Dynamic Structural Evolution of Metal-Metal Bonding Network in Monolayer WS2

Layered transition metal dichalcogenides (TMDs) exist in a range of crystal phases with distinct electronic character. Some crystal phases are known to exhibit unique in-plane anisotropy characterized by a periodic distortion of the lattice and a formation of metal metal bonding network. Here, we report in situ observation of dynamic structural evolution in the one-dimensional zigzag chains of single layer WS2 induced by electron beam irradiation. Metastable zigzag chains of tungsten atoms are found to undergo reorganization of metal-metal bonds, resulting in emergence of tetramer clusters and zigzag chains with a new orientation. Our first-principles calculations reveal a small (similar to 0.1 eV per formula unit) activation energy barrier for monolayer WS2 zigzag chain reorientation and a metastable transition state in the form of tetramer clusters. We further show that local tetramer clusters can be induced and stabilized by local electronic charging effects. The formation of local tetramer clusters indicate that this dynamic structural evolution is not a charge density wave phenomenon; we find instead that these lattice changes are a response to electronic instabilities that weaken the W-S bonds in the zigzag phase. Our findings shed light on the origin of structural instabilities and phases in two-dimensional materials, and constitute a step further toward their potential uses in phase change applications.