Kinetics and atomic mechanisms of rapid semiconductor-to-metal transitions in monolayer TMDCs*

Rapid and controllable transitions between semiconducting (H) and metallic (T') phases of monolayer transition-metal dichalcogenides are of interest for 2D electronics. However, theoretical studies have been limited to calculations of thermodynamic stability of H and T' phases, while experimental investigations have uncovered only slow thermally-activated transitions that occur over $10^3-10^4$ seconds. Here, we demonstrate, through a combination of DFT and non-adiabatic QMD, softening of phonon modes located at the Brillouin zone boundary, thus exposing a hitherto unknown low barrier pathway for the H-T' phase transformation. We compare the fast kinetics of this new mechanism to previous strategies for improving the H-T' phase transition by quantifying phase transition activation barriers in strained, charge- and donor-doped monolayers using NEB. We discuss implications of this pathway in enabling fast phase transitions through irradiation for use in 2D electronics and non-volatile memories.