Defect-Mediated Phase Transitions and Structural Dynamics in Single-Layer VSe₂

Atomic-scale elucidation of phase transition pathways in two-dimensional (2D) materials is practically necessary for achieving desired architectures in next-generation devices, yet it remains hindered by insufficient understanding of defect-mediated kinetics. Here, we study the behavior of Se defects in mediating phase transitions in single-layer (SL) VSe₂ grown on Au(111), achieved through controlled thermal annealing and selenium (Se) replenishment. Using scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we find that the initial SL VSe₂ is a 1T phase featuring a substrate-induced moiré superstructure and that several annealing stages lead to two defective, Se-poor phases with an increased in-plane lattice constant. Density functional theory (DFT) calculations, which align closely with experimental findings, provide insights into the atomic structures of the defective V selenide compounds. Analysis of the calculated phase transition pathway reveals that the formation of Se defects is directly correlated with the annealing temperature, and the density of Se defects plays an important role in stabilizing the 2H phase in the streaked domain and modulating the phase transition. This study highlights the potential of properly controlling the density of Se defects in VSe₂ to manipulate the ratio of different phases within streaked domains, enabling tunable phase-engineered 2D van der Waals structures.