Structural and electronic properties of bulk and ultrathin layers of V2O5 and MoO3

The structural and electronic properties of bulk and ultrathin films of V2O5 and MoO3 layered oxides have been studied with first-principles density functional theory calculations including Van der Waals dispersion corrections. The U parameter in the DFT+ U approach has been determined in order to properly reproduce geometry, band-gap, macroscopic dielectric constant, and formation enthalpies of the two materials. The mono-, and multilayers are cleaved along the < 0 0 1 > and < 0 1 0 > stable crystallographic orientations for V2O5 and MoO3, respectively. Three layers are needed in order to recover bulk-like properties of V2O5 and MoO3. Spin-orbit effects have been incorporated in our simulations, and they result in marginal effects (similar to 20-30 meV) on the electronic band-gap of V2O5 and a more pronounced (similar to 200 meV) effect on the band gap of bulk MoO3. We also discuss the importance of including local field effects for the reproduction of the anisotropy of the dielectric constant, which reflects the crystal structure of these materials.