Articles

publications

  • 2012
    • Christiane Ziegler, Christine Müller
    • Proteine auf medizinischen und technischen Substraten
    • Nachrichten aus der Chemie, Volume 60, Issue 5, pages 561–565 (2012)
    • Abstract >>
    • Bevor sich auf einer festen Oberfläche ein Biofilm bildet, adsorbieren in der Regel Proteine. Ein solches Adsorbat zu charakterisieren, erfordert die Kombination mehrerer oberflächenanalytischer Methoden, die alle ihre spezifischen Grenzen haben. Oft gelingt es nur in kleinen Schritten und über Modellsysteme, ein Proteinadsorbat auf einer realen Oberfläche zu beschreiben.
    • Stefan Lach, Anna Altenhof, Kartick Tarafder, Felix Schmitt, Md. Ehesan Ali, Michael Vogel, Jens Sauther, Peter M. Oppeneer, Christiane Ziegler
    • Metal–Organic Hybrid Interface States of A Ferromagnet/Organic Semiconductor Hybrid Junction as Basis For Engineering Spin Injection in Organic Spintronics
    • Advanced Functional Materials, Volume 22, Issue 5, pages 989–997 (2012)
    • Abstract >>
    • Electrons in organic semiconductors (OSC) possess remarkably long spin relaxation times. Hybrid spintronic devices that combine OSC with ferromagnetic (FM) substrates are therefore expected to provide a route to devices with improved and new functionalities. A crucial role is played by the FM-OSC interface which governs the spin injection into the OSC. Using spin-resolved photoelectron spectroscopy and ab initio calculations we study here such possible injection channels in metal phthalocyanines (MPc). We report the first direct observation of the successful engineering of different spin-selective hybrid interface states at the Fermi level of a FM-OSC hybrid junction only by changing the central metal atom of a MPc. Our results demonstrate that tailoring the chemical interaction at the FM-OSC interface is a promising way to modify the spin injection channels and thus the spin injection capability.
    • A. Lüders, C. Müller, K. Boonrod, G. Krczal, Ch. Ziegler
    • Tomato Bushy Stunt Viruses (TBSV) in Nanotechnology Investigated by Scanning Force and Scanning Electron Microscopy
    • Colloids and Surfaces B: Biointerfaces, Volume 91, 154-161 (2012)
    • Abstract >>
    • Spherical plant viruses like the tomato bushy stunt virus (TBSV) allow for multiple applications in nanotechnology due to their shape. In this article, different types of the virus were created by extending coat protein (CP) at carboxylic termini with 2 different charged amino acids by point mutation. The obtained CPs carried 6 aspartic acid (negative charge) and 4 histamine (positive charge) residues. The ability of TBSV to form self assembled monolayers with large ordered areas on native and chemically modified mica will be presented. The structural differences between layers formed by the wild type and by the genetically modified types will be discussed in detail.
  • 2011
    • H. Gnaser, J. Lösch, A. Orendorz, Ch. Ziegler
    • Temperature-dependent grain growth and phase transformation in mixed anatase-rutile nanocrystalline TiO2 films
    • Phys. Status Solidi A 208 Nr. 7, 1635-1640 (2011)
    • Abstract >>
    • Nanocrystalline TiO2 films were prepared from colloidal suspensions using a mixture of anatase and rutile TiO2 particles. Their structure was examined by X-ray diffraction (XRD). The as-prepared specimens exhibit the presence of both anatase and rutile crystallites with average sizes of ∼30 and ∼50 nm, respectively, as determined from their characteristic diffraction peaks in XRD. Isochronal annealing of the films for 90 min in air at temperatures of up to 1270 K effected clear structural changes: between 470 and 870 K, the crystallite sizes do not change, with the crystallites remaining in their original phase. For annealing temperatures Ta ≥ 970 K the anatase and rutile grain sizes increase, while above ∼1170 K, the anatase phase disappears and only crystallites in the rutile phase of TiO2 are observed. Their size increases further with increasing annealing temperature and reaches ∼150 nm at 1270 K. Concurrently, the weight fraction of rutile changes from an initial value of ∼0.09 (for 470−870 K) to unity (above 1170 K). At Ta = 970 K, the increase of the duration of annealing from 90 to 360 min leads to a moderate increase of the crystallite size. The results indicate that the transformation of anatase particles is governed by the concurrent occurrence of interface and surface nucleation processes.
    • O. Köhler, P. Kowalewski, Ch. Ziegler
    • Lithographically Structured Biological Recognition Structures for Immunosensing and Single Cell Analytics
    • Physics Status Solidi A 208 Nr. 6, 1275-1278 (2011)
    • Abstract >>
    • The preparation of structured antibody layers was shown. For this purpose, a (3-aminopropyl)-trimethoxysilane (APTMS) monolayer on a silicon wafer was pholithographically structured. To the APTMS structures, IgG antibodies can easily be attached via glutaraldehyde coupling. To prevent unspecific bonding to the bare silicon substrate, a commercial blocking agent has to be used.
    • A. Blau, A. Murr, S. Wolff, E. Sernagor, P. Medini, G. Iurilli, Ch. Ziegler, F. Benfenati
    • Flexible, all-polymer microelectrode arrays for the capture of cardiac and neuronal signals
    • Biomaterials 32, 1778-1786 (2011)
    • Abstract >>
    • Microelectrode electrophysiology has become a widespread technique for the extracellular recording of bioelectrical signals. To date, electrodes are made of metals or inorganic semiconductors, or hybrids thereof. We demonstrate that these traditional conductors can be completely substituted by highly flexible electroconductive polymers. Pursuing a two-level replica-forming strategy, conductive areas for electrodes, leads and contact pads are defined as microchannels in poly(dimethylsiloxane) (PDMS) as a plastic carrier and track insulation material. These channels are coated by films of organic conductors such as polystyrenesulfonate-doped poly(3,4-ethylenedioxy-thiophene) (PEDOT:PSS) or filled with a graphite-PDMS (gPDMS) composite, either alone or in combination. The bendable, somewhat stretchable, non-cytotoxic and biostable all-polymer microelectrode arrays (polyMEAs) with a thickness below 500 μm and up to 60 electrodes reliably capture action potentials (APs) and local field potentials (LFPs) from acute preparations of heart muscle cells and retinal whole mounts, in vivo epicortical and epidural recordings as well as during long-term in vitro recordings from cortico-hippocampal co-cultures.
    • H. Gnaser, J. Lösch, A. Orendorz, C. Ziegler
    • Temperature-dependent grain growth and phase transformation in mixed anatase-rutile nanocrystalline TiO2 films
    • physica status solidi (a), Volume 208, Issue 7, pages 1635–1640 (2011)
    • Abstract >>
    • Nanocrystalline TiO2 films were prepared from colloidal suspensions using a mixture of anatase and rutile TiO2 particles. Their structure was examined by X-ray diffraction (XRD). The as-prepared specimens exhibit the presence of both anatase and rutile crystallites with average sizes of ∼30 and ∼50 nm, respectively, as determined from their characteristic diffraction peaks in XRD. Isochronal annealing of the films for 90 min in air at temperatures of up to 1270 K effected clear structural changes: between 470 and 870 K, the crystallite sizes do not change, with the crystallites remaining in their original phase. For annealing temperatures Ta ≥ 970 K the anatase and rutile grain sizes increase, while above ∼1170 K, the anatase phase disappears and only crystallites in the rutile phase of TiO2 are observed. Their size increases further with increasing annealing temperature and reaches ∼150 nm at 1270 K. Concurrently, the weight fraction of rutile changes from an initial value of ∼0.09 (for 470−870 K) to unity (above 1170 K). At Ta = 970 K, the increase of the duration of annealing from 90 to 360 min leads to a moderate increase of the crystallite size. The results indicate that the transformation of anatase particles is governed by the concurrent occurrence of interface and surface nucleation processes.
