Publikationen

The injection of water beneath liquid diethylene triamine in a glass cuvette leads to an unexpected phase evolution behavior of the two liquids. The space and time dependent developments of the molecular structure and the underlying transport associated with mixing of the two liquids are monitored by optical imaging and scanning Brillouin microscopy. Apparently, results obtained by either experimental technique lead to disparate interpretations. Whereas optical imaging suggests the existence of a two phase structure, which disappears within a few hours, acoustic microscopy indicates the evolution of a more gradually evolving and longer-lived three phase structure. According to molecular acoustics, the transport of diethylene triamine into water and vice versa behaves strongly asymmetric in time. An attempt is made to reconcile the observed optical and acoustic manifestations of the mixing process on the basis of molecular complex formation.

Metal oxide nanostructures offer interesting possibilities to design functional surfaces for biosensing applications, for instance, through higher surface area leading to enhanced immobilization of biomolecules, which increases the detection limit. Herein, an amplified electrochemical sensing method has been presented for the detection of DNA based on the readout resulting from chemical oxidation of guanine on nanoscaled metal oxides (TiO2, SnO2 and Fe3O4) obtained by chemical vapor deposition (CVD) onto pencil graphite electrode (PGE) as electrochemical transducer. The proposed strategy is suitable to produce cost-effective disposable sensor elements enabling quantitative detection of nanomolar concentrations of DNA. When preparing these metal oxide surfaces by CVD onto PGEs, the various experimental conditions; such as, the effect of different concentrations of 20 mer-bases DNA oligonucleotide (ODN20), and the surface pretreatment steps were Studied to obtain better surface properties for DNA immobilization. The detection limit estimated for signal-to-noise ratios >3 corresponds to 21.3, 519 and 45.8 nmole/ml ODN20 concentrations for PGEs modified with TiO2, SnO2 and Fe3O4 films, respectively. The electrochemical detection of DNA onto metal oxide@PGEs is discussed together with the application potential.

Highly crystalline CoFe2O4 nanoparticles (<d> ~ 5.6 nm) were synthesized by thermal decomposition of (i) single molecular precursor CoFe2(OtBu)8 and (ii) stoichiometric mixture of individual metal precursors. The precursors were thermolyzed in a high-boiling solvent (Docosane, bp. 369 °C) to produce uniformly dispersed cobalt ferrite nanoparticles in the case of molecular pre-cursor, whereas bimodal distribution of nanoparticles was obtained in the multi-source synthesis. TG/DSC and HR-TEM reveraled single- and multi-step decomposition profiles and differential nucleation steps in the two cases, which influence size dispersion and phase purity.

Increasing requirements concerning the powder quality for fine-grained ferroelectric ceramics or for low temperature coating technologies provoke further improvements in particle characterization methods. So far, a correct quantitative measurement of the ferroelectric properties for as synthesized particles is lacking and only already sintered ceramics can be measured precisely. We report on a measurement setup to characterize films made of as synthesized ferroelectric particles. The chosen measurement setup enables the definite determination of ferroelectricity in such nondensified particle-based structures. A centrifugally casted BaTiO3 particulate film with 50% porosity is used for characterization. The correct value of the remanent polarization for this porous film obtained by the new method is almost half the value estimated by conventional measurement setups. Compared to a sintered BaTiO3 ceramic the achieved remanent polarization is much lower (10%). At the same time a higher maximum field has to be applied to the sample as lower fields act on the individual particles because of the porosity of this film.

In 1999, the International Research Training Group 'GRK532' was founded as a pilot project for cross-border European postgraduate education along the German/French/Luxembourg borders. The project consists of an interdisciplinary research programme on synthesis, isolation and characterization of new materials accompanied by an ambitious continuous and consecutive education programme between research groups from chemistry, pharmacy, physics, biochemistry, materials science and analytical chemistry. The education programme aims at qualifying doctoral students in international, interdisciplinary and soft skills during their thesis work. Key elements of the education programme are lectures, hands-on workshops, scientific presentations and discussions in English. Alumni receive a certificate of successful participation, which is signed by the German and French speakers and the president of the Deutsch-Französische Hochschule/Université Franco-Allemande [Franco-German University].

