Synthesis and Structure-Dependent Optical Properties of ZnO Nanocomb and ZnO Nanoflag

The structure-dependent optical properties of ZnO nanostructures have attracted considerable attention due to their fascinating optoelectronic properties and great structural diversity. Novel ZnO nanocomb and ZnO nanoflag have been successfully synthesized by chemical vapor deposition (CVD) method using Au nanoparticles (NPs) as the catalyst at the deposition temperatures of 900 and 950 °C, respectively. X-ray diffraction and high-resolution transmission electron microscopy results show that the ZnO nanocomb handle and its teeth grow in [01̅11] and [0001] orientations, respectively, while the ZnO nanoflag sheet and its pole grow along [0001] and [21̅1̅0] orientations, respectively. Au NPs as well as deposition temperature played an important role in the growth of the nanocomb handle and nanoflag pole. Synchrotron-based scanning transmission X-ray microscopy (STXM) reveals the thickness distribution and the crystallinity of ZnO nanocomb and ZnO nanoflag. For the near-surface emission, photoluminescence and cathode luminescence spectra of these two ZnO nanostructures show band gap emission from both nanocomb and nanoflag but green emission from only ZnO nanocomb. Synchrotron-based two-dimensional X-ray absorption near-edge structure–X-ray excited optical luminescence (2D XANES–XEOL) further reveals that the green (defect) emissions come from both the surface and bulk of nanostructures. In the ZnO nanocomb, the O excitation channel contributes more favorably to the band gap emission compared to the defect emission, while the Zn excitation channel contributes less favorably to the band gap emission than the defect emission. Meanwhile, ZnO nanoflag displays an excellent crystallinity with weak defect emission; the Zn and O excitation channels both contribute predominantly to the band gap emission.