The effect of substrate-induced strain in polycrystalline ZnO thin films on different substrate, e.g., GaN epilayer, sapphire (0001), quartz glass, Si(111)∕SiO2, and glass deposited by sol-gel process, has been investigated by x-ray diffraction, scanning electron microscope, electrical resistivity, and photoluminescence measurements. A strong dependence of orientation, crystallite size, and electrical resistivity upon the substrate-induced strain along the c axis has been found. The results of structural and morphological studies indicate that relatively larger tensile strain exists in ZnO deposited on sapphire and glass, while a smaller compressive strain appears in film deposited on GaN and the strain is relaxed in larger crystallite size. The electrical resistivity of the films increases exponentially with increasing strain. The excitonic peak positions are found to shift slightly towards lower energy side with increasing strain. The analysis shows that GaN being a closely lattice-matched substrate produces ZnO films of better crystallinity with a lower resistivity.
Graphene quantum dots (GQDs) synthesized by a direct chemical method have been used in combination with ZnO nanowires (NWs) to demonstrate their potential as a solar harvesting material in photovoltaic cells exhibiting an open circuit voltage of 0.8 V. The excited state interaction between the photoexcited GQDs and the ZnO NWs has been verified from the charge-transfer process by both emission spectroscopy and photovoltaic measurements. This work has implications for less expensive and efficient next generation solid-state solar cells.
The anomalous photocurrent decay in aqueous solution grown ZnO nanowires (NWs) under steady ultraviolet light illumination have been investigated. The photocurrent growth-decay measurements using the above-band and subband gap light excitation energies in the as-grown and annealed NWs show that while a VZn-related defect complex is formed by the surface adsorbed H2O molecules, a faster carrier trapping by the surface adsorbed O2 molecules and a slower carrier recombination at the defect, Zni cause the photocurrent decay under steady illumination supported by the results of the photocurrent spectra and photoluminescence measurements. The predicted mechanism has been explained through a model.
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