X. Y. Chen

Effect of ZnO Nanoparticle Properties on Dye-Sensitized Solar Cell Performance

We have investigated the effect of ZnO nanoparticle properties on the dye-sensitized solar cell performance. Nanoparticles with different sizes and optical properties were considered. We found that there is a complex relationship between native defects, dye adsorption, charge transport and solar cell performance. The presence of a high concentration of nonradiative defects was found to be detrimental to photovoltaic performance, whereas for radiative defects, samples displaying orange-red defect emission exhibited better performance compared to samples with green defect emission (when the samples had similar emission intensities). Detailed discussion of the nanoparticle properties and their relationship with dye adsorption, electron injection, electron lifetime, electron transport time, and solar cell performance is given.

Electrical properties of ZnO nanorods studied by conductive atomic force microscopy

ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, we present a complementary electrical characterization of individual upright standing and lying ZnO nanorods using conductive atomic force microscopy (C-AFM). Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps. The current maps were recorded simultaneously with the topography acquired by contact mode AFM. Further, C-AFM was utilized to determine the local current-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs. Current-voltage characterization revealed a characteristic similar to that of a Schottky diode. Detailed discussion of the electrical properties is based on local I-V curves, as well as on the 2D current maps recorded from specific areas.

The influence of the ZnO seed layer on the ZnO Nanorod/GaN LEDs

We have studied the influence of the seed layer on the growth of ZnO nanorods on GaN by vapor deposition and the performance of the p-GaN/ZnO nanorod light emitting diodes (LEDs). The seed layer had a significant influence on the orientation and density of the ZnO nanorods as well as on the current-voltage curves of the devices, while optical properties exhibited a weaker dependence on the seed layer. A uniform and bright yellow electroluminescence was observed in all the devices, while the photoluminescence spectra exhibited a prominent UV emission and a weak green emission.

Recent progress in hydrothermal synthesis of zinc oxide nanomaterials.

Hydrothermal synthesis is of considerable interest due to its low cost, simplicity and relatively low growth temperature (typically below 200 °C). Since the synthesis is performed in aqueous solutions (no organic solvents), it can also be safe and environmentally friendly (depending on precursor chemicals). Consequently, it has been a subject of intense research in recent years. In this article, we review recent progress in hydrothermal synthesis of zinc oxide nanomaterials, with focus on practical relevance for a variety of applications.

Photoresponse from single upright-standing ZnO nanorods explored by photoconductive AFM

Summary Background: ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, photoconductive atomic force microscopy (PC-AFM) has been applied to investigate transient photoconductivity and photocurrent spectra of upright-standing ZnO nanorods (NRs). With a view to evaluate the electronic properties of the NRs and to get information on recombination kinetics, we have also performed time-resolved photoluminescence measurements macroscopically. Results: Persistent photoconductivity from single ZnO NRs was observed for about 1800 s and was studied with the help of photocurrent spectroscopy, which was recorded locally. The photocurrent spectra recorded from single ZnO NRs revealed that the minimum photon energy sufficient for photocurrent excitation is 3.1 eV. This value is at least 100 meV lower than the band-gap energy determined from the photoluminescence experiments. Conclusion: The obtained results suggest that the photoresponse in ZnO NRs under ambient conditions originates preferentially from photoexcitation of charge carriers localized at defect states and dominates over the oxygen photodesorption mechanism. Our findings are in agreement with previous theoretical predictions based on density functional theory calculations as well as with earlier experiments carried out at variable oxygen pressure.

Effect of annealing on the performance of CrO3/ZnO light emitting diodes

Heterojunction CrO3/ZnO light emitting diodes have been fabricated. Their performance was investigated for different annealing temperature for ZnO nanorods. Annealing in oxygen atmosphere had significant influence on carrier concentration in the nanorods, as well as on the emission spectra of the nanorods. Surprisingly, annealing conditions, which yield the lowest band edge-to-defect emission ratio in the photoluminescence spectra, result in the highest band edge-to-defect emission ratio in the electroluminescence spectra. The influence of the native defects on ZnO light emitting diode performance is discussed.

Nitrogen doped-ZnO/n-GaN heterojunctions

Nitrogen-doped ZnO nanorods were prepared by electrodeposition using two different Zn precursors (zinc nitrate and zinc acetate), while all other growth conditions (dopant precursor, concentration, growth temperature, and bias) were identical. We have shown that the precursor used affects the properties of the ZnO nanorods, and that the presence of rectifying properties in n-GaN/N:ZnO heterojunctions is strongly related to the use of nitrate precursor for ZnO growth. The difference in the properties of ZnO obtained from two precursors is attributed to the differences in native defect and impurity concentrations, which could affect the electronic properties of the samples.

Optical Properties of Oxide Nanomaterials

This chapter discusses different methods of optical characterization of various metal oxide materials. Photoluminescence (room temperature, temperature-dependent, and time-resolved), cathodoluminescence, electroluminescence, UV-Vis spectroscopy, and Raman spectroscopy are discussed.

Growth of Triangular ZnO Nanorods by Electrodeposition

Electrodeposition of ZnO nanorods on GaN-based substrates from different precursors was studied. We found that in the initial stage of the growth, triangular nanorods are formed and that they are transformed into hexagonal nanorods for longer growth time. The precursor used mainly determines the duration of the initial growth stage; i.e., slower growth along [0001] direction results in longer period for transition from triangular to hexagonal nanorod shape. Growth mechanisms of the triangular nanorods and their transformation into hexagonal nanorods are discussed.

Ce-doped ZnO nanorods by electrodeposition

We have investigated the use of different cerium precursors for fabrication of Ce:ZnO nanorods by electrodeposition. ZnO nanorods fabricated with different Ce precursors had similar morphology, but very different optical properties. The influence of annealing in oxygen at different temperatures on the optical properties of Ce:ZnO nanorods was also studied.