Investigation of the defect centers in novel zinc oxide (ZnO) nano-particles by in-situ electron paramagnetic resonance (EPR) and optical spectroscopy

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Current research interest in nano-structured ZnO is motivated by novel applications as electrode material for dye-sensitized solar cells [1], light-emitting diodes [2], photoluminescence [3], thermoelectrics [4], printable electronics [5], varistors [6, 7], sensors [8] or nano-generators [8]. All of these applications exploit the semi-conducting properties of ZnO, which is a direct wide-bandgap (3.4 eV) n-type semiconductor, and take advantage of the fact that desired material properties may be tailored by means of controlling the defect structure by reducing the crystal size or doping. Despite its advantages, the lack of fundamental knowledge about intrinsic defects and doping ions presents an obstacle to the development of practical devices which requires p-type conductivity for high performance. The difficulty to make reliable p-type ZnO is closely related to the intrinsic and extrinsic defects such as interstitials, vacancies and doping ions, respectively.Although various ZnO compounds have been extensively investigated by many researchers over the last decades, there still exists controversy in various properties and some issues have even remained unclear. Things become more complicated and controversial when the crystallite size is confined to the nano-dimension. Although metal ion doping and compound morphology play a decisive role to control materials properties, the synthesis method defines an additional, most crucial role that markedly impacts nature and amount of intrinsic defects or impurities.The proposed research will provide a fundamental understanding of defects in ZnO that could lead to reliable p-type ZnO. The results of these electronic, optical and electrical characterization investigations could give the basis for the industrial and economic manufacturing of p-type ZnO. This project will provide physical understanding of the defect structures in bulk particles, nano-particles, quantum dots and bulk sintered ceramics (varistor) form of ZnO.Goals of the proposed research include: 1) Synthesize pure and metal-ion doped ZnO nano-particles and quantum dots by various routes. 2) Understand the fundamental structural (XRD; TEM, FTIR) properties of synthesized ZnO materials and investigate the electronic (EPR, ENDOR), optical (PL) and electrical (impedance) properties of intrinsic and extrinsic defects in them. 3)Elucidate the surface defects and the role of hydrogen in ZnO nano-particles.4) Understand the quantum confinement effects in ZnO quantum dots.5)Tune the band structure or band gap states of ZnO by controlling the defects. 6)Understand the effect of metal oxide additives on the electronic, electrical and optical properties of ZnO-based varistor.