National Institute Of Materials Physics - Romania

The hydrogen storage material lithium borohydride was infiltrated from solvent solution into the pores of the carbon replica of MSU-H mesoporous silica by the incipient wetness method. Different amounts of lithium borohydride up to 40 % from the weight of the high surface area carbonic material were used. By this method was achieved a good dispersion of the hydride into the carbonaceous support. The hydrogen desorption starts at temperatures as low as 150 degrees C for the sample with 8 wt% LiBH4 dispersed onto the MSU-H carbon replica while pure LiBH4 does not release hydrogen below 300 degrees C. After rehydrogenation, a lower amount of hydrogen is desorbed due to the fact that increasing temperature up to 400 degrees C a part of hydride segregates outside the pores of the carbonaceous support.

In this paper we present electron paramagnetic resonance (EPR) and Mossbauer (transmission method, TMS and measurements in the scattering method, CEMS) experiments on Fe-57 (0.11at.%) doped nanocrystalline anatase-type TiO2, synthesized by a hydrothermal method. Different Fe3+ and Fe2+ ions positions, with various hyperfine interactions evidenced in Mossbauer spectra (MS), confirm a partial magnetic ordering at room temperature. The magnetic hyperfine fields in the TMS spectra, better resolved at lower temperatures, do not change essentially with temperature. The first CEMS measurements, carried out on iron-doped TiO2 nanoparticles, reveal a larger disorder in the surface particles layer. The temperature dependence of the double integral EPR spectral intensity, proportional with the sample susceptibility, shows an anomalous behaviour. It suggests the bound magnetic polaron (BMP) mechanism for the magnetic ordering.

The formation and crystallization of disordered nanosized ZnO resulting from the thermal decomposition of nanocrystalline hydrozincite [Zn-5(CO3)(2)(OH)(6)] has been Observed and investigated during pulse annealing experiments Up to 625 degrees C in air or vacuum by electron paramagnetic resonance of trace amounts of substitutional Mn2+ impurity ions, in correlation with X-ray diffraction and transmission electron microscopy measurements. The mesoporous structure of the disordered ZnO, which initially forms in air and vacuum at 225 and 175 degrees C, respectively, further transforms into nanocrystalline ZnO of increasing particle size and improved lattice quality at higher annealing temperatures. The crystallization process, which does not affect the concentration of the substitutional impurity ions, as well as the simultaneous presence of both disordered and crystalline phases, should be considered in further applications of the resulting nanosized ZnO.

A mesoporous structure of self-assembled nanocrystals of cubic ZnS doped with Mn2+ ions with a homogeneous distribution of pores of similar size was synthesized at room temperature by a surfactant-assisted liquid-liquid reaction. The component nanocrystals exhibit a high crystallinity and a tight size distribution centered at 2 nm, as well as the narrowest Electron Paramagnetic Resonance (EPA) spectra linewidth and the best resolution reported so-far, effects attributed to self-assembling. The observed EPA spectra consist of lines from the substitutional Mn2+(I) and surface Mn2+(II) and Mn2+(III) centers. Here we show that, in contrast with previous reports, our EPA spectra are highly sensitive to structural changes during pulse annealing in vacuum up to 500 degrees C. The changes are related to the transformation of the surface Mn2+ centers in new Mn2+ centers, attributed to an oxidation process in which the thermal decomposition of the Tween 20 additive, also observed by EPA, seems to be involved. We have also been able to observe, for the first time by EPR spectroscopy, the formation of the ZnO phase and the nanocrystals size increase, which occur during annealing up to 500 degrees C, structural changes confirmed by XRD and TEM observations on the samples previously investigated by EPR.

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn2+ pulse annealed in vacuum and in air, up to 500 A degrees C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid-liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn2+(I) centers. No spectra from Mn2+ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 A degrees C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 A degrees C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn2+-type centers in a hexagonal ZnO structure at T similar to 300 A degrees C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.

Silica has been derivatized with aminopropyl groups which subsequently have been chemically modified with N-methoxy-2,6-dinitroaniline derivatives to yield a solid which changes its color in the presence of arginine or lysine, from yellow to blue. Other amino-acids (such as glycine, valine, cysteine, cystine, alanine, phenylalanine, histidine) do not showed this behavior. The same procedure may be applied to silica nanoparticles. The new materials were characterized by UV-Vis, IR, and TEM.

