Large-scale synthesis of single-phase, high-quality GaN nanocrystallites

Large-scale synthesis of high-quality GaN nano-crystallites has been achieved by direct reaction of a 4:1 molar Ga/Ga2O3 mixture with ammonia at 950degreesC. X-ray diffraction, transmission electron microscopy, selected-area electron diffraction and high-resolution transmission electron microscopy revealed that the produced GaN nanocrystallites were single hexagonal wurtzite structure with an average particle size around 45 nm. A sharp near band edge emission peak and a blue light emission peak were observed in photoluminescence spectroscopy. The synthesis approach is simple and easy to be commercialized.

Photoluminescence of doped ZnS nanoparticles under hydrostatic pressure

The pressure dependence of the photoluminescence from ZnS : Mn2+, ZnS : Cu2+, and ZnS : Eu2+ nanoparticles were investigated under hydrostatic pressure up to 6 GPa at room temperature. Both the orange emission from the T-4(1) - (6)A(1) transition of Mn2+ ions and the blue emission from the DA pair transition in the ZnS host were observed in the Mn-doped samples. The measured pressure coefficients are -34.3(8) meV/GPa for the Mn-related emission and -3(3) meV/GPa for the DA band, respectively. The emission corresponding to the 4f(6)5d(1) - 4f(7) transition of Eu2+ ions and the emission related to the transition from the conduction band of ZnS to the t(2) level of Cu2+ ions were observed in the Eu- and Cu-doped samples, respectively. The pressure coefficient of the Eu-related emission was found to be 24.1(5) meV/GPa, while that of the Cu-related emission is 63.2(9) meV/GPa. The size dependence of the pressure coefficients for the Mn-related emission was also investigated. The Mn emission shifts to lower energies with increasing pressure and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS : Mn2+ nanoparticles than in bulk. Moreover, the absolute pressure coefficient increases with the decrease of the particle size. The pressure coefficients calculated based on the crystal field theory are in agreement with the experimental results. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Magnetic properties of tin-doped magnetite nanoparticles

Structural and magnetic characteristics of Fe3-xSnxO4 (x < 0.3) nanoparticles synthesized using the precipitation exchange method have been investigated by X-ray diffraction, transmission electron microscope, Mossbauer spectra, X-ray photoelectron spectroscopy and magnetization measurement. The mean particle dimension decreases from 8 to 6 nm, the lattice parameters enlarge, the saturation magnetization decreases, as well as the magnetization and the coercive field increase, with increasing tin-content. The paramagnetic property of the specimens indicates that the replacement of Fe3+ by Sn4+ on the octahedral sites of Fe3O4 causes a progressive lowering of the Curie temperature and the Curie temperatures of the materials are all lower than that of crystallite tin-doped magnetite. This striking debasing is due to the lessening of the grain size. This is the smallest size reported thus far for paramagnetic tin-doped magnetite particles. (c) 2006 Elsevier B.V. All rights reserved.

Structure and visible luminescence of ZnO nanoparticles

ZnO nanoparticles were synthesized in ethanolic solution using a sol-gel method. The structural and optical properties were investigated by X-ray diffraction, transmission electron microscopy, UV absorption, and photoluminescence. After annealing at 200 degrees C, the particle size is increased and the peak of defect luminescence in the visible region is changed. A yellow emission was observed in the as-prepared sample and a green emission in the annealed sample. The change of the visible emission is related to oxygen defects. Annealing in the absence of oxygen would increase oxygen vacancies. (c) 2006 Elsevier Ltd. All rights reserved.

Amorphous silicon nanoparticles in compound films grown on cold substrates for high-efficiency photoluminescence

Silicon nanoparticles have been fabricated in both oxide and nitride matrices by using plasma-enhanced chemical vapour deposition, for which a low substrate temperature down to 50 degreesC turns out to be most favourable. High-rate deposition onto such a cold substrate results in the formation of nanoscaled silicon particles, which have revealed an amorphous nature under transmission electron microscope (TEM) examination. The particle size can be readily controlled below 3.0 nm, and the number density amounts to over 10(12) cm(-2), as calculated from the TEM micrographs. Strong photoluminescence in the whole visible light range has been observed in the as-deposited Si-in-SiOx and Si-in-SiNx thin films. Without altering the size or structure of the particles, a post-annealing at 300 degreesC for 2 min raised the photoluminescence efficiency to a level comparable to the achievements with nanocrystalline Si-in-SiO2 samples prepared at high temperature. This low-temperature procedure for fabricating light-emitting silicon structures opens up the possibility of manufacturing integrated silicon-based optoelectronics.

