Substrate-free growth, characterization and growth mechanism of ZnO nanorod close-packed arrays

ZnO nanorod close-packed arrays are successfully fabricated in a substrate-free manner by a citric acid assisted annealing process at a low growth temperature of 400?°C. Each nanorod of ZnO nanorod close-packed arrays grows along the [0001] direction and is single crystalline with an average diameter of 50?nm, and an average length of 0.5?μm. The aspect ratio is 10. The ZnO nanorod close-packed arrays show a strong exciton absorption peak at 372?nm in UV-visible absorption spectra, exhibiting a blue-shift relative to the bulk exciton absorption (387?nm). Finally, a new growth mechanism is proposed for the substrate-free preparation of ZnO nanorod close-packed arrays by a citric acid assisted annealing process.