Chuan Beng Tay

Green luminescence from Cu-doped ZnO nanorods: Role of Zn vacancies and negative thermal quenching

Bright and stable structured green luminescence (GL) is achieved from solution-grown Cu-doped ZnO nanorods. Dependence of photoluminescence on the annealing parameters reveals that GL is correlated with creation of Zn vacancies (VZn) and then formation of Cu dopants at Zn sites (CuZn). High internal quantum efficiency (43%) of the GL can be sustained up to 240 K due to negative thermal quenching. In contrast to the poor stability of defects-related visible emission, the structured GL shows good stability with respect to sample heating. Cu-doped ZnO nanorods with strong and stable GL have potential applications in visible light display and lighting.

ZnO Coaxial Nanorod Homojunction UV Light‐Emitting Diodes Prepared by Aqueous Solution Method

The fabrication of a p-shell/n-core coaxial nanorod ZnO homojunction light-emitting diode by inexpensive solution method is demonstrated. The p-type conductivity of the ZnO shell arises from the incorporation of potassium while the n-type conductivity of the core is due to unintentional doping.

Universal photoluminescence evolution of solution-grown ZnO nanorods with annealing: important role of hydrogen donor

Poor near-band-edge emission (NBE) prohibits the application of solution-grown ZnO nanorods in optoelectronics, thus their photoluminescence (PL) was studied with respect to post-annealing temperature and duration. A universal behavior was revealed: NBE was enhanced by one order (or two) of magnitude after annealing in air (or H2) at about 425 °C for 30 min, while the enhancement factor starts to decrease after annealing at higher temperatures. The evolution of PL was mainly ascribed to annealing-induced activation and dissociation of hydrogen donor, which was identified as HO by both PL and Raman analyses. Results from nanorods with different diameters and annealing gases further support this assignment. The results provide new insights to understand and optimize the properties of solution-grown ZnO.

ZnO nanorods with low intrinsic defects and high optical performance grown by facile microwave-assisted solution method.

Vertically aligned ZnO nanorods were grown at 90 °C by both microwave synthesis and traditional heated water bath method on Si (100) substrate with a precoated ZnO nanoparticle seed layer. A detailed comparison in the morphology, defects, and optical properties of the ZnO nanorods grown by the two methods across the pH range of 10.07-10.9 for microwave synthesis and conventional heated water bath method was performed using scanning electron microscopy, photoluminescence, and X-ray photoelectron spectroscopy. The results show that the microwave route leads to more uniformly distributed nanorods with a lower density of native defects of oxygen interstitials and zinc vacancies. The microwave synthesis presents a promising new approach of fabricating metal oxide nanostructures and devices toward green applications.

Empirical Model for Density and Length Prediction of ZnO Nanorods on GaN Using Hydrothermal Synthesis

We present an empirical model based on the solubility of zinc and temperature to explain the observed variations in density and length of hydrothermally grown zinc oxide (ZnO) nanorods on GaN substrate as reported by Le et al. [J. Cryst. Growth, 293, 36 (2006)]. The ionic equilibrium of the growth solution is solved numerically. By using the experimental data and calculated equilibrium concentrations of species in the solution, expressions were formulated to represent the dependence of the ZnO nanorod density and length on the solubility of zinc and temperature. Activation energies of 0.77 and -2.11 eV were found for the length and density, respectively. The proposed model can be used to predict the ZnO nanorod density and length on GaN substrate within the growth temperature range of 333 to 423 K, precursor solution pH between 9.7 and 10.6, and a degree of saturation 20 < S < 60.

Investigation of Morphology and Photoluminescence of Hydrothermally-Grown ZnO Nanorods Grown on Substrates Pre-Coated with ZnO Nanoparticles

We show that the rod-like morphology of hydrothermally-grown ZnO nanorods can be obtained on any substrate, pre-coated with ZnO nanoparticles, when pH<7.3 or pH>10.1. When pH>10.1, three types of morphology, from uniform nanorods to large clustered rods, can be distinguished based on the solubility of zinc. In line with classical thermodynamics, the density of nanorods increases when the solubility of zinc decreases. We also report that the orange defect peak photoluminescence can be reduced, while the UV peak increased by decreasing the pH of the growth solution.

Origin and Quenching of Novel ultraviolet and blue emission in NdGaO3: Concept of Super-Hydrogenic Dopants

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO3 (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)”, where each Ce4+ ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.85 nm and 1.15 nm whereas the predicted theoretical value of SHD radius for NdGaO3 is ~1.01 nm.