Effect of surface defects on the visible emission from ZnO nanoparticles
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S.P. Singh National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India
R.S. Katiyar Department of Physics, University of Puerto Rico, San Juan, Puerto Rico 00931-3343 (Received 29 August 2006; accepted 20 December 2006)
The luminescent property of ZnO nanoparticles prepared using the wet chemical method has been investigated. The ZnO nanoparticles in the range 5–10 nm exhibit hexagonal Wurtzite structure, and the photoluminescence (PL) spectrum at room temperature shows a broad visible luminescence band and insignificant near-bandgap emission. The broad green luminescence is dominant at both room and boiled off liquid-nitrogen temperature, while the ultraviolet band edge emission is strongly quenched. The prepared ZnO nanoparticles have residual intermediate compound on the surface in the form of an acetate group, which acts as defect centers for the emission of green luminescence. A trace amount of zinc hydroxide is observed in one of the samples and is found to further enhance the intensity of the green luminescence. Raman scattering studies on nanoparticles indicate that the acetate/hydroxyl groups are loosely bound on the surface and are not present in the interior of the ZnO crystal structure I. INTRODUCTION
Zinc oxide, a wide band gap (3.3 eV) semiconductor, is a very promising material because of its wide range of properties. Its application areas are diverse: ultraviolet (UV) light emitters, lasers, varistors, transparent high power electronics, piezo-electric transducers, chemical sensors, and phosphors for flat panel display devices.1–4 Nanostructures are receiving a great deal of attention in this context because of their exceptional optical and electrical properties compared to their bulk counterpart, arising from the confinement of electrons and phonons in nanostructures.5–8 In recent years, the development of high-luminescence efficiency phosphor, operating at low voltage is the focus of flat panel display industry. In this regard, ZnO phosphor powders are of immense interest because they are abundant in nature and non-toxic, and have unique luminescence properties. The ZnO luminescence spectrum consists of a sharp UV emission band centered around 380 nm and a broad green emission centered around 510–530 nm.9 The narrow UV band edge emission is generally attributed to the radiative an-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0321 2404
J. Mater. Res., Vol. 22, No. 9, Sep 2007
nihilation of excitons, whereas the mechanism behind the origin of broad green luminescence is still under debate, especially in the nanostructures. For a long time, the origin of the green luminescence has been attributed to the oxygen vacancies or zinc interstitials10–12 or to the defect centers located at the surface of the nanoparticles.13 Guo et al.14 have shown that deep surface traps are responsible for the presence of green luminescence and could be efficiently quenched by the passivation of surface defects by the organic capping agent pol
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