National Institute Of Materials Physics - Romania

The modification of the ideal lattice parameter in porous silicon has been determined by X-ray diffraction. A contraction effect has been detected in several samples. There was shown that the position of the photoluminescence band is related to the modification of the lattice parameter. The quantum theory of confinement in wires was used for the interpretation of the experimental data.

The reversible photostructural effects observed in the amorphous thin films and in chalcogenide glasses based on arsenic have been explained with the help of computer relaxed models. Two photostructural mechanisms have been established: the incorporation of the chalcogenide molecular fraction into the disordered layers and the change of the relative positions of the atoms in the layers without destroying the bonding network. In the frame of this atomic scale mechanisms it is possible to explain the reversible photodarkening and the photoanisotropy observed in the chalcogenide thin Alms and bulk glasses.

Surface enhanced Raman scattering (SERS) is being increasingly used for the study of the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin, arising from the excitation of surface plasmons in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, for which available experimental data are scarce. The chemical process originates from the increase in the polarizability of the molecules at the metal surface under the action of the incident radiation, which leads to the formation of new chemical bonds with the atoms of the metal support. The present work was devoted to the study of the SERS spectra of poly(3-hexylthiophene) deposited on rough Ag and Au supports by evaporating the solvent from a solution of known concentration. The experiments revealed the existence of a chemical surface effect. The results obtained show that the SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds of the type MeX (Me = Ag or Au, X = Cl or O). The SERS measurements reported here reveal an increase in the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. It was observed for the first time by SERS spectroscopy that the doping of 3-PHT with FeCl3 leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. (C) 1998 John Whey & Sons, Ltd.

Surface-enhanced Raman scattering (SERS) has lately proved to be an effective technique which has led to its increasing application for studying the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin arising from the excitation of surface plasmons (SPs) in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, on which only scarce experimental data are available at present. The chemical process originates in an increased polarizability of the molecules at the metal surface under the action of an incident radiation, as a result of which new chemical bonds are formed with the atoms of the metal support. This paper is devoted to the study of the SERS spectra under 1064 nm excitation for two forms of polyaniline, one of the emeraldine base type and the other of the emeraldine salt type. Our purpose has been to identify the chemical effects at the polymer/metal interface that are involved in specific modifications of the SERS spectra. The following results concerning SERS spectrum variation with metal support type (AE, Au and Cu) have been obtained. An oxidation of emeraldine base taking place at the polymer/metal interface has been revealed. In the case of emeraldine salt, an oxidized form of polyaniline, the SERS spectra remain unchanged regardless of the changes in the metal support, which proves the stability of this form. A new result has been obtained as SERS spectroscopy revealed an electron delocalization process taking place in the emeraldine salt molecule as a result of an electron transfer from metal to the polymer molecule occurring on the level of quinoid rings. The concept of a rough Raman-active surface has been clarified by using a parameter Wa, similar to the grating groove depth/grating periodicity ratio which occurs when a diffraction grating is used as optic coupler. (C) 1998 Elsevier Science S.A. All rights reserved.

The polarization behaviour of Armco iron in solutions of HCl with and without inhibitors has been studied by potentiostatic method. At lower overvoltage values the dissolution process is controlled by activation, while at higher overvoltage values the dissolution process is controlled by diffusion. The inhibition of this metal corrosion in the aqueous solutions of HCl has been studied using three surfactants. Under the critical micelle concentration the inhibition of those three surfactants is negligible. At a concentration higher than critical micelle concentration, the inhibiting action of surfactants increases rapidly. The process of inhibition was attributed to the formation of the adsorbed film on the metal surface, that protects the metal against the corrosive agents.

GaN thin films were deposited by laser ablation of liquid Ga target in nitrogen reactive atmosphere. An Nd-YAG laser (lambda = 1.06 mu m, tau(FWHM) = 10 ns) of 0.35 J energy/pulse was used as laser source. The nitrogen pressure was varied in the range of 10(-2) to 10(-1) mbar. As substrates, we used (0001) sapphire plates and (100) Si wafers, coated or uncoated with ZnO as buffer layers, and heated below 300 degrees C. Different analysis techniques evidenced the characteristics of the deposited films. SIMS profiles corresponding to N and Ga in-depth distribution carried out the presence of layers of the order of 130-150 nm, with uniform distribution of Ga and N inside the layer. XPS studies evidenced the Ga-N bonding. The Nls signal contains as main peak the one centered at 397.3 eV and corresponding to Ga-N bond. From the distance between the photoelectron Ga 3d peak and the Auger Ga LMM peak, the calculated Auger parameter of 1083.9 eV corresponds to the one reported in literature for GaN compound (1084.05 eV). Both techniques evidenced an oxygen contamination below 5%. XRD recorded spectra show the presence of a peak assigned to (002) GaN crystalline orientation. Optical absorption spectroscopy studies in the UV-visible range evidenced a high transparency (over 80% transmission) for the deposited films. The energy band gap obtained from the absorption spectra was found to be larger than 3.6 eV. (C) 1998 Elsevier Science B.V.

Amorphous multilayers based on Se and CdSe alternated sublayers were succesfully prepared by thermal vacuum evaporation and laser ablation with the periodicity of 22 nm. The multilayer structure is stable up to similar to 70 degrees C. In the samples prepared by laser ablation a large contraction of the multilayer stacking occurs by annealing. Amorphous SiOx/CdSe multilayers with the periodicity of similar to 15 nm were successfully prepared by thermal vacuum evaporation. The structure is stable up to 400 degrees C annealing temperature. Pulse excimer laser irradiation of the SiOx based multilayer produces an expansion of the interlayer distance with similar to 0.33%.

Hardness, X-ray diffraction and transmission as well as infrared reflection measurements on virgin and ultraviolet (UV)-irradiated amorphous films of the GexAs40-xS60 system have been performed. The UV light induces film softening for x < 19 and film hardening for x > 19. The structure remains amorphous but the distance characteristic of medium range order decreases for x < 19 and increases for x > 19. Photo-contraction of the films (up to similar to 12%) has been revealed from IR spectra and X-ray transmission experiments. A main effect of UV light is the release of sulphur. (C) 1998 Elsevier Science B.V. All rights reserved.

The photoluminescence (PL) decay measurements were performed on porous silicon films. It was observed that the two components of FL, one of them fast (ns) and the other slow (mu s or ms sometimes) have different contributions to PL signal, depending on the wavelength of the excitation light. The slow component of PL was in details investigated. Time decay cures for different excitation (337.1 nm, 470 nm and 550nm) and emission (550, 650, 700, 800 and 860 nm) wavelengths and also for different excitation intensities were taken. All decay curves were fitted with a stretched exponential. The slow component of PL was proposed to be attributed to the radiative recombination on surfaces.

The effect of electron beams on polyoctenamer is investigated by viscosimetry, swelling and electron spin resonance spectroscopy. From these experimental investigations it has been deduced that the main degradation process in polyoctenamer is its crosslinking during electron beam irradiation in air, at room temperature. (C) 1997 Elsevier Science B.V.