Photoluminescence study of ZnO nanowires with Zn residue

ZnO nanowires were grown by heat treatment of thermally evaporated Zn thin films at 430 and 500 °C. Thermal annealing at 430 °C leads to incomplete oxidation of Zn forming ZnO/Zn composite films. As a result of the Zn residue, there is decrease in optical transmittance along with a significant variation in photoluminescence properties. Although, Zn diffusion in ZnO does not produce additional phonon modes in Raman scattering, the enhancement of LO phonon mode due to electric field induced Raman scattering is observed. The blue shift of blue emission and the red shift in the band edge absorption at photoluminescence excitation monitoring blue emission for ZnO/Zn film is correlated with charge transfer from Zn to ZnO. The photoluminescence excitation peak position at shorter wavelength monitoring blue, red and green remains unchanged, albeit band edge excitation peak shows significant red shift for ZnO/Zn thin films. This indicates that the excitation resonance at shorter wavelength is not caused by the usual electronic band to band or defect transition but by the plasmons in ZnO. The results are important for fabrication of ZnO based MESFET and LED displays.