    • M. Wilhelmi, C. Müller, C. Ziegler, M. Kopnarski
    • BSA adsorption on titanium: ToF-SIMS investigation of the surface coverage as a function of protein concentration and pH-value
    • Anal. Bioanal. Chem. 400, 697-701 (2011)
    • Abstract >>
    • The adsorption properties of bovine serum albumin (BSA) on pure titanium (99.99%+) were studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). For this purpose, films consisting of BSA were prepared on cleaned titanium sheets in a phosphate buffered solution under different conditions and analysed by ToF-SIMS. The dependence of the surface coverage on the concentration of the protein solution could be determined from the secondary ion mass spectrometry signals. The results agree satisfactorily with the corresponding results obtained by XPS. The observed adsorption behaviour can be fitted by the well-known formula describing a Langmuir isotherm. The Langmuir constant for pH 7 was found to be K=17499lg−1.
    • Michael Vogel, Felix Schmitt, Jens Sauther, Benedikt Baumann, Anna Altenhof, Stefan Lach, Christiane Ziegler
    • Photoionization cross section weighted DFT simulations as promising tool for the investigation of the electronic structure of open shell metal-phthalocyanines
    • Anal. Bioanal. Chem. 400, 673-678 (2011)
    • Abstract >>
    • The valence band structure of different metal-phthalocyanines was investigated by comparing ultraviolet photoelectron spectra at different excitation energies with simulated spectra that take the different photoionization cross-sections at these energies into account. The Kohn-Sham eigenvalue spectra, derived from density functional theory calculations, using hybrid exchange-correlation functionals, were weighted with the photoionization coefficients in accordance with the used excitation energy. By applying these techniques, the differences in the photoelectron spectra using He I and He II radiation can be reproduced and investigated. It will be shown that the 3d-orbitals of the used metal central atom of these molecules have a major influence. The changes at different excitation energies were studied for Fe, Co, and Cu central atoms to describe the chemical tailoring effects.
    • Felix Schmitt, Jens Sauther, Stefan Lach, Christiane Ziegler
    • Characterization of the Interface Interaction of Cobalt on Top of Copper- and Iron-Phthalocyanine
    • Anal. Bioanal. Chem. 400, 665-671 (2011)
    • Abstract >>
    • The electronic structure of the interface between ferromagnetic cobalt and the organic semiconductors copper- (CuPc) and iron-phthalocyanine (FePc) was investigated by means of photoemission spectroscopy (UPS, IPES, and XPS). These metal-phthalocyanine (MePc) molecules have an open shell structure and are known to show promising properties for their use in organic spintronics. In spintronic devices, the interface between ferromagnetic electrode and the organic layer determines the spin injection properties and is hence important for the quality of, e.g., a possible spin-valve device. For this purpose, cobalt was deposited onto the MePcs, such as in devices with ferromagnetic top contacts. The reported investigations reveal a diffusion of cobalt into the organic layers and chemical reactions at the interface.
    • D. Chylarecka, T. K. Kim, K. Tarafder, K. Müller, K. Gödel, I. Czekaj, Ch. Wäckerlin, M. Cinchetti, Md. E. Ali, C. Piamonteze, F. Schmitt, J.-P. Wuestenberg, Ch. Ziegler, F. Nolting, M. Aeschlimann, P. M. Oppeneer, N. Ballav, T. A. Jung
    • Indirect Magnetic Coupling of Manganese-Porphyrin to Ferromagnetic Cobalt Substrate
    • The Journal of Physical Chemistry C 115, 1295-1301 (2011)
    • Abstract >>
    • The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule−substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co−Mn−Cl) from the Co surface.
    • Christine Müller, Johanna Wald, Wiebke Hoth-Hannig, Natalia Umanskaya, Daniel Scholz, Matthias Hannig, Christiane Ziegler
    • Protein Adhesion on Dental Surfaces – a Combined Surface Analytical Approach
    • Anal. Bioanal. Chem. 400, 679-689 (2011)
    • Abstract >>
    • Protein adsorption is a field of huge interest in a number of application fields. Information on protein adhesion is accessible by a variety of methods. However, the results obtained are significantly influenced by the applied technique. The objective of this work was to understand the role of adhesion forces (obtained by scanning force spectroscopy, SFS) in the process of protein adsorption and desorption. In SFS, the protein is forced to and retracted from the surface, even under unfavorable conditions, in contrast to the natural situation. Furthermore, adhesion forces are correlated with adhesion energies, neglecting the entropic part in the Gibbs enthalpy. In this context, dynamic contact angle (DCA) measurements were performed to identify the potential of this method to complement SFS data. In DCA measurements, the protein diffuses voluntarily to the surface and information on surface coverage and reversibility of adsorption is obtained, including entropic effects (conformational changes and hydrophobic effect). It could be shown that the surface coverage (by DCA) of bovine serum albumin on dental materials correlates well with the adhesion forces (by SFS) if no hydrophobic surface is involved. On those, the entropic hydrophobic effect plays a major role. As a second task, the reversibility of the protein adsorption, i.e., the voluntary desorption as studied by DCA, was compared to the adhesion forces. Here, a correlation between low adhesion forces and good reversibility could be found as long as no covalent bonds were involved. The comparative study of DCA and SFS, thus, leads to a more detailed picture of the complete adsorption/desorption cycle.
  • 2010
    • Oliver Köhler, Iris Steinmann, Christiane Ziegler
    • Lithographic Structures with Attached Antibodies for Columnar Immunosensors
    • Sensor Letters 8, 747-751 (2010)
    • Abstract >>
    • Columnar biosensors are a new type of electromechanical sensors which can be mass produced, allow easy quantification of the sensor signal, and have a relatively high Q factor. To use these columns for biosensing purposes, the column caps have to be coated selectively with antibodies which act as a receptor layer. Here two different coating strategies are shown: In the first case, the wafer surface was photolithographically structured in such a way, that the column caps were coated with a protein-adherent layer. In the second approach, the whole surface except the column caps was coated with a protein-repellent layer. After coating the column cap surfaces in a subsequent step with anti-A antibodies, selective attachment of erythrocytes of blood group A was observed.
