Distinct Reconstruction Patterns and Spin-Resolved Electronic States along the Zigzag Edges of Transition Metal Dichalcogenides*

Two-dimensional transition metal dichalcogenides are a new class of materials exhibiting various intriguing physical, chemical, and mechanical properties. Integration of such materials for potential device applications will necessarily encounter creation of different boundaries. Using first-principles approaches, here we investigate the structural, electronic, and magnetic properties along two inequivalent M- or X-terminated zigzag edges of MX$_{\mathrm{2}}$ (M$=$Mo, W; X$=$S, Se). Along the M-terminated edges, we discover a previously unrecognized but energetically strongly preferred (2x1) reconstruction pattern, which is universal for all the MX$_{\mathrm{2}}$, characterized by place exchanges of the outmost X and M edge atoms. In contrast, the X-terminated edges undergo a more moderate (2x1) or (3x1) reconstruction for MoX$_{\mathrm{2}}$ or WX$_{\mathrm{2}}$, respectively. We further use the prototypical examples of zigzag MoX$_{\mathrm{2}}$ nanoribbons to demonstrate that the M- and X-terminated edges possess distinctly different electronic and magnetic properties, which can be exploited for a broad range of spintronic and catalytic applications