Electronic structure and reactivity of defect MoS2: I. Relative stabilities of clusters and edges, and electronic surface states

The defect structure of 2H-MoS2 is examined by density functional methods, and the results are compared with experiment where available. The defects considered range from edge structures to clusters of decreasing size to the single MoS2 molecule. Stability of the edges is found to be in the order (101̄x) (most stable)>(12̄1x)>(12̄10)>(101̄0), where the inclination index x=3 or 4. A large relaxation energy is associated with the reconstruction of the edge from ideal geometry of a cut through the 2H-MoS2 crystal: for the (101̄x) edge, energy of 0.63 eV per MoS2 molecular formula is released upon a concerted movement of exposed S atoms that increases the coordination of the edge Mo atoms from 4 to 5. The electronic band structure calculation identifies surface states, which have a metallic character for the (101̄x) edge and a narrow gap semiconducting character for the (12̄1x) edge. For reference purposes, periodic band structure calculation of a single 2-D sheet of MoS2 by the LCAO DFT method used in this work yields practically identical results to earlier LAPW DFT calculations of Park et al. [J. Chem. Phys. 111 (1999) 1636]. The HOMO of the (101̄x) edge is a surface state that penetrates beyond the first layer of edge Mo atoms. The effective mass of an electron in this state is calculated to be 1.9 me, compared to 4.1 me for a hole in the HOMO of the 2-D sheet. Clusters of (MoS2)n (n=7, 3, 2, 1) also show large relaxation energies from the 2H-MoS2 geometry, with the main tendency to increase the coordination of the exposed Mo atoms by inward movement. The difference in energies of the spin singlet and triplet states increases from near zero for (MoS2)7 to 0.53 eV for the single MoS2 molecule, in excellent agreement with the data for MoS2 in frozen argon matrix reported by Liang and Andrews [J. Phys. Chem. A 106 (2002) 6945].