    • Christine Richter, Martina Reinhardt, Stefan Giselbrecht, Daniel Leisen, Vanessa Trouillet, Roman Truckenmüller, Axel Blau, Christiane Ziegler, Alexander Welle
    • Spatially controlled cell adhesion on three-dimensional substrates
    • Biomedical Microdevices 12, 787-795 (2010)
    • Abstract >>
    • The microenvironment of cells in vivo is defined by spatiotemporal patterns of chemical and biophysical cues. Therefore, one important goal of tissue engineering is the generation of scaffolds with defined biofunctionalization in order to control processes like cell adhesion and differentiation. Mimicking extrinsic factors like integrin ligands presented by the extracellular matrix is one of the key elements to study cellular adhesion on biocompatible scaffolds. By using special thermoformable polymer films with anchored biomolecules micro structured scaffolds, e.g. curved and µ-patterned substrates, can be fabricated. Here, we present a novel strategy for the fabrication of µ-patterned scaffolds based on the “Substrate Modification and Replication by Thermoforming” (SMART) technology: The surface of a poly lactic acid membrane, having a low forming temperature of 60°C and being initially very cell attractive, was coated with a photopatterned layer of poly(L-lysine) (PLL) and hyaluronic acid (VAHyal) to gain spatial control over cell adhesion. Subsequently, this modified polymer membrane was thermoformed to create an array of spherical microcavities with diameters of 300 µm for 3D cell culture. Human hepatoma cells (HepG2) and mouse fibroblasts (L929) were used to demonstrate guided cell adhesion. HepG2 cells adhered and aggregated exclusively within these cavities without attaching to the passivated surfaces between the cavities. Also L929 cells adhering very strongly on the pristine substrate polymer were effectively patterned by the cell repellent properties of the hyaluronic acid based hydrogel. This is the first time cell adhesion was controlled by patterned functionalization of a polymeric substrate with UV curable PLL-VAHyal in thermoformed 3D microstructures.
    • J. Wald, C. Müller, M. Wahl, W. Hoth-Hannig, M. Hannig, M. Kopnarski, Ch. Ziegler
    • ToF-SIMS investigations of adsorbed proteins on dental Titanium
    • Physica Status Solidi A 207, 831–836 (2010)
    • Abstract >>
    • Two model proteins, bovine serum albumin (BSA) and lysozyme (LSZ), were used to study adsorption behavior on dental titanium by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Protein films were prepared as single component layers and as mixtures of the two proteins. Additionally, the behavior of consecutive adsorption of both proteins was investigated. The obtained data were analyzed by means of principal component analysis (PCA). Dental titanium samples with a layer consisting of simply one protein could be separated from each other by selecting amino acid related peaks and using PCA or samples coated with a mixture of both proteins, the differentiation was also possible. The partial displacement of LSZ molecules by BSA was shown in the case of simultaneous adsorption. The consecutive adsorption of BSA and subsequently LSZ resulted in layers with a bigger amount of LSZ than of BSA compared to the layers obtained by simultaneous adsorption.
    • C. Müller, A. Lüders, W. Hoth-Hannig, M. Hannig, Ch. Ziegler
    • Initial bioadhesion on dental materials as a function of contact time, pH, surface wettability and isoelectric point
    • Langmuir 26, 4136-4141 (2010)
    • Abstract >>
    • The adsorption of bovine serum albumin (BSA) on surfaces of dental enamel and of dental materials was investigated by scanning force spectroscopy. This method provides adhesion forces which can be measured as a function of contact time between protein and surface, pH, wettability, and isoelectric point of the surface. Whereas the chosen ceramic and composite materials resemble very well the adhesion on natural enamel, a much stronger adhesion was found for the more hydrophobic surfaces, that is, gold, titanium, poly(methyl methacrylate) (PMMA), and poly(tetrafluoroethylene) (PTFE). On hydrophilic surfaces, adhesion is mainly influenced by the electrostatic forces between protein and surface. However, the conformational change of BSA at pH values above pH 8 has to be taken into account. On the very hydrophobic PTFE surface, the special interface structure between PTFE and water plays an important role which governs BSA adhesion.
  • 2009
    • Axel Blau, Tanja Neumann, Christiane Ziegler, Fabio Benfenati
    • Replica-molded poly(dimethylsiloxane) culture vessel lids attenuate osmotic drift in long-term cell culturing
    • Journal of Biosciences 34, 1, 59-69 (2009)
    • Abstract >>
    • An imbalance in medium osmolarity is a determinant that affects cell culture longevity. Even in humidified incubators, evaporation of water leads to a gradual increase in osmolarity over time. We present a simple replica-moulding strategy for producing self-sealing lids adaptable to standard, small-size cell-culture vessels. They are made of polydimethylsiloxane (PDMS), a flexible, transparent and biocompatible material, which is gas-permeable but largely impermeable to water. Keeping cell cultures in a humidified 5% CO2 incubator at 37°C, medium osmolarity increased by +6.86 mosmol/kg/day in standard 35 mm Petri dishes, while PDMS lids attenuated its rise by a factor of four to changes of +1.72 mosmol/kg/day. Depending on the lid membrane thickness, pH drifts at ambient CO2 levels were attenuated by a factor of 4 to 9. Comparative evaporation studies at temperatures below 60°C yielded a 10-fold reduced water vapour flux of 1.75 g/day/dm2 through PDMS lids as compared with 18.69 g/day/dm2 with conventional Petri dishes. Using such PDMS lids, about 2/3 of the cell cultures grew longer than 30 days in vitro. Among these, the average survival time was 69 days with the longest survival being 284 days under otherwise conventional cell culture conditions.
    • Petra Schneider, Claudia Steitz, Karl H. Schäfer, Christiane Ziegler
    • Hot embossing of pyramidal micro-structures in PMMA for cell culture
    • Phys. Status Solidi A, 206: 501–507 (2009)
    • Abstract >>
    • For the investigation of the effect of 3-dimensional structures upon cell attachment and growth, polymeric micropyramids with dimensions comparable to the size of cells were fabricated. Moulds for the hot embossing of the polymers were manufactured in silicon by photolithography and wet chemical etching. For an accurate casting the moulds need very smooth surfaces, which were achieved by using KOH/IPA for crystallographic etching. A PTFE-like layer was deposited by a RIE process from C4F8 plasma to form an anti-sticking layer on the mould surfaces to result in an easy release of the embossed substrates. Preparation methods and embossing parameters were established and optimised to emboss pyramidal structures with different base lengths and distances from 1 μm to 40 μm into PMMA. The imprinted polymeric microstructures were characterised with profilometry and SEM. First cell culture experiments were performed using neuronal and epithelial cell types on the micropyramidal surfaces to investigate their influence upon the cell-surface interactions. While both cell types attached nicely on either plane or microstructured PMMA surfaces, especially the neurite outgrowth was influenced by the pyramidal structures. The effect of the pyramidal design upon cell development might be of potential interest for the use in cell culture and tissue engineering.
    • Mirko Cinchetti, Kathrin Heimer, Jan-Peter Wüstenberg, Oleksiy Andreyev, Michael Bauer, Stefan Lach, Christiane Ziegler, Yongli Gao, Martin Aeschlimann
    • Determination of spin injection and transport in a ferromagnet/organic semiconductor heterojunction by two-photon photoemission
    • Nature Materials 8, 115 - 119 (2009)
    • Abstract >>
    • A fundamental prerequisite for the implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Here, we consider a model system consisting of a high-quality interface between the ferromagnet cobalt and the OSC copper phthalocyanine (CuPc). We focus on interfacial effects on spin injection and on the spin transport properties of CuPc. Using spin-resolved two-photon photoemission, we have measured directly and in situ the efficiency of spin injection at the cobalt–CuPc interface. We report a spin injection efficiency of 85–90% for injection into unoccupied molecular orbitals of CuPc. Moreover, we estimate an electron inelastic mean free path in CuPc in the range of 1 nm and a 10–30 times higher quasi-elastic spin-flip length. We demonstrate that quasi-elastic spin-flip processes with energy loss less than or equal to200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.
