Structural Phase Transformations in Photoexcited Transition Metal Chalcogenide Monolayers Studied Using Combined Pump-Probe Experiments and Non-Adiabatic Molecular Dynamics Simulations*

Optical control of structural phases in two dimensional chalcogenides is a promising route for precise functionalization of these materials for electronic, optical and catalytic applications. In this study, we use ab initio time dependent density functional theory simulations supported by ultrafast electron diffraction measurements to understand the electronic structure of electronically-excited MoTe₂ crystals as well as the resulting atomic dynamics responsible for transformation between the H and T’ crystal structures. Specifically, we identify a nesting of the excited-state Fermi surface that leads to softening of phonon modes at the Brillouin zone boundary. This modulation of the ionic potential energy surface exposes a low activation-energy-barrier pathway for the H-T’ phase transformation in this family of materials. This example of excitation-driven phase transformation has important advantages over other reported phase transformation mechanisms that rely on thermal and chemical driving forces or electron-doping.