Glass-ceramics containing RE3+-doped BaF2 nanocrystals (RE = Eu, Sm, Dy, Ho and Pr) with the size below 10 nm size have been made by using the controlled crystallization at higher temperatures of the RE3+-doped SiO2-BaF2 xerogels. Photoluminescence measurements have indicated the incorporation of the RE3+ ions in both silica network and in the BaF2 nanocrystals. Thermoluminescence measurements have shown a peak whose position depends on the nature of RE3+-dopant as it follows: 140 degrees C (for Ho3+, Dy3+), 340 degrees C (for Sm3+) and 370 degrees C (for Eu3+); in Eu3+-doped SiO2 glass the TL peak is shifted to 383 degrees C. The peaks in glass-ceramics were assigned to the recombination of the electrons thermal released from the RE3+-electron traps located in both glass-matrix and BaF2 nanocrystals. Within the series the trivalent lanthanide ions act as increasingly deeper electron trapping centres. (C) 2011 Elsevier B.V. All rights reserved.

The aim of this study is to investigate the micrometer and submicrometer scale structuring of silicon by liquid chlorine and fluorine precursors with 200 fs laser pulses working at both fundamental (775 nm) and frequency doubled (387 nm) wavelengths. The silicon surface was irradiated at normal incidence by immersing the Si (111) substrates in a glass container filled with liquid chlorine (CCl(4)) and fluorine (C(2)Cl(3)F(3)) precursors. We report that silicon surfaces develop an array of spikes with single step irradiation processes at 775 nm and equally at 387 nm. When irradiating the Si surface with 400 pulses at 330 mJ/cm(2) laser fluence and a 775 nm wavelength, the average height of the formed Si spikes in the case of fluorine precursors is 4.2 mu m, with a full width at half maximum of 890 nm. At the same irradiation wavelength chlorine precursors develop Si spikes 4 mu m in height and with a full width at half maximum of 2.3 mu m with irradiation of 700 pulses at 560 mJ/cm(2) laser fluence. Well ordered areas of submicrometer spikes with an average height of about 500 nm and a width of 300 nm have been created by irradiation at 387 nm by chlorine precursors, whereas the fluorine precursors fabricate spikes with an average height of 700 nm and a width of about 200 nm. Atomic force microscopy and scanning electron microscopy of the surface show that the formation of the micrometer and sub-micrometer spikes involves a combination of capillary waves on the molten silicon surface and laser-induced etching of silicon, at both 775 nm and 387 nm wavelength irradiation. The energy-dispersive x-ray measurements indicate the presence of chlorine and fluorine precursors on the structured surface. The fluorine precursors create a more ordered area of Si spikes at both micrometer and sub-micrometer scales. The potential use of patterned Si substrates with gradient topography as model scaffolds for the systematic exploration of the role of 3D micro/nano morphology on cell adhesion and growth is envisaged. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3619856]

We studied the effect of temporally pulse shaping upon the properties of thin layers synthesized by pulsed laser deposition with fs laser pulses generated by a Ti-sapphire laser source. We showed that the film morphology and structure can be gradually modified when applying mono-pulses of different duration or passing to a sequence of two pulses of different intensities. (C) 2011 Elsevier B.V. All rights reserved.

The smart-cut (TM) process is based on inducing and processing structural defects below the free surface of semiconductor wafers. The necessary defects are currently induced by implantation of light elements such as hydrogen or helium. An alternative softer way to induce shallow subsurface defects is by RF-plasma hydrogenation. To facilitate the smart-cut process, the wafers containing the induced defects need to be subjected to an appropriate thermal treatment. In our experiments, (0 0 1) Si wafers are submitted to 200 and 50 W hydrogen RF-plasma and are subsequently annealed. The samples are studied by transmission electron microscopy (TEM), before and after annealing. The plasma-introduced defects are {1 1 1} and {1 0 0} planar-like defects and nanocavities, all of them involving hydrogen. Many nanocavities are aligned into strings almost parallel to the wafer surface. The annealing is performed either by furnace thermal treatment at 550 degrees C, or by in situ heating in the electron microscope at 450, 650 and 800 degrees C during the TEM observations. The TEM microstructural studies indicate a partial healing of the planar defects and a size increase of the nanometric cavities by a coalescence process of the small neighbouring nanocavities. By annealing, the lined up nanometric voids forming chains in the as-hydrogenated sample coalesced into well-defined cracks, mostly parallel to the wafer surface.