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Contents lists available at ScienceDirectJournal of Luminescencejournal homepage: www.elsevier.com/locate/jluminOxygen vacancy mediated temperature dependent emission behavior oflocalized bound excitons in ZnO nanorodsXiangdong Meng a, ? , Yuxue Zhou a , Xianghua Zeng a , Xiaobing Chen a , Yanqiu Chu ba College of Physics Science and Technology, Yangzhou University, Yangzhou, Jiangsu 225002, PR Chinab School of Science, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, PR ChinaA R T I C L E I N F OKeywords:Oxygen vacancies defectsBound excitonsPhotoluminescenceZnO nanorodsA B S T R A C TThe present work reports on the tunable optical emission from oxygen vacancies in ZnO nanorods. The commongreen luminescence at room temperature originated from oxygen vacancy defects can be mediated by simplycontrolling pH values of alkaline solutions without the introduction of any intentional dopants. The peak en-ergies of the green band are found to be monotonously tuned in the range from 2.17 to 2.47 eV, which couldpromote the applications of tunable wavelength ZnO optoelectronic devices. The temperature dependent be-havior of exciton transitions and oxygen vacancy recombinations in ZnO nanorods is investigated in detail. It issuggested that the localized bound exciton (LBX) attached near the lateral facets of ZnO nanorods could surviveup to nearly room temperature due to the large binding energy.1. IntroductionIn recent years, ZnO has attracted increasing attention in view of itsdirect wide band gap (3.37 eV) and large free exciton (FX) bindingenergy (approximately 60 meV). Wurtzite ZnO has been of considerablesignif i cance for the promising device applications such as short wave-length light emission diodes (LED), surface acoustic wave devices,transparent electrodes and so on [1,2]. One-dimensional (1D) ZnOnanostructures with diversif i ed morphology such as nanotubes, nano-wires, nanobelts, and nanorods have been studied widely which can beused as potential building block in nanoelectronic and nanooptoelec-tronic devices [3–6]. Specif i cally, ZnO nanorods in the form of hybridor heterojunctions have shown extraordinary application prospects forphotodetectors, gas sensors and photovoltaic devices in recent reports[7–10].A photoluminescence (PL) spectroscope is a nondestructive andsensitive tool to investigate the native point defects in ZnO which areusually assumed to be responsible for the ubiquitous broad visibleemission. The native point defects such as oxygen vacancies (V O ),oxygen interstitials (O i ), zinc vacancies (V Zn ), zinc interstitials (Zn i ) andoxide antisites (O Zn ) are suggested as candidates responsible for theorigin of the omnipresent green luminescence band in ZnO PL spectra[11]. Although the exact structural origin of the green band remainscontroversial, V O recombination is convincingly accepted by many re-searchers [12–14]. The peak positions and brightness of the green lu-minescence in ZnO can be varied by doping with dif f erent atoms,suitable for practical applications in light emission devices [15–18].Besides visible emission, high crystalline quality ZnO thin f i lms andnanomaterials usually exhibit UV emission. The near band edge (NBE)free excitons (FX), some bound excitons (BX) and donor-acceptor-pair(DAP) are usually concluded as origin of UV emissions in temperaturedependent ZnO PL spectra [19,20]. In the present work, the modulationof V O in ZnO nanorods was achieved by readily varying pH values inaqueous solutions without the introduction of any doping, which wouldresult in the monotonous shift of the green band. In addition, thetemperature behavior investigation on the exciton transitions and V Orecombinations is carried out.2. ExperimentHigh purity anhydrous zinc acetate [Zn(CH 3 COO) 2 , Zn(Ac) 2 ] andammonia were used without further purif i cation for the undoped ZnOnanorods synthesis. In a typical synthesis, an aqueous solution con-taining adequate quantity of Zn(Ac) 2 was prepared and then drippedinto the ammonia to regulate the solution pH as 8.5, 9, 9.5, 10 and 10.5,respectively. The resulting suspension was transferred into a stainlessautoclave, and held at 140 °C for 24 h. The nanorods products wereobtained by carefully f i ltering and washing the precipitates with deio-nized water.The size and morphology of ZnO nanorods were measured by usinga JSM-6700F f i eld-emission scanning electron microscope (FE-SEM)and a JEOL2010 high resolution transmission electron microscopehttps://doi.org/10.1016/j.jlumin.2017.11.036Received 21 July 2017; Received in revised form 19 November 2017; Accepted 21 November 2017? Corresponding author.E-mail address: mengxd@yzu.edu.cn (X. Meng).Journal of Luminescence 195 (2018) 201–208Available online 22 November 20170022-2313/ © 2017 Elsevier B.V. All rights reserved.T