Luminescence in ZnO Quantum Particles
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Wet-chemical and electrochemical routes were used to synthesize zinc oxide nanoparticles showing peculiar luminescent properties. ZnO-I are the nanoparticles that show only band gap ultraviolet (UV) emission, whereas ZnO-II type nanoparticles show the defect green luminescence along with the UV emission. Photoluminescence excitation (PLE) spectra of ZnO-I nanocrystals exhibit a single feature located at about 368 nm whereas ZnO-II nanocrystals show two features in PLE: a prominent feature at about 368 nm which is size dependent and a hump at 325 nm. These results are discussed in detail in view of the luminescence behavior in ZnO quantum particles. I. INTRODUCTION
ZnO is a direct, wide band gap semiconductor that has recently been used to fabricate ultraviolet (UV) laser diodes.1,2 The nanocrystalline materials have applications in semiconductor nanostructure lasers,3 light-emitting diodes,4 and solar cells.5 Consequently, quantum-confined ZnO nanoparticles have been under intense investigation with emphasis on the luminescent properties.6–16 Zinc oxide shows luminescence spectra with one or several of the following bands: sharp ultraviolet emission band centered around 380 nm, a green emission band centered around 530 nm along with the weak emission bands centered around 640 nm, and infrared (IR) emission band centered around 730 nm. Various different mechanisms were proposed12–20 to explain these luminescence features in ZnO. It is believed17 that narrow UV band is due to the radiative annihilation of excitons or band gap luminescence, whereas oxygen vacancies play a vital role in existence of broad green luminescence. The reliable investigations of quantum size effects in the nanoclusters are limited by the availability of “good quality” nanoclusters with narrow size distribution and electronically well-passivated surface. ZnO quantum particles have been synthesized by various chemical and electrochemical methods. Mahamuni et al.15 and Guo et al.21 have reported the synthesis routes to obtain ZnO quantum particles with quenched green luminescence. The presence of green luminescence was attributed to the deep surface traps that could be well-passivated by the organic capping agent poly vinyl pyrrolidone (PVP). Recently, Zhou et al.22 have shown that the presence of
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 3, Mar 2004
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Zn(OH)2 shell on core ZnO nanoparticle is responsible for the occurrence of green luminescence from ZnO. van Dijken et al.23 propose that oxygen vacancies are responsible for the existence of green luminescence. Monticone et al.24 as well as Mo et al.25 account for the green luminescence in terms of the oxygen vacancies or zinc interstitial sites. In general, it is believed that the luminescence properties of ZnO quantum particles are very sensitive to its surface states, which in turn are largely dependent on the synthesis method. ZnO quantum particles showing UV lu
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