K. H. Tam

Defect emissions in ZnO nanostructures

Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

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.

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.

Non-destructive characterization of vertical ZnO nanowire arrays by slow positron implantation spectroscopy, atomic force microscopy, and nuclear reaction analysis

ZnO nanorods, grown by a hydrothermal method, have been characterized by slow positron implantation spectroscopy (SPIS) and atomic force microscopy (AFM). It has been demonstrated that such non-destructive characterization techniques can provide a comprehensive picture of the nanorod structure (including its length, shape, orientation, and seed layer thickness), as well as provide additional information about defects present in the structure. Nanorods were also characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD), and it was found that the SPIS/AFM combination is more sensitive to the nanorod orientation and the thickness of the seed layer. To obtain still more information about defects in the nanorods, as well as to confirm the findings on the sample structure, nuclear reaction analysis (NRA) was performed and a large concentration of bound hydrogen was found. The results obtained by different characterization techniques are discussed.

ZnO nanorods by hydrothermal method for ZnO/GaN LEDs

Zinc oxide (ZnO) has been attractive for optoelectronics application due to its wide band gap (Eg=3.37 eV) and large exciton binding energy (~60 meV) characteristics. However, p-type doping of ZnO is still controversial and problematic. Therefore, there is considerable interest in fabrication of n-ZnO/p-GaN heterojunction LEDs. In this work, we fabricated the LEDs consisting of n-ZnO nanorod arrays on p-GaN substrate. ZnO nanorod arrays were fabricated by a hydrothermal method. Hydrothermal methods have the advantage that they are simple, inexpensive and environmentally friendly. However, nanorods fabricated by hydrothermal methods typically have large numbers of defects due to the low growth temperature (90degC). The defect related photoluminescence (PL) is significantly affected by annealing, and under suitable conditions it can be entirely eliminated. As-grown nanorods exhibit UV emission and large yellow defect emission which is likely due to the presence of OH groups. The PL spectra can be significantly improved by annealing the nanorods at 200degC under Ar flow. Therefore we investigated the influence of argon annealing of ZnO nanorods on the performance of ZnO/GaN LEDs, as well as the influence of annealing environments. Devices with ZnO rod length ~ 250 nm were fabricated and the results obtained are discussed.

Tailoring and modifications of a ZnO nanostructure surface by the layer-by-layer deposition technique

Different ZnO nanostructures have been modified using the layer-by-layer polyelectrolyte deposition process. The polymer multilayers were deposited on free standing ZnO tetrapods, ZnO tetrapods on a substrate and ZnO nanorod arrays. In addition, attachment of metallic (Au) nanoparticles to the ZnO nanostructure surface using layer-by-layer deposition was demonstrated. The properties of the ZnO nanostructures with modified surfaces were investigated by electron microscopy, absorption and photoluminescence measurements. A linear increase in polymer thickness with the number of polymer multilayers was confirmed by absorption and transmission electron microscopy. The technique can be readily extended to different nanoparticles and different morphologies of ZnO.

ZnO nanorod heterojunctions and LEDs

Zinc oxide (ZnO) nanorods are of great interest due to their potential use in optoelectronic devices. Hydrothermal synthesis and electrodeposition are two very promising techniques for ZnO growth as they are simple and inexpensive and also involve low fabrication temperature, favoring the use of large area and/or flexible substrates. In this work, we investigated the influence of the seed layer on the morphology and optical properties of ZnO nanorods, as well as the influence of post-fabrication treatments, such as annealing under different conditions. To further demonstrate the potential use of ZnO nanorods in optoelectronic devices, light emitting diodes (LEDs) were also studied in this paper. We have focused on heterojunction devices with ZnO as an n-type layer, since heterojunction ZnO LEDs typically exhibit better performance compared to homojunctions. We have investigated devices using GaN and NiO as a p-type material, and in addition hybrid organic/inorganic devices were also prepared.

Hydrothermal growth of periodic ZnO nanorod arrays using polystyrene sphere templates

Zinc Oxide (ZnO) photonic crystals (PCs) are of great interest in recent years due to its potential applications in optoelectronic devices. However, most of the growth temperatures in fabricating ZnO nanostructures in periodic structures reported up to date are very high (>500°C), which is not favorable for low cost manufacture. In this work, a method involving low growth temperature (90°C), hydrothermal synthesis, was used to fabricate ZnO nanostructures. Self-assembly of polystyrene (PS) spheres was used to fabricate periodic arrays. PS spheres were self-assembled on the silicon (Si) or fluorine-doped tin oxide (FTO) substrate with ZnO seed layer derived from zinc acetate. After crosslinking the PS sphere layer, hydrothermal growth was performed. The influence of growth conditions (concentration, addition of PEI, etc.) on fabricated ZnO nanorod array has been studied. Then, PS spheres were removed by annealing in air, and fabricated ZnO nanorod arrays were studied by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. Periodicity of the array was strongly substrate dependent (better on Si than on FTO). Light emitting diodes were also fabricated in order to demonstrate the potential use of this ZnO periodic array.

Defect emission in ZnO nanorods fabricated by hydrothermal method

Zinc oxide (ZnO) is a wide bandgap semiconductor which has great potential for a variety of practical applications. Hydrothermal growth of ZnO nanostructures is of particular interest because it is low cost, low temperature and environmentally friendly method. It is well‐known that ZnO nanorods fabricated by hydrothermal method exhibit prominent defect related emission. In this work, we studied the morphology and optical properties of ZnO nanorods with different lengths fabricated with different seed layers. Effects of annealing at different temperatures and in different atmospheres were investigated. It was found that the defect emission was strongly affected by the annealing conditions. The origin of the defect emission and the observed differences after annealing are discussed.

InN nanorods growth: Influence of temperature, catalyst, and gas flow rate

One dimensional (1‐D) semiconductor nanostructures are attracting lots of attention due to their novel properties different from bulk and their potential application in nanodevices. Indium nitride (InN) is of particular interest for various optoelectronics and electronic applications, especially for field effect transistors due to its large drift velocity. However, the synthesis and characterization of InN nanostructures have not been studied comprehensively. In this work, the growth of InN nanorods was studied. It was found that the morphology of the nanorods was strongly dependent on the NH3 flow rate and the catalyst used. Based on the obtained results, the possible growth mechanisms of InN nanorods are discussed.