Restructure ZnO Nanorods by Strategic Annealing with Method of Alternately Controlling Oxygen Pressure
Feng-Ming Chang1*, Sanjaya Brahma1, Jing-Heng Huang1, Kuang-Yao Lo1
1Department of Physics, National Cheng Kung University, Tainan, Taiwan
* presenting author:Feng-Ming Chang,
“Post-annealing” is a common process in ZnO nano-structure to restructure and repair defects since self-assembly ZnO material is nearly a crystalline structure and can be healed defects by enough thermal budge. There are many reports about the evolution of structure quality of ZnO nanorods with annealing at different temperature or at different gas (pressure). However, there is some inconsistent among optical analyses (PL and XPS) to determine the actual structure variation of ZnO nano-structure. Most query is the role of oxygen gas in the treatment of post-annealing.
We investigate the mechanism of evolution of surface and bulk structure with strategic annealing conditions. For this, we designed a series of "double-annealing" experiments at 800˚C by tuning oxygen pressure alternately (vacuum or different oxygen pressure), and analyzed photoluminescence (PL) to investigate the bulk/surface defects and X-ray photoelectron spectroscopy (XPS) to study the first few layers on the surface (the scan depth is about 5 nm). With varied conditions of double annealing, the change of the oxygen (zinc) vacancy or interstitial concentration, both on the surface and in the bulk of the ZnO nanorods, will revealed in XPS and PL spectra according to their resolution. PL spectra indicate the defects in ZnO nanorods would be eliminated at high temperature annealing but tiny visible spectra in PL reveal the evolution of defects after strategic annealing, which is well consistent with Zn 2p XPS. The second annealing treatment with vacuum environment can suppress and eliminate the formation of oxygen interstitials or antisites under high annealing temperature which provides enough thermal budge to heal or remove these defects. The peak of the visible region in PL shift from the orange-red luminescence to the green-blue luminescence, meanwhile, Zn 2p in XPS shift from the chemical sate of ZnO (1022 eV) to chemical sate of Zn (1020 eV). We have built an effective model to explain the relation between annealing at different condition and the change of oxygen defects.

Keywords: Zinc oxide, nanorods, photoluminescence, x-ray photoelectron spectroscopy, defects