Influences of Cd-Substitution and Intrinsic Vacancies on the Electronic Structures and Optical Properties of ZnO Nanotubes

Abstract

The electronic structures and optical properties of the defect-free armchair and zigzag ZnO nanotubes (NTs) as well as ZnO NTs containing defects have been investigated using the first-principles projector augmented wave potential within density functional theory framework. Simultaneously, the relevant properties for the corresponding ZnO sheets are also exhibited for comparison. Attributing to the influence of curvature, the band gaps of the defect-free, Cd substitutional, and O deficient ZnO NTs are decreased, as compared with the corresponding sheets. Moreover, for both ZnO sheets and ZnO NTs, the band gap of the substitutional Cd case is smaller than that of the defect-free case, by contraries, O vacancy case is larger. In particular, the Zn vacancy could introduce magnetism in both ZnO sheet and ZnO NTs. Curvature-induced drifting of the conduction bands towards the Femi level allows the electronic excitations from the valence band to conduction band at Γ-point through optical absorption in the visible region. The near-band-edge emission of the Cd-doped ZnO NTs shows a slight red shift, while the presence of the O vacancy contributes strongly to optical absorption in the visible region. This finding is useful for the design of new generation of materials with improved solar radiation absorption.