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)