National Institute Of Materials Physics - Romania

Photoluminescence properties of Er3+-doped calcium niobium gallium garnet (CNGG) disordered crystals have been studied in the range of 200-2000 nm at 10-290 K. In addition to emission bands due to Er3+, a broad emission band is observed at 470 nm. From the emission spectra that were obtained by various excitations, level assignment has been established for the emission bands. Different temperature dependences are observed between the emission bands at 522 and 543 nm. This is explained using non-radiative phonon relaxation between the H-2(11/2) and S-4(3/2) states. Besides the 1535 nm Er3+ emission band, near infrared emission bands are observed at 860 and 1240 nm, which are attributed to the transitions from the S-4(3/2) state to the (4)vertical bar(13/2) and vertical bar 4(11/2) states, respectively.

The paper reports the first observation of cooperative optical absorption and emission processes in Yb3+:Sc2O3 transparent laser ceramics. The intensity of cooperative processes in highly transparent ceramics Yb3+:Sc2O3 with up to 30 at.% Yb scales well with the data existing on single crystals and indicates that the distribution of the Yb3+ ions is random, without regions of agglomeration. (c) 2007 Published by Elsevier B.V.

The nano-crystalline PbS/n-Si heterostructure was studied using photocurrent spectra, I-V and C-V characteristics and admittance spectroscopy. The frequency dependent junction capacitance and conductance measurements show the presence of two contributions: first, a defect related mechanism which we attribute to a deep trap level with a large cross-section in nano-crystals with smaller sizes around 5 nm at the interface with Si and a second mechanism with an activation energy of about 250 meV, attributed to carrier transport in relatively large PbS grains of about 15 nm in size. (c) 2007 Elsevier B.V. All fights reserved.

We study the effect of annealing on structure, morphology and optical properties of nanocrystalline films of GaN, GaN:O and GaN:Mn prepared by ion assisted deposition on silicon, quartz and glassy carbon substrates. Blisters and holes having diameters proportional with the thickness of the film were observed in GaN:O deposited on silicon and glassy carbon. The Mn excess in GaN:Mn turns through annealing into MnxNy islands. The degree of short- and intermediate-range order in the films was investigated by micro-Raman spectroscopy, extending to about 3 nm for gallium oxynitride films annealed to 973 K and to more than 10 nm in GaN samples. A diminished oxygen content following the annealing procedures on GaN:O samples is noticed from the reduced intensity of the oxygen mode at 1000 cm(-1) in the Raman spectra. This observation is supported with X-ray photoemission spectroscopy measurements. The presence of oxygen at concentrations above 15at.% in the films leads to an abrupt nanocrystalline-amorphous transition. (C) 2007 Elsevier B.V. All rights reserved.

High dielectric materials have gained an important position in microwave electronics by reducing the size and cost of components for a wide range of applications from mobile telephony to spatial communications. Ba(Zn1/3Ta2/3)O-3 (BZT) is an A(B'B '')O-3 type perovskite material, showing ultra high values of the quality factor Q. Ceramic-based BZT dielectric materials were prepared by solid state reaction. The samples were sintered at temperatures in the range 1400 divided by 1600 degrees C for 4 h. Compositional, structural and morphological characterization were performed by using XRD, SEM and EDX analysis. The dielectric properties were measured in the microwave range (6 divided by 7 GHz). An additional annealing at 1400 degrees C for 10 h has improved some dielectric parameters. For samples sintered at temperatures higher than 1500 degrees C, the permittivity values were obtained in the interval 30 divided by 35 and almost do not change the value after the annealing. The Q x f product substantially increases up to about 135,000 GHz, exhibiting a low temperature coefficient of the resonant frequency (tau(f)) in microwaves. The best parameters were obtained for the samples sintered at 1600 degrees C with additional annealing. The achieved high values of the Q x f product recommend these materials for microwave and millimeter wave applications. (C) 2007 Elsevier B.V. All rights reserved.

The catalytic effect of the addition of lanthanum oxide (La2O3), in the range 0.5-2.0 mol%, on the hydrogen storage properties of MgH2 prepared by ball milling has been studied. The addition of La2O3 reduces the formation during milling of the metastable orthorhombic gamma-MgH2 phase. The desorption rate of samples with 1 and 2 mol% La2O3 comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La2O3. The presence of LaH3 after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La2O3 gives a lower hysteresis factor compared with sample with 2 mol%. (C) 2006 Elsevier B.V. All rights reserved.

Fine-grained ceramics of the Al2O3-TiO2 system were synthesised by reactive sintering of sol-gel precursors (Al- and Ti-alkoxides). The thermal behaviour of the as-prepared xerogels was examined by thermal analysis and X-ray powder diffraction. Preliminary results concerning powder consolidation into bulk ceramic parts using spark plasma sintering (SPS) are discussed. (c) 2007 Elsevier B.V. All rights reserved.

Photoluminescence (PL) properties of the composites based on zinc oxide (ZnO) and single-walled carbon nanotubes (SWNTs) prepared by hydrothermal synthesis are studied in this paper. The emission and excitation spectra of nanometric ZnO powder are dramatically influenced by the adsorption of different molecules coming from the environment. Therefore, a special attention is given to reveal the influences of adsorption/de-sorption process on photoluminescence properties of zinc oxide nanoparticles. A quenching effect of the intrinsic PL is observed regularly when the ZnO is synthesized in the presence of SWNTs, i.e. when a ZnO/SWNTs composite is formed. A distinct feature of the ZnO/SWNTs composite, is an emission band with a maximum at 405-450 nm.

Dark noise plays an important role in electronic devices, especially in detectors. Performance of infrared detectors is reflected by parameters such as detectivity and specific detectivity, strongly related to the noise spectral density. This work summarizes the results on characterization of high-performance PbS photo detectors, using a certification pending method developed by the laboratory, in agreement to the standard ISO/CEI 17025:2005. The method is validated for a resistor of known resistance, by comparing the Johnson thermal noise using two different methods: first, the fast Fourier transform measured with a Stanford Research Systems SR785 Spectrum Analyzer and second using a Stanford SR830 lock-in amplifier which measures the rms noise for a certain frequency. We studied the influence of physical dimensions on the performance of thin film polycrystalline PbS photoresistors in the planar geometry. Measurements were performed to compare the dark noise of PbS photoresistors with that in high quality metal thin film resistors. The laboratory setup used for the measurements is appropriate to accurately determine typical parameters of infrared detectors such as spectral responsivity, blackbody responsivity, specific detectivity, and noise spectral density.

A novel method for compensating the noise of the electronic balance used in lengthy experiments, such as studying the temperature dependence of liquid crystal density, is presented. It is shown, by numerical simulation of density measurement that the method can reduce both drift and random noise errors. This is accomplished using a stepwise temperature ramp and using arctangent type functions as basis vectors for modeling the signal variations whose coefficients are determined by numerical regression at every temperature step. Drift is derived from the overall deviation from the modeled signal and noise is reduced by the averaging effect of the numerical regression. The simulations show that in the considered case the proposed drift compensation method can give more accurate results than traditional methods. From the point of view of signal processing, the method resembles a numerical lock-in amplifier method, but as opposed to that, it uses a non-cyclic stepwise signal modulation.