simulated synthesis of LMs using CVD and the Rice group has synthesized large single crystal monolayer, bilayer and multilayer transition metal dichalcogenides (MoS2, MoSe2, WSe2, MoTe2) and characterized them using Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM).

The USC group has identified a three-stage reduction-sulfidation pathway that leads to the formation of MoS2 monolayer from predeposited atomically thin films of MoO3. The high-temperature self-reduction of the MoO3 surface leads to the formation of unsaturated Mo surface sites, which act as initiation sites for the sulfidation reaction. Subsequent sulfidation leads to the formation of Mo-S-Mo and Mo-S2 atomic motifs representative of the MoS2 monolayer

Self reduction: A predeposited MoO3 layer (on an inert Al2O3 substrate) undergoes self-reduction at high temperatures leading to O2 evolution


Sulfidation of the predeposited MoO3 surface by S2 vapor leads to further reduction and a partially sulfidized surface


Subsequent sulfidation of the Mo-O-S surface leads to the formation of atomic motifs characteristic of the MoS2 monolayer

 

In a separate study, the USC team used quantum molecular dynamics simulations to identify atomic scale mechanisms involved in the synthesis of MoS2 clusters by the gas-phase sulfidation of MoO3 by gaseous H2S. Specifically the atomic configurations along reduction and sulfidation pathways will be used to benchmark empirical reactive force-fields (like ReaxFF) for large-scale computational synthesis simulations.

Reaction pathway for the gas-phase synthesis of MoS2 clusters from MoO3 and H2S reactants

 

Publications

  1. Computational Synthesis of MoS2 Layers by Reactive Molecular Dynamics Simulations: Initial Sulfidation of MoO3 Surfaces. S. Hong, A. Krishnamoorthy,  P. Rajak, S. Tiwari, M. Misawa, F. Shimojo, R. Kalia, A. Nakano, P. Vashishta. Submitted
  2. Quantum Molecular Dynamics Study on Sulfidation of Molybdenum Oxide. C. Sheng, S. Hong, A. Nakano, R. Kalia, F. Shimojo, P. Vashishta. In preparation.

 

PIs involved in this research

  • Priya Vashishta (USC)
  • Pulickel Ajayan (Rice)