Surface Plasmon Mediated Photoluminescence Properties Of Nanostructured ZnO Films

Zinc oxide (ZnO) is a compound semiconductor with direct and wide band gap. It has a band gap of 3.37 eV and an exciton binding energy of 60 meV at room temperature, high thermal stability and excellent optical properties. At present, ZnO has become a new hot spot followed GaN. Although the study of ZnO has been made great processes both in the experimental and theoretical aspects, it is necessary to further study the origin of green emission, the improvement of quality of growth of ZnO, and the mechanism of surface plasmon mediated the luminescence of ZnO,In this thesis, the pure ZnO thin films with different nanostructures deposited on the sapphire substrates were prepared by a chemical vapor deposition method. A vacuum DC sputtering system was used to deposit Au nanaoparticles on the surface of ZnO nanostructured films. The crystalline structure of ZnO was characterized by X-ray diffractometer (XRD). The results show that all as-prepared ZnO thin films are wurtzite hexagonal structure, and have the strongest peak of ZnO (002). It indicated that the as-grown ZnO films have the perfected orientation growth along c axis.The surface morphologies and roughness of samples were measured by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The results showed that the surfaces of samples were not smooth by SEM. The roughness of samples was different by AFM ananlysis results.The photoluminescence (PL) spectra of ZnO thin films were measured by fluorescence spectrometer. The PL spectra of uncoated ZnO thin films consist of two emission peaks. One is the near band edge emission at around 380 nm, and another is deep level emission due to defects. Moreover, the intensity of UV emission is relatively weaker compared with that of visible emission. The PL spectra of Au-coated ZnO thin films prepared at different temperatures also consist of UV emission band and visible emission band. However, the UV emission intensities of Au-coated samples are enhanced compared with that of un-coated samples. The visible emission intensities of samples were suppressed to some constants. In the case of significant changes in PL of ZnO films, we proposed the following physical mechanism. The electric field of the incident light coupled with Au nanoparticle surface electrons, the surface plasmons were created at the interface between ZnO nanosturcutrues and Au nanoparticles. After light illuminated, the electric field near Au nanoparticles was strongly enhanced. Therefore, the dramatic increase of intensity of excitation source resulted in the improvenment of excitation rate in the excitation process. The electrons located at defect states transferred to conduction band by Au nanoparticles. The charge transfer caused the increase in electron density in conduction band. Finally, UV emission from ZnO was enhanced. Meanwhile the decrease in electron density in defect level resulted in the suppression of the visible emission.The absorption spectra of Au-coated ZnO films were measured by UV-VIS-NIR spectrophotometer. The experimental results showed that a strong absorption peak at ~ 500 nm occurred in visible region. This absorption peak resulted from the surface plasmon resonance of Au nanopariticles. To investigate the vibration modes of ZnO, Raman scattering spectroscopy was used.