First-Principles Calculations of the Electronic Structure and Optical Properties of Yttrium-Doped ZnO Monolayer with Vacancy

The electronic structures and optical characteristics of yttrium (Y)-doped ZnO monolayers (MLs) with vacancy (zinc vacancy, oxygen vacancy) were investigated by the first-principles density functional theory. Calculations were performed with the GGA+U (generalized gradient approximation plus U) approach, which can accurately estimate the energy of strong correlation semiconductors. The results show that the formation energy values of Y-doped ZnO MLs with zinc or oxygen vacancy (V-Zn, V-O) are positive, implying that the systems are unstable. The bandgap of Y-V-Zn-ZnO was 3.23 eV, whereas that of Y-V-O-ZnO was 2.24 eV, which are smaller than the bandgaps of pure ZnO ML and Y-doped ZnO MLs with or without V-O. Impurity levels appeared in the forbidden band of ZnO MLs with Y and vacancy. Furthermore, Y-V-Zn-ZnO will result in a red-shift of the absorption edge. Compared with the pure ZnO ML, ZnO MLs with one defect (Y, V-Zn or V-O), and Y-V-Zn-ZnO, the absorption coefficient of Y-V-O-ZnO was significantly enhanced in the visible light region. These findings demonstrate that Y-V-O-ZnO would have great application potential in photocatalysis.