National Institute Of Materials Physics - Romania

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.

In hydrogenated high-purity Si, the vacancy-oxygen (VO) center is shown to anneal already at temperatures below 200 degrees C and is replaced by a center, identified as a vacancy-oxygen-hydrogen complex, with an energy level 0.37 eV below the conduction-band edge and a rather low thermal stability. At long annealing times, the process is reversed and the concentration of the latter defect is reduced, while the VO center partly recovers. The divacancy (V-2) center anneals in parallel with the initial annealing of the VO center, and the loss in V-2 exhibits a one-to-one proportionality with the appearance of a hole trap 0.23 eV above the valence-band edge attributed to a divacancy-hydrogen (V2H) center.

Self assembled InGaAs/GaAs quantum dots (QD) have a great potential for high performance optoelectronic devices such as low threshold laser diodes, infrared detectors, modulators, memories. In order to characterize the behavior of the QD system, we use two p(+)-n structures grown epitaxially on GaAs under similar conditions. The first structure acts as reference while in the second structure a single QD self-assembled layer is introduced in the middle of the n-GaAs matrix layer. The structure is designed such that for OV applied bias the QD layer lies outside the depleted region. When the reverse bias is increased, the charge from the QD system is removed and the depletion layer moves further into the GaAs matrix material. The electronic structure of the QD is investigated using two methods: photoluminescence, in order to characterize the transition energies between electron and hole levels in the QD system and capacitance spectroscopy in order to study the electron levels in the conduction band only. In addition, admittance spectroscopy spectra are measured in order to characterize the carrier transport mechanism. There is no evident step in the capacitance versus frequency behavior at room temperature in the range 1 Hz-1 MHz, indicating a large carrier cross section caption and/or a low activation energy for the carrier transport between the dot system and the wetting layer and GaAs barriers. The plateau in the C-V behavior, due to charging or discharging of the QD system is modeled using the solution of a Poisson equation and the resulting energy of the electron states within the conduction band and QD size distribution are correlated with results from photoluminescence studies, which involve transitions between energy levels both from the conduction and valence bands.

The thermal stability of two amino acid-(tyrosine and tryptophan) coated magnetite and their corresponding precursors, [Fe-2(III) Fe-II(Tyr)(8)]center dot 9H(2)O and [(Fe2FeII)-Fe-III(Trp)(2)(OH)(4)](NO3)(2)center dot 8H2O (where tyrosine=Tyr and tryptophan=Trp), was analyzed in comparison with free amino acids. The complexes present a lower thermal stability relative to the free ligand, due to the catalytic effect introduced by the iron cation and the presence of NO3- groups. The presence of NO3- group determines also a different degradation's stoichiometry of the amino acid anion comparative with the one expressed by the free ligand molecule. The amino acid bonded to magnetite decomposes in two steps, its presence inducing an increasing of gamma-Fe2O3-->alpha Fe2O3 conversion temperature.

Intercalated PbI2 compounds were prepared by exposing crystalline films and crystalline powders of lead iodide to pyridine or iodine vapor at room temperature. Correlated studies of optical absorption, Raman scattering and low temperature photoluminescence (PL) reveal different properties in comparison with those of pure crystalline PbI2 powder. Depending of the nature of intercalated molecules (organic or inorganic) the basic semiconducting properties of PbI2 are dramatically modified. The main signature of iodine intercalated PbI2 consists in a luminescence band peaking at 2.24 eV appearing at 77 K under 350 nm excitation light. In the case of pyridine intercalated PbI2 a new intense band at about 3.3 eV in the absorption spectrum is observed. The the PL spectra of pyridine intercalated PbI2 at 77 K change substantially when the excitation wavelength. The Raman spectra confirm the presence of pyridine between the PbI2 layers.

PbZrO3 powders have been prepared by an alkoxide-based sol-gel route, starting from lead acetate, zirconium n-butoxide, and n-butanol as a solvent, and hydrolysed with different amounts of water in neutral and alkaline medium. The local environment of Zr and Pb atoms was pursued from the sol to the dried (150 degrees C) and heated (400 degrees C) powders, by extended x-ray absorption fine structure (EXAFS). The analysis of the sol revealed links between Pb and Zr, and even more links between Zr and Zr. The metal neighbourhoods in the dried powders are not influenced by the hydrolysis conditions. Pb-Zr correlations are gradually lost from the sol to the dried and heated powders, while the loss of Zr-Zr correlations is considerably lower.

in the last decade, considerable research effort was directed to the deposition of multilayer films with layer thicknesses in the nanometer range (superlattice coatings), in order to increase the performance of various cutting tools and machine parts. The goal of the present work was to investigate the main microstructural, mechanical and wear resistance characteristics of a superlattice coating, consisting of alternate multilayer ZrN/TiAIN films, with various bilayer periods (5 divided by 20 nm). The coatings were deposited by the cathodic arc method on Si, plain carbon steel and high speed steel substrates to be used as wear resistance surfaces. The multilayer structures were prepared by using shutters placed in front of each cathode (Zr and Ti + Al). The characteristics of multilayer structures (elemental and phase composition, texture, Vickers microhardness, thickness, adhesion, and wear resistance) were determined by using various techniques (AES, XPS, XRD, microhardness measurements, scratch, and tribological tests). A comparison with the properties of ZrN and TiAIN single-layer coatings was carried out.

The effect of ammonium polymolybdate (APM) on the corrosion of carbon steel in a solution of 1 M HCl was studied by weight loss and Mossbauer spectrometry. Inhibition efficiencies (P) have been obtained from weight measurement. The inhibition efficiencies increased with increase APM concentration. Mossbauer spectrometry shows that a superficial compound is formed on the electrode surface as a result of corrosion. (c) 2007 Elsevier B.V. All rights reserved.

This work describes the preparation of HfO2. thin films by the sol-gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by 'dip-coating' with a pulling out speed of 5 cm min(-1). The films densification was achieved by thermal treatment for 10 min at 100 degrees C and 30 min at 450 degrees C or 600 degrees C, with a heating rate of 1 degrees C min(-1). The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4-0.7 mu m), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO2 layer at the coating-substrate interface. (C) 2007 Elsevier B.V. All rights reserved.

The mixed oxide system 0.1SnO(2)-0-9 alpha-Fe2O3 (Sn1) was synthesized using the hydrothermal method and characterized using X-ray diffraction (XRD), Mossbauer spectroscopy and transmission electron microscopy (TEM). Sensing properties in both CH4 and CO were recorded as a function of temperature, humidity and concentration. The results were compared with those obtained for SnO2 (Sn10). It has been observed that Sn1 displays superior stability and sensitivity for methane as compared to Sn10. For both Sn1 and Sn10, the sensitivity to methane and carbon monoxide is influenced by temperature. In contradistinction to Sn10, Sn1 presents a reduced sensitivity to water vapors. A model based on the simultaneous evaluation of the electrical resistance and workfunction was elaborated to describe the selectivity of Sn1. (c) 2007 Elsevier B.V. All rights reserved.