  • 2008
    • Adam Orendorz, Christiane Ziegler, Hubert Gnaser
    • Photocatalytic decomposition of methylene blue and 4-chlorophenol on nanocrystalline TiO2 films under UV illumination: A ToF-SIMS study
    • Applied Surface Science 255, 1011-1014 (2008)
    • Abstract >>
    • The photocatalytic degradation of methylene blue and 4-chlorophenol on nanocrystalline TiO2 (nc-TiO2) under UV irradiation was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Nanocrystalline TiO2 films were prepared from suspensions containing TiO2-crystallites of different average sizes, the smallest one being 12 nm. The organic substances (either methylene blue or 4-chlorophenol) were applied to these films. The specimens were studied in the pristine state and upon UV exposure. The UV illuminations were carried out both under atmospheric conditions and in situ under ultrahigh vacuum in the ToF-SIMS instrument. Distinct mass signals from the parent molecules and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is significantly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signals are strongly diminished whereas fragmentation products are identified to be present at the TiO2 surfaces. UV irradiations carried out under different vacuum conditions in the ToF instrument (ultrahigh vacuum, air or oxygen adsorption) indicate that varying ambient conditions may influence the photocatalytic reaction on the nanocrystalline TiO2 films.
    • J. Wüsten, S. Berger, M. Salomo, A. Mönnich, M. Bauer, S. Lach, M. Aeschlimann, Ch. Ziegler
    • Hot-electron dynamics in thin films of sodium-doped perylene-3,4,9,10-tetracarboxylic dianhydride
    • Physical Review B 78, 195326 (2008)
    • Abstract >>
    • Time-resolved two-photon photoemission of the doping process of perylene-tetracarboxylicdianhydride (PTCDA) thin films with sodium gives insight into the dynamics of hot electrons in this system. Interaction with Na results in a charge transfer from Na to PTCDA and increases the lifetime of optically excited electrons. A similar increase in lifetime can be observed for ultrathin PTCDA layers on Ag(111). Both results are discussed in the one-electron picture, treated in “classical” photoemission theory, and in the many-electron picture as commonly considered for strongly correlated systems.
    • F. Bernsmann, N. Lawrence, M. Hannig, C. Ziegler, H. Gnaser
    • Protein films adsorbed on experimental dental materials: ToF-SIMS with multivariate data analysis
    • Anal. Bioanal. Chem. 391, 545 (2008)
    • Abstract >>
    • The proteins lysozyme, amylase, and bovine serum albumin (BSA) were adsorbed on two experimental dental materials, made of fluoroapatite particles embedded in polymer matrices, and on silicon wafers. The protein films were prepared as single-component layers, as binary mixtures, and as double layers. These systems were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the multivariate data analysis technique of discriminant principal-component analysis (DPCA). During adsorption of a single protein film on to the solid surfaces, the three proteins could be clearly distinguished by the scores of their mass spectra after selection of amino acid-related peaks and DPCA. Furthermore, very similar results were obtained on the two different fluoroapatite substrates. For samples coated with binary layers of two proteins adsorbed simultaneously, it was found for both substrate types that BSA shows the strongest ability to adsorb followed by lysozyme, while amylase has the smallest ability. By contrast, the consecutive adsorption of two protein layers showed a strong influence of substrate type on the adsorption ability of the proteins.
    • A. Orendorz, C. Ziegler, H. Gnaser
    • Photocatalytic decomposition of methylene blue and 4-chlorophenol on nanocrystalline TiO2 films under UV illumination: A ToF-SIMS study
    • Appl. Surf. Sci. 255, 1011 (2008)
    • Abstract >>
    • The photocatalytic degradation of methylene blue and 4-chlorophenol on nanocrystalline TiO2 (nc-TiO2) under UV irradiation was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Nanocrystalline TiO2 films were prepared from suspensions containing TiO2-crystallites of different average sizes, the smallest one being 12 nm. The organic substances (either methylene blue or 4-chlorophenol) were applied to these films. The specimens were studied in the pristine state and upon UV exposure. The UV illuminations were carried out both under atmospheric conditions and in situ under ultrahigh vacuum in the ToF-SIMS instrument. Distinct mass signals from the parent molecules and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is significantly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signals are strongly diminished whereas fragmentation products are identified to be present at the TiO2 surfaces. UV irradiations carried out under different vacuum conditions in the ToF instrument (ultrahigh vacuum, air or oxygen adsorption) indicate that varying ambient conditions may influence the photocatalytic reaction on the nanocrystalline TiO2 films.
    • Tanja Neumann, Christiane Ziegler, Axel Blau
    • Multielectrode array recordings reveal physiological diversity of intrinsically photosensitive retinal ganglion cells in the chick embryo
    • Brain Research, Volume 1207, Pages 120–127 (2008)
    • Abstract >>
    • Intrinsically photosensitive retinal ganglion cells (ipRGCs) play important roles in non-image forming photoreception and participate in the regulation of the circadian rhythm and the pupillary light reflex. The aim of the present work was to characterize the light response of ipRGCs at two developmental stages of the embryonic chick. The electrophysiological study was based on comparative multielectrode array recordings from acute retinal slices. To ensure that light was the only source of excitation, intercellular activity modulation by gap junctions and chemical synapses was inhibited by carbenoxolone and bafilomycin A1, respectively. Action potentials evoked by blue light were detected as early as day 13 of embryonic development, which is notably earlier than the completion of the maturation process of functional rods and cones. Three different response types were distinguished by their response latency and sensitivity to different illumination intensities. At this point it is not clear whether these types just represent different maturation stages or have different morphologies and functions with respect to the non-image forming visual system and circadian entrainment.
    • Falk Bernsmann, Nicole Lawrence, Matthias Hannig, Christiane Ziegler, Hubert Gnaser
    • Protein films adsorbed on experimental dental materials: ToF-SIMS with multivariate data analysis
    • Analytical and Bioanalytical Chemistry, Volume 391, Number 2, Seiten 545-554 (2008)
    • Abstract >>
    • The proteins lysozyme, amylase, and bovine serum albumin (BSA) were adsorbed on two experimental dental materials, made of fluoroapatite particles embedded in polymer matrices, and on silicon wafers. The protein films were prepared as single-component layers, as binary mixtures, and as double layers. These systems were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the multivariate data analysis technique of discriminant principal-component analysis (DPCA). During adsorption of a single protein film on to the solid surfaces, the three proteins could be clearly distinguished by the scores of their mass spectra after selection of amino acid-related peaks and DPCA. Furthermore, very similar results were obtained on the two different fluoroapatite substrates. For samples coated with binary layers of two proteins adsorbed simultaneously, it was found for both substrate types that BSA shows the strongest ability to adsorb followed by lysozyme, while amylase has the smallest ability. By contrast, the consecutive adsorption of two protein layers showed a strong influence of substrate type on the adsorption ability of the proteins.
  • 2007
    • A. Orendorz, A. Brodyanski, J. Lösch, L.H. Bai, Z.H. Chen, Y.K. Le, Ch. Ziegler, H. Gnaser
    • Phase Transformation and Particle Growth in Nanocrystalline Anatase TiO2 Films Analyzed by XRD and Raman Spectroscopy
    • Surface Science, Volume 601, Issue 18, Pages 4390–4394 (2007)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films were prepared from colloidal suspensions using particles with a nominal size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO2 (e.g., the Eg, B1g and A1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ∼13 nm to ∼125 nm between 470 K and 1220 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO2 occurs gradually in the temperature range 1220–1320 K and the average crystallite size increases to ∼160 nm.
