National Institute Of Materials Physics - Romania

A model is considered in which the bonds of a honeycomb lattice are covered by rodlike molecules of types AA, BB, and AB. Neighboring molecular ends have three-body and orientation-dependent interactions. The model is shown to be equivalent to a spin-1/2 Ising model on the same lattice with a field, but with only pairwise interactions. Symmetric and asymmetric coexistence surfaces for the separation into an AA-rich and a BB-rich phase are calculated exactly. (C) 2004 American Institute of Physics.

alpha-Fe2O3-In2O3 mixed oxide nanoparticles system has been synthesized by hydrothermal supercritical and postannealing route, starting with (1-x)Fe(NO3)(3)(.)9H(2)O(.)xIn(NO3)(3)(.)5H(2)O aqueous solution (x = 0-1). X-ray diffraction and Mossbauer spectroscopy have been used to study the phase structure and substitutions in the nanosized samples. The concentration regions for the existence of the solid solutions in the alpha-Fe2O3-In2O3 nanoparticle system together with the solubility limits of In3+ ions in the hematite lattice and of Fe3+ ions in the cubic In2O3 structure have been evidenced. In general, the substitution level is considerably lower than the nominal concentration x. A justification of the processes leading to the formation of iron and indium phases in the investigated supercritical hydrothermal system has been given. (C) 2004 Elsevier Ltd. All rights reserved.

Nanoparticles of barium zirconate were obtained by the sol-gel method starting from different barium sources and zirconium n-butoxide in butanol. The intermediate wet gels were obtained by hydrolysis of the precursors with water excess, at room temperature. The dried gels were characterized by DTA and IR techniques and were thermally treated in air to obtain crystalline products. XRD and TEM methods proved the formation of barium zirconate nanopowders. According to XRD and TEM data, better results were obtained when starting with barium nitrate than with barium acetate precursors.

We present photoconductivity and Raman scattering data obtained from composite films of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes at different weight concentrations from 0% to 64%. It is found that the introduction of nanotubes in the PPV precursor polymer solution, heated at 300degreesC to perform conversion into PPV, yields drastic modifications in both the structural features of the composite components and in the electronic properties of the composites. The PPV polymer matrix becomes more disordered due to the introduction of nanotubes which induce a shortening of the polymer conjugated segments as shown by Raman scattering spectra. In addition, these spectra yield information about the evolution from small bundles to thick bundles of single-walled nanotubes as function of their concentration x. Photoconductivity data show that the percolation regime begins at x=2%, indicating that a migration network for the photogenerated charges is established above this threshold. By using a model based on distributions of PPV conjugated lengths and their changes as function of x, we calculate the Raman scattering band shapes and their relative intensities. The theoretical results lead to a comprehensive interpretation of experimental data.

A binary telluride (CdTe) as well as a ternary telluride (PbSeTe) were electrodeposited in the pores of a track-etched polycarbonate membrane, resulting nanowires and nanotubes. Either acid or alkaline solutions were used for preparing CdTe compound, whereas the ternary PbSe1-xTex compound was deposited from an acid solution, only. The investigation of electrode mechanisms was performed by cyclic voltammetry. The nanostructures were visualized using scanning electron microscopy (SEM).

Polycarbonate porous membranes were prepared by etching the ion tracks contained in foils irradiated in the fluence range of 1 ion/sample to 10(8) ions/cm(2). The membranes were used as templates for the preparation of tubes employing auto-catalytic copper deposition. A three-step process was used: pre-activation, activation and copper deposition. The deposition bath employed formaldehyde as reducing agent and tartrate as complexing agent. Copper nucleation was studied in order to optimize the activation steps thus allowing the preparation of thin tubes. The external diameters of the tubes ranged from 300 to 2000 nm. The inner diameters varied as a function of the time-of-exposure to the electroless bath. The length of the tubes was 30 mum, corresponding to the thickness of the foils. (C) 2004 Elsevier B.V. All rights reserved.

The temperature T variation of the normalized vortex-creep activation energy U* determined in standard magnetization relaxation experiments for Pb2Sr2Y0.53Ca0.47Cu3O8+delta single crystals with random point disorder exhibits a maximum, which moves to lower-T values by increasing the external magnetic field oriented parallel to the c axis. The nonmonotonic U*(T) dependence is related to the change of the vortex pinning barriers involved in the creep process across the order-disorder transition in the vortex system (accompanied by the occurrence of the second magnetization peak), in a dynamic scenario. The decrease of U* with decreasing T in the low-T region is caused by the shift of the current density J range probed in standard magnetization measurements toward the critical current density, and the significant U*(J) variation in the elastic-creep domain. The dynamic approach is confirmed by the behavior of highly disordered top-seeded melt-grown YBa2Cu3O7-delta crystals at low T, for which no second magnetization peak appears, and U* does not depend on T.

Second harmonic generation from n-GaAs(100) electrodes in sulfuric acid solution has been investigated at an incident wavelength of 1064 nm over a potential range where Faradaic processes are excluded. In the p-in/p-out polarization configuration, the rotational anisotropy in the nonlinear response was found to depend both on the doping level and the applied potential. The effects of the applied potential on the isotropic amplitude of the SHG signal turned out to be closely related to the population of surface states as revealed by impedance spectroscopy, indicating that these states could play a significant role in second harmonic generation. (C) 2004 Elsevier Ltd. All rights reserved.

Pyroelectric current spectroscopy (PCS) was used to investigate the homogeneity of the Nb doped lead zirconate-titanate Pb(Zr-0.92 Ti-0.08)O-3, (PZT) ceramics intended to be used in the infrared (IR) pyroelectric detectors manufacturing. The non-homogeneous composition in the ceramic volume is reflected by the occurrence of several peaks in the PCS spectra, suggesting the presence of PZT phases with different Zr/Ti ratio, thus having different transition temperatures. The non-homogeneity was confirmed by compositional analysis performed using an Energy Dispersive X-ray Spectrometer (EDAX) attached to a Scanning Electron Microscope (SEM). PCS can be a useful tool in optimizing the calcination and sintering parameters (temperature and time) during ceramic preparation. (C) 2004 Elsevier B.V. All rights reserved.

The electronic mechanisms induced by the UV exposure of thin films of polyvinyl alcohol doped with pairs of mixed valence metal ions were studied in relation to their optical behaviour by Mossbauer spectroscopy and optical absorption. The results obtained definitely point to the role of each element from the pair in the electronic mechanism involved, with influence on the optical properties regarding applications in real-time holography and integrated optics.