Study On Fabrication And Optical Properties Of Several Semiconductor And Rare Earth Nanomaterials

Due to small size effect, quantum confinement effect and surface effect, nanomaterials with unique physical and chemical properties have stimulated great interest and have been investigated extensively. ZnO, ZnSe and CdS, having wide band gap and large exciton binding energy, are important II–IV group direct band semiconductors. These semiconductors have been widely applied in many fields, including nanolasers, solar cell, light-emitting diodes, field-effect transistors, photodetectors, gas sensors, optical switches, etc. The gadolinium oxide doped with europium (Gd₂O₃:Eu³⁺) possess excellent luminescent characteristics and high thermal stability, which are useful in luminescent lighting and optoelectronic devices, including fluorescent lamps, high definition televisions, plasma display panels, flat panel displays, etc. In this dissertation, ZnO, ZnSe and CdS nanostructures have been achieved by the simple thermal evaporation methods. Their morphologies, structures, growth mechanism and optical properties have been investigated in detail. Nanocrystalline Gd₂O₃:Eu³⁺ have been synthesized by low temperature combustion method. The luminescent properties of the nanocrystals were studied. The main research results are shown as followed:(1) Novel ZnO tetrapod and multipod nanostructures were successfully synthesized in bulk quantity through thermal evaporation method. The ZnO nanostructures were of high purity, and were well-crystallized with wurtzite structure. The preferred growth direction of legs was found to be the [0001] direction. Possible growth mechanisms were proposed for the formation of the ZnO nanostructures. Room temperature photoluminescence spectra showed that the as-synthesized ZnO nanostructures had a strong green emission centered at 495 nm and a weak ultraviolet (UV) emission at 383 nm. The UV emission corresponds to the near band edge emission and the green emission probably originates from surface defects and oxygen vacancies. ZnO nanowires and star-like ZnO nanostructures were also synthesized in bulk quantity by thermal evaporation method. Optical properties of these ZnO nanostructures were studied.(2) ZnSe nanowires with zinc-blende structure were synthesized by a simple hydrogen assisted thermal evaporation method. Structure analysis indicated that the nanowires have periodically twinned structures and grown along the [111] direction. Photoluminescence spectra revealed that the nanowires have a near band edge emission at 462 nm and a broad defect-related deep level emission band extending from 500 to 680 nm. Anti-Stokes photoluminescence was observed for the first time in ZnSe nanowires excited by 488 nm laser. Optical waveguide behavior and surface photovoltage (SPV) of ZnSe nanowires was also investigated.(3) By using a simple one-step H2-assisted thermal evaporation method, high quality CdS nanostructures have been successfully fabricated in large scale. The as-synthesized CdS nanostructures consisted of nanobelts and nanosaws with a hexagonal wurtzite structure. The deposition temperature played an important role in defining the size and morphology of the CdS nanostructures. A vapor-liquid-solid (VLS) combined with vapor-solid (VS) growth mechanism was proposed to interpret the formation of CdS nanostructures. Room temperature photoluminescence measurements showed intense green emission centered at 512nm for both nanobelts and nanosaws, which is the band-to-band emission of CdS. The waveguide behavior of both types of CdS nanostructures was observed and discussed. CdS nanobelts exhibited excellent optical waveguide property.(4) Nanocrystalline Gd2O3 doped with europium was synthesized by the facile combustion method. In glycine-nitrate combustion reaction, the structure changed from cubic to monoclinic by varying the glycine-to-metal nitrate (G/M) molar ratio. The influences of glycine and annealing temperatures on the grain size, crystallinity and luminescent properties were clarified. Nanocrystalline Gd2O3:Eu3+ with cubic phase was also prepared by modified combustion synthesis using citric acid as the fuel. Nanocrystalline Gd2O3:Eu3+ with cubic structure showed a dominant emission peak at about 612 nm, being attributed to 5D0→7F2 transition.