The Optical Properties And Applications Of The Cd-doped ZnO

ZnO is an important metal oxide semiconductor material with a wide direct band gap. At room temperature, the band gap and exciton binding energy of ZnO were 3.37 e V and 60 me V, respectively. In recent years, the structure and potential application of ZnO, such as the piezoelectric effect, optoelectronics, photocatalysis, photovoltaic, gas sensing characteristics and so on, has attracted much research interest in which the optical properties and related application is one of the most attracted topic. Researchers found that intrinsic ZnO semiconductor was of some shortcomings in applied research, but the results of large number of experiments have shown that the optical properties of the intrinsic semiconductor of ZnO can be improved by proper doping which is of great significance for the study of the optical properties of ZnO.We studied the theoretical basis and the potential applications of optical properties of ZnO and Cd-dopped ZnO through the theoretical simulation and experiment.In theoretical research, we used first-principles calculation method. Cd atoms were doped to substitute Zn atoms in 2 × 2 × 2 supercell of ZnO so that the intrinsic ZnO and Cd-dopped ZnO unit cell was optimized. Then we calculated the band structures, density of states, the number of population, the dielectric function and the absorption coefficient of the optimized architecture. Meanwhile we analysed the electronic structures and optical properties of them. It is shown that calculation results of pure ZnO was consistent with the experimental results of previous report. After the incorporation of Cd, cell unit is slightly expanded; And a new and narrow energy band occured at-7 e V. At the same time, the bottom of the conduction band was lower and the band gap was narrowed; the imaginary part of the dielectric function was red shift overall and the strength of dielectric peak was increased in low-energy area while reduced in high-energy area. The absorption edge was red-shift and the absorption coefficient was increased in the ultraviolet.In experiment research, we prepared ZnO and ZnO/Cd（where Cd / Zn molar ratio 0.02, 0.04, 0.06） nanorodes successfully by low temperature hydrothermal method using Zn（NO₃） ₂ · 6H₂ O and Cd（NO₃） 2 · 4H₂ O as raw material, X-ray diffraction（XRD） and field emission scanning electron microscope（SEM） were applied to characterize the crystal structure and surface morphology of the samples. Then the nano-thin film device was fabricated by these samples. We used a spectrophotometer to measure the gas sensing and optical properties of nano-thin film devices at room temperature by exposing to 1000 ppm of four kind of typical reducing gases（hydrogen, ethanol, acetone and ammonia）. The results show that the value of ZnO lattice constants a and c is gradually increased with increasing Cd doping concentration which was consistent with the result of theoretical calculations. The ZnO nanomaterials were nanorods with hexagonal structure before and after Cd doping. ZnO/Cd nanorods with the greater surface area compared with pure ZnO nanorod. The samples were best selective to ammonia at room temperature for both pure and Cd doped ZnO nanomaterials. Pure ZnO nanomaterials thin film device was not sensitive to hydrogen（sensitivity 1.002）, and it was more sensitive to ethanol and acetone（sensitivity 1.031 and 1.041 respectively）, while it was most sensitive to ammonia,（sensitivity 1.245）. The sensitivity of all the test gas to nanomaterials thin film device was enhanced after Cd doped. In particular, when the concentration of Cd was 2%, the sensitivity was the strongest to hydrogen, ethanol, acetone and ammonia, and the sensitivity was 1.08, 1.211, 1.114 and 1.786, respectively.