    • S. Hafizovic, F. Heer, T. Ugniwenko, U. Frey, A. Blau, C. Ziegler, A. Hierlemann
    • A CMOS-based microelectrode array for interaction with neuronal cultures
    • Journal of Neuroscience Methods, Volume 164, Issue 1, Pages 93–106 (2007)
    • Abstract >>
    • We report on the system integration of a CMOS chip that is capable of bidirectionally communicating (stimulation and recording) with electrogenic cells such as neurons or cardiomyocytes and that is targeted at investigating electrical signal propagation within cellular networks in vitro. The overall system consists of three major subunits: first, the core component is a 6.5 mm × 6.5 mm CMOS chip, on top of which the cells are cultured. It features 128 bidirectional electrodes, each equipped with dedicated analog filters and amplification stages and a stimulation buffer. The electrodes are sampled at 20 kHz with 8-bit resolution. The measured input-referred circuitry noise is 5.9 μV root mean square (10 Hz to 100 kHz), which allows to reliably detect the cell signals ranging from 1 mVpp down to 40 μVpp. Additionally, temperature sensors, a digital-to-analog converter for stimulation, and a digital interface for data transmission are integrated. Second, there is a reconfigurable logic device, which provides chip control, event detection, data buffering and an USB interface, capable of processing the 2.56 million samples per second. The third element includes software that is running on a standard PC performing data capturing, processing, and visualization. Experiments involving the stimulation of neurons with two different spatio-temporal patterns and the recording of the triggered spiking activity have been carried out. The response patterns have been successfully classified (83% correct) with respect to the different stimulation patterns. The advantages over current microelectrode arrays, as has been demonstrated in the experiments, include the capability to stimulate (voltage stimulation, 8 bit, 60 kHz) spatio-temporal patterns on arbitrary sets of electrodes and the fast stimulation reset mechanism that allows to record neuronal signals on a stimulating electrode 5 ms after stimulation (instantaneously on all other electrodes). Other advantages of the overall system include the small number of needed electrical connections due to the digital interface and the short latency time that allows to initiate a stimulation less than 2 ms after the detection of an action potential in closed-loop configurations.
    • J. Wüsten. K. Heimer, S. Lach, Ch. Ziegler
    • Alkali Metals in Perylene-3,4,9,10-Tetracarboxylicdianhydride Thin Films
    • J. Appl. Phys. 102, 023708 (2007)
    • Abstract >>
    • n-type doping of the molecular organic semiconductor perylene-3,4,9,10-tetracarboxylicdianhydride (PTCDA) by sodium, potassium, and cesium was carried out. The chemical properties of the doping processes were investigated by means of x-ray photoemission and infrared absorption spectroscopy. Simultaneously the evolution of the occupied electronic states around the transport gap was monitored by ultraviolet photoemission spectroscopy. It was found that the doping ratio depends on the ionization energy of the alkali metal, in particular if compared with the highest occupied molecular orbital ionization energy of the formed alkali-PTCDA complex. Additionally, only in the case of cesium doping, an averaged ratio of two alkali metal atoms per PTCDA was found at the surface. In the case of sodium and potassium, averaged surface doping ratios of only 1.3±0.1 alkali metal atoms per PTCDA molecule can be reached. However, in the bulk phase, nearly complete doping can be reached by all three alkali metals.
    • M. Herrscher, Ch. Ziegler, D. Johannsmann
    • Shifts of frequency and bandwidth of quartz crystal resonators coated with samples of finite lateral size
    • J. Appl. Phys. 101, 114909 (2007)
    • Abstract >>
    • Recently, the viscoelastic properties of polymeric materials were probed by pushing a hemispherical cap of the respective material against the front surface of a quartz crystal resonator and measuring the induced shift of frequency and bandwidth, Δf and ΔΓ, as a function of the contact area, Ac.1 The shift of the resonance parameters was found to be proportional to the contact area, with the constant of proportionality containing the sample’s shear modulus. Confining the contact area to a small spot in the center of the plate is central to this approach, because the resonator would otherwise be overdamped. However, more detailed experiments have shown that there are small deviations from the proportionality of the frequency shift to the contact area. In particular, the ratio of ΔΓ and Δf (the “D-f ratio”), which should reflect intrinsic material properties, was found to slightly depend on contact area. Employing a finite element method simulation, two hypotheses were tested for the nontrivial area dependence of Δf and ΔΓ, which are, first, scattering of the acoustic wave from the acoustically heterogeneous surface and, second, a change of the lateral amplitude distribution, (“energy trapping”) induced by loading the crystal in the center only. It can be concluded that scattering affects the D-f ratio only at very small contact radii (<10 μm), whereas in the range of millimeter-sized contacts, the change of energy trapping dominates. Employing a perturbation analysis, a relation of the form Δf ≈ αAc(1+βAc) is found, where α contains the viscoelastic parameters and the term in brackets is the nontrivial correction. Such a linear dependence agrees well with the results from simulation and experiment. An improved estimate of the material’s shear modulus is obtained by analyzing the ratios Δf/Ac and ΔΓ/Ac as a function of contact area Ac, and extrapolating these values to the limit Ac→0.
    • A. Orendorz, A. Brodyanski, J. Lösch, L.H. Bai, Z.H. Chen, Y.K. Le, C. Ziegler, H. Gnaser
    • Structural investigation of pristine and annealed nanocrystalline TiO2 thin films by X-ray diffraction and Raman spectroscopy
    • Physica Status Solidi (c) 4, 1822 (2007)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films were prepared from colloidal suspensions using particles with a nominal size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO2 (e.g., the Eg, B1g and A1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ∼13 nm to ∼125 nm between 470 K and 1220 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO2 occurs gradually in the temperature range 1220 – 1320 K and the average crystallite size increases to ∼160 nm. The Eg vibration in anatase films reveals a blue shift and an asymmetric peak broadening towards higher frequencies for smaller particle size/lower annealing temperatures. A power-law dependence between the Raman peak width and the inverse of the crystallite size is found.
    • J. Sauther, J.Wüsten, S. Lach, Ch. Ziegler
    • Gas phase and bulk photoemission spectra of highest occupied molecular orbitals of pi-conjugated organic molecules
    • physica status solidi (c), Volume 4, Issue 6, pages 1844–1851 (2007)
    • Abstract >>
    • The highest occupied molecular orbital (HOMO) of the pi-conjugated organic molecule 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) was investigated via bulk and gas phase ultraviolet photoemission spectroscopy. As a result of the absence of various photoemission linewidth broadening effects which occur in the case of condensed matter, the measurements on single molecules allow an estimate of the contributions of these effects to the total linewidth. Distinctive peaks resulting from vibrational excitations can be recognized and thus a comparison with other spectroscopy methods is possible. Furthermore, spectra of the HOMOs of 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) and 1,8-naphthalene-dicarboxylic anhydride (NDCA) were also recorded and compared to the results for PTCDA, showing a decrease in linewidth with decreasing molecule size.
