Reversible Manipulations of Triangular-Shaped Mirror Twin Boundary Loops in Ultrathin NiTe₂

High-density mirror twin boundaries (MTBs) embedded in two-dimensional (2D) transition metal dichalcogenides (TMDCs) have emerged as fascinating platforms for exploring charge density wave and Tomonaga-Luttinger liquid-related issues. However, the reversible manipulation of high-density MTBs in 2D TMDCs remains challenging. Herein, we report the first fabrication of high-density MTB loops in ultrathin 1T-NiTe₂ on the SrTiO₃(001) substrate, by postannealing as-grown 1T-NiTe₂ under Te-deficient conditions. This formation process is found to be mediated by the generation, accumulation, and assembly of Te vacancies into triangular vacancy loops in ultrathin 1T-NiTe₂, according to on-site scanning tunneling microscopy/spectroscopy (STM/STS) characterizations combined with density functional theory (DFT) calculations. Unique charge density modification is also observed to be correlated with the length of the one-dimensional MTBs. Overall, this work should inspire further investigations of the formation mechanism and exotic physical properties of one-dimensional electron systems in ultrathin TMDCs.