High Aspect Ratio Ternary Zn1-xCdxO Nanowires by Electrodeposition for Light-Emitting Diode Applications

We present a combined experimental and computational approach to study Zn1-xCdxO nanowires (NWs) and their integration in light-emitting diode (LED) structures. Self-standing Zn1-xCdxO NWs have been electrodeposited on fluorine-doped tin oxide and p-GaN substrates. The electrochemical behavior has been studied, and the reaction mechanism is discussed. Low-dimensional Zn1-xCdxO structures have been obtained for CdCl2 concentrations in the deposition bath lower than 6 mu M whereas at higher concentration it is admixed with crystallized CdO and the aspect ratio of the wires is decreased. According to scanning electron microscopy observations, the Zn1-xCdxO NWs have a higher aspect ratio (> 30) than pure ZnO NWs (similar to 20) grown in similar conditions. Analyses show that the ZnO is doped with cadmium incorporated within ZnO NWs and that Cd doping increases with increasing Cd(II) content in the deposition bath. X-ray diffraction studies show increased lattice parameters in Cd-alloyed ZnO NWs. Photoluminescence studies on pure ZnO and Zn1-xCdxO NWs show the near band-edge emission red shifted by 3-7 nm as a function of Cd(II) concentration (4 or 8 mu M in the electrolyte). The structural and optical properties of the prepared materials have been interpreted using density functional theory (DFT) to computationally simulate the effect of Cd substitution for Zn in the ZnO lattice. DFT calculations show that the crystal lattice parameters increase with the partial replacement of Zn atoms by Cd and that the band gap enlargement is due to the increased lattice parameters. We demonstrate the possibility to tailor the electroluminescence emission wavelength by cadmium doping in ZnO nanowires integrated in Zn1-xCdxO NWs/p-GaN heterojunction based LED structures. Reported results are of great interest for the research on band gap engineering, of low-dimensional zinc oxide by doping/alloying NWs and for wavelength-tunable LED applications.