Takamasa Ishigaki

Effect of hydrogen doping on ultraviolet emission spectra of various types of ZnO

A pulse-modulated plasma irradiation technique was applied to hydrogenation of ZnO. Three kinds of ZnO samples were employed to investigate the electronic state of hydrogen in ZnO. Secondary-ion-mass-spectroscopy analysis using isotope tracer revealed that the surface layer to 100 nm was doped with hydrogen after the irradiation and its concentration was in the order of 1016 cm−3. The efficiency of band edge emission was increased by the hydrogenation. However, the the degree of the improvements depended on impurity and defect concentration in the original samples. It was concluded that hydrogen in ZnO passivates deep donor and acceptor states by electron transfer from hydrogen to the defects.

Passivation of active recombination centers in ZnO by hydrogen doping

The effect of hydrogen doping on luminescence properties of ZnO was investigated. Hydrogen was incorporated in the ZnO crystal by irradiation with an inductively coupled plasma (ICP), in particular, the pulse modulated mode operation of ICP, and the luminescence spectra and hydrogen concentration of the resultant samples were analyzed. A hydrogenated region of 20–100 nm was formed at the sample surface by the irradiation and the concentration of hydrogen was 1017–1018 cm−3. Hydrogen doping improved the ultraviolet emission efficiency of all the samples, and the degree of improvement depended on the initial state (impurity concentration) of the original samples. The most significant improvements were recorded for the sample lightly contaminated with Cu, Al, and Li. The correlation between impurity concentration and hydrogen doping effects is discussed from the viewpoint of charge transfer between hydrogen and the other impurities.

Enhancement and patterning of ultraviolet emission in ZnO with an electron beam

An intense enhancement of ultraviolet (UV) emission was observed in various kinds of ZnO samples that were prepared using a wet chemical method when they were under electron-beam irradiation. The UV emission can increase to more than two times its initial value, whereas the visible emission reduces to a negligible value. We suggest that this enhancement effect mainly results from electron-stimulated desorption of adsorbed water. Applying this effect, we have developed a simple technique of directly writing submicrometer UV emission patterns in ZnO with an electron beam without changing the material’s surface morphology.

Defect-Mediated Photoluminescence Dynamics of Eu3+-Doped TiO2 Nanocrystals Revealed at the Single-Particle or Single-Aggregate Level†

Lanthanide-doped materials are finding use in a wide variety of applications in optics as gain media for amplifiers and lasers and as biolabels, white-light emitters, and full-color phosphors for displays. Since direct excitation of the parity-forbidden intra-f-shell lanthanide ion crystal-field transitions is inefficient, it is anticipated that the luminescence of lanthanide ions incorporated in a wide-band-gap semiconductor lattice (e.g., ZnO and TiO2) could be sensitized efficiently by exciton recombination in the host (Figure 1). Recently, we synthesized Eu-doped TiO2 (TiO2:Eu ) nanocrystals by Ar/O2 radio-frequency thermal plasma oxidation and observed bright red emission either by exciting the TiO2 host with UV light of shorter wavelength than 405 nm or by directly exciting Eu at a wavelength beyond the absorption edge (405 nm, 3.06 eV) of TiO2. Various types of defect states have been considered to play an important role in energy transfer between TiO2 and the activating Eu ions. For example, with increasing annealing temperature, the photoluminescence (PL) intensity of visible emissions due to Eu ions increases at first but then decreases and reaches a maximum at an annealing temperature of 700 8C. In this respect, the luminescence of Eu depends critically on the locations of dopants in the host. However, the mechanism of the energy-transfer process from the defect energy levels of the host to dopants has not yet been clarified owing to several difficulties, such as the inhomogeneous distribution of ions in the material. Single-molecule (single-particle) fluorescence spectroscopy has already yielded new insight into the photophysics and photochemistry of inorganic and organic nanocrystals. There are, however, only a few reports on the PL behavior of lanthanide-doped materials. We have now investigated the PL dynamics of undoped TiO2 and TiO2:Eu 3+

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Abstract Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC2H5)5, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.

