Kewei Wu

Surface Passivation Effect on the Photoluminescence of ZnO Nanorods

We report an investigation of the impact of surface passivation on the optical properties of ZnO nanorods. Al2O3 coating and hydrogen plasma treatment were used to passivate the surface states. It was found that Al2O3 coating led to the suppression of the deep level emissions, while hydrogen plasma treatment completely quenched the deep level emissions. It was confirmed that the surface states of the as-grown ZnO nanorod arrays indeed contributed to the deep level emissions. Evidence was also provided that shows surface states have a greater impact on the green emission than the orange emission and may cause the negative thermal quenching behavior. Moreover, the passivation effect was confirmed by the changes of the O 1s and Zn 2p spectra.

Enhanced near band edge emission of ZnO via surface plasmon resonance of aluminum nanoparticles

The enhanced near band edge emission from a ZnO thin film and nanorod array by capping aluminum nanoparticles has been studied by photoluminescence spectra. The enhancement is attributed to the resonant coupling between the bandgap transition of the semiconductor and the surface plasmon of metal nanoparticles. It is also found that the Al nanoparticles support the surface plasmon from the deep-UV to the visible region with different annealing temperatures. This cost-effective approach is useful for manufacturing highly efficient optoelectronic devices.

Metal enhanced photoluminescence from Al-capped ZnMgO films: The roles of plasmonic coupling and non-radiative recombination

Aluminium (Al) enhanced emission is investigated by temperature-dependent photoluminescence (PL) from ZnMgO films grown by metal-organic chemical vapor deposition. The PL enhancement is sensitive to both the thickness of Al film and the temperature. The enhancement ratio increases from 3.5 to ∼51 when temperature decreases from room temperature to 20 K. Increased surface plasmon (SP) coupling and decreased non-radiative recombination rate with decreasing temperature are suggested to account for the giant enhancement. The coupling between the ZnMgO excitons and the SP resonance is confirmed by time-resolved PL.

Influence of p-type and n-type dopants on the magnetic properties of ZnCuO based diluted magnetic semiconductor thin films

Cu-doped ZnO (ZnCuO), Cu–Ga co-doped ZnO (Zn(Cu,Ga)O) and Cu–Na co-doped ZnO (Zn(Cu,Na)O) diluted magnetic semiconductor thin films have been deposited on (0 0 0 1) sapphire substrates by pulsed laser deposition. All the films have a hexagonal wurtzite ZnO structure with a high c-axis orientation. Hall-effect measurements showed that the ZnCuO film is n-type, in which the carrier concentration is greatly enhanced by the addition of Ga, while the introduction of Na resulted in p-type behaviour with a high resistivity. The magnetic hysteresis measurement indicated that the pure ZnCuO film is ferromagnetic at room temperature. Doping of Ga (n-type dopant) does not have a significant effect on the room temperature ferromagnetism. However, the room temperature ferromagnetic behaviour is destroyed by the addition of Na (p-type dopant). X-ray photoelectron spectroscopy analyses revealed that there exist more oxygen defects in the ZnCuO, Zn(Cu,Ga)O films than in the Zn(Cu,Na)O film. It was suggested that the interaction of Cu2+ with the oxygen defect in an n-type environment is responsible for the ferromagnetism observed in the ZnCuO system.

Structural and optical properties of ZnCdO/ZnO multiple quantum wells grown on sapphire substrates using pulsed laser deposition

High quality Zn0.92Cd0.08O/ZnO multiple quantum wells with smooth interfaces have been prepared on c-plane sapphire substrates by pulsed laser deposition. The periodic structure has been characterized by scanning transmission electron microscope and energy dispersive x-ray spectroscopy line scans. The temperature dependent photoluminescence of Zn0.92Cd0.08O/ZnO exhibits an inconspicuous S-shaped property due to a combined effect of the slightly disordered ZnCdO alloy. We can observe both quantum confinement effects and quantum-confinement Stark effect in the quantum wells. We can modulate the well emission energy from 2.90 to 3.085 eV by varying the well thickness at room temperature.

Vibronic Fine Structures on the {\sim}3.0 eV Violet Emission in Ion-Implanted ZnO Nanorods

Unusual vibronic fine structures of the ~3.0 eV violet emission from ion-implanted ZnO nanorods have been investigated by photoluminescence (PL). A set of equally separated peaks with energy spacing of 72 meV, which corresponds to the longitudinal optical (LO) phonon energy of ZnO, were well resolved in the low-temperature PL spectra. The overall emission band can be perfectly described on the basis of the LO phonon-assisted transition with the Huang–Rhys factor of 1.98, indicating intermediate electron-phonon coupling. The thermal quenching of PL gives rise to an activation energy of ~160 meV, which is attributed to the energy level of acceptor-like defects introduced by implantation.