    • F. Heer, S. Hafizovic, T. Ugniwenko, U. Frey, W. Franks, E. Perriard, J.-C. Perriard, A. Blau, C. Ziegler, A. Hierlemann
    • Single-chip microelectronic system to interface with living cells
    • Biosensors and Bioelectronics, Volume 22, Issue 11, Pages 2546–2553 (2007)
    • Abstract >>
    • A high degree of connectivity and the coordinated electrical activity of neural cells or networks are believed to be the reason that the brain is capable of highly sophisticated information processing. Likewise, the effectiveness of an animal heart largely depends on such coordinated cell activity. To advance our understanding of these complex biological systems, high spatiotemporal-resolution techniques to monitor the cell electrical activity and an ideally seamless interaction between cells and recording devices are desired.Here we present a monolithic microsystem in complementary metal oxide semiconductor (CMOS) technology that provides bidirectional communication (stimulation and recording) between standard electronics technology and cultured electrogenic cells. The microchip can be directly used as a substrate for cell culturing, it features circuitry units per electrode for stimulation and immediate cell signal treatment, and it provides on-chip signal transformation as well as a digital interface so that a very fast, almost real-time interaction (2 ms loop time from event recognition to, e.g., a defined stimulation) is possible at remarkable signal quality. The corresponding spontaneous and stimulated electrical activity recordings with neuronal and cardiac cell cultures will be presented.The system can be used to, e.g., study the development of neural networks, reveal the effects of neuronal plasticity and study cellular or network activity in response to pharmacological treatments.
    • A. Orendorz, A. Brodyanski, J. Lösch, L.H. Bai, Z.H. Chen, Y.K. Le, C. Ziegler, H. Gnaser
    • Phase transformation and particle growth in nanocrystalline anatase TiO2 films analyzed by X-ray diffraction and Raman spectroscopy
    • Surf. Sci. 601, 4390 (2007)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films were prepared from colloidal suspensions using particles with a nominal size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO2 (e.g., the Eg, B1g and A1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ˜13 nm to ˜125 nm between 470 K and 1220 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO2 occurs gradually in the temperature range 1220 1320 K and the average crystallite size increases to ˜160 nm.
    • A. Orendorz, A. Brodyanski, J. Lösch, L. H. Bai, Z. H. Chen, Y. K. Le, C. Ziegler, H. Gnaser
    • Structural investigation of pristine and annealed nanocrystalline TiO2 thin films by X-ray diffraction and Raman spectroscopy
    • physica status solidi (c), Volume 4, Issue 6, pages 1822–1829 (2007)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films were prepared from colloidal suspensions using particles with a nominal size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO2 (e.g., the Eg, B1g and A1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ∼13 nm to ∼125 nm between 470 K and 1220 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO2 occurs gradually in the temperature range 1220 – 1320 K and the average crystallite size increases to ∼160 nm. The Eg vibration in anatase films reveals a blue shift and an asymmetric peak broadening towards higher frequencies for smaller particle size/lower annealing temperatures. A power-law dependence between the Raman peak width and the inverse of the crystallite size is found.
    • K. Heimer, J. Wüsten, S. Lach, and Ch. Ziegler
    • Interaction of alkali metals with perylene-3,4,9,10-tetracarboxylicdianhydride thin films studied by IR spectroscopy
    • J. Chem. Phys. 126, 164709 (2007)
    • Abstract >>
    • In order to clarify the doping behavior of different alkali metals in perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), Fourier transform infrared spectra of PTCDA thin films doped with sodium, potassium, and cesium were measured and compared. Furthermore the vibrational properties were calculated using density-functional theory and these calculated vibrational frequencies were assigned to the experimental IR modes of the thin films.
  • 2006
    • J. Wüsten, Th. Ertl. Ch. Ziegler
    • Electron transport in pristine and alkali metal doped perylene-3,4,9,10- tetracarboxylicdianhydride (PTCDA) thin films
    • Phys. Rev. B 74, 125205 (2006)
    • Abstract >>
    • Time-resolved two-photon photoemission of the doping process of perylene-tetracarboxylicdianhydride (PTCDA) thin films with sodium gives insight into the dynamics of hot electrons in this system. Interaction with Na results in a charge transfer from Na to PTCDA and increases the lifetime of optically excited electrons. A similar increase in lifetime can be observed for ultrathin PTCDA layers on Ag(111). Both results are discussed in the one-electron picture, treated in “classical” photoemission theory, and in the many-electron picture as commonly considered for strongly correlated systems.
    • A. Orendorz, A. Brodyanski, J. Lösch, L.H. Bai, Z.H. Chen, Y.K. Le, C. Ziegler, H. Gnaser
    • Structural transformations in nanocrystalline anatase TiO2 films upon annealing in air
    • Surface Science, Volume 600, Issue 18, Pages 4347–4351 (2006)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films were prepared from colloidal suspensions using particles with a size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO2 (e.g., the Eg, B1g and A1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ∼13 nm to ∼90 nm between 720 K and 1170 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO2 occurs in the temperature range 1170–1320 K and the average crystallite size increases to ∼160 nm.
    • D. Then, A. Vidic, Ch. Ziegler
    • A highly sensitive self-oscillating cantilever array for the quantitative and qualitative analysis of organic vapor mixtures
    • Sensors and Actuators B: Chemical, Volume 117, Issue 1, Pages 1–9 (2006)
    • Abstract >>
    • A setup for gas and liquid sensing applications was developed and operated which is based on both the frequency shifts and the bending of microcantilevers. To drive the cantilevers an electrostatic and magnetic actuation was developed with a closed feed-back loop which forces the cantilever to oscillate always at its resonance frequency. The oscillation is detected via the beam-deflection technique. By measuring the dc-signal of the photodiode the static bending of the cantilever can be monitored simultaneously. The closed feed-back loop propagates a very stable oscillation at the resonance frequency and gives a strong increase of the quality factor compared to the system without this feed-back loop.For the highlysensitiveidentification of gases and organicvapors the cantilevers are functionalized by polymer coatings to detect specific chemical interactions. The polymer coatings begin to swell after exposure to analyte vapor. Additionally to the mass increase, a change of surface stress between the coating and the cantilever surface occurs, resulting in a bending of the cantilever. With this setup it is possible to measure concentrations of analytes in the lower ppm-range. The data evaluation was done by multivariate data analysis. The sensor responses were used in principal component regression (partial least squares, PLS) for the qualitative and quantitative analysis and prediction of vapors under investigation. The results obtained with different volatile organic compounds (VOCs) are presented to introduce this promising approach for easy-to-use and highlysensitive sensor systems.
    • Hubert Gnaser, Adam Orendorz, Christiane Ziegler, Elisabeth Rowlett, Wolfgang Bock
    • TOF-SIMS study of photocatalytic decomposition reactions on nanocrystalline TiO2 films
    • Applied Surface Science, Volume 252, Issue 19, Pages 6996–6999 (2006)
    • Abstract >>
    • Nanocrystalline TiO2 is known to be a very efficient photocatalyst. In order to elucidate the details of reaction pathways occurring on the surface, nanocrystalline TiO2 films (with 12 nm average crystallite size) were covered by methylene blue (MB) and studied, both in the pristine state and upon UV exposure, by TOF-SIMS. Distinct mass signals related to the MB parent molecule (m = 284.1 amu) and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is pronouncedly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the amount of the parent molecule is diminished whereas intermediate reaction products are identified to be present at the TiO2 surfaces. Eventually, the parent molecule and the characteristic fragment species disappear completely upon extended exposure to UV light.
