Relaxation in the (001) surface of MoS2: Band structure calculations

The (001) surface of MoS2 has been experimentally observed to relax: the top-most atomic layer relaxes by 1.6%, while the first van der Waals spacing contracts by 3%. Fenske-Hall band structure calculations on a 6-layer slab have indicated a flow of electron density to sulfur atoms at the surface. Possible reasons for relaxation of the topmost layer include (i) increased bonding to Mo atoms just beneath the surface; (ii) increased Mo-surface S electrostatic interaction; (iii) a decrease in surface S orbital energies, and hence the total energy; and (iv) partial restoration of the band gap. As for the contraction of the first van der Waals spacing, the calculations indicate that the electrostatic repulsion between the top two layers is reduced; in addition, some factors arising from the general movement of electron density to the surface appear to favor stronger van der Waals interactions.