G.A. Hirata

Physical properties of Y2O3:Eu luminescent films grown by MOCVD and laser ablation

Abstract Luminescent Y2O3:Eu3+ thin films were deposited on sapphire, polycrystalline Al2O3 and indium tin oxide coated glass or sapphire substrates by two different techniques: metallorganic chemical vapor deposition (MOCVD) and laser ablation. Microcrystalline Y2O3:Eu films were grown in a MOCVD chamber by decomposing and reacting yttrium and europium organometallic precursors in an oxygen atmosphere at low pressures (1–10 mTorr) and low substrate temperatures (500–700°C). The as-deposited films showed the characteristic red fluorescence spectrum of Y2O3:Eu with the main peak centered about 611 nm wavelength. The as-deposited films averaged 1.0 μm in particle size and 2.0 μm in thickness. Post-deposition annealing treatments in the temperature range 900–1200°C enhanced the luminescent intensity of the films. The as-deposited laser ablated oxide films were amorphous and required annealing at temperatures higher than 800°C to observe luminescence, which occurred in conjunction with crystallization. The as-deposited films averaged 500 nm in thickness and after post-annealing at 1000°C were composed of 15–200 nm grains.

Anisotropy in the compressive mechanical properties of bovine cortical bone and the mineral and protein constituents

The mechanical properties of fully demineralized, fully deproteinized and untreated cortical bovine femur bone were investigated by compression testing in three anatomical directions (longitudinal, radial and transverse). The weighted sum of the stress-strain curves of the treated bones was far lower than that of the untreated bone, indicating a strong molecular and/or mechanical interaction between the collagen matrix and the mineral phase. Demineralization and deproteinization of the bone demonstrated that contiguous, stand-alone structures result, showing that bone can be considered an interpenetrating composite material. Structural features of the samples from all groups were studied by optical and scanning electron microscopy. Anisotropic mechanical properties were observed: the radial direction was found to be the strongest for untreated bone, while the longitudinal one was found to be the strongest for deproteinized and demineralized bones. A possible explanation for this phenomenon is the difference in bone microstructure in the radial and longitudinal directions.

Neodymium-doped nanoparticles for infrared fluorescence bioimaging: The role of the host

The spectroscopic properties of different infrared-emitting neodymium-doped nanoparticles (LaF3:Nd3+, SrF2:Nd3+, NaGdF4: Nd3+, NaYF4: Nd3+, KYF4: Nd3+, GdVO4: Nd3+, and Nd:YAG) have been systematically analyzed. A comparison of the spectral shapes of both emission and absorption spectra is presented, from which the relevant role played by the host matrix is evidenced. The lack of a “universal” optimum system for infrared bioimaging is discussed, as the specific bioimaging application and the experimental setup for infrared imaging determine the neodymium-doped nanoparticle to be preferentially used in each case.

Investigation of the physical properties of a blue-emitting phosphor produced using a rapid exothermic reaction

The blue-emitting phosphor cerium activated yttrium silicate, (Y1� mCem)2SiO5, was prepared via a novel synthesis technique called combustion synthesis. Combustion synthesis involves a highly exothermic redox reaction between metal nitrates and an organic fuel to produce a solid powder. The combustion synthesis parameter, the fuel-to-oxidizer ratio, has a direct effect on the physical properties of the as-synthesized powders due to the reaction temperature being dependent on the variation of this ratio. Thus, varying the fuel-to-oxidizer ratio produced powders with varying crystallite sizes, carbon contamination and surface areas, which in turn affected the luminescent efficiencies of the as-synthesized powders. The reaction temperature was found to reach a maximum with a 60% fuel rich mixture, with the powders exhibiting the largest crystallite sizes, smallest surface area and carbon contamination, and highest luminescent efficiency in the as-synthesized state. As-synthesized powders exhibit a high degree of porosity due to the large amount of gas to solid formed. To more accurately predict the specific surface areas of porous powders, the standard geometrical model used for gas absorption measurements was modified for porous particles and found to more accurately predict the specific surface area of highly porous powders. # 2002 Elsevier Science B.V. All rights reserved.

XPS and HRTEM characterization of cobalt-nickel silicide thin films

Abstract We studied by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) films of Co–Ni/p-Si deposited by PLD on Si(100) substrates. They were thermally treated in vacuum to promote silicide formation. By means of XPS in-depth profiles, it was observed that the deposited metal film contains more Co than Ni. The Co and Ni 2p transitions present shifts characteristic of silicide at respective ranges of 778.3–778.6 and 853.2–853.6 eV, while the Si2p transition appears at 99.2–99.5 eV, as determined by XPS. By means of HRTEM, nanocrystalline regions belonging to CoSi2, Ni2Si and NiSi2 structures were identified. Some grains of CoSi2 are large in size, more than 20 nm in diameter, while Ni2Si and NiSi2 nanocrystals are of the order of 10 nm. There are several regions where no crystalline ordering seems to be apparent. The SiO2 layer acted as an effective diffusion barrier suppressing mobility of metal into the Si(100) substrate. The observed tendencies of the Co and Ni concentrations as a function of depth agree with a model of CoSi and NiSi structure separation and subsequent formation of CoSi2 and NiSi2.