    • F. Heer, S. Hafizovic, W. Franks, A. Blau, Ch. Ziegler, A. Hierlemann
    • CMOS Microelectrode Array for Bidirectional Interaction With Neuronal Networks
    • IEEE Journal of Solid-State Circuits 41, Number 7, 1620-1629 (2006)
    • Abstract >>
    • A CMOS metal-electrode-based micro system for bidirectional communication (stimulation and recording) with neuronal cells in vitro is presented. The chip overcomes the interconnect challenge that limits today's bidirectional microelectrode arrays. The microsystem has been fabricated in an industrial CMOS technology with several post-CMOS processing steps to realize 128 biocompatible electrodes and to ensure chip stability in physiological saline. The system comprises all necessary control circuitry and on-chip A/D and D/A conversion. A modular design has been implemented, where individual stimulation- and signal-conditioning circuitry units are associated with each electrode. Stimulation signals with a resolution of 8 bits can be sent to any subset of electrodes at a rate of 60 kHz, while all electrodes of the chip are continuously sampled at a rate of 20 kHz. The circuitry at each electrode can be individually reset to its operating point in order to suppress artifacts evoked by the stimulation pulses. Biological measurements from cultured neuronal networks originating from dissociated cortical tissue of fertilized chicken eggs with amplitudes of up to 500 muVpp are presented
    • S. Hafizovic, F. Heer, W. Franks, F. Greve, A. Blau, C. Ziegler, A. Hierlemann
    • CMOS Bidirectional Electrode Array for Electrogenic Cells
    • Proc. IEEE-MEMS 2006, Istanbul, Turkey (2006)
    • Abstract >>
    • We report on a CMOS-based microelectrode-array chip (6.5 by 6.5 mm2) for bidirectional communication (stimulation and recording) with electrogenic cells such as cardiomyocytes or neurons targeted at investigating electrical signal propagation within cellular networks in vitro. The integration of on-chip circuitry, which includes analog signal amplification and filtering stages, analog-to-digital converters, a digital-to-analog converter, stimulation buffers, temperature sensors, and a digital interface for data transmission notably improves the overall system performance. Additionally, the interconnect challenge that limits the size of currently used microelectrode arrays is overcome. Measurements with cardiomyocytes and neuronal cells were successfully carried out, and the circuitry characterization evidenced a total equivalent input noise of 11.7 µ VRMS(0.1 Hz -100 kHz) at a gain of 1,000.
    • A. Orendorz, A. Brodyanski, J. Lösch, L.H. Bai, Z.H. Chen, Y.K. Le, C. Ziegler, H. Gnaser
    • Structural transformations in nanocrystalline anatase TiO2 films upon annealing in air
    • Surf. Sci. 600, 4347 (2006)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films were prepared from colloidal suspensions using particles with a size of 12 nm. Their structure was examined by Raman spectroscopy and X-ray diffraction (XRD). The as-prepared specimens exhibit exclusively features due to the anatase phase of TiO2 (e.g., the Eg, B1g and A1g vibration modes in Raman spectroscopy and the characteristic diffraction peaks in XRD). Isochronal annealing of the films in air at temperatures of up to 1320 K effected clear structural changes, observed both in Raman and XRD: the crystallite size increases from ˜13 nm to ˜90 nm between 720 K and 1170 K, with the crystallites remaining in the anatase phase. A phase transition to the rutile phase of TiO2 occurs in the temperature range 1170 1320 K and the average crystallite size increases to ˜160 nm.
    • H. Gnaser, A. Orendorz, C. Ziegler, E. Rowlett, W. Bock
    • TOF-SIMS study of photocatalytic decomposition reactions on nanocrystalline TiO2 films
    • Appl. Surf. Sci. 252, 6996 (2006)
    • Abstract >>
    • Nanocrystalline TiO2 is known to be a very efficient photocatalyst. In order to elucidate the details of reaction pathways occurring on the surface, nanocrystalline TiO2 films (with 12 nm average crystallite size) were covered by methylene blue (MB) and studied, both in the pristine state and upon UV exposure, by TOF-SIMS. Distinct mass signals related to the MB parent molecule (m = 284.1 amu) and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is pronouncedly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the amount of the parent molecule is diminished whereas intermediate reaction products are identified to be present at the TiO2 surfaces. Eventually, the parent molecule and the characteristic fragment species disappear completely upon extended exposure to UV light.
  • 2005
    • Yong Men, Hubert Gnaser, Christiane Ziegler, Ralf Zapf, Volker Hessel, Gunther Kolb
    • Characterization of Cu/CeO2/γ-Al2O3 Thin Film Catalysts by Thermal Desorption Spectroscopy
    • Catalysis Letters, Volume 105, Numbers 1-2 (2005)
    • Abstract >>
    • Thermal desorption spectroscopy (TDS) under ultra high vacuum (UHV) condition has been used to investigate the desorption characteristics of Cu/CeO2/γ-Al2O3 thin film catalysts coated onto the microchannel of a microreactor. TDS results demonstrate that surface desorption profiles and chemical properties (acid–base and redox properties) are remarkably influenced by the catalyst composition, i.e. the loading of copper and ceria. The enhanced basicity with the increase of ceria loading and the decrease of copper loading is evident from the shifted desorption maximum of CO2 in TDS spectra. Three oxygen species, ranging from weakly bound oxygen desorbed at low temperature to the strongly held lattice oxygen desorbed at high temperature, are easily discernible and clearly identified by O2 TDS spectra, depending on the catalyst compositions. The concomitant thermal desorption of O2, CO2, and H2O at low temperature indicates the unique chemical properties of copper/ceria catalyst with appropriate copper and ceria contents. The observed low-temperature feature is ascribed to the role of porthole of copper/ceria interfacial area for several desorbed species. The weakly bound oxygen species is attributed to the enhanced abundance of copper/ceria interfacial anionic vacancies created by the intimate contact between copper and ceria entities and its impact on steam reforming of methanol (SRM) reaction is tentatively discussed in terms of reverse oxygen spillover.
    • J. Wüsten, Th. Ertl, S. Lach, Ch. Ziegler
    • Post deposition purification of PTCDA thin films
    • Applied Surface Science, Volume 252, Issue 1, Pages 104–107 (2005)
    • Abstract >>
    • The decomposition of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules during evaporation of unpurified raw material in ultra high vacuum was studied. The fragments were identified by mass spectrometry and the influence of these fragments and further contaminations of the raw material on the electronic structure of PTCDA thin films was measured by photoemission spectroscopy. Annealing of contaminated PTCDA films was tested as cheap and easy to perform method for (partial) post deposition purification of the contaminated films.
    • Steffen Berger, Kathrin Heimer, H.G. Mack, Christiane Ziegler
    • IR and SFM study of PTCDA thin films on different substrates
    • Applied Surface Science, Volume 252, Issue 1, Pages 81–84 (2005)
    • Abstract >>
    • FT-IR spectroscopy and SFM were used to investigate the growth of thin films of the organic semiconductor 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) deposited by vacuum sublimation onto various substrates, i.e. Ag(111) layers on mica, KBr(100), mica, oxidized Si, and TiO2 nanoparticles on Si. Layer thicknesses of PTCDA varied from 10 to 1500 nm.The anhydride vibrations of PTCDA differ for the used substrates, which can be connected to the orientation of the molecules relative to the substrate surface and the film morphology as detected in the SFM pictures.
    • N. Schwender, K. Huber, F. Al Marrawi, M. Hannig, Ch. Ziegler
    • Initial bioadhesion on surfaces in the oral cavity investigated by scanning force microscopy
    • Applied Surface Science, Volume 252, Issue 1, Pages 117–122 (2005)
    • Abstract >>
    • Scanning force microscopy (SFM) was used to measure the adhesion forces between BSA, a saliva protein, and two dental surfaces, natural enamel and a filling material (Dyract AP™). Measurements were taken in phosphate buffered aqueous solutions (PBS). Forces were resolved down to the piconewton regime. The dependency of the adhesion force on the interaction time, pH-value and substrate surface was monitored. In a further step, surface samples were fixed on an enamel brace and carried for a defined time in the oral cavity. The formed biofilm, called pellicle, shows a different morphology on the different substrates. This can be explained by the above-mentioned substrate dependence of the adhesion force.
