National Institute Of Materials Physics - Romania

Soft and hard type piezoceramic sintered samples were subjected to neutron irradiation in fluxes up to, 10(18) n/cm(2) and the changes of the basic properties brought about by irradiation were investigated. The results showed that intermediate irradiation levels of up to 10(15) n/cm(2) do not change significantly the properties, but higher levels changed them. Thus the electromechanical coupling factor, the dielectric permittivity and losses decreased while the mechanical quality factor increases with increasing neutron flux for both types of materials. Changes of the lattice constants a and c were also observed. The main defects produced by neutron irradiation are oxygen vacancies but they are not entirely responsible for the modification of the materials properties. Some other factors such as domain pinning and lattice deformation can contribute to the properties degradation.

The glassy, amorphous and disordered chalcogenide materials, important for optoelectronic applications, are discussed. The main optical and optoelectronic phenomena, specific to these materials are shown, and the applications based on these phenomena are evidenced.

X-ray absorption spectroscopy has been used to get information on the structure of chalcogenide glasses. The data analysis was focused on the near edge X-ray absorption spectrum. The experimental As and Se K-edge absorption spectra of thin films of As2Se3 and As4Se4 have been measured in Beijing synchrotron at the beam line, using a double crystal monochromator with Si (111) crystals. The near-edge absorption curves were resolved in two components: the strong white line at the energy threshold and the first oscillatory part following the white line at approximately 7-8 eV above the K-edge, which undergoes changes in shape and position as a function of the modifications in the atomic structure induced by the change of composition. The XANES characteristic for the two types of films allows to conclude that the local disorder around As and Se is different. The non-stoichiometric composition, As4Se4, exhibits more disorder than the stoichiometric one.

A layer of doped CdS nanocrystals was deposited on the surfaces of nanopores of polyethyleneterephtalate (PET) membranes. Optical absorption measurements were used to determine the bandgaps of the nanocrystals; CdS:Mn2+ and CdS:Mn2+:Cu+ show a blue and red shift, respectively, relative to the band gap of nanostructurated CdS. Photoluminescence spectra of the doped and undoped CdS samples exhibit wide bands covering the visible spectrum range.

We investigated the dc magnetization curves of overdoped La2-xSrxCuO4 single crystals in the low-temperature T range, with the external magnetic field H oriented along the c axis. It was found that the onset field for the second magnetization peak, associated with the order-disorder transition in the vortex system, increases significantly with decreasing T in the low-T region, where the superconductor parameters are independent of T. The upward curvature of the order-disorder transition line determined in standard magnetization measurements at low T is explained by considering the reduction of the actual pinning energy due to the macroscopic currents induced in the sample. A simple energy balance equation for the dynamic conditions generated in the widely performed magnetization studies is proposed.

We report some spectroscopic arguments showing the guest-host interactions in the case of the composites containing Jacobsen salen complex and nanoporous molecular sieves of different types: SBA-15, MCM-48, MCM-41. The complex was embedded by impregnation, but care was given for washing out the weakly anchored part. Siliceous nanoporous materials lead to a weaker interaction than A1 containing forms. Thermal analysis has shown that at least in the case of the composite containing MCM-41 the embedded complex still contain chloride ions, therefore the bonding to the surface was apical. In situ experiments indicate that the thermal stability of the embedded complex is higher than in the free state. The differences among the composites coming from the structure of the molecular sieves are discussed.

Arsenic is the main component of many chalcogenide glasses. Due to its triple co-ordination arsenic has the tendency to form two-dimensional networks in both crystalline and amorphous state. Besides the layered form of arsenic it exists molecular arsenic. Small molecules are characteristic to this form of arsenic. Large arsenic clusters have been not identified up to now in amorphous films. A modelling study was carried out in order to demonstrate the possibility to find or to prepare fullerene-like configurations based on arsenic as well as nanotubes (or nanowires) of various diameters.

Single-walled carbon nanotubes (SWNTs) under a moderate non-hydrostatic pressure, of 0.58 GPa, undergo a non-reversible transformation. Due to the plastic deformations, structural defects and carbon nanotube fragments of different size are produced. Short fragments of spherical or ellipsoidal form, behaving as closed-shell fullerenes are observed both by Raman spectroscopy and photoluminescence (PL). The main vibrational indicative of the fragments of shorter length is the band at similar to 1458 cm(-1) that is regularly observed in the Raman spectra of fullerene. Similar to fullerenes self assemblies, the interaction between the nanotube fragments is noticed in the Raman spectrum by the band at similar to 94 cm(-1) that reveals an inter-particle vibration mode. When the nanotubes are dispersed into host matrix, as aromatic hydrocarbons (AHs) with isolated or condensed phenyl rings, supplementary mechanico-chemical reactions take place. For AHs with isolated phenyl rings, like biphenyl or p-terphenyl, a chemical functionalization of SWNT fragments is demonstrated by the appearance of a new Raman band at 1160 cm(-1). In PL spectra, the interaction of SWNTs with AHs, is noticed by a detailed vibronic structure appearing in the high energy side of the emission spectrum, that increase with the weight of SWNTs in the AHs/SWNTs mixtures.

We use the Landauer-Buttiker formalism to study the mesoscopic Fano effect in Aharonov-Bohm rings with an embedded two-dimensional noninteracting dot. The magnetic field dependence of the dot levels leads to a global shift of the Fano lines which becomes important for small ring/dot area ratios. As the magnetic field is varied the Fano dips move periodically from one side of the peak to the other, as reported by Kobayashi [Phys. Rev. Lett. 88, 256806 (2002)]. We show that this effect appears due to a specific magnetic control of the difference between the phase of the single nonresonant path via the free arm of the ring and the global phase of all trajectories involving resonant tunnelings through the dot.

In this article the obtaining and characterization of a mixture of copper sulfide (CuS, Cu9S, CuS2) is presented. The characterization was made by IR Spectroscopy, X-ray Powder Diffraction (XRD) and Transmission Electron Microscopy (TEM). The sulfide mixture represents an aggregate of small particles with an average diameter of 20-30 nm and particles with the diameter of about 500 nm. The smaller particles have the cuboidal morphology and correspond to CuS2; the bigger ones have the hexagonal morphology and correspond to CuS, and Cu9S8.