Abstract
Although, post annealing is an efficient way to annihilate/restructure deficiencies in self-assembly (SA) ZnO nanorods (ZNRs), the detailed investigation about the surface properties of annealed SA-ZNRs is a long standing issue and the major discrepancy is mainly due to single step annealing. We demonstrate the strategic two step annealing process to create reliable structural configuration in SA-ZNRs during the first round of annealing at 800 °C in vacuum (VA process), and create intrinsic defects in the second step of annealing in oxygen rich atmosphere (OA process) to correlate the formation of the defects related to green/orange-red emission. SA-ZNRs annealed in VA-OA processes reveal positive correlations between the oxygen flow rate and formation of oxygen interstitials (Oi) and zinc vacancies (VZn). The OA-VA processes exhibit the relation of residual Oi and additional Vo. According to VA-OA and OA-VA processes, we propose that the green emission in ZnO annealed in oxygen poor/rich condition is mainly due to the formation of Vo/VZn and annealing at oxygen rich condition creates Oi that lead to strong orange-red emission. Rather than O1s, we propose a reliable method by considering the peak shift of Zn2p in XPS to inspect the ZnO matrix, which has good interdependence with the characteristics of PL.
Highlights
Zinc oxide (ZnO) based semiconductor devices has attracted enormous attention among researchers because of its versatile applications in a variety of research fields such as optoelectronics, piezoelectronics, solar cells, and have been reviewed extensively[1,2,3,4]
The room temperature PL spectra (Fig. 1(a)) of single annealed ZnO nanorods (ZNRs) shows two distinct peaks: (a) A strong UV emission originating from the recombination of free excitons and is considered as the characteristic signature of wurtzite ZnO, (b) the visible luminescence centered at 558 nm is usually attributed to all possible surface defects dominated by OH− ions produced usually in ZnO grown by wet chemical method[24]
This peak assignment is doubtful and we repeat that the position/ broadness of the peak may be related to the unstable surface defects generated during synthesis, surface dangling bonds, precipitates/segregations/organic residues and surface OH− ions that virtually suppress the intrinsic defects in ZNRs
Summary
Zinc oxide (ZnO) based semiconductor devices has attracted enormous attention among researchers because of its versatile applications in a variety of research fields such as optoelectronics, piezoelectronics, solar cells, and have been reviewed extensively[1,2,3,4]. There are plenty studies about the generation and annihilation of deficiencies via annealing, but the instability properties of self-assembly (SA) ZNRs induce the difficulties on the analyses of surface states and related deficiencies via annealing[8] The presence of these defects suppresses the ultraviolet (UV) emission efficiency in photoluminescence[8,9] that degrade the optical properties of ZnO, and inhibit its application in optoelectronic devices. We strongly believe that the first few layers over the surface of ZNRs fabricated by wet chemical synthesis consists of unstable surface defects, surface dangling bonds, precipitates/segregations/organic residues and surface OHions that virtually suppress all signals from the intrinsic defects of ZnO
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