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.