RF nitrogen plasma irradiation of metal-doped TiO2 nanowire arrays as an effective technique for improved light transmission and optical bandgap manipulation

Abstract TiO2 NWs doped with V, Cr and Sn ions are subjected to RF generated nitrogen plasma and characterized for variance in light transmission and optical bandgap. Structurally, the introduction of Sn4+ in the TiO2 lattice causes defect broadening along the direction of individual nanowires, causing lattice distortion. This is different to Cr3+ and V3+ doping, which etch TiO2 {110} and {111} faces, causing increases in nanowire diameter. Nitrogen plasma irradiation affects no visible change in structure, but does reduce the areal density of the nanowires, subsequently yielding improved light transmission through the array. Tauc transformations of diffuse reflectance data show that the combination of metal-doping and plasma irradiation induce a decrease of the optical bandgap of the TiO2 structures. This bandgap decrease is mainly attributed to deep-lattice nitrogen doping, resulting in optical band narrowing by localized N 2p states, created by β-N species present in the plasma.