Investigations on shape and chemical composition of one-dimensional magnetite nanostructures grown by a catalystassisted vapor phase procedure are reported. Intrinsic crystal chemistry (preferred growth of most stable surfaces) could be modulated by seeding the magnetite growth through Au nanoclusters, which led to elongated nanostructures (VLS mode); however, the structures have similar facets as observed in uncatalyzed growth. Geometric and energetic contributions to the evolution of the predominately observed {111} surface facets are discussed on the basis of high-angle annular dark field (HAADF) images and electron energy loss spectroscopy (EELS). The Fe:O stoichiometry in magnetite nanowire was determined by EELS, which manifested the reproducibility of nanowire growth by molecule-based CVD and the slightly nonstoichiometric nature of magnetite (Fe3O4-0.15). In combination with HAADF-TEM techniques, Au nanoclusters were identified on the surface of single-crystalline nanowires, which ably result from the surface diffusion of the catalyst (Au) material. In addition, core-shell SnO2/Fe3O4 1 D nanostructures were fabricated by sequential deposition of Sn and Fe precursors. Cross-sections of the coaxial nanostructures revealed polycrystalline magnetite shells on single-crystalline SnO2 wires constituted by well-defined single-crystalline facetted grains of slightly nonstoichiometric magnetite.

Grafting of poly(acrylic acid) (PAA) onto all aluminum surface was successfully achieved by free radical polymerization of acrylic acid using typical radical initiators, benzoyl peroxide and 2,2'-azobisisobutyronitrile. Both spotlike and brush morphologies were achieved. A complete coverage of PAA oil in aluminum surface was then achieved by using a thermal chemical vapor deposition process. The PAA thickness was determined by ellipsometry and the superficial chemical composition by X-ray photoelectron spectroscopy (XPS). Grazing angle Fourier transform infrared (FTIR) spectroscopy confirmed the presence of carboxylic acid groups on the surface, and the contact angle Measurements revealed a decreasing free surface energy of aluminum due to the polymer surface covering.

Mixed-metal tert-butoxide, [CoGa2(OtBu)8], was employed in the chemical vapor deposition (CVD) and sol-gel processes to obtain thin films and nanoparticles of spinel CoGa2O4 phase, respectively. The appropriate Co:Ga ratio and intact vaporization (115-120 °C / 10-2 Torr) of the molecular source produced crystalline deposits of CoGa2O4 at relatively low temperatures (~ 500 °C). A clean transformation of the heterometal precursor in spinel oxide of definite composition was supported by TG/DTA analysis that showed no weight loss above 470 °C. The SEM images of CoGa2O4 films showed homogeneous morphology and dense microstructure constituted by nanometric grains (<d>, ~ 35 nm). Hydrolytic decomposition of the precursor produced gels that upon heat-treatment (400-1200 °C) formed nanoscaled spinel. For comparison, CoGa2O4 was also prepared by complexing Co2+ and Ga3+ ions with glycolate ligands, significant agglomeration effect, broader size dispersion and amorphous domains were observed indicating that low-temperature synthesis of monophasic materials following conventional chemical approaches is hampered by thermodynamic impediments. The UV/Vis spectra of CoGa2O4 particles exhibit characteristic peaks corresponding to 4A2(F) à 4T1(P) transition in the divalent cobalt cation. The magnetization data of the CoGa2O4 nanoparticles showed as expected an antiferromagentic behavior.

We demonstrate that illuminating metal oxide gas sensors with ultra-violet light is a viable alternative not only to activate but also to modulate their response towards oxidizing gases. Here, the performance of individual monocrystalline SnO2 nanowires to NO2 at room temperature as function of the flux and the energy of photons is studied. The results reveal that nearly identical responses, similar to thermally activated sensor surfaces, can be achieved by choosing the optimal illumination conditions. On the basis these results, a qualitative model to explain the response of these sensors towards oxidizing gases is proposed. This finding paves the way to the development of conductometric gas sensors operating at room temperature.