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.

Growth and formation mechanism of shape-selective preparation of ZnO structures: correlation of structural, vibrational and optical properties

A shape-selective preparation method was used to obtain highly crystalline rod-, needle-, nut-, and doughnut-like ZnO morphologies with distinct particle sizes and surface areas.

Controlled compensation via non-equilibrium electrons in ZnO

Doping wide-band-gap semiconductor with impurities always accompanied spontaneous compensation of opposite charged intrinsic defects, which lead to invalid control of the type of free carriers. We demonstrate an effectual route to overcoming such detrimental defects formation during doping by suppressing Fermi level shifting using non-equilibrium carriers gathering on the polar epitaxial surfaces. Non-equilibrium carriers are generated by ultraviolet light excited interband transitions (photon energy greater than bandgap). Because the p-type dopants are compensated by non-equilibrium electrons at metal-polar surfaces, donor-type native defects are inhibited. This new doping strategy provides an attractive solution to self-compensation problems in wide–band-gap semiconductors with spontaneous polarization of the future.