Insight into electronic structure and optical properties of Nb and F co-doped SnO2 with hybrid functional theoretical method

Structural, electronic and optical properties of Nb and F co-doped SnO2 were mainly investigated using density functional theory (DFT). Hybrid density functional theory was employed to obtain band structure, density of states (DOS) and optical properties. The calculated lattice parameters of pure SnO2 by PBEsol functional (a = 4.774 Å and c = 3.218 Å) are in good agreement with experimental date (a = 4.742 Å and c = 3.206 Å). When co-doping with Nb&F, the length of octahedron (4.136 Å) closely matches with pure SnO2 (4.112 Å) in direction of perpendicular to the c-axis. The direct band gaps of 3.80 eV, 3.38 eV 3.63 eV, 3.23 eV, 3.06 eV have been obtained for pure, F, Nb, Nb&F and Nb&2F doped SnO2 respectively, and the dispersed conduction band after doping does not significantly affect the electron effective mass of the conduction band minimum (CBM). A study of band alignment shows the clearly increasing valence band maximum (VBM) and the slightly decreasing CBM occur after doping. Analyses of the charge density difference and Bader charge suggest that Sn–O bond shows covalent characteristic, but the degree of ionization decreases in following order: Nb > Nb&F > Nb&2F > SnO2 > F. In terms of optical properties after doping, the optical absorption side slightly red or blue shift, and the reflectivity in the visible spectrum is greatly low.