Growth Mechanism and Photonic Behaviours of Nanoporous ZnO Microcheerios

Nanoporous ZnO microcheerios were fabricated in a controlled fashion by oxidizing the DC sputtered metallic Zn crystals. The growth mechanism and photonic behavior of the materials with such a nano/microstructure were investigated and characterized in detail. It was demonstrated that the large surface energy caused by the growth steps around the equator of the metallic Zn crystals with a bi-hexagonal pyramid structure is essential for the growth of the cheerios. In particular, the nanoporous structure acts as a scaffold to provide an enormous surface area for further oxidation, resulting in composition gradients that drive Zn atoms from the core of crystals to the surface of the scaffold via the ZnO/Zn interface by self-diffusing. Such oriented diffusion behavior gives rise to the formation of ZnO microcheerios, which consist of nanowhiskers with radial orientations, having an interesting photonic behavior. As a result a high efficiency exciton emission and a visible emission that covers a wide span of wavelengths from 450 to 900 nm are observed. It is demonstrated that the visible spectra consist of five individual emissions dominated by various mechanisms. Such emissions can also be tuned precisely by manipulating the defect chemistry and the size of nanopores with the controlled synthesis process.