Publikationen

Two photoremovable protecting groups, namely, nitroveratryloxycarbonyl (NVo) and diethylamino-coumarin-4-yl (DEACM), have been tested for wavelength-selective, independent removal. The chromophores were attached to the amine group of aminopropyltriethoxysilane and used for the modification of silica surfaces. A photolytic experiment on the photosensitive layers allowed us to identify the irradiation conditions for the selective cleavage of the chromophores. UV measurements revealed that the photolabile DEACM group can be cleaved off with UV light at 412 nm without damaging the NVo group. The NVo group could then be removed at 365 nm. Masked irradiation of substrates modified with a I: I molar mixture of both silanes allowed the generation of bifunctional patterns after the selective cleavage of DEACM and NVo in a sequential irradiation process. The deprotection reaction was confirmed by coupling two different fluorescent dyes to the liberated amine groups. The expected two-color pattern could be observed by fluorescence microscopy.

Plastic deformation of micron and sub-micron scale specimens is characterized by intermittent sequences of large strain bursts (dislocation avalanches) which are separated by regions of near-elastic loading. In the present investigation we perform a statistical characterization of strain bursts observed in stress-controlled compressive deformation of monocrystalline molybdenum micropillars. We characterize the bursts in terms of the associated elongation increments and peak deformation rates, and demonstrate that these quantities follow power-law distributions that do not depend on specimen orientation or stress rate. We also investigate the statistics of stress increments in between the bursts, which are found to be Weibull distributed and exhibit a characteristic size effect. We discuss our findings in view of observations of deformation bursts in other materials, such as face-centred cubic and hexagonal metals.

Topographical structures were created on the surface of multi-walled carbon nanotube-based coatings deposited on borosilicate glass using the direct laser interference patterning (DLIP) technique. Films made by multi-walled carbon nanotubes (MWNTs) dispersed in antimony-doped tin oxide (ATO) matrix and networks of MWNTs with both low and high adherence to the substrates were irradiated with one single laser pulse. Due to the high absorption coefficient of ATO, the film was completely removed at the interference maxima positions leading to periodic arrays of high quality on macroscopic areas. Additionally, increase of the laser fluence has produced wider ablated regions. Irradiation of high adherent networks of MWNTs produced a periodic porous structure, what has been attributed to the presence of adherence promoters in the film. On the other hand, MWNT networks with low adhesion to the substrate were strongly removed at the interference maxima positions. In this case, however, the fabricated periodic structures presented several defects that result from the poor adherence of the film to the substrate.

Multi-walled carbon nanotubes (MWNTs) dispersed in surfactants were investigated as additional conductive elements in an antimony-doped tin oxide (ATO) matrix. Dispersion of MWNTs in a cationic surfactant was more efficient than in anionic or non-ionic surfactants. Precursor solutions as well as coatings of ATO and ATO/MWNT composites were characterized. The effect of the addition of MWNTs on the optical, electrical and mechanical properties of the original ATO coatings was demonstrated. Low concentrations of MWNTs are sufficient to decrease the resistivity of conducting ATO films by a factor of 16, with preserved transparency (90%) in the visible range. Fewer ATO nanoparticles and lower sintering temperatures than those presented in the literature lead to films with comparable resistivities and even higher transparency, with reduced manufacturing costs. The addition of MWNTs has also increased the hardness of the ATO coatings from HB to 2H-4H, depending on the MWNT concentration employed.

Thin films of multi-walled carbon nanotubes (MWNTs) were prepared by spin coating on borosilicate substrates. The precursor solution was a dispersion of MWNTs in water containing hexadecyl-trimethyl-ammonium-chloride (HDTAC), a cationic surfactant. In order to enhance the adhesion of the films to the substrate, different concentrations of an adhesion promoter were added to the precursor dispersion. Parameters such as the optimum concentration of MWNTs in the networks, temperature of heat treatment, transparency in the visible range, hardness and adherence of the films were investigated. The structural, optical, electrical and mechanical characterization of the thin films is demonstrated in this work.

Ionic liquid was used as additive in photosensitive photopolymerizable materials. Ionic liquid as additive in photopolymerizable hologram material increases the sensitivity, the resolution, and the refractive index modulation of the material efficiently, which present strong diffusion after stopping the exposure due to the diffusion-controlled polymerization in ionic liquid. The diffraction efficiency was raised up to the theoretical maximum of 34% in the thin hologram.

Fundamental processes of wear include the rupture of single chemical bonds and the displacement of atoms or small clusters by mechanical action. Experimental studies of such processes have become feasible with the development of scanning probe microscopy. The small volume affected in these experiments overlaps with the size scale of large atomistic simulations, making a direct comparison possible. The complexity of real-world wear processes is reduced in most nanometer-scale experiments, for example, by probing surfaces of single crystals or by establishing and maintaining carefully controlled environments, including ultraclean conditions. The studies address the onset and topography of wear, the formation of debris structures, the interplay of mechanical and chemical action, the role of ultrathin films, the role of crystal defects in wear processes, and temporal and thermal effects.

The work function difference between single layer and bilayer graphene grown epitaxially on 6H-SiC(0001) has been determined to be 135+-9 meV by means of the Kelvin probe force microscopy. Bilayer films are found to increase the work function as compared to single layer films. This method allows an unambiguous distinction between interface layer, single layer, and bilayer graphene. In combination with high-resolution topographic imaging, the complex step structure of epitaxial graphene on SiC can be resolved with respect to substrate and graphene layer steps.

The underlying matrix plays an important role in the adhesion, proliferation and differentiation processes of Caco-2 cells. When culturing these cells for pharmaceutical purposes it is essential to know the influence of different supports on morphological and functional cell parameters. The impact of polystyrene, Matrigel-coated polystyrene, glass and nanostructured Easy-To-Clean (ETC01) slides was investigated over time by real-time quantitative reverse transcription polymerase chain reaction, enzymatic assays and immunofluorescent staining techniques. Compared to polystyrene, ETC01 slides induced cellular activities towards functional differentiation after short cultivation times. Glass significantly accelerated the differentiation process up to day 10 in culture, while Matrigel-coating had no significant benefit. By day 21 postseeding, the phenotype had equalized as indicated by constant brush border enzyme activity and villin mRNA expression masking the initial differences between the supports. The accelerated differentiation on specific matrices could be advantageous as it may enable cultured monolayers to be used earlier, and has to be considered when interpreting and comparing results.

Validation of reference gene expression stabilities is a prerequisite for reliable normalization of qPCR data. The present study assessed the variation of six reference genes (ACTB, GAPDH, B2M, HPRT1, SDHA, YWHAZ) in Caco-2 cells under the influence of different growth supports and cultivation periods. Genes were ranked according to their stability using the geNorm software. To verify the influence of reference gene selection, ALPI gene expression during differentiation was quantified using the most or the least stable reference gene for normalization. Experimental conditions significantly affected the expression levels of reference genes. Whereas GAPDH and ACTB were revealed as most stable genes, SDHA was the least stable one. The extent of ALPI gene expression was significantly changed by the selection of the reference gene. This study provides a basis for qPCR studies related to both the differentiation process of Caco-2 cells and the elucidation of cell behaviour influenced by surface modifications.