Rapid Synthesis of Nanocrystalline ZnGa 2 O 4 Phosphor at Low Temperature
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Rapid Synthesis of Nanocrystalline ZnGa2O4 Phosphor at Low Temperature Suresh D. Kulkarni1,2 and S. A. Shivashankar1,2 1 2
Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore, India. Materials Research Centre, Indian Institute of Science, Bangalore, India.
ABSTRACT A novel microwave-assisted synthesis technique was used for the rapid preparation of nanocrystalline ZnGa2O4 at two different temperatures. The crystalline spinel oxide is formed at temperatures as low as 100 oC within few minutes, at a high yield of 96%, requiring no postsynthesis annealing. The as-prepared samples are polycrystalline and phase-pure as verified by XRD, with a crystallite size of ~5 nm. Polycrystalline ZnGa2O4 substituted with Mn2+, Cr3+, Cu2+, and Co2+ was also similarly prepared. All samples are highly monodispersed, as measured by TEM. The ZnGa2O4 nanocrystals without further surface modification can be readily dispersed in chloroform to form a fully transparent colloidal solution, using which the bandgap of ZnGa2O4 was determined to be ~4.5 eV. The entire synthesis procedure, including solution preparation, microwave irradiation, and centrifugation takes about 30 minutes, which is faster than any procedure reported for a complex oxide like ZnGa2O4, as well as one with a small thermal budget. Photoluminescence shows a broad emission extending from 330 nm to 800 nm, which is surmised to be due to the defect structure in the oxide produced. INTRODUCTION Advanced material preparation and processing are essential to address current microelectronics challenges. Commercialization calls for the need for economical, simple and effective methods for material fabrication. Zinc gallate, ZnGa2O4, an important fluorescent compound semiconducting oxide with a wideband gap, has been recognized as an attractive candidate for phosphor host material for flat panel displays. Due to its excellent thermal and chemical stability [1], ZnGa2O4 phosphors are preferred over sulfide-based phosphors. As such, zinc gallate has attracted considerable research attention, as evidenced by the large number of publications in recent years [2-5]. It is also being investigated for white light emission in addition to blue, green, and red emission when doped with suitable activators[2, 6]. Production of ZnGa2O4 phosphor by solid-state reaction involves multi-step processes, high sintering temperatures, and relatively long times that are undesirable from both technological and environmental points of view. The grinding involved in such a process may damage phosphor surfaces, resulting in loss of emission intensity. The phosphors so obtained are of micrometer size[7], hence light output is strongly affected (decreases) by scattering at grain boundaries. Furthermore such ceramic processes do not give control of nanostructure, grain size, or size distribution, as these methods are limited to micrometer-size mixing. Further, the volatility at high temperature of ZnO, one of the reactants, suggests the need for preparation of ZnGa2O4 phosphor at lowe
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