Charge density wave in a SnSe₂ layer on and the effect of surface hydrogenation

We have carried out an investigation using density functional theory (DFT) of the atomic and electronic structures of SnSe₂ layers on the surface and hydrogenation of this surface. We have considered a (2 × 2) SnSe₂ superstructure oriented along the diagonal direction of the surface periodicity, for which scanning tunneling microscopy (STM) measurements have recently been reported. In the band structure calculations, while the s-p character surface state originating from each SnSe₂ layer is determined, there is an additional half-filled surface state in the fundamental band gap region due to the Sn adatom. At the M̄ point in the Brillouin zone, a charge density wave (CDW) partial gap opening of ∼0.1 eV occurs between these surface states close to the Fermi level. Here, the CDW gap is caused by two reasons; (i) Fermi surface nesting, due to the inequivalent electron pockets at the M̄ point, and (ii) the out of plane weak electron-phonon coupling regime due to the mean-field (MF) theory (2Δ/k_BT^MF = 3.52). Upon hydrogen adsorption on the surface, we have obtained a β-phase SnSe layer and SeH₂ molecule with a bond angle of ∼90°. The hydrogenated surface pushes the surface state associated with the SnSe₂ layer into the Si projected bulk band continuum. After SeH₂ desorption, the work function drops from 5.20 eV to 4.39 eV.