First-principles Study Of The Electronic Structure And Optical Properties Of Ce-doped ZnO, Ce/N Codoped TiO₂, And Transparent Conducting Oxides In₄Sn₃O₁₂ And In₄Ge₃O₁₂

Wide band gap semiconductor materials have been attracting people's attention due to their special physical properties and widely application. At ambient conditions, ZnO crystallizes is the wurtzite structure and is a wide band gap semiconductor with a direct bandgap of 3.4 eV. In comparison with other optoelectronic materials, for example, GaN, ZnO has a lot of advantages such as low static dielectric constant, large optical coupling coefficient, high chemical stability, excellent piezoelectric, and optoelectric properties. The exciton binding energy of ZnO is up to 60 meV, much higher than that of GaN (which is between 21 meV and 25 meV), so devices made from ZnO can operate more efficiently at a high temperature. ZnO has a broad application prospects in ultraviolet (UV) light-emitting diodes, solar cells, liquid crystal displays, gas sensors, UV semiconductor lasers, and transparent conductice film. In the past decades, it was found that by doping different elements into ZnO crystal can change and make its properties play a better performance.The transparent conductive oxides exhibit excellent photoelectric properties such as wide band gap, low electrical resistivity, and high transmittance in the visible spectrun region (390 nm– 760 nm, or about 1.63 eV– 3.18 eV). They are widely used in commercial applications, including solar cell, flat panel displays, electroluminescence display, electroluminescent color display, special functional coating windows, and other optoelectronic devices areas. By changing the composition of the compounds, we can adjust their electrical, optical, chemical, and physical properties to meet the needs at some special occasions.TiO₂ is a promising metal oxide material for application in photocatalysts, water splitting and solar cells due to its excellent physical and chemical properties, such as high oxidative power, long term stability, nontoxicity, and low cost. However, the energy conversion efficiency of anatase TiO₂ is low owing to its wide intrinsic band gap of about 3.2 eV; therefore, TiO₂ absorbs only a small fraction of solar light in the ultraviolet region, and the quantum yield is low due to a rapid recombination between active electrons and holes. Thus, great efforts have been made to modify the electronic properties of TiO₂ in order to extend its optical absorption edge into visible light range and enhance its photocatalytic activity.In this paper, we firstly introduces the first-principle calculation method which basing on the density functional theory (DFT). Density functional theory calculations by using both generalized gradient approximation (GGA) method and the GGA with considering strong correlation effect (GGA+U) were performed to elucidate the effect of Ce 4f orbit on the electronic structure and optical properties of ZnO. It is found that after the cerium incorporation, a new localized band appears between the valence and the conduction bands, which corresponds to the majority spin of Ce 4f states. It is this localized band that constructs a bridge between the valence and conduction states, which will improve the optical performance of ZnO. ZnO:Ce is a degenerate semiconductor. The mismatch of the majority and minority spin for the Ce 4f states, Ce 5d states, and the spin-polarized holes in O 2p states induced by Ce doping leads to the presence of the magnetic order for ZnO:Ce. We also study the band structure and optical properties of ZnO:Ce with lacking one electron and two electrons, respectively. With the deficiency of the electrons, the Fermi level moves downward. The magnetism disappears when the system lacks two electrons. The analysis of optical properties shows that ZnO:Ce is a promising dielectric material and has potential applications in optoelectric devices.The cation ordering of In₄Sn₃O₁₂ and In₄Ge₃O₁₂ is explored by means of first-principles calculations. It is found that the valence band maximum of the materials is determined by the hybridzation of d states of metal elements and O 2p states; the conduction band minimum is occupied by the admixture of the O 2p states, In 5s states. and Sn 5s or Ge 4s states, respectively. The two compounds are direct band gap semiconductors. The low intensity of the absorption coefficient, reflectivity, and loss function show that they are good transparent conductive oxides.The electronic structure and optical properties of anatase TiO₂, N, Ce monodoped and Ce/N codoped TiO₂ have been investigated by using DFT with GGA+U method. It is found that for Ce/N codoped TiO₂, the top of the valence band shifts significantly to high energy range, which is occupied by the minority spin N 2p states, while leaving the conduction band edge almost unchanged determining by the admixture of Ce 4f and Ti 3d states. The magnetism of N monodoped and Ce/N codoped TiO₂ origin from the mismatch of the majority and minority spin of O 2p, N 2p, and Ce 4f states. The stronger interaction between the dopants and the ions in TiO₂ is the main reason for the narrowing of the band gaps and the Fermi levels move upwards. The narrowing of the band gaps are about 0.47 eV, 0.21 eV, and 1.05 eV for N, Ce monodoped, and Ce/N codoped TiO₂, respectively. The red-shift of Ce/N codoped is the most obviously, which satisfies the requirement for water splitting. Thus, we predict that Ce/N codoping is one of the best choice for enhancing the photoelectrochemical activity of TiO₂.