National Institute Of Materials Physics - Romania

MOS-like structures were obtained by chemical deposition of a polycrystalline PbS thin film on top of a silicon dioxide/Si substrate. Gold ohmic electrodes in coplanar configuration were subsequently deposited by vacuum evaporation on PbS surface (drain and source electrodes). The gate aluminum electrode was deposited on the back of the Si substrate. The dependence of the photoconductive signal, generated in the PbS film, on the gate voltage was studied for wavelengths ranging between 800 nm and 3000 nm at room temperature as well as at low temperatures. It was found that the relative variation of the signal could be as high as 50% for gate voltages ranging between -30 V and +30 V. Two possible mechanisms are proposed to explain the signal variation with the gate voltage: 1) Variation of the depleted region's thickness in the PbS film, that leads to a variation of the conduction channel's resistance (the reference resistance called, also, the dark resistance), 2) The possible variation of the majority carriers (holes) life-time due to the electron blocking at the PbS/oxide interface when positive gate voltages are applied on the back electrode. Integrated IR detectors with controlled sensitivity in the 800-3000 nm range can be manufactured at a relatively low cost using the PbS/oxide/Si MOS-like structure.

CuBa2Ca3Cu4O12-y (Cu:1234) high-temperature superconductors (HTS) doped with up to 2% Zn were grown using the high-pressure synthesis technique. Magnetization loops of the samples were measured at various temperatures between 5 and 77.3 K and magnetic fields up to 14 T. Critical current densities J(c) of the samples were estimated using the critical state model. The results show that Zn-induced pinning centers increase J(c) of Cu:1234 several times, depending on field and temperature. From the experimentally determined field-temperature region in which a higher Zn concentration lead to a higher J(c), we suggest the existence of a cross-over from quite efficient, extended (in the c-axes direction) pinning centers to point-like (inefficient) pinning centers at a certain temperature, depending on field. This effect can be attributed to the fact that, unlike other HTS, in Cu: 1234 there is a second critical temperature T-c2 of about 70-80 K (in zero field, and 50-60 K in 15 T), related to the over-doping of pyramidal basal plane (outer CuO2 plane). (C) 2002 Elsevier Science Ltd. All rights reserved.

Nanocrystalline silicon is studied with a view to obtaining a new photonic material. Non-equilibrium electronic processes in such materials play a significant role. We have studied trapping phenomena in nanocrystalline porous silicon and nanocrystalline silicon-based Multi-Quantum Wells structures by means of Optical Charging Spectroscopy method, which is a very good and sensitive method. We have also analyzed the modeling of the processes that occur during our measurements. This modeling allows us to separate the relative contribution of the different types of discharge currents that can appear: ohmic conduction currents of either equilibrium or non-equilibrium carriers, displacement currents, diffusion currents and tunneling currents through insulating layers (in Multi-Quantum Wells structures). It also allows us to increase the accuracy of the determination of the experimental trap parameters and to determine parameters that are not directly measurable. The model can be applied to other nanocrystalline semiconductors and can be easily generalized for other high resistivity materials.

Glassy composition As25Te35Si40 was investigated by microhardness, differential scanning calorimetry, X-ray diffraction, electrical conductivity and Raman spectroscopy measurements. A continuous random network model of the glass structure has been developed.

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.