R.E. Kroon

Effect of fuel content on luminescence and antibacterial properties of zinc oxide nanocrystalline powders synthesized by the combustion method

Nanoscale ZnO powders were synthesized via the combustion method using zinc nitrate hexahydrate as the source (oxidant) material and urea, and glycine or citric acid monohydrate as fuels. The effect of the relative fuel to oxidant ratio on the characteristics of ZnO particles produced with urea as the fuel was also investigated. X-ray diffraction analysis revealed that the ZnO nanocrystalline particles were successfully synthesized by combustion and the crystallite size was different depending on the fuel. The surface morphology showed a dramatic change as the fuel changed in the synthesis process. The chemical states of the synthesized ZnO powders were investigated using X-ray photoelectron spectroscopy, which allowed an assessment of the Zn and O related defect concentrations. The optical bandgap varied from 3.12 to 3.14 eV for different fuels and it decreased to 3.10 eV in the fuel rich case for urea. The maximum defect level photoluminescence emission was observed for the ZnO powder synthesized using urea as the fuel, for the stoichiometric fuel mix. All products exhibited similar antibacterial effects; however they had a greater effect on Staphylococcus aureus compared to Escherichia coli.

Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method.

Mixed oxides of zinc and cadmium with different proportions were deposited on ordinary glass substrates using the sol-gel spin coating method under optimized deposition conditions using zinc acetate dihydrate and cadmium acetate dihydrate as precursors. X-ray diffraction patterns confirmed the polycrystalline nature of the films. A combination of cubic CdO and hexagonal wurtzite ZnO phases was observed. The oxidation states of Zn, Cd and O in the deposited films were determined by X-ray photoelectron spectroscopic studies. Surface morphology was studied by scanning electron microscopy and atomic force microscopy. The compositional analysis of the thin films was studied by secondary ion mass spectroscopy. The transmittance of the thin films was measured in the range 300-800nm and the optical bandgap was calculated using Taucs plot method. The bandgap decreased from 3.15eV to 2.15eV with increasing CdO content. The light emission properties of the ZnO:CdO thin films were studied by photoluminescence spectra recorded at room temperature. The current-voltage characteristics were also assessed and showed ohmic behaviour. The resistance decreased with increasing CdO content.

Influence of Bi doping on the structure and photoluminescence of ZnO phosphor synthesized by the combustion method

Bismuth doped ZnO (BZO) phosphors have been synthesized by the combustion method. The effect of Bi doping up to 4mol% on the structural, morphological, optical and photoluminescence (PL) properties have been investigated. X-ray diffraction analysis revealed that the BZO phosphors had the hexagonal wurtzite structure. The nanocrystallite size decreased from 75 to 38nm as the Bi concentration increased up to 3mol%, but then increased slightly for 4mol% Bi. The chemical states of the synthesized BZO phosphors were investigated using X-ray photoelectron spectroscopy and revealed the presence of both Bi3+ and Bi2+ charge states. The surface morphology showed spherical grains with some small particle agglomeration. The grain agglomeration and irregular shapes increased with increasing Bi concentration in the BZO phosphor. The absorption spectra were calculated from the reflection spectra using the Kubelka-Munk function and a blue shift in the absorption was obtained. The optical bandgap varied from 3.08 to 3.11eV for increasing Bi doping concentration. The PL spectra showed a blue emission at 410-500nm and a broad red peak at 650nm. These peaks are attributed to oxygen related defects in the ZnO host. The addition of Bi decreased the red emission and enhanced the blue emission.

Structural, optical and photoluminescence properties of Eu3 + doped ZnO nanoparticles

The structure, particle morphology and luminescent properties of europium (Eu3+) doped ZnO nanoparticles (NPs) prepared by co-precipitation method are discussed. When excited using a 325nm He-Cd laser, undoped ZnO NPs exhibited weakly the well-known ultraviolet excitonic recombination emission (at ~384nm) and strongly broad band visible emissions associated with defects (at ~600nm). In addition, the ZnO NPs exhibited green emission at ~600nm associated with defects when excited using a monochromatized xenon lamp. Upon Eu3+ doping line emissions attributed to 5D0→7F1,2,3,4 transitions of Eu3+ ions were observed when the materials were excited using a monochromatized xenon lamp. The exchange interaction mechanism is identified as the cause for concentration quenching of the luminescence of Eu3+ doped ZnO NPs in this study.

Twinning in copper deformed at high strain rates

Copper samples having varying microstructures were deformed at high strain rates using a split–Hopkinson pressure bar. Transmission electron microscopy results show deformation twins present in samples that were both annealed and strained, whereas samples that were annealed and left unstrained, as well as samples that were unannealed and strained, are devoid of these twins. These deformation twins occurred at deformation conditions less extreme than previously predicted.

