Yu Hang Leung

Different origins of visible luminescence in ZnO nanostructures fabricated by the chemical and evaporation methods

We prepared ZnO nanostructures using chemical and thermal evaporation methods. The properties of the fabricated nanostructures were studied using scanning electron microscopy, x-ray diffraction, photoluminescence, and electron paramagnetic resonance (EPR) spectroscopy. It was found that the luminescence in the visible region has different peak positions in samples prepared by chemical and evaporation methods. The samples fabricated by evaporation exhibited green luminescence due to surface centers, while the samples fabricated by chemical methods exhibited yellow luminescence which was not affected by the surface modification. No relationship was found between green emission and g∼1.96 EPR signal, while the sample with yellow emission exhibited strong EPR signal.

Green, yellow, and orange defect emission from ZnO nanostructures: Influence of excitation wavelength

ZnO commonly exhibits luminescence in the visible spectral range due to different intrinsic defects. In order to study defect emissions, photoluminescence from ZnO nanostructures prepared by different methods (needles, rods, shells) was measured as a function of excitation wavelength and temperature. Under excitation at 325nm, needles exhibited orange-red defect emission, rods exhibited yellow defect emission, while shells exhibited green defect emission. Obvious color change from orange to green was observed for needles with increasing excitation wavelengths, while nanorods (yellow) showed smaller wavelength shift and shells (green) showed no significant spectral shift. Reasons for different wavelength dependences are discussed.

Magnetic properties of Mn doped ZnO tetrapod structures

ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn diffusion doping was performed at two different temperatures (600 and 800 °C). The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray fluorescence, x-ray diffraction (XRD), superconducting quantum interference device magnetometer, and photoluminescence. Diffusion doping resulted in the increase of the size of tetrapods, but no new peaks were found in XRD spectrum. Mn doped ZnO tetrapod structures were found to be ferromagnetic with Curie temperature ∼50 K, and showed large coercive field (∼3500 Oe for 800 °C sample, ∼5500 Oe for 600 °C sample).

Visible photoluminescence in ZnO tetrapod and multipod structures

The properties of ZnO tetrapod and multipod structures were investigated using scanning electron microscopy, x-ray diffraction, photoluminescence (PL), and electron paramagnetic resonance (EPR) spectroscopy. While there is relationship between g=1.96 EPR and green PL in some of the samples, this is not the case for all the samples. Therefore, the commonly assumed transition between a singly charged oxygen vacancy and photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt, Appl. Phys. Lett. 68, 403 (1996)] does not explain the green emission in all ZnO samples. The green emission likely originates from surface defects.

Toxicity of Metal Oxide Nanoparticles: Mechanisms, Characterization, and Avoiding Experimental Artefacts

Metal oxide nanomaterials are widely used in practical applications and represent a class of nanomaterials with the highest global annual production. Many of those, such as TiO2 and ZnO, are generally considered non-toxic due to the lack of toxicity of the bulk material. However, these materials typically exhibit toxicity to bacteria and fungi, and there have been emerging concerns about their ecotoxicity effects. The understanding of the toxicity mechanisms is incomplete, with different studies often reporting contradictory results. The relationship between the material properties and toxicity appears to be complex and diifficult to understand, which is partly due to incomplete characterization of the nanomaterial, and possibly due to experimental artefacts in the characterization of the nanomaterial and/or its interactions with living organisms. This review discusses the comprehensive characterization of metal oxide nanomaterials and the mechanisms of their toxicity.

Mechanisms of Antibacterial Activity of MgO: Non‐ROS Mediated Toxicity of MgO Nanoparticles Towards Escherichia coli

The toxicity of metal oxide nanomaterials and their antimicrobial activity is attracting increasing attention. Among these materials, MgO is particularly interesting as a low cost, environmentally-friendly material. The toxicity of MgO, similar to other metal oxide nanomaterials, is commonly attributed to the production of reactive oxygen species (ROS). We investigated the toxicity of three different MgO nanoparticle samples, and clearly demonstrated robust toxicity towards Escherichia coli bacterial cells in the absence of ROS production for two MgO nanoparticle samples. Proteomics data also clearly demonstrate the absence of oxidative stress and indicate that the primary mechanism of cell death is related to the cell membrane damage, which does not appear to be due to lipid peroxidation.

Strategies for improving the efficiency of semiconductor metal oxide photocatalysis

Photocatalysis is of significant interest for a wide range of applications related to energy and environment, such as pollutant degradation and hydrogen production. We will provide a review of the relationship between photocatalyst properties and its photocatalytic performance, as well as the strategies for the enhancement of photocatalytic activity, in particular under solar/ambient/visible illumination. Common applications of photocatalysts will then be reviewed, and we will summarize existing problems and areas requiring further improvements.

Influence of annealing on stimulated emission in ZnO nanorods

Vertically aligned ZnO nanorod arrays with rod lengths in the range of 200–1500nm were fabricated by a hydrothermal method. No stimulated emission was observed in as grown nanorods. Annealing of the rods in forming gas and oxygen significantly affected their optical properties and enabled the achievement of stimulated emission. The lowest lasing threshold and defect emission as well as the longest spontaneous emission decay times were obtained for nanorods annealed in oxygen flow. This indicates that interstitial oxygen, which is commonly assumed to be the cause of yellow-green defect emission, is not the dominant defect in hydrothermally grown nanorods.

Time-resolved photoluminescence from ZnO nanostructures

Different ZnO nanostructures (tetrapods, shells, rods, and highly faceted rods) were characterized by photoluminescence (PL) and time-resolved PL measurements. It was found that different nanostructures exhibit very different optical properties in terms of defect emission and decay times of the spontaneous emission. No correlation was found between the PL decay times and defect emission intensities and defect emission positions. The short decay times of the UV emission are most likely due to nonradiative defects that are correlated with the crystalline quality and do not contribute to the visible emission. Neither short PL decay times nor intense defect emissions rule out achievement of stimulated emission.

Time-resolved photoluminescence study of the stimulated emission in ZnO nanoneedles

ZnO nanoneedles were fabricated by thermal evaporation of Zn nanoparticles at 800 °C and atmospheric pressure. The samples showed strong ultraviolet photoluminescence and weak orange defect luminescence. Time-resolved photoluminescence (TRPL) was measured using the Kerr-gated fluorescence technique in order to probe the ultrafast carrier dynamics in exciton-exciton scattering and electron hole plasma (EHP) regimes. In both regimes, the decay time of the photoluminescence is very fast (∼1ps). Even though no structure is detected in the time-integrated spectra of the EHP emission, the TRPL reveals the coexistence of the excitons and free carriers. Possible reasons for the observed phenomena are discussed.