Valence control and periodic structures in Cu-doped ZnO nanowires

Abstract

Cu2+- and Cu1+-dominant Cu-doped ZnO (CZO) nanowires (NWs) were prepared by annealing CuxO-coated ZnO NWs synthesized via chemical vapor deposition (CVD) and chemical bath deposition (CBD), denoted as CVD-CZO and CBD-CZO, respectively. Transmission electron microscopy (TEM) analysis showed that Cu doping caused periodic undulations along the surfaces of both the CVD-CZO and CBD-CZO NWs, which are associated with alternating compressive and tensile strain. X-ray diffraction (XRD) patterns showed that the (0002) peak of the CVD-CZO NWs can be deconvoluted to a major peak shifted toward a higher angle, and a minor one toward a lower angle. These findings are consistent with the variation of d(0002) observed from TEM, but CBD-CZO NWs shifted only toward a lower angle. X-ray photoelectron spectra showed that the dominant substitution impurities in the CVD-CZO and CBD-CZO NWs were Cu2+ and Cu1+, respectively, which agrees with the XRD results. Cathodoluminescence spectra of the CVD-CZO NWs exhibited strong blue-green and weak red emissions, while those of the CBD-CZO NWs showed weak blue-green emissions. The distinct luminescences are attributed to different types of defect associated with the valences of dopants, which were caused by the interaction of diffused Cu ions with intrinsic defects within pristine CVD- and CBD-synthesized ZnO NWs.