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.