National Institute Of Materials Physics - Romania

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]

Rare-earth doped xerogels (Eu3+, Sm3+, Ho3+, Pr3+) were prepared by using the sal-gel method and their magneto-optical and optical properties have been studied. The Magnetic Circular Dichroism (MCD) spectra are quite similar to those recorded in the RE-doped fluorozirconate glasses; the fine structures shown by the MCD spectra are better resolved compared to the optical absorption spectra. The MCD technique has been correlated with optical measurements in order to investigate the site symmetry in the particular case of Eu3+-doped xerogel and oxyfluoride glasses. In the xerogel, coordination symmetry around the Eu3+ ions is close to (D) under bar (3h) and is lower in the oxyfluoride glass. (C) 2011 Elsevier B.V. All rights reserved.

Nanocrystalline Ho-doped BaTiO3, with average nanocrystals size of 20 nm, have been prepared using a sol-gel combustion technique. The structural and morphological properties of the powders have been investigated by X-ray powder diffraction and high resolution transmission electron microscopy. Chemical states of the holmium on the Ba0.97Ho0.03TiO3 ceramic surface were analyzed using X-ray photoelectron spectroscopy. Furthermore, their photoluminescence properties were analyzed.

In the so-called 'step-shape' angular spin distribution model for layered systems, the non-collinear directions of the atomic magnetic moments are confined to the film plane and form a homogeneous fan spanning inside an (in-plane) angular interval Delta phi centered at an angle phi(0). A general approach for deriving the two parameters phi(0) and Delta phi via Fe-57 Mossbauer spectroscopy measurements is discussed. The analysis extends our previously reported treatment, which assumed that the angular aperture Delta phi develops symmetrically versus a fixed direction phi(0) (e.g., the in-plane easy axis of magnetization) oriented either along or perpendicular to the in-plane projection of the Mossbauer gamma-ray direction. The proposed approach is also applicable for those cases when not only the spin aperture Delta phi is changing but also the aperture center phi(0) is rotating under the influence of different external parameters, such as applied field, temperature, stress, etc. The method is suitable for applications to nanoscale layered heterostructures with in-plane uniaxial or unidirectional magnetic anisotropy. The method is applied to experimental data obtained on a 2-nm thick defected Fe layer with in-plane magnetic texture. (C) 2011 Elsevier B.V. All rights reserved.

Clean Si(001) single crystal surfaces provided different surface reconstructions: p(1 x 2) and c(n x 2) (n = 4, 6) at room temperature. The in situ oxidation of these surfaces was followed by Auger electron spectroscopy and by X-ray photoelectron spectroscopy. It is found that, in similar ultrahigh vacuum conditions, much faster contamination (about 500 times) occurs when the samples are investigated by AES than by XPS, owing to the interaction of the electron beam with the sample surface. With the sample subject to the AES investigation, the contamination occurs by forming >Si2C=O complexes based on the Si dimers. During XPS, reaction with water molecules from the residual gas should also be taken into account.

PbS micro- and nanoparticles were synthesized by a simple precipitation reaction of lead nitrate with thioacetamide in hydrosoluble polymer water solutions. The effects of four water soluble polymers: polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and poly-N-vinyl pyrrolidone (PVP) on the PbS crystallites morphology and structural properties were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that for the PbS particles obtained in the PVA. PEG and PVP, the (2 0 0) diffraction peak of the nanocrystals becomes dominant. The highest texture in the [2 0 0] direction was observed for the crystallites obtained in the presence of PVP. Polydisperse PbS particles with cubic morphology and size ranging from 100 nm to several microns are obtained in the case of PAM and PEG. Monodisperse cubic PbS crystallites with an average size of 200 nm are formed in the presence of PVA and PVP. (C) 2011 Elsevier B.V. All rights reserved.

A microscopic theory for optical properties of self-assembled quantum-dot systems is presented. For nitride-based material systems, the single-particle states are determined from atomistic tight-binding (TB) calculations. This includes not only localized quantum-dot states but also delocalized wetting-layer states, since both contribute to the optical properties and are coupled via interaction processes. The TB wave functions are used to calculate matrix elements for optical dipole transitions as well as Coulomb and LO-phonon interaction of carriers. Absorption and gain spectra are investigated via a solution of the semiconductor Bloch equations including Coulomb interaction effects as well as dephasing due to carrier interaction with LO-phonons. Numerical results are presented for InN/GaN QD-WL systems and allow to investigate the excitation-density dependence of the optical spectra for this particular system. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Layered double hydroxide materials with Zn2+ and Mg2+ as bivalent cations and Al3+ and Ga3+ as trivalent cations were studied. The anions to balance the positive charge of the network were carbonate and/or nitrate. The samples were prepared in laboratory and routinely characterized. Here the investigations by Fourier transform infrared spectra and thermogravimetric measurements are detailed. On the basis of calibration curves for carbonate and nitrate ions obtained from IR spectra of the corresponding salts, the sample analysis of the interlayer anions was performed quantitatively.

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 results of the electric and magnetic measurements performed on PbZr0.2Ti0.8O3-BiFeO3 symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO3 films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments. (C) 2011 Elsevier B.V. All rights reserved.