RESUMEN
The development of thin-film thermoelectric applications in sensing and energy harvesting can benefit largely from suitable deposition methods for earth-abundant materials. In this study, p-type copper oxide thin films have been prepared on soda lime silicate glass by direct current (DC) magnetron sputtering at room temperature from a pure copper metallic target in an argon atmosphere, followed by subsequent annealing steps at 300 °C under various atmospheres, namely air (CuO:air), nitrogen (CuO:N) and oxygen (CuO:O). The resultant films have been studied to understand the influence of various annealing atmospheres on the structural, spectroscopic and thermoelectric properties. X-ray diffraction (XRD) patterns of the films showed reflexes that could be assigned to those of crystalline CuO with a thin mixed Cu(I)Cu(II) oxide, which was also observed by near edge X-ray absorption fine structure spectroscopy (NEXAFS). The positive Seebeck coefficient (S) reached values of up to 204 µV K-1, confirming the p-type behavior of the films. Annealing under oxygen provided a significant improvement in the electrical conductivity up to 50 S m-1, resulting in a power factor of 2 µW m-1 K-2. The results reveal the interplay between the intrinsic composition and the thermoelectric performance of mixed copper oxide thin films, which can be finely adjusted by simply varying the annealing atmosphere.
RESUMEN
The effects of Cr on local environment and electronic structure of rutile TiO2 are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities CrTi0 and CrTi1- as well as Cr-oxygen vacancy complex 2CrTi + VO are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO2 at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t2g states of the Cr ions in order to reach the stable oxidation state of Cr3+. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO2 cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO2 are very sensitive to the concentration of Cr.
