National Institute Of Materials Physics - Romania

The changes produced by anodic and natural oxidation upon the trap parameters in nanocrystalline porous silicon were compared. To put them in evidence, we used optical charging spectroscopy The same trapping It levels (with the same activation energies) were observed after both oxidation processes. In comparison with flesh samples, a new trapping level (the deepest one) appears and the concentrations of the surface traps strongly diminish.

A new method for the preparation of BaFCl:Eu2+ has been developed. A coprecipitation chemical reaction between BaCl2 and NaF acidified aqueous solution has been used. Doping with Eu2+ was carried out by adding EuF3 during preparation time. A thermal treatment in vacuum similar to those used in the sintering process of supraconductive ceramics was used in order to accomplish the chemical reaction and to improve the homogeneity of europium ion distribution. Finally, a fine powder consisting of microcrystalline, 4-5 mu m grains was obtained. The product has been checked by X-ray diffractometry and characterised by optical methods. Photoluminescence (PL) measurements attest the europium impurification and oxygen contamination during preparation, which has a great importance for the photostimulability properties of the compound. Photostimulated luminescence (PSL) of the X-irradiated samples shows good performance as X-ray storage phosphor. (C) 2000 Elsevier Science B.V. All rights reserved.

The paper presents the study of paramagnetic centres formed by gamma -rays irradiation of polycristalline (NHa)(2)SeO4 and (HH4)(2)SeO3 at 77 K and at room temperature. From the e.s.r. spectra and from the behaviour on heating NH3+ radical as well as radicals from selenium: SeO4-, SeO3-, SeO2- were put into evidence. The spin density values on oxygen and selenium orbitals were calculated. A mechanism of formation of these species is suggested.

Changes in the ambient humidity induce significant, long-termed variations both in conductance and in the calibration characteristics of SnO2 gas sensors for different reducing gases. These gas-dependent variations depend both on humidity changes (initial and final humidity) and on the time spent in each humidity. A previous knowledge of the conductance and calibration transients, together with a permanent information about humidity and about the time spent in each humidity, should result in significant improvements in the operation of electronic noses equipped with SnO2 gas sensors. (C) 2000 Elsevier Science S.A. All rights reserved.

Graded ferroelectric (GF) structures were obtained by successive deposition of Pb1-3y/2Lay(Zr0.4Ti0.6)O-3 films with different La contents on Pt/TiO2Si substrates. The method used for deposition was sol gel. The final structure consists of four layers, starting from the Pt electrode to the top gold electrode: (1) no La; (2) 4% La; (3) 8% La; and (4) 12% La. Ferroelectric properties of the GF structure were investigated, and an anomalous shift of the hysteresis loop along the polarization axis was found. The sense of this shift depends on the initial direction of the ferroelectric polarization. The presence of hysteresis displacement "up" or "down" can be explained if the presence of a built-in charge in the GF structure is admitted. The built-in potential produced by this charge is computed using a simple theoretical model. (C) 2000 American Institute of Physics. [S0003-6951(00)01040-8].

Surface-enhanced Raman scattering (SERS) is a process with origins, electromagnetic and chemical. The electromagnetic enhancement consists of the excitation of surface plasmons in the metallic support of the thin film. With only the electromagnetic enhancement mechanism, the surface spectra should not differ from volume Raman spectra. However, between SERS and volume Raman spectra, there are differences resulting from the chemical reactions taking place at the polymer/metal interface, intermediated by solvent molecules, that finally depend on the types of polymers and metallic supports. Polyaniline (PAN) is an excellent material to emphasize the chemical component of SERS. This is due to its particular structure with a repeating unit that contains two entities at different weights-a reduced state and an oxidized state-that, in turn, react differently with a metallic substrate. SERS spectra depend on the oxidizing properties of the metal surface, which involves an intermediate compound of the types Ag2O and Au2O3 when N-methyl-2-pyrrolidinone is used as the solvent. This article presents new results concerning the surface chemical effects that produce variations of the PAN SERS spectra. The SERS spectra of the PAN emeraldine base (PAN-EB) layered on Au support are characterized by a semiquinoid structure that we believe is induced on the intermediate compound Au2O3. In the presence of H2SO4, the SERS spectra change gradually as the degree of acid protonation doping increases. The SERS spectra of the fully protonated PAN-EB are identical to those obtained on PAN emeraldine salt (PAN-ES) synthesized by cyclic voltammetry in an acid medium and are invariable with the type of metallic support. The SERS spectra show that the emeraldine salt can be partially or totally deprotonated with water or NH4OH. The deprotonation is complete for the Ag support and partial for the Au support. The SERS spectra of the fully protonated PAN-EB are characterized by a double band with maxima at about 1330 and 1370 cm(-1). Although the generation process of positive charge on the macromolecular chain of PAN-EB doped in the presence of (C4H9)(4)NBF4 is similar to that due to protonic acid doping, involving cation addition (C4H9+ or H+ ions, respectively) in SERS spectra, the complex band situated at about 1330-1370 cm(-1) no longer appears. The doping of PAN-EB with FeCl3 produces two polymer cm forms: a salt type characterized by a protonated structure similar to that found for PAN-ES and a base type similar to the leucoemeraldine form. (C) 2000 John Wiley & Sons, Inc.

The room-temperature compressibility and stability of single-phase icosahedral Ti-Zr-Ni alloys were investigated for pressures up to 30 GPa, using synchrotron radiation energy dispersive diffraction. The icosahedral Ti-Zr-Ni single phase is stable over this pressure range, no phase transition being noticed. The pressure-volume data were least-squares fitted to a Birch-Murnaghan equation of state to obtain the zero-pressure bulk modulus B-0 = (173+/-5) GPa and its pressure derivative B-0' = 2.3+/-0.5.

Photodarkening relaxation under light exposure of a-As2Se3 amorphous films doped with 0.5 at.% of metals Sn, Mn, Sm or Dy and a-AsSe films doped with Sn up to 10.0 at.% Sn was studied in dependence on the impurity and thermal treating. Both factors reduce photodarkening and the degree of reduction depends on the type of impurity. The relaxation process may be described by a stretched exponential with the dispersion parameter 0.4 less than or equal to alpha 1 and time constant increasing with embedding of impurity or thermal annealing. The results are discussed in the frame of the recently forwarded "slip-motion" model of photodarkening in chalcogenide glasses.

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ce islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 degrees C), therefore the diffusion length of adatoms is high (similar to 100 mu m) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38-1.55 mu m. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up Bo 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed.

Sodium borate (Na2O.4B(2)O(3)) and copper doped glasses were investigated with respect to the effects of thermal annealing below the softening temperature. X-ray diffraction, optical spectroscopy and electron spin resonance methods were used. Annealing of the glass network results in a partial healing of the structural defects and a redistribution of the borate structural units, so that copper atoms situated in tetrahedral coordination in as-quenched samples adopt a distorted octahedral configuration after annealing.