Photoluminescence Enhancement From Ag-ZnO Nanostructures

ZnO quantum dots(QDs)have attracted much attention due to their advantages,such as wide band gap(3.37 eV),large exciton binding energy(60 meV),large specific surface area and obvious quantum-size effect.These merits make them as promise materials for ultraviolet optoelectronic devices.In this dissertation,the photoluminescence enhancement from Ag-ZnO nanostructures has been investigated.The related results are as follows:Ag nanostructures such as nanoparticles and nanowires were prepared by polyol chemical reduction methods.The average size of Ag nanoparticles is 200 nm,and the Ag nanowires are 160 nm in diameters and 3-15?m in lengths.The Ag nanostructures exhibit single crystal as measured by high resolution Transmission Electron Microscopy.The Morphology of Ag nanostrctures were influenced by concentration ratio of AgNO3 and PVP,growth time and synthesis temperature.Different Ag nanostructures have different absorption peaks.ZnO QDs were synthesized by sol gel method.The sizes of ZnO QDs were controlled by varying reaction temperatures.The 5 nm ZnO QDs has a band-edge emission(365 nm)and a defect-related emission(520 nm).The emission properties of Zn O QDs depend on synthetic temperature,storage time and temperature.The Ag nanoparticles were dropped on the surface of Si substrate firstly,and then drop ZnO QDs on it,and the mixed samples were prepared.The sample with one drop of Ag nanoparticles and three drop of ZnO QDs exhibits strong emission enhancement.The enhancement ratio is 7-fold for band-edge emission at 364 nm and 40-fold for defect-related emission at 520 nm.However,the emission enhancement of Ag nanowires on ZnO quantum dots is weak.To reveal the enhancement mechanism,the experimental and theoretical absorption and scattering spectra of Ag nanoparticles were analysed.Two plasmon resonances named transverse dipolar mode(L1)and longitudinal dipolar mode(L2)were observed in the calculated spectra of Ag nanoparticles.The resonance wavelength of these two modes appears red-shift and blue-shift phenomenon with the decrease of the aspect ratio of Ag nanoparticles.Moreover,the ZnO inclusion can make the resonance wavelength of Ag nanoparticles significantly red shifted,and increase the difference of the L1 and L2 resonance wavelength.The L1 resonance wavelength matches the defect related emission of ZnO,which implies that the transverse dipolar mode of Ag nanoparticles leads to the enhancement of ZnO quantum dot defect emission.However,the quadrupole mode of Ag nanoparticles accounts for the enhancement of ZnO quantum dots ultraviolet emission.The enhanced electric field distribution of Ag nanoparticles under the excitation of 364 nm and 520 nm was obtained by the finite difference time domain method,and the enhancement mechanism was verified.This work may give a new approach to design ZnO based nano light-emitting devices.