Pressure dependence of Mn2+ luminescence in differently sized ZnS : Mn nanoparticles

The pressure behavior of Mn2+ emission in the 10-, 4.5-, 3.5-, 3-, and 1-nm-sized ZnS:Mn2+ nanoparticles is investigated. The emission shifts to lower energies with increasing pressure, and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS:Mn2+ nanoparticles than in bulk. The pressure coefficient increases with the decrease in particle size with the 1-nm-sized particles as an exception. Pressure coefficient calculations based on the crystal field theory are in agreement with the experimental results. The pressure dependence of the emission intensity is also size dependent. For nanoparticles 1 and 3 nm in size, the luminescence intensity of Mn2+ decreases dramatically with increasing pressure, while, for bulk and particles with average sizes of 3.5, 4.5, and 10 nm, the luminescence intensity of Mn2+ is virtually unchanged at different pressures. The bandwidth increases faster with increasing pressure for smaller particles. This is perhaps due to the fact that there are more Mn2+ ions at the near-surface sites and because the phonon frequency is greater for smaller particles. These new phenomena provide some insight into the luminescence behavior of Mn2+ in ZnS:Mn2+ nanoparticles.

In-situ synthesis of AlN powders and composite AlN powders with yttrium addition

Using Al-Mg and Al-Mg-Y alloys as raw materials and nitrogen as gas reactants, AIN powders and composite AIN powders by in-situ synthesis method were prepared. AIN lumps prepared by the nitriding of Al-Mg and Al-Mg-Y alloys have porous microstructure, which is favorable for pulverization. They have high purity, containing 1.23 % (mass fraction) oxygen impurity, and consisted of AIN single phase . The average particle size of AIN powders is 6.78 mum. Composite AlN powders consist of AlN phases and rare, earth oxide Y2O3 phase. The distribution of particle size of AIN powders shows two peaks. In view, of packing factor, AIN powders with such size distribution can easily be sintered to high density.

High-frequency ferromagnetic resonance on ultrafine cobalt particles

We report on high-frequency (300-700 GHz) ferromagnetic resonance (HF-FMR) measurements on cobalt superparamagnetic particles with strong uniaxial effective anisotropy. We derive the dynamical susceptibility of the system on the basis of an independent-grain model by using a rectangular approach. Numerical simulations give typical line shapes depending on the anisotropy, the gyromagnetic ratio, and the damping constant. HF-FMR experiments have been performed on two systems of ultrafine cobalt particles of different sizes with a mean number of atoms per particles of 150 +/- 20 and 310 +/- 20. In both systems, the magnetic anisotropy is found to be enhanced compared to the bulk value, and increases as the particle size decreases, in accordance with previous determinations from magnetization measurements. Although no size effect has been observed on the gyromagnetic ratio, the transverse relaxation time is two orders of magnitude smaller than the bulk value indicating strong damping effects, possibly originating from surface spin disorders.

Structure, fluorescence and stability of CdS nanoparticles prepared in air

CdS nanoparticies were prepared in air and their stability by air annealing was studied. A small change in crystal structure and particle size was observed by air annealing, but a rapid reduction in fluorescence was found. Through investigation, it is revealed that it is the surface change or reconstruction rather than the variation of the size or structure that decreases the fluorescence. The emission of the particles consists with two peaks which are dependent on the excitation energy. The two peaks are considered to be arisen from "two" different sizes of nanoparticles and may be explained in terms of selectively excited photoluminescence. Finally we discuss why the discrete state of nanoparticles are able to be resolved in the photoluminescence excitation spectrum, but could not be differentiated in the absorption spectrum.

