Understanding the role of rare-earth metal doping on the electronic structure and optical characteristics of ZnO

Abstract

ZnO has been a very appealing candidate for optoelectronics, energy conversion, and photocatalysis due to several unique features, including high electron mobility, a wide direct bandgap with a large exciton binding energy, and electrochemical activity. However, ZnO is incapable of utilizing visible light, which comprises around 43% of the solar spectrum, rendering it inefficient for solar cells and photocatalysis. Herein, using density functional theory (DFT) based on the first principles method, we demonstrated that doping ZnO with rare earth (RE) metals may greatly enhance its optical absorption and photoconductivity in the visible solar spectrum. First, we report the structural properties and formation energy of all the RE-doped ZnO. Thankfully, the formation energy of all the RE doped ZnO samples is negative, indicating that all the samples are energetically favorable, and could be synthesized in the laboratory. Secondly, we studied the mechanical properties of all the RE-doped ZnO samples, and it has been realized that all the RE-doped ZnO samples meet the mechanical stability criteria and are highly ductile, therefore suitable for optoelectronics. Going forward, the key optical properties such as optical absorption, optical conductivity, dielectric function, and reflectivity of pure and RE-doped ZnO were analyzed. The large experimental bandgap of ZnO results in an apparent zero absorption, as well as poor photoconductivity in the visible spectrum. However, Ce, Pm, Sm, Eu, Gd, and, most notably, Nd doping significantly increase the absorption, and optical conductivity of ZnO in the visible range, as well as the overall dielectric constant thus increasing the photovoltaic and photocatalytic efficiencies. Finally, we examined the electronic band structure and density of states (DOS) of pure and RE-doped ZnO to better understand how metal doping affects its optical properties. The inclusion of Ce, Nd, Pm, Sm, Eu and Gd decreases the effective bandgap and induces a redshift in the conduction band, hence facilitating the transfer of photoelectrons to the conduction band. In contrast, La and Pr doped samples, show poor absorption, photoconductivity, and dielectric constant, even lower than pure ZnO. Overall, this research may aid scientists and engineers in optimizing ZnO-based optoelectronic, photocatalytic, and photovoltaic device efficiencies.