Near white light emission from ZnO nanostructures
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Near white light emission from ZnO nanostructures Sanjaya Brahma1, P. Jaiswal1, K. K. Nanda1, L. M. Kukreja3, S.A. Shivashankar1,2 1 Materials Research Centre, 2Centre for Nanoscale Science and Engineering, Indian Institute of Science, Bangalore-560012, 3Raja Ramanna Centre for Advanced Technology, Indore-452013, India ABSTRACT We report white light emission from ZnO nanostructures in powder form, prepared by microwave irradiation-assisted chemical synthesis, in the presence of a structure directing agent. Determination of their crystallinity, actual shape, and orientation was made using X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and optical properties have been studied through photoluminescence (PL), measured using HeCd laser (325 nm) as the excitation source. There is a noticeable variation in the luminescence correlated with variation of process parameters, such as microwave power, duration of irradiation, and the type/concentration of surfactants. The CIE (Commission Internationale l’Eclairage) diagram shows that the luminescence lies in yellow region of the color space. As the luminescence from the powder of ZnO lies in the yellow region, it is possible to produce white light from the powder of ZnO by using a blue laser as the excitation source. INTRODUCTION Lighting for residential, industrial and commercial applications using conventional light sources is very energy-intensive, because of its widespread use, often around the clock. Lighting, as provided today, not only consumes a very significant amount of energy but is also highly inefficient. For example, incandescent light bulbs convert about 5% of the electricity they use into visible light, though the efficiency of compact fluorescent lamps (a phosphor-coated gas discharge tube) is significantly higher (~20%). A survey reveals that worldwide, grid-based electric lighting consumed about 2,650 TWh of electricity in 2005, which amounts to some 19% of total global electricity consumption [1]. Production of electric power from widely used conventional resources (such as from coal, natural gas, and petroleum) is one of the biggest causes of greenhouse gas emissions. The energy consumed to supply lighting throughout the world produce green house emission of ~1900 megatonnes of CO2 per year [2]. To reduce the “carbon footprint” of electric lighting, different approaches have been taken, with steps to replace incandescent light bulbs with fluorescent tubes, compact fluorescent lamps, inorganic LEDs and organic LEDs. With increasing emphasis on “green technologies”, considerable emphasis is being placed on energy conservation, specifically on energy-efficient production of white light from solid state devices. Solid state lighting has been reviewed comprehensively by Humphreys and Schubert [3,4]. Inorganic LEDs and organic OLEDs emit light of single color in a narrow band of wavelengths. However, there is need for white light for a broad range of applications, and it is also reported that solid state white lig
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