Publikationen

Iron oxide films have been deposited on Si(100) substrates by chemical vapour deposition (CVD) of iron(Ill) tert-butoxide ([Fe(OtBu)3]2) in the temperature range 350-450 °C. The precursor flux and substrate temperature were varied to control the phase composition, average grain size and film thickness. The nature of substrate and deposition temperature markedly influence the morphology and iron-oxygen stoichiometry in the CVD deposits. Phase transformations in iron oxide films were achieved through precise local and periodic heating of the films by interfering laser beams. The interaction of iron oxide films with short laser pulses (Nd:YAG, 355 nm) induced partial transformation of hematite (α-Fe2O3) to magnetite (Fe3O4) or magnetite to wustite (Fe1-xO), respectively. The phase characterization and morphology of the hematite and magnetite films were investigated before and after laser irradiation by X-ray diffractometry, high resolution scanning electron microscopy and white light interferometry.

A total of 81 doped and undoped anatase nano-particles were synthesised by a precipitation/co-precipitation process followed by a hydrothermal treatment to obtain increased visible light photocatalytic activity. The screening process was performed utilising a high throughput analysis system based on the photometric monitoring of the photocatalytic degradation of organic dyes (Rhodamine B, Malachite Green, Acid Blue 29). Photocatalytically active coatings were prepared with selected catalysts with high and low rankings from the screening. Degradation experiments with stearic acid could confirm the varying grades of visible light activity as seen in the screening process.

Complexation reactions of titanium tetraethoxide [Ti(OEt)4] and titanium tetra-n-butoxide [Ti(OBun)4] with 3-pentenoic acid (PA) and allylacetoacetate (AAA), in a 1 : 1M ratio, were studied in ethanol solution at room temperature. 13C-NMR and FTIR spectra showed that all PA and AAA completely reacted with both titanium alkoxides. Hydridosilane compounds such as triethoxysilane and triethylsilane were added to titanium chelate complexes in a 1 : 1M ratio. The investigation of products by 13C- and 29Si-NMR and FTIR showed additions of SiH to the CC double bond. The hydrolysis of titanium-PA and AAA complexes, by water in 1 : 4 ratios, resulted in released PA in an amount of 10% and AAA of 20%. The stability of hydrolyzed products was investigated by 13C-NMR, 29Si-NMR, and FTIR.

Titanium carbide/amorphous-carbon (TiC/a-C:H) nanocomposite coatings deposited by pulsed unbalanced reactive magnetron sputtering have been investigated in terms of structure, chemical and phase composition by AFM, TEM, XPS and XRD analyses. Subject to total carbon content, metallic titanium, titanium carbide and amorphous-carbon phases were found in the deposited coatings, which contributed to the observed microstructures and morphologies. The specific resistivity of nanocomposite coatings scales up with increasing amount of matrix-forming carbon. Hardness profiles of the different compositions revealed that nearly stoichiometric TiC films with average crystallite size of 70 nm exhibit the maximum hardness, whereas the lowest friction coefficient (µ < 0.1) was found in films rich in amorphous-carbon and containing smaller TiC nanocrystallites (<d> similar to 10 nm).

We present results on the structural and magnetic properties of highly crystalline α-Fe2O3 nanoparticles of average size similar to 200 nm, synthesized from a novel sol-gel method using metal alkoxide precursor. These particles are multi-domain, showing the weak ferromagnetic antiferromagnetic (WF-AF) transition (i. e., the Morin transition) at T-M = 256 (2) K. Mossbauer measurements revealed a jump in hyperfine parameters (HP's) at T similar to T-M, which also displays thermal hysteresis upon cooling or heating the sample. The analysis of HP's as a function of temperature allowed us to discard temperature gradients as well as the coexistence of WF/AF phases as possible origins of this hysteretic behaviour. Instead, the hysteresis can be qualitatively explained by the small size and high-crystallinity of the particles, which hinder the nucleation of the WF or AF phases yielding metastable states beyond T-M.

Mixed-metal oxides with the composition Y3ScxGa5-xO12 (x = 2.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, and 3.0) have been prepared by an aqueous sol-gel method. The effects of scandium substitution on the garnet phase formation were studied by IR spectroscopy and X-ray powder diffraction (XRD). The XRD data indicate that single-phase Y3ScxGa5-xO12 ceramic samples were obtained for x = 2.0, 2.1, 2.2, 2.25, 2.3, and 2.4. The results also show that the formation of Y3ScxGa5-xO12 garnets depends on the molar ratio of scandium and gallium in the investigated composition, and consequently on the mean cationic radius at the Al3+ sites. The variation of lattice parameters for the Y3ScxGa5-xO12 phases with different x is reported.

