Cameliu Himcinschi

Comparison of techniques to characterise the density, porosity and elastic modulus of porous low-k SiO 2 xerogel films

A range of mesoporous xerogel low-k dielectric films were prepared and characterised using complementary techniques: Laser-generated surface acoustic waves, ellipsometric porosimetry, Rutherford backscattering and nanoindentation. The density, porosity, pore size distribution, cumulative surface area, elastic modulus and hardness of the films were measured as well as their dielectric constants. Dielectric constant values of k = 1.7-2.3 were measured for samples with porosities of 36-55%. Mean pore radii values of 2.2-4.2 nm and surface areas of 280-240 m3 cm-3 were also obtained. Using porosity and mean film density values determined using different techniques, the film skeletal density of these samples were calculated to be ≈ 1.4 g cm-3, almost 40% lower than that of dense SiO2. The elastic moduli of the films were found to be E < 4 GPa.

Substrate influence on the optical and structural properties of pulsed laser deposited BiFeO3 epitaxial films

Epitaxial BiFeO3 films pulsed laser deposited on SrTiO3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations.

Optical and magneto-optical study of nickel and cobalt ferrite epitaxial thin films and submicron structures

Epitaxial films and ordered arrays of submicron structures of nickel and cobalt ferrites were deposited on Nb doped SrTiO3 by pulsed laser deposition. X-Ray diffraction and atomic force microscopy showed that the films have a good crystalline quality and smooth surfaces. A larger number of phonon bands was observed in the polarization dependent Raman spectra of the ferrite films than expected for the cubic spinel structures. This is explained by short range ordering of the Ni2+ (or Co2+) and Fe3+ cations at the octahedral sites inducing a lowering of the symmetry. The same behavior was also observed in the Raman spectra measured for the submicron structures, suggesting the same cation distribution as in the films. The diagonal components of the dielectric function for nickel and cobalt ferrites are determined from ellipsometry in the 0.73–5 eV photon energy range. The absorption edge was analyzed using a bandgap model and the energies for the indirect and direct optical transitions were calculated. It was...

Phonon Raman spectra of colloidal CdTe nanocrystals: effect of size, non-stoichiometry and ligand exchange

Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs.PACS: 63.20.-e, 78.30.-j, 78.67.-n, 78.67.Bf

Synthesis of nanostructured chitin–hematite composites under extreme biomimetic conditions

Chitin of poriferan origin is a unique and thermostable biological material. It also represents an example of a renewable materials source due to the high regeneration ability of Aplysina sponges under marine ranching conditions. Chitinous scaffolds isolated from the skeleton of the marine sponge Aplysina aerophoba were used as a template for the in vitro formation of Fe2O3 under conditions (pH ∼ 1.5, 90 °C) which are extreme for biological materials. Novel chitin–Fe2O3 three dimensional composites, which have been prepared for the first time using hydrothermal synthesis, were thoroughly characterized using numerous analytical methods including Raman spectroscopy, XPS, XRD, electron diffraction and HR-TEM. We demonstrate the growth of uniform Fe2O3 nanocrystals into the nanostructured chitin substrate and propose a possible mechanism of chitin–hematite interactions. Moreover, we show that composites made of sponge chitin–Fe2O3 hybrid materials with active carbon can be successfully used as electrode materials for electrochemical capacitors.

Novel nanostructured hematite–spongin composite developed using an extreme biomimetic approach

The marine sponge Hippospongia communis (Demospongiae: Porifera) is a representative of bath sponges, which possess characteristic mineral-free fibrous skeletons made of a structural protein – spongin. This fibrous skeleton is mechanically robust, resistant to acidic treatment, and thermally stable up to 160 °C. Due to these properties, we decided to use this biological material for the first time for the hydrothermal synthesis of hematite (α-Fe2O3) via catalyzed hydrolysis of FeCl3 to obtain a hematite–spongin composite. The material obtained was studied with Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HR-TEM), X-ray Photoemission Spectroscopy (XPS) and Raman spectroscopy. The α-Fe2O3–spongin-based composite was tested for its potential application as an anode material in a capacitor. The results indicate that components constructed using this novel composite material have a positive effect on the capacitance of energy storing devices.

Optical properties and molecular orientation in organic thin films

The optical properties and the molecular orientation in thin films of 3,4,9,10perylenetetracarboxylic dianhydride (PTCDA) and N,N � -dimethyl-3,4,9,10perylenetetracarboxylic diimide (DiMePTCDI) were studied by means of variable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM), near edge x-ray absorption fine structure (NEXAFS) spectroscopy, and infrared (IR) and Raman spectroscopy. VASE reveals that both kinds of film exhibit a strong optical anisotropy. For PTCDA, the optical constants are found to have much higher values in the substrate plane than perpendicular to it. While th ea nisotropy measured in the substrate plane on passivated GaAs(100) is very small for PTCDA a giant anisotropy is observed for DiMePTCDI. This difference in the optical properties is attributed to the different orientation of molecules in the thin organic films. While the PTCDA molecules lie flat on the substrate with their molecular plane parallel to the substrate surface, the DiMePTCDI molecules are tilted with respect to the substrate surface and are predominantly oriented with their long axis parallel to the [011] direction of the substrate as confirmed by VASE, NEXAFS, and Raman and IR results.

Characterization of silica xerogel films by variable-angle spectroscopic ellipsometry and infrared spectroscopy

Silica xerogel films with low dielectric constants were prepared using a sol-gel spin coating method. The as-prepared films were further treated by hexamethyldisilazane to achieve the hydrophobization of the pore surfaces, by replacing hydrophilic silanol groups with hydrophobic trimethylsilyl (TMS) groups. The thickness and optical constants of the films were derived from variable-angle spectroscopic ellipsometry measurements. The determined refractive index decreases from 1.271±0.008 to 1.188±0.003 (values at 632.8 nm) while the porosity increases from 40.4 to 58.6% with the process parameters used. The Maxwell-Garnet approximation was used to relate the ellipsometric data to porosity. The IR absorption bands of CH species in TMS groups reveal that the surface area of the pores is larger in the samples with lower porosity.

Optical properties of epitaxial BiFeO3 thin films grown on LaAlO3

Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3 eV as compared to the rhombohedral films. Optical bandgaps of 3.10 eV and 2.80 eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100 nm as was confirmed also by XRD investigations.

Raman spectra and dielectric function of BiCrO3: Experimental and first-principles studies

We present the complex dielectric function of BiCrO3 thin films in the energy range of 0.73–9.8 eV determined using spectroscopic ellipsometry. By analyzing the absorption onset region, it is shown that the optical bandgap of BiCrO3 is indirect, with a value of 2.27 eV. The imaginary part of the BiCrO3 dielectric function, ɛ2, calculated using density functional theory in the generalized gradient approximation with an Hubbard potential of 3 eV agrees well with the experimentally determined one. Raman spectra of both polycrystalline and epitaxial thin films of BiCrO3 are reported. The temperature dependent Raman measurements indicate a structural phase transition at ∼400 K which was confirmed also by x-ray diffraction investigations.