Diverse Spin-Polarized In-Gap States at Grain Boundaries of Rhenium Dichalcogenides Induced by Unsaturated Re–Re Bonding

Grain boundaries (GBs) are important structural defects that have significant influence on the electronic structure and magnetic properties of transition-metal dichalcogenides (TMDs). However, reports on GBs in lower-symmetry T″-structure TMDs, especially on their local electronic structures, are rare. Here, we report a systematic study of intrinsic coherent GBs in monolayer ReSe2, a representative T″-structure TMD with anisotropy and intriguing electrical properties, via a combination of atomic-resolution scanning transmission electron microscopy (STEM) imaging and first-principles calculations. These coherent GBs feature a coherent Se sublattice and unchanged Re–Se (and Se–Re) local coordination. Based on the saturation of the Re–Re bond, the GBs are divided into three categories: those with saturated Re–Re bonds (3 Re–Re bonds per Re atom), denoted as α-GBs; those with truncated Re4 chains but intact Re4 clusters (β-GBs); and those with broken Re4 clusters (γ-GBs). The intact configurations of the α-GBs enable them to form easily, and they possess semiconductor characteristics inherited from the pristine ReSe2 monolayer. Interestingly, the introduction of both β-GBs and γ-GBs leads to the emergence of local magnetic moment, arising from the Re 5dxz atomic orbitals around the boundaries. Moreover, the GBs with broken Re–Re bonds (β- and γ-GBs) exhibit subtle differences in spin-polarized in-gap states, demonstrating the strong dependency of the electronic properties on the precise atomic structure of the GBs. These results are of fundamental importance in understanding lower-symmetry TMDs and the structure–property relationships in two-dimensional materials.