Geminal Mirror Twin Boundaries in H-Phase NbTe₂

Grain boundaries (GBs) in transition metal dichalcogenides (TMDs) significantly influence their physicochemical properties. Mirror twin boundaries (MTBs), a special GB type, reveal one-dimensional quantum features such as topological states and charge density waves. However, the large-scale fabrication of well-aligned MTBs remains challenging. Here, we present a simple solution method to introduce high-density paired MTBs in monolayer 1H-NbTe₂. Using atomic-resolution scanning transmission electron microscopy (STEM), we identify two "MTB" types─metastable Nb-oriented 4|4E and Te-oriented 4|4P, forming aligned MTBs with quantized spacings. The formation mechanism of paired MTBs is hypothesized to involve specific intralayer atomic rearrangements within a distorted 1T-phase, followed by H-phase core coalescence, facilitated by strain-induced energy barrier reduction and electron doping stabilization. Density functional theory (DFT) calculations indicate that paired MTBs stabilize metastable H-phase, enabling the coexistence of superconductivity and nontrivial band topology. The approach of our group to fabricating high-density paired MTBs advances boundary engineering in TMDs for designing functional quantum devices.