Yangfan Lu

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.

Controllable growth and optical properties of ZnO nanostructures on Si nanowire arrays

In this paper, two types of hybrid semiconductors made up of silicon nanowires and ZnO nanostructures, namely Si/ZnO core–shell nanowire arrays and ZnO quantum dots (QDs)-decorated Si nanowire arrays, have been prepared by combining metal-assisted wet-chemical etching and metal–organic chemical vapor deposition (MOCVD). We demonstrate that ZnO QDs and thin ZnO layers can be grown on Si nanowires in a controlled manner by varying growth parameters including working pressure and growth time. Meanwhile, porous silicon and porous Si/ZnO nanowire arrays have also been fabricated. The morphology and optical properties of both hybrid nanostructures have been carefully investigated for their potential applications in nanowire optoelectronics. A quantum confinement effect in ZnO QDs was confirmed by the blue-shifted photoluminescence. Porous Si/ZnO core–shell nanowires display a very broad emission band throughout the entire visible light range.

A new type of p-type NiO/n-type ZnO nano-heterojunctions with enhanced photocatalytic activity

A new type of NiO/ZnO nano-heterojunction was developed by the template-assisted approach. The p–n nano-heterojunctions were formed between the NiO and ZnO nanoparticles in the ultrathin shell of the NiO/ZnO hollow nanospheres. The grain size of the NiO/ZnO nano-heterojunctions is in the 10 nm scale, and the specific surface area is higher than 100 m2 g−1. Moreover, the ultrathin shell makes the distribution of the NiO/ZnO nano-heterojunctions to arrange in a single-layer structure rather than a stacking arrangement. Therefore, this new type of the NiO/ZnO nano-heterojunctions could almost completely separate the electron and hole to the surface rather than being wasted in the materials. As a result, the photocatalytic activity of the p-type NiO/n-type ZnO nano-heterojunctions for the degradation of rhodamine B (RhB) was much higher than that of ZnO. In particular, the p-type NiO/n-type ZnO hollow nanospheres with different Ni/Zn molar ratios exhibited diverse catalytic activity, the mechanism of which has been discussed in detail.

Synthesis of ZrO2:Fe nanostructures with visible-light driven H2 evolution activity

In this paper, the ZrO2:Fe nanostructures with precisely controlled Fe doping contents are obtained by using a template method. The characterizations obviously show that the as-prepared samples have hollow sphere-like morphology and high crystalline quality. Furthermore, the band gap of the ZrO2:Fe nanostructures is facilely tunable by controlling the Fe content. In addition, the density-functional theory (DFT) calculation reveals that the formation of an impurity band in the band gap narrows the band gap of the Fe-doped ZrO2 nanostructures. The visible-light driven photocatalytic activity of ZrO2 nanostructures could be remarkably enhanced by doping the Fe impurity. This can be attributed to the red shift of the absorption edge and the trapping effect of the ZrO2:Fe nanostructures. The research results provide a general and effective method to synthesize different photocatalysts with enhanced visible-light driven H2 evolution activity.

Violet emission in ZnO nanorods treated with high-energy hydrogen plasma.

Violet photoluminescence was observed in high-energy hydrogen-plasma-treated ZnO nanorods at 13 K. The photoluminescence spectrum is dominated by a strong violet emission and a shoulder attributed to excitonic emission. The violet emission shows normal thermal behavior with an average lifetime of about 1 μs at 13 K. According to the time-resolved and excitation density-dependent photoluminescence, it was found that the violet emission is determined by at least two emitting channels, which was confirmed by annealing experiments. Evidence was also given that the violet emission is related to hydrogen. We suggested that the hydrogen-related complex defects formed under high-energy hydrogen plasma treatment are responsible for this violet emission.

Dual-acceptor p-type behaviour in ZnO films grown by plasma-assisted metalorganic chemical vapour deposition

Nominally undoped and N-doped ZnO thin films were grown by plasma-assisted metalorganic chemical vapour deposition. P-type conductivity was confirmed by Hall-effect measurements, not only in the N-doped but also in the nominally undoped ZnO. The zinc vacancy and extrinsic nitrogen acceptor states were identified by low-temperature photoluminescence, with the energy level located at 270 meV and 180 meV above the valence-band maximum, respectively. An evident increment in the oxygen as well as nitrogen concentration in the p-type ZnO : N layer was well confirmed by secondary ion mass spectroscopy.

Electronic Band Structures and Native Point Defects of Ultrafine ZnO Nanocrystals.

Ultrafine ZnO nanocrystals with a thickness down to 0.25 nm are grown by a metalorganic chemical vapor deposition method. Electronic band structures and native point defects of ZnO nanocrystals are studied by a combination of scanning tunneling microscopy/spectroscopy and first-principles density functional theory calculations. Below a critical thickness of ∼1 nm ZnO adopts a graphitic-like structure and exhibits a wide band gap similar to its wurtzite counterpart. The hexagonal wurtzite structure, with a well-developed band gap evident from scanning tunneling spectroscopy, is established for a thickness starting from ∼1.4 nm. With further increase of the thickness to 2 nm, VO-VZn defect pairs are easily produced in ZnO nanocrystals due to the self-compensation effect in highly doped semiconductors.