Spectroscopic Investigation of Up-Conversion Properties in Green Emitting BaMgF4:Yb3+,Tb3+ Phosphor

In this work we have comprehensively studied the up-conversion (UC) properties of BaMgF4:Yb3+,Tb3+ phosphor for the first time. BaMgF4:Yb3+,Tb3+ phosphors were prepared by a simple and low cost precipitation method. To determine the influence of dopant concentration on luminescence properties, the corresponding UC luminescence spectra of BaMgF4:Yb3+,Tb3+ phosphors were studied under NIR excitation. Emission spectra under NIR excitation reveal the vital role of Tb3+ concentration in spectral tuning from the blue to green region. The UC decay curves were also studied to explore the possible energy transfer (ET) mechanisms between Yb3+ and Tb3+. The results reported here are expected to provide an approach for better understanding ET mechanisms in many Yb3+/Tb3+ codoped UC phosphors. This study will be helpful in applications where precisely defined optical transitions is an essential criterion.

Energy Transfer Mechanisms and Optical Thermometry of BaMgF4:Yb3+,Er3+ Phosphor

Motivated from our previous studies on the upconversion properties of BaMgF4:Yb3+,Tb3+ phosphor, here we investigated the upconversion properties of BaMgF4:Yb3+,Er3+ phosphor. We demonstrate a two-way versatile approach for the fine-tuning of emission from green to the red region, by varying the dopant concentration and adjusting the pulse width of an infrared laser. The mechanism involved in tuning the emission color by laser power and pulse width variation was illustrated in detail. The temperature dependent upconversion spectra were studied by analyzing the fluorescence intensity ratio of the thermally coupled levels. The maximum sensitivity obtained is 83.29 × 10-4 K-1 at 583 K, which is much higher than the temperature sensitivity reported for other fluoride based materials. Moreover, the influence of the excitation power density on the ability of the phosphor for temperature sensing was also investigated. We obtained a maximum (∼415 K) temperature detection at 2563 mW laser power. The obtained results illustrate the potential use of BaMgF4:Yb3+,Er3+ phosphor in an optical thermometer due to its highly sensitive temperature detection ability.

Parallelogram‐Shaped and Triangular Defects in GaAs

Faulted triangular defects on the {111} planes, bounded along the (110) directions, are shown to develop after annealing in Be-implanted Si doped GaAs. These defects have not been reported for Be implantation studies using semi-insulating GaAs, and are thought to be planar Si precipitates. Similar defects have been reported by other workers near the highly compensated p-n junction region in diffused GaAs, and also in highly Si-doped GaAs. Prismatic parallelogram-shaped loops on the {110} planes, comprised of excess Ga and As interstitials created during implantation and bounded by perfect 90° dislocations along the (112) directions, are also identified in the implanted samples annealed at 500 °C. These loops are replaced by triangular loops on the {110} planes in samples annealed at 600 °C and higher. A model for the triangular loop formation, based on differing stabilities of the α- and P-type 90° dislocations, is proposed. To our knowledge, this is the first report of triangular loops on the {110} planes in GaAs.

Analysis of secondary phases in InSbBi thin films

Abstract The secondary phases formed during the epitaxial growth of InSbBi by metal–organic vapor-phase epitaxy is reported. The material was grown on InSb and GaAs substrates at 455°C and was characterised using scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (TEM) and X-ray diffraction. The formation of secondary phases was found to be critically dependent on the V/III ratio. A V/III ratio slightly below stoichiometry lead to InBi and In 2 Bi phases forming on the surface, whereas a V/III ratio slightly above stoichiometry produced BiSb phases. Cross-sectional TEM revealed the formation of large Bi inclusions within layers grown on InSb substrates. The Bi inclusions ranged in size from 0.5 to 1 μm and were distributed throughout the epilayer. Bi–Ga inclusions were observed in the case of layers grown on GaAs substrates.

Imaging the atomic structure and local chemistry of platelets in natural type Ia diamond

In the past decades, many efforts have been devoted to characterizing {001} platelet defects in type Ia diamond. It is known that N is concentrated at the defect core. However, an accurate description of the atomic structure of the defect and the role that N plays in it is still unknown. Here, by using aberration-corrected transmission electron microscopy and electron energy-loss spectroscopy we have determined the atomic arrangement within platelet defects in a natural type Ia diamond and matched it to a prevalent theoretical model. The platelet has an anisotropic atomic structure with a zigzag ordering of defect pairs along the defect line. The electron energy-loss near-edge fine structure of both carbon K- and nitrogen K-edges obtained from the platelet core is consistent with a trigonal bonding arrangement at interstitial sites. The experimental observations support an interstitial aggregate mode of formation for platelet defects in natural diamond.The accurate structure of the platelet defects in diamond is now resolved by transmission electron microscopy, and, out of all the proposed models, it agrees well with the zigzag atomic model.