National Institute Of Materials Physics - Romania

Experimental demonstration of low power laser induced optical nonlinearities in two different types of specially designed materials: dye doped liquid crystals and copolymers with new pendant azobenzene moieties, is presented. Saturation of the nonlinear refractive index change is observed for some of the samples. The origin of the huge nonlinearity and its saturation in the continuous wave laser regime and at low power levels are analyzed.

Tl+-doped lithium fluoride crystals exhibit very interesting properties concerning the formation of colour centres after X-rays irradiation. The presence of Tl+ ions, even in small concentrations, increases the number of traps which stabilize the H-centre aggregates. These H-centre aggregates become smaller and their number increases, inducing two sets of trapping levels. Photoluminescence measurements after subsequent thermal bleaching of the thermoluminescence peaks show a faster decreasing of the F-3(+) centres, in three steps, compared with the F-2 ones. The F-H recombination processes may induce the excitation-emission transitions of Tl+-ions.

Nanoparticles of interest in nanotechnology (design of nanodevices) and medicine (controlled-release of drugs) can be produced both by anorganic and organic synthesis. The biogenetic production is now of high interest due to simplicity of the procedures and their versatility. Several species of bacteria and plants are able to synthesize nanoparticles or to help in the process of their production. The paper gives an overview on the biogenetic production of nanomaterials.

Tin chalcogenides SnX(2) and SnX, where X = S, Se and Te present a particularly interest for their electronic properties and applications in gas sensors. The state of tin in these materials is important for understanding of the sensing effect and improvement of the sensor performances. Mossbauer spectroscopy is a widely used technique for the analysis of the local electronic structure or chemical bonding in solids. In this paper we applied Mossbauer technique for the investigation of bulk and thin films of SnSe(2) chalcogenide. The films of SnSe2 chalcogenide were obtained by the methods: PLD ("Pulsed laser deposition") and PED ("Pulsed electron deposition"). Mossbauer measurements were performed by transmission (TMS), respectively conversion electron spectroscopy (CEMS). By CEMS spectroscopy surfaces, coatings and thin films containing Sn can be studied on substrates and to various depths up to 1000 nanometers.

Phase change materials are the most promising materials for innovation in the computer memory industry. Of great importance is the understanding of the processes that take place during the switching from the high resistivity state to the low resistivity state. In this paper we have tried to find out the correlations between the crystallo-chemical parameters and the switching properties of these materials. The crystallo-chemical parameters, average electronegativity, and glass forming ability (n) over bar(k) over bar/(Z) over bar have been calculated for a large number of tellurium chalcogenides. The switching parameters have been correlated with the physical properties of these materials: resistivity, activation energy, and average electronegativity. Change of the chemical bonding during switching would be responsible for the outstanding properties of the phase change materials.

Recent results have shown that size-effects in nanostructured materials may enable much more efficient thermoelectric devices. In this paper, the electrochemical preparation of Bi(1.95)Te(2.7)Se(0.35) and Bi(2.15)Te(2.55)Se(0.3) submicron wires arrays with nanosized grain structure is reported. X-ray diffraction (XRD) analysis has indicated the presence of nanosized crystallites in the structure of the wires; high resolution transmission electron microscopy (HRTEM) investigations have confirmed the formation of crystallites with geometrical dimensions of 5-14 nm and random orientations in the structure of these wires. These wires were further analyzed by field emission scanning electron microscopy (FESEM). The measurement of the transport properties showed that a highly degenerate semiconductor with a large thermopower (-125 mu V K(-1) at 300 K) and a low resistivity was obtained. (C) 2009 Elsevier Ltd. All rights reserved.

Technological process of interference lithography using two-layer chalcogenide photoresist were investigated. Top As(40)S(30)Se(30) layer is photoresist with a high selectivity and can be used for recording of interference pattern and formation of first lithographic mask. Second, more thick As(4)Ge(30)S(66) layer, almost is not sensitive to light, but dissolves in weak (0.05%) water solution of KOH. That, optimizing the etchant solutions for both layers, exposure and time of etching it is possible to carry out the technological process of formation of the lithographic mask with high modulation and with the groove form close to rectangular. This technology has been used for the fabrication of one- and two-dimensional periodic structures. Using two layer As(40)S(30)Se(30)-As(4)Ge(30)S(66) photoresist, we have fabricated the diffraction gratings and two-dimensional periodic structures with elements of submicron size. Relief parameters and diffractions properties of the obtained structures and their dependence on etching time are studied.

Matrix assisted photo-amorphization effect has been observed in thin chalcogenide films of composition As40S30Se30 deposited by thermal evaporation on silicon wafer substrate covered by silver. The initial films contain Ag2S and Ag4SeS crystallites embedded in an amorphous matrix. Illumination by a cold light halogen lamp induces the disappearance of the crystalline fraction and leads to significant changes in the diffraction pattern of the amorphous films.

Electrochemical polymerization of indole in a LiClO4/CH3CN solution on a single-walled carbon nanotubes (SWNTs) film was studied by cyclic voltammetry, Raman scattering and FTIR spectroscopy. Comparing the cyclic voltammograms recorded on a blank Pt electrode with those obtained when carbon nanotubes films were previously deposited onto the Pt electrode, a down-shift of the indole reduction peak potential in the latter case was observed. Raman spectroscopy studies indicate that the electrochemical deposition of polyindole (PIN) onto the SWNT film results in a breaking of SWNT bundles into individual tubes. A covalent functionalization of SWNTs with PIN in the doped state is demonstrated by FTIR spectroscopy, when an increase in the intensity of the absorption band at 1045 cm(-1) is observed. Besides, Raman and FTIR studies performed on samples electrochemically prepared and thereafter post-chemically reacted with an NH4OH solution, indicate both a roping process of individual tubes with PIN as a binding agent and a strong steric hindrance effect. (C) 2009 Elsevier B.V. All rights reserved.

Composites based on carbon nanotubes and ZnO particles with needle shapes were prepared for applications in energy storage. Depending on the temperature (85 or 25 degrees C) at which the reaction between NaOH and ZnCl(2) was carried out, particles with two different morphologies: needle-shaped (NS) and double-pyramid-shaped (DPS), respectively, are obtained. Scanning electron microscopy, photoluminescence, UV-Vis absorption spectroscopy, x-ray diffraction and Raman light scattering studies reveal that the NS and DPS particles belong to ZnO with wurtzite (WZ) structure and epsilon-Zn(OH)(2) as precursors of ZnO, respectively. Using the ZnO/carbon nanotube composite as a negative electrode and an electrolytic solution containing LiPF(6), the charge-discharge characteristics of rechargeable lithium ions cells were determined. Additional information concerning the electrochemical reactions at the interface of the two electrodes was obtained by cyclic voltammetry.