Mo-edge reconstructions in MoSe2 and MoS2 : Reexamination of the mechanism

For zigzag nanoribbons of transition metal dichalcogenides (TMDCs), experiments show a band gap with different gap size at the Mo edges, while theoretical models often predict metallic edges. Such an inconsistency could be attributed to the inadequate understanding of the possible mechanisms of edge reconstructions. Here, we revisit the mechanisms of different edge reconstructions along the Mo edge of $\mathrm{Mo}{X}_{2}$ ($X=\mathrm{Se}$, S) zigzag nanoribbons by first-principles calculations. We demonstrate that the valency of $\frac{1}{3}$ edge Mo increases to 5+ with $X$ adatom reconstruction, or all edge Mo decreases to 3+ with a Mo trimer reconstruction. Based on these Mo valence reconstructions, thermodynamically stable reconstructions are proposed for the Mo edge in $6\ifmmode\times\else\texttimes\fi{}$ and $4\ifmmode\times\else\texttimes\fi{}$ periodicities, which are comparable with the recent experimental observation of a small energy gap (of 0.36 eV) at the Mo edge of zigzag $\mathrm{Mo}{\mathrm{Se}}_{2}$ nanoribbons. The edge Mo valency reconstruction leads to quasi-one-dimensional Peierls distortion and charge- and spin-density waves at the edge. These findings should be applicable to other TMDCs.