EPR, thermo and photoluminescence properties of ZnO nanopowders

https://doi.org/10.1016/j.saa.2011.06.048Get rights and content

Abstract

Nanocrystalline ZnO powders have been synthesized by a low temperature solution combustion method. The photoluminescence (PL) spectrum of as-formed and heat treated ZnO shows strong violet (402, 421, 437, 485 nm) and weak green (520 nm) emission peaks respectively. The PL intensities of defect related emission bands decrease with calcinations temperature indicating the decrease of Zni and Vo+ caused by the chemisorptions of oxygen. The results are correlated with the electron paramagnetic resonance (EPR) studies. Thermoluminescence (TL) glow curves of gamma irradiated ZnO nanoparticles exhibit a single broad glow peak at ∼343 °C. This can be attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centers. The trapping parameters of ZnO irradiated with various γ-doses are calculated using peak shape method. It is observed that the glow peak intensity increases with increase of gamma dose without changing glow curve shape. These two characteristic properties such as TL intensity increases with gamma dose and simple glow curve structure is an indication that the synthesized ZnO nanoparticles might be used as good TL dosimeter for high temperature application.

Highlights

• Nanocrystalline ZnO powders have been synthesized by combustion method. • PL and TL intensities of defect related emissions have been studied. • The results are correlated with the EPR spectroscopy and discussed.

Introduction

ZnO is an n-type II–VI semiconductor and has rapidly emerged as a promising optoelectronic material due to its large direct band gap of 3.37 eV, low power threshold for optical pumping at room temperature, highly efficient UV emission resulting from a large exciton binding energy at room temperature. This large exciton binding energy provides excitonic emission more efficiently even at higher temperature. Thus wurtzite structured ZnO is of potential importance for its application in light emitting diodes (LEDs), laser diodes (LDs) and ultra-violet (UV) photo detectors [1], [2], [3], [4].

The possibility of tailor making bulk material properties by varying size, structure and composition of constituting nanoscale particles makes them candidates for various important applications in the field of materials research. Zhou et al. [5] synthesized ZnO nanoparticles by gel-template combustion method and investigated the influence of heating conditions on size and photoluminescence of ZnO nanoparticles. Fan et al. [6] synthesized ZnO nanorods with hexagonal structures by the hydrothermal method at different conditions and studied their room temperature emission spectra and fluorescent dynamics. Salavati-Niasari et al. [7] prepared ZnO nanotriangles by thermal decomposition and studied their photocatalytic activity. Risti et al. [8] synthesized ZnO nanopowders by sol–gel synthesis and Raman spectra of ZnO particles were interpreted taking into account the nanosize effect. Chen et al. [9] prepared ZnO nanoparticles by direct precipitation method, Aimable et al. [10] synthesized ZnO nanoparticles by polymer-assisted precipitation in mild hydrothermal conditions and the influence of soft templates (organic species) to control size and size distribution in the final product was investigated.

In the present study, we report the photoluminescence (PL) and thermoluminescence (TL) properties of spherical shaped nanocrystalline ZnO powders prepared via low temperature solution combustion method. Thermoluminescence (TL), also known as thermally stimulated luminescence (TSL) is widely used to study defects in insulators and semiconducting materials. Moreover, this method is successfully applied in the field of radiation dosimetry. ZnO is inert to environmental conditions, nontoxic and insoluble in water. In spite of these characteristics, there is not much information related to the potential application of ZnO in TL dosimetry. The lack of interest of ZnO as dosimetric material is due perhaps to its other important applications. However, all the past works have been carried out on beta and X-ray irradiation [11], [12], [13] and there are no reports on γ-ray irradiation. In this paper, we report for first time the thermo luminescent properties of ZnO nanopowders irradiated by γ-rays. We have also examined the intrinsic defects in ZnO like zinc or oxygen vacancies present in this material using Electron paramagnetic resonance (EPR) spectroscopy and the results are discussed in detail.

Section snippets

Experimental

Fig. 1 shows the flow chart for the preparation of ZnO nanopowders. All the chemical reagents used in the present experiments were of analytical grade and used without further purification. Stoichiometric amounts of Zn(NO3)2·6H2O and the fuel C2H6N4O2 were dissolved in doubled distilled water in a cylindrical petri dish and heated in muffle furnace set at 300 °C. A detailed synthesis procedure of ZnO nanopowders and its characterization (PXRD, SEM, TEM, FTIR, UV–vis and Raman) is available in

Photoluminescence (PL) studies

Fig. 2 shows the PL spectrum of the as-formed ZnO nanopowders. Upon 254 nm excitation, the emission spectrum of ZnO has the asymmetric curve in the visible region, which implies the superposition of multiple emission bands. Gaussian curve fitting was applied to deconvolute the PL curve. The PL spectrum shows a violet emission peak at ∼402 nm, blue emission peaks at ∼421, 437, 485 nm and weak green emission peak at ∼525 nm. The violet emission corresponds to the near band edge emission of the wide

Conclusions

Hexagonal wurtzite structure ZnO nanopowders were prepared by combustion technique. The PL spectrum of ZnO nanopowders gives the near band edge UV emission and defect related blue and green emissions. EPR studies also confirm the formation of defects in ZnO nanopowders. TL glow curves of gamma irradiated ZnO nanoparticles shows a broad peak at ∼343 °C, which is attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial

Acknowledgements

Dr. B.M. Nagabhushana gratefully acknowledges Visvesvaraya Technological University, Belgaum, for the financial support (VTU/2009-10/A-9/11714) to carryout this research work. Dr. H. Nagabhushana thanks Dr. S.C. Sharma, Vice-chancellor, Tumkur University, Tumkur, for constant encouragement and support. A. Jagannatha Reddy express his gratitude to Dr. M. Suguna, for fruitful discussions regarding nanomaterials.

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