Characterization of undoped and doped ZnO Nanowires for Optoelectronic Applications

Metal oxide nanostructures such as ZnO have a huge impact due to its unique properties. This work focuses on the synthesis and characterization of ZnO NWs. In particular, Rietveld Refinement method is implemented to perform a deeper structural characterization of the nanostructure’s properties. Undoped and doped NWs (Ca, Eu, Ga) morphological, structural and optical properties were studied, varying the post-annealing temperature (in undoped nanowires) from 300 °C to 700 °C and doping concentration (in doped NWs) from 1 to 5 mol% on both microwave and conventional oven. The structural and morphological characterization of undoped NWs by SEM showed that the microwave synthesis promoted the formation of wurtzite ZnO nanowires and flower-like structures, while conventional synthesis formed more dispersed nanowires. In both cases average dimensions of 5 μm of length and near 1 μm of diameter were observed with a very broad size distribution. The synthesis of doped NWs revealed morphology changes when compared to the undoped ones, and in particular for the europium doped NWs, Eu(OH)3 was formed attached to ZnO NWs. UV-Vis spectroscopy was also performed to determine the band gap. Differences were observed between undoped and doped ZnO NWs and between not annealed ZnO NWs and the annealed ones with band gap values between 3.1 and 3.2 eV, as also reported in literature. Rietveld Refinement method was implemented on these NWs to study the evolution of the crystal structure of NWs. The lattice parameters were determined, and the results showed a decrease of the c/a ratio with increasing annealing temperature in undoped NWs. However, the overall tendency does not follow a standard behavior in doped ZnO NWs. No major differences were found when comparing microwave and conventional synthesis in the doped material. A protocol for Rietveld Refinement was developed as a tool for a deeper microstructural characterization of nanocrystalline structures