Transparent conductive oxides (TCO) having a wide bandgap, high transparency, and conductivity are necessary materials for fabricated photovoltaic heterostructure solar cells, transparent conducting electrodes, window materials, displays, etc. Today, the most widely used TCO are indium tin oxide (ITO), which has suitable characteristics. However, ITO has some negative factors such as the limited deposits of indium in the Earth's crust causing a constant increase in its cost, and also high toxicity and environmental hazard industrial-scale production. These factors are the reasons for replacing ITO with more safe, economically profitable, and affordable materials. Zinc oxide doped by donor impurities of Al, Ga, or In is a promising material for future technologies of electronics and optoelectronics. In economic terms, aluminum is the most favorable donor impurity. [1] Al-doped ZnO (ZnO:Al) overcomes all aforementioned negative factors. The material is nontoxic, the prevalence of raw materials in the Earth's crust, high stability to hydrogen plasma, and temperature changes. ZnO has a wide direct band gap (%3.34 eV at room temperature) that allows to be highly transparent (%85-95%) in a wide range of wavelengths (300-1000 nm). In several of our previous articles, we have comprehensively investigated the effect of deposition technological parameters on the structural, optical, and electrical properties of zinc oxide films. [2][3][4][5] However, the problem of small electroactivity (EA) for introduced donor impurities into the ZnO lattice still exists. [6] EA of the impurity is a very important characteristic of a doped semiconductor. The conductivity of semiconductor doping by donor impurity (in our case Al in ZnO) is determined not only by the mobility of free carriers but also their concentration, and, hence, the EA of donor impurity. The EA means the number of conductivity electrons that which donor impurity atom delivers to the conduction band (EA ¼ 100% means that each Al ion, substituted Zn ion in cation sublattice deliver one electron into the conduction band). [7] Really EA < 100%, because of different reasons. When ZnO is doped with aluminum, the intrinsic
In nanocomposite layers consisting of Co nanoparticles in Al2O3 matrix the thermoelectric power was studied in a wide range of Co content. These layers revealed the giant thermoelectric power in a magnetic field for Co content below percolation threshold. This unusual phenomenon has been explained by hopping electron transport through the media which consist of both, nonmagnetic and magnetic centers in the conditions of temperature gradient.
Atmospheric pressure metal-organic chemical vapor deposition was used to synthesize Ag-containing ZnO nanostructures of different morphology on Si substrates coated by Ag. Ag from Ag/Si substrates and Ag from silver acetylacetonate after its decomposition were used as a catalyst for ZnO nanocrystal growth for deposition of ZnO nanostructures with different morphologies. We investigated the relation of the structural parameters and chemical composition probed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy with the photoluminescence (PL) properties and electron-phonon coupling (EPC) reflected in the multi-phonon Raman spectra. The Raman and PL spectra were studied at different powers of the exciting laser radiation (Pexc). The spectral position and width of the phonon Raman peaks and the near bandgap PL (NBPL) band at low Pexc are supposed to be determined by the structural quality of the surfaces/boundaries of the crystallites. The intensity of the near-bandgap and defect-related PL and the magnitude of the EPC are additionally affected by the dopant concentration. Because of the large crystallite size (>30 nm, determined from XRD), the effects of phonon or electron confinement are negligible in these nanostructures. The behaviour of the position and width of phonon and PL bands with increasing Pexc indicates that the heat dissipation in the film, which is dependent on the nanostructure morphology and Ag content, plays an important role. In addition, the cytotoxicity of ZnO:Ag nanostructures was investigated by using monolayer cell cultures of epithelioid origin MDBK (Madin-Darby bovine kidney) and MDCK (Madin-Darby canine kidney) cells at a MTT assay revealing that the level of silver doping of ZnO nanostructures, their morphology, and geometric dimensions determine their toxic effects.
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