National Institute Of Materials Physics - Romania

Polymerization-depolymerization processes induced by irradiation with alternate high and low intensity light control the nano-scale extension of disordered layer-like units in arsenic chalcogenides. These processes were simulated in the frame of a Monte-Carlo-Metropolis method. A model for photodarkening and photobleaching phenomena in layered chalcogenides was proposed.

The connection between the high-resolution spectral disordering of the rare earth-doped garnets and the manifestation or the energy transfer in the global or selective emission decay is discussed. It is shown that the identification of the pair satellites and the investigation of their decay properties are important for determination or verification of the mechanisms of interaction that determine the energy transfer in simply activated or in sensitized crystals. The competition of various energy transfer processes in de-excitation of the metastable levels such as down-conversion and up-conversion in Nd: YAG under high intensity pump is discussed. It is shown that the energy transfer parameters determined from the emission decay could give a consistent description of data on quantum efficiency or heating effects. (C) 2002 Elsevier Science B.V. All rights reserved.

In this paper the results of electrical measurements of the mass-loading influence on AT-cut quartz and Y-cut langasite resonators are compared with those obtained by X-ray topography analysis of the same resonators. The mass-loading,effects on resonator characteristics have been studied using the Ballato's transmission-line analogs of the trapped-energy resonators vibrating in thickness-shear mode. This study revealed that the mass-loading influence on electrical parameters of langasite resonators is smaller than in the case of quartz resonators. 5MHz Sawyer AT-cut quartz and Y-cut langasite resonators with Au electrodes, having various diameters and thicknesses were used in experiments. The results of the X-ray topography investigations are in agreement with the electrical measurements previously performed.

The electron density at the gold nuclei in metallic gold and in the cubic intermetallic compounds CsAu, RbAu, AuMg, AuZn, AuCd, AuSc, AuAl2, AuGa2, AuIn2, AuSn2 and AuSb2 have been calculated using the Wien97 computer code. The electron densities thus obtained and the experimental values of the isomer shifts define a linear correlation spanning nearly the whole range of isomer shifts observed for the 77 keV transition of Au-197. From the slope of this correlation one obtains Delta = (4.6 +/- 0.2).10(-3) fm(2) for the change of the mean square nuclear charge radius accompanying 77 keV transition in Au-197.

Since the discovery of carbon nanotubes, Raman Scattering Spectroscopy and transmission Electron Microscopy have been proved to be powerful for a detailed investigation of their properties. In TEM, a direct observation can be made, whereas Raman Scattering is used extensively to probe the different samples via their vibrational properties. In this paper, we report on recent results obtained in both single-walled carbon nanotubes (SWNTs), multiwalled-carbon nanotubes (MWNTs) and PMMA/nanotubes composites. In addition, in the case of SWNTs, we present studies carried out in Raman scattering when SERS (Surface Enhanced Raman Scattering) conditions are used. In particular, we put in evidence an increased state of disorder at the interface nanotubes/metallic support when the nanotube film thickness is decreased. Strong degradations can be observed primarily on metallic tubes with a concomitant formation of C-60-like molecules. In the case of MWNTs, calculations of interactions between concentric tubes lead to the occurrence of low frequency vibrational modes, in good agreement with experiments.

The electromechanical and piezoelectric properties of PT-type ceramics can be modified both by A or B site substitution in the perovskite, and by varying the porosity of the material. In this work rare earth modified lead titanate material with the general formula (Pb(0.88)Nd(0.036)Dy(0.044))(Ti(0.98)Mn(0.02))O(3), was prepared by the ceramic technique. Sintering was carried out at 1260degreesC and 1170degreesC, for 5 hours. The microstructural analysis was performed on fresh fracture surface, in order to observe the true porosity of the material and morphology and dimensions of grains. A scanning electron microscope equipped with wavelength dispersive X-ray spectrometer and energy dispersive X-ray spectrometer was used. Materials with round shaped and agglomerated particles, smaller than 1 m, and pores with the maximum diameter of about 7 m were obtained. The effect of the pores on the values of electromechanical and piezoelectric properties is discussed.

The chemical deposition of palladium on n and p-GaAs from an aqueous solution of PdCl2, in strong acid medium can be an alternative technique for contacts on GaAs. This paper presents the variations of palladium films thickness (measured by an interferometric method) as a function of solution concentration, reaction time and substrate conductivity. There are presented data concerning the adhesion of palladium films (measured by a mechanical method) related to the film thickness. From a comparative study concerning the adhesion of evaporated films such as: Au/n-GaAs, Ag/n-GaAs, AnGe/n-GaAs and Pd/n-GaAs, it results the conclusion that the adhesion of palladium films are of the same order of magnitude as the metallic films evaporated without an annealing procedure.

We have succeeded in growing nanocrystals of Ag structured like fractals in several solid matrices (NaG, KBr and KCl). These are shown to be diffusion-limited aggregates when fractal dimensions were evaluated. Depending upon the crystal in which the fractal structures were obtained, the fractal dimensions of the Ag nanoclusters were found to be between 1.73 and 1.81. (C) 2002 Elsevier Science B.V. All rights reserved.

Tl-0 (6 p(1)) irradiation centers are proposed as new paramagnetic probes for structural phase transition investigations. The P2(1)cn --> C1 c1 phase transition in Rb-2 ZnCl4 single crystals is monitored using the Tl-0 and the well-known Tl2+ (6 s(1)) paramagnetic probes. The anomalous temperature dependence exhibited by the EPR spectra of both types of centers in the C1 c1 phase was analyzed considering a soft mode contribution.

Polyaniline-fullerene (PA-C-60) composites have been studied by Surface Enhanced Raman scattering (SERS) spectroscopy and X-ray diffraction. They were obtained by mixing solutions of polyaniline-emeraldine base (PA-EB) and C-60, as well as by chemical synthesis from aniline, sulfuric acid and potassium dichromate, with addition of C-60. The Raman bands peaking at 1330-1370 cm(-1). associated with a protonated structure Were used as indicative for changes of the PA-EB phonon spectrum resulting from C-60 doping. The two types of compounds show different SERS spectra, also dependent on the metallic support used (An or Ag). Variation of the SERS spectra with the type of metallic support is related to a chemical interface interaction between composite and metal. In mixture samples, a doped polymeric chain (with ionically attached C-60(-)) was evidenced by an increased fraction of quinoid rings. The SERS spectra of the chemically synthesized PA-C-60 reveals the existence of two polymeric structures: a doped PA (doped with C-60(-) ions) and an undoped one, the latter with a structure of 'pendant chain' type. Structural (XRD) data reveal the presence of C-60 nano-zones, with a lattice parameter increase of 0.3-0.6%, attributed to slight oxidation. Detailed analysis of the fcc(111) line asymmetry suggests, in mixture samples, a boundary zone with an 'expanded' lattice. induced by ionic interface effects. (C) 2002 Elsevier Science Ltd. All rights reserved.