Structure and visible luminescence of ZnO nanoparticles

ZnO nanoparticles were synthesized in ethanolic solution using a sol-gel method. The structural and optical properties were investigated by X-ray diffraction, transmission electron microscopy, UV absorption, and photoluminescence. After annealing at 200 degrees C, the particle size is increased and the peak of defect luminescence in the visible region is changed. A yellow emission was observed in the as-prepared sample and a green emission in the annealed sample. The change of the visible emission is related to oxygen defects. Annealing in the absence of oxygen would increase oxygen vacancies. (c) 2006 Elsevier Ltd. All rights reserved.

Influence of 5A-Type Zeolite Powder on Tetrahydrofuran Hydrate Formation and Dissociation Process

Visual observations of tetrahydrofuran (THF) hydrate formation and dissociation processes with 5A-type zeolite powder were made at normal atmospheric conditions and below zero temperature by microscope. Results indicate that 5A-type zeolite powder can promote THF hydrate growth. At the same time, in the presence of 5A-type zeolite, agglomerated crystals and vein-like crystals of THF hydrate were also formed. SA-type zeolite powder increases the crystallization temperature and decreases the dissociation temperature. The particle size distribution of 5A-type zeolite powder influences THF hydrate formation and its dissociation characteristics significantly.

Nano-Ag on vanadium dioxide. I. Localized spectrum tailoring

We report on the utilization of localized surface plasmon resonance (LSPR) of Ag nanoparticles to tailor the optical properties Of VO2 thin film. Interaction of nano-Ag with incident light yields a salient absorption band in the visible-near IR region and modifies the spectrum Of VO2 locally. The wavelength of modification occurs in a limited spectral region rather than affects the full spectrum. The wavelength of modification shows a strong dependence on the metal nanoparticle size and shifts toward the red as the particle size or the mass thickness of nano-Ag increases. Also, we found that the wavelength can be shifted into the IR further by introducing a thin layer of TiO2 onto the nano-Ag. Interestingly, with the help of LSPR effects the VO2 film exhibits an anomalous thermochromic behavior in the modification wavelength region, which may be useful in optical switching applications.

Nano-Ag on vanadium dioxide. II. Thermal tuning of surface plasmon resonance

Thermal tuning of the localized surface plasmon resonance (LSPR) of Ag nanoparticles on a thermochromic thin film of VO2 was studied experimentally. The tuning is strongly temperature dependent and thermally reversible. The LSPR wavelength lambda(SPR) shifts to the blue with increasing temperature from 30 to 80 degrees C, and shifts back to the red as temperature decreases. A smart tuning is achievable on condition that the temperature is controlled in a stepwise manner. The tunable wavelength range depends on the particle size or the mass thickness of the metal nanoparticle film. Further, the tunability was found to be enhanced significantly when a layer of TiO2 was introduced to overcoat the Ag nanoparticles, yielding a marked sensitivity factor Delta lambda(SPR)/Delta n, of as large as 480 nm per refractive index unit (n) at the semiconductor phase of VO2.

激光衍射法与吸管法对东北黑土区土壤粒径分布测定的差异性研究

目前激光衍射法(laser diffraction method,LD)逐步被用于土壤颗粒粒径分布(particle size distribution, PSD)的分析,为了对比LD法和吸管法测定东北黑土区土壤PSD的差异性,采用LD法和吸管法分别对东北黑土区宾州河流域36个土壤剖面不同层次178个土壤样品的PSD值进行了测定与分析。结果表明,同吸管法相比,LD法低估了土壤的黏粒含量,平均低估幅度19.69%,而高估了土壤的粉粒和砂粒含量,平均高估幅度分别为14.66%和5.13%。LD法所得PSD结果依据美国土壤质地分类制判定的土壤质地,相对于吸管法总体由粉黏质偏向粉砂质方向。建立了LD法与吸管法测定PSD结果的转换模型,将LD法测定的PSD结果利用转换模型校正后,其测定的各土壤粒级同吸管法相比,准确度达96.97%～98.71%,判定土壤质地的准确率也达83.15%。