Structures comprising a xerogel, doped with lanthanide ions (erbium, terbium and europium), embedded in porous anodic alumina (PAA) have been fabricated and their optical and electrical characteristics have been studied. Erbium photoluminescence at 1.53 µm from titania xerogel/PAA was found to increase with the number of xerogel layers and erbium. concentration for the excitation wavelength 532 nm, matching the area of transparency of both titania xerogel and PAA. Visible green and red electroluminescence was observed for terbium- and europium-doped In2O3 and SnO2 xerogels embedded in porous anodic alumina. The improvement of the electrical properties of the xerogel/PAA cell is discussed, taking into account the observed ability of conducting ln2O3:Sn (ITO) nanoparticles to penetrate into the anodic alumina pores.

We report here on dialkyldithiocarbamates complexes of the type [C6H3(CH3)S2MS2CNR2] (M = As and Bi: R = CH3, C2H5, and CH2-CH2), prepared by the reaction of toluene-3,4-dithiolatoarsenic(III) and -bismuth(III) chlorides [C6H3(CH3)S2MCl] (M = As and Bi) with sodium/ammonium salts of dialkyldithiocarbamates [XS2CNR2] (X = Na and NH4: R = CH3, C2H5 and CH2-CH2). The new derivatives were characterized by melting point, elemental analysis (C, H, N, S and As/Bi) and spectroscopic studies (IR, 1H and 13C NMR). Single crystal X-ray diffraction analyses of toluene-3.4-dithiolatoarsenic(III) dimethyldithiocarbamate and toluene-3,4-dithiolatoarsenic(III) pyrrolidine-dithiocarbamate revealed a monodentate chelating mode of the dimethyl as well as the pyrrolidine-dithiocarbamate moieties in these complexes.

A BaSnO3 powder with a crystallite size of 27.6 nm has been prepared through a hydrothermal reaction of a peptised SnO2∙xH2O and Ba(OH)2 at 250°C and the following crystallization of this hydrothermal product at 330°C. The peptisation of the SnO2∙xH2O gel is dependent on the pH value. Through peptisation the mean particle size of SnO2∙xH2O in the aqueous solution has been decreased by a factor of 100 to 8 nm. A limited agglomeration in the sol-prepared powder has been observed under the microscope. The structure evolution and crystallisation behaviours of the sol-prepared powders were investigated by TG-DTA, IR and XRD. The BaSn(OH)6 phase in the as-prepared powder transforms into an amorphous phase at 260°C, from which the BaSnO3 particles nucleate and grow with an increase in temperature. The single-phase BaSnO3 powder has been obtained at a temperature as low as 330°C. This sol-prepared powder is more sinter-reactive than the get-prepared powder and can be sintered to a ceramic with 90.7% of the theoretic density.

Nanocrystalline NiGa2O4 films were deposited on silicon substrates by the CVD of a new heterometal alkoxide, [NiGa2(OtBu)8]. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) showed a single-step decomposition behavior for the molecular precursor at low temperature (240°C), suitable for a CVD process. [NiGa2(OtBu)8] is monomeric in solid state with a tetrahedral Ni2+ center coordinated by two monoanionic {Ga(OtBu)4}-moieties. Despite an adequate vapor pressure, the gas-phase transport of [NiGa2(OtBu)8] is susceptible to the distance and geometry of the effective diffusion path. Investigations on CVD deposits obtained using different transport pathways (reservoir –> substrate) show that gas-phase travel through a long (39 cm) and angular reactor tube induces fragmentation of the heterometal compound, which yields minor amounts of Ni, NiO, and Ga2O3 (amorphous), besides the target (NiGa2O4) composition. Shortened reservoir –> substrate path length (13 cm) produced stoichiometric NiGa2O4 films, apparently due to reduced collision probabilities and a laminar flow. In both cases, the chemical composition was determined using energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS), whilst the structure was evaluated using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). From these, the advantages and limitations of the single molecular source in the growth of NiGa2O4 films could be determined.