Publications

Two-dimensional (2D) arrays of nanometre scale holes were opened in thin SiO2 layers on silicon by electron beam lithography and chemical etching. Oxidized silicon wafers with a 5 nm thick SiO2 layer on top were used in this respect. Pattern transfer involved either only removal of SiO2 or a two-step process of oxide removal and anisotropic silicon chemical etching to form nanometre scale silicon V-grooves. The size of the holes in the photoresist layer varied in the range 40-80 nm, depending on the exposure dose used. The smallest holes in the oxide were about 50 nm in diameter, while in V-grooves the smallest width was approximate to70 nm. 2D arrays of Ge dots or Ge/Si hetero-nanocrystals were selectively grown on these patterned silicon wafers. In small windows only one Ge island per hole was nucleated.

The photoluminescence (PL) dynamics of terbium-exchanged zeolites X was investigated upon laser excitation at 355 nm. The results evidenced the presence of at least two terbium main environments with PL lifetimes varying between 391-411 and 753-770 mus. The two-site nature of terbium distribution in zeolites X permitted a quantitative analysis of the migration process of terbium ions inside the pores and cavities upon dehydration in air at 200 degreesC. Besides the increase of the PL lifetimes with about 30% and 80% compared to those of the hydrated zeolite, a fraction of almost 30% of terbium ions was estimated to migrate from the supercages to the neighboring sodalites or hexagonal prisms. Our results evidenced for the first time the capability of time-resolved luminescence spectroscopy in quantitatively tracking for the intrazeolitic migration of lanthanides. (C) 2004 Elsevier B.V. All rights reserved.

In-plane angular magnetoresistivity Deltarho(ab)(anis) measurements were made on Y1-xPrxBa2Cu3O7-delta single crystals in the pseudogap region. For xgreater than or equal to0.2 single crystals, Deltarho(ab)(anis)(theta) displays a deviation from the typical quasiparticle contribution (proportional tosin(2)theta) for temperatures smaller than a certain value T-phi in the pseudogap region. This deviation is consistent with a flux-flow type contribution to angular magnetoresistivity, indicating the presence of vortexlike excitations above the zero-field critical temperature in the pseudogap region.

The SmCo5 hard magnetic phase was added to magnetoresistive granular Cu-Fe alloys in order to investigate the influence of the presence of a hard magnetic phase on the magnetoresistive properties of a granular alloy that contains a soft magnetic phase. Cu-80(Sm0.17Co0.83)(x)Fe20-x ribbons, with x = 20, 15, 10, 5, obtained by melt spinning, were investigated. The ribbons are composed of magnetic Fe, SmCo5, and Co precipitates embedded in a Cu matrix. In the as-quenched state, the magnetic interactions between magnetic precipitates lead to the formation of magnetic coherent regions and the magnetoresistance effect is only observed at high field (>1T). After annealing, the strength of interactions decreases and a magnetoresistance effect is observed at low field (<1T). The largest magnetoresistance effect (16%) is observed at 5K for the Cu-80(Sm0.17Co0.83)(10)Fe-10 alloy annealed at 450degreesC.

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.