A new combustion synthesis method for GaN:Eu3+ and Ga2O3:Eu3+ luminescent powders

New low-temperature methods to produce GaN:Eu and Ga 2 O 3 :Eu (0 ≤ x ≤ 1) highly-luminescent powders are presented. These procedures yield finely divided powders through exothermic reactions between the precursors. The preparation of Eu-doped Ga 2 O 3 powders was achieved using a new combustion synthesis technique (hydrazine/metal-nitrate method). The process starts with aqueous solutions of Eu(NO 3 ) 3 and Ga(NO 3 ) 3 as the precursors and hydrazine as (non-carbonaceous) fuel. A spontaneous combustion reaction occurs by increasing the temperature to between 150 and 200 °C in a closed vessel filled with argon, and produces (Eu x Ga 1-x ) 2 O 3 directly. The preparation of Eu-doped GaN uses the ammonium hexafluoro-metal method. The powders present strong luminescence associated with the dopant. A sharp and strong GaN luminescence is observed, indicative of high purity and crystallinity as determined by low-temperature cathodoluminescence. The composition and powder morphology have been studied using energy dispersive spectroscopy and scanning electron microscopy.

Millimeter-Long Carbon Nanotubes: Outstanding Electron-Emitting Sources

We are reporting the fabrication of a very efficient electron source using millimeter-long and highly crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.17 V/μm and reach threshold emission at 0.24 V/μm. In addition, these electron sources are very stable and can achieve a peak current density of 750 mA cm(-2) at only 0.45 V/μm. In order to demonstrate intense electron beam generation, these devices were used to produce visible light by cathodoluminescence. Finally, density functional theory calculations were used to rationalize the measured electronic field emission properties in open carbon nanotubes of different lengths. The modeling establishes a clear correlation between length and field enhancement factor.

Luminescence enhancement in Eu3+-doped α- and γ-Al2O3 produced by pressure-assisted low-temperature combustion synthesis

Intense red luminescence in Eu3+-doped gamma (γ) and alpha (α) alumina (Al2O3) phosphors obtained by direct and indirect combustion synthesis at low-temperatures is reported. γ and α-(Al1−xEux)2O3 are easily produced by combustion synthesis at 280 °C in the range of x=0.001–0.06 at. %. The well-defined direct synthesis allows europium ions to incorporate into the α or γ alumina lattice in spite of the large size difference between these ions and the aluminum cations in Al2O3. These materials yield a strong fluorescence at room temperature due to f–f transition lines within Eu3+ (4f6) electron emission configuration. Furthermore, from luminescence measurements, it is deduced that Eu3+ occupy low-symmetry sites in the Al2O3 lattice.

A novel method for the synthesis of sub-microcrystalline wurtzite-type InxGa1−xN powders

A novel method to synthesize wurtzite-type gallium–indium nitride powders (InxGa1 − xN, x = 0, 0.5, 1) with small particle size, high purity and high crystallinity has been developed. The method produces finely divided powders via the pyrolysis reaction of a complex salt (ammonium hexafluoroindium-gallate, (NH4)3InxGa1 − xF6) in an ultrahigh purity ammonia flow inside of a quartz tubular reactor at relatively low temperature, 630 °C. The conditions of the process avoid the formation of metallic indium, oxides or fluorides. Scanning electron microscopy and X-ray diffraction analysis performed on these sub-micron particles of InxGa1 −xN show an hexagonal wurtzite-type structure, which is very similar to pure InN produced by the same technique.

Microstructural properties of Eu-doped GaN luminescent powders

GaN powders doped with europium have been prepared using Eu and Ga nitrates and N2H4 as reactants. The resulting particles have dimensions ranging from 0.5 to 1.0 μm. The crystalline structure was studied by transmission electron microscopy, and it consisted of single crystals with a hexagonal (wurtzite) structure containing small cubic domains (zinc blende) and a high density of stacking faults, all aligned along the [0001] and 〈111〉 directions, respectively. Cathodoluminescence measurements show strong light emission in the red region. This luminescence corresponds to transitions of Eu with the strongest emission in the 611 nm line, which is associated to the Eu3+ 4f transition from 5D0 to 7F2. These results demonstrate the feasibility of GaN:RE powders for luminescent applications.