Effect of additives on photocatalytic activity of titanium dioxide powders synthesized by thermal plasma

Abstract TiO 2 nanopowder was synthesized from titanium tetrachloride in a thermal plasma reactor. To improve photocatalytic activity, silicon tetrachloride and iron(III) acetylacetonate were added to the plasma reactor. The photocatalytic activity of prepared pure TiO 2 (T-powder), Si-doped TiO 2 (ST) and Fe-doped TiO 2 (FT) powder was evaluated by photodegradation of acetaldehyde. Decomposition efficiency of T-powder under UV-light was increased with the content of anatase. A small amount of Si-dopant improved the photocatalytic activity, while excessive Si-dopant over 2% decreased the photocatalytic activity of ST-powder because of the reduction of active sites on the catalyst. FT-powder was tested for the photocatalytic activity under visible light and UV-light. Decomposition efficiency increased with the addition of Fe-dopant because of suppression of electron–holes recombination. However, excessive Fe-dopant by over 15% inhibited the crystallization of anatase and acted as recombination center, leading to decrease the decomposition efficiency.

Control of particle size and phase formation of TiO2 nanoparticles synthesized in RF induction plasma

TiO2 nanoparticles have been synthesized in this work via Ar/O2 RF thermal plasma oxidation of atomized liquid precursors containing titanium tetrabutoxide and diethanolamine. Quench gases (Ar or He), either injected from the shoulder of the reactor (transverse injection) or injected counter to the plasma plume from the bottom of the reactor (counter-flow injection), are used to affect the quench rate and therefore the particle size and phase constituent of the resultant powders. The experimental results show that counter-flow injection is more effective in reducing the particle size, while He is more effective than Ar. As a result, well-dispersed TiO2 nanopowders with controllable phase structure (up to ~90% of anatase) and average particle size (down to 20 nm) are obtained. The experimental results are well supported by numerical analysis on the effects of the quench gas on flow pattern and temperature field of the thermal plasma as well as trajectory and temperature history of the particles.

Generation of pulse-modulated induction thermal plasma at atmospheric pressure

The radio frequency induction thermal plasma of sufficiently high electric power for materials processing has been successfully generated with a pulsemodulated operating condition. A solid-state amplifier, which supplies the electric power with a nominal frequency of 1 MHz, was employed for the pulsing plasma generation. The Ar–H2 plasma was generated at a high power level of 17 kW at atmospheric pressure. Typically, the plasma remained stable until the pulse duty factor went down to 30%, when the period of the high power level was 5 ms and the low power level was about 6 kW.

Oxygen Diffusion in Single- and Poly-Crystalline Zinc Oxides

Abstract18O diffusion coefficients were measured in zinc oxide ceramics using a secondary ion mass spectrometer. The results are interpreted as indicating extrinsic behavior. The values of the lattice diffusion coefficients with higher valence dopants compared with zinc ions are greater than lower valence dopant such as lithium ions. Using the data at deeper depth, the grain boundary diffusivity of oxide ions was also evaluated. Although the lattice diffusion coefficients varied by two orders of magnitude, the products of grain boundary width and grain boundary diffusion coefficient were less sensitive to the type of dopants.

Phase formation in molybdenum disilicide powders during in-flight induction plasma treatment

The in-flight modification of MoSi{sub 2} powders has been carried out by using an Ar{endash}H{sub 2} induction plasma. Reactor pressure, powder feed rate, and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. Metastable hexagonal structure of {beta}{endash}MoSi{sub 2} is the major phase observed in the Ar{endash}H{sub 2} induction of plasma-treated molybdenum disilicide powders, while the stable phase of tetragonal structure of {alpha}{endash}MoSi{sub 2} usually retains no less than 30 wt.{percent}. Depending on the experimental condition and the deviation from stoichiometry in raw materials, low silicides, Mo{sub 5}Si{sub 3} and Mo{sub 3}Si, and free Si were observed. {copyright} {ital 1997 Materials Research Society.}