    • Adam Orendorz, Jens Wüsten, Christiane Ziegler, Hubert Gnaser
    • Photoelectron spectroscopy of nanocrystalline anatase TiO2 films
    • Applied Surface Science, Volume 252, Issue 1, Pages 85–88 (2005)
    • Abstract >>
    • Nanocrystalline TiO2 (anatase) films were prepared using either colloidal suspensions or a sol–gel route. The electronic structure of these films was analyzed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Apart from pristine films, films containing defects introduced by annealing under ultra-high vacuum conditions or by ion bombardment were investigated. Generally, annealing in the temperature range up to 720 K results in no significant changes in the XPS and UPS spectra as compared to the pristine state, indicating that the amount of defect formation is too low to be observable by these techniques. On the other hand, ion irradiation causes the appearance of distinct defect states; these could be identified in agreement with previous data from photoemission studies on rutile and anatase single crystals. From UPS, a valence-band width of ∼4.6 eV was determined for the nanocrystalline anatase films.
    • J. Wüsten, S. Berger, K. Heimer, S. Lach, Ch. Ziegler
    • Interaction of alkali metals with perylene-3,4,9,10- tetracarboxylic–dianhydride thin films
    • J. Appl. Phys. 98, 013705 (2005)
    • Abstract >>
    • n doping of the molecular organic semiconductor perylene-3,4,9,10-tetracarboxylic–dianhydride (PTCDA) is often achieved by use of alkali metals as dopants. This doping process is commonly performed in two steps. In the first the dopant is evaporated onto the surface of the PTCDA film. As it has been believed that the dopant shows an inhomogeneous diffusion profile through the layer with most of the dopant accumulated in the first few layers, a subsequent annealing step has been performed in order to reach a homogeneous distribution of the dopant in the whole layer. In this paper experimental results concerning chemical composition ((angle resolved) X-ray photoemission spectroscopy, secondary-ion-mass spectrometry, Fourier transform infrared spectroscopy), electronic structure (ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy), as well as electrical properties (conductivity, Seebeck coefficient) are shown before and after doping and before and after annealing. These results suggest that the deposited dopant is redistributed and partially removed during the annealing step. A model for the dopant distribution is suggested.
    • Y. Men, H. Gnaser, C. Ziegler, R. Zapf, V. Hessel, G. Kolb
    • Characterization of Cu/CeO2/γ-Al2O3 Thin Film Catalysts by Thermal Desorption Spectroscopy
    • Catal. Lett. 105, 35 (2005)
    • Abstract >>
    • Thermal desorption spectroscopy (TDS) under ultra high vacuum (UHV) condition has been used to investigate the desorption characteristics of Cu/CeO2/γ-Al2O3 thin film catalysts coated onto the microchannel of a microreactor. TDS results demonstrate that surface desorption profiles and chemical properties (acid–base and redox properties) are remarkably influenced by the catalyst composition, i.e. the loading of copper and ceria. The enhanced basicity with the increase of ceria loading and the decrease of copper loading is evident from the shifted desorption maximum of CO2 in TDS spectra. Three oxygen species, ranging from weakly bound oxygen desorbed at low temperature to the strongly held lattice oxygen desorbed at high temperature, are easily discernible and clearly identified by O2 TDS spectra, depending on the catalyst compositions. The concomitant thermal desorption of O2, CO2, and H2O at low temperature indicates the unique chemical properties of copper/ceria catalyst with appropriate copper and ceria contents. The observed low-temperature feature is ascribed to the role of porthole of copper/ceria interfacial area for several desorbed species. The weakly bound oxygen species is attributed to the enhanced abundance of copper/ceria interfacial anionic vacancies created by the intimate contact between copper and ceria entities and its impact on steam reforming of methanol (SRM) reaction is tentatively discussed in terms of reverse oxygen spillover.
    • A. Orendorz, J. Wüsten, C. Ziegler, H. Gnaser
    • Photoelectron spectroscopy of nanocrystalline anatase TiO2 films
    • Appl. Surf. Sci. 252, 85 (2005)
    • Abstract >>
    • Nanocrystalline TiO2 (anatase) films were prepared using either colloidal suspensions or a sol gel route. The electronic structure of these films was analyzed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Apart from pristine films, films containing defects introduced by annealing under ultra-high vacuum conditions or by ion bombardment were investigated. Generally, annealing in the temperature range up to 720 K results in no significant changes in the XPS and UPS spectra as compared to the pristine state, indicating that the amount of defect formation is too low to be observable by these techniques. On the other hand, ion irradiation causes the appearance of distinct defect states; these could be identified in agreement with previous data from photoemission studies on rutile and anatase single crystals. From UPS, a valence-band width of ˜4.6 eV was determined for the nanocrystalline anatase films.
    • B. Huber, A. Brodyanski, M. Scheib, A. Orendorz, C. Ziegler, H. Gnaser
    • Nanocrystalline anatase TiO2 thin films: preparation and crystallite size-dependent properties
    • Thin Solid Films 472, 114 (2005)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films with crystallites of either of three different sizes (nominally 6, 12, and 20 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These suspensions were painted onto the respective substrates, dried in air and then calcined at 720 K in air for 1 h; the resulting films have a whitish appearance and are several μm thick. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. X-ray diffraction (XRD) and cross-section transmission electron microscopy (TEM) data demonstrated that two of the films are solely composed of the anatase phase whereas in the third also the brookite phase of TiO2 might be present. The average crystallite size was derived from the width of the XRD diffraction peaks and was found to agree within ∼5% with the nominal size. The elemental composition and the amount of impurities in the different films was investigated by secondary-ion mass spectrometry. Whereas the Ti/O concentration is constant throughout all films, the presence of contaminants is substrate-dependent: while in films deposited on silicon or sapphire the atomic fraction of impurities (most prominently Na and K) is in the range of ∼10−4, for glass substrates an about 100 times higher level of those species is observed, probably the result of outdiffusion during the calcination step or during the reduction of the samples.A detailed study of the influence of the sample temperature T and the ambient oxygen pressure p(O2) on the DC electrical conductivity σ of the films was performed. For all films, a power-law dependence of σ on p(O2), σ∝p(O2)−n, was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=470 K, n amounts to 1.31, 1.15 and 0.56 for the 6-, 12-, and 20-nm films, respectively. For reduced films, an exponential dependence of σ on T−1 was determined, yielding activation energies EA with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes. Impedance spectroscopy was utilized to determine the frequency-dependent complex resistance of the various films. In all cases, a pure RC behavior was found: whereas the magnitude of R strongly depends on the crystallite size, the sample temperature and the ambient oxygen pressure, C exhibits little variations and falls in the range ∼18 to ∼25 pF. Using thermal desorption mass spectrometry, distinct desorption peaks for O2 species were observed which shift to higher temperature values with increasing heating rate. An activation energy for that desorption process of ∼0.9 eV was obtained thereof for a 6-nm nanocrystalline film.
    • Bernd Huber, Alexander Brodyanski, Michael Scheib, Adam Orendorz, Christiane Ziegler, Hubert Gnaser
    • Nanocrystalline anatase TiO2 thin films: preparation and crystallite size-dependent properties
    • Thin Solid Films, Volume 472, Issues 1–2, Pages 114–124 (2005)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films with crystallites of either of three different sizes (nominally 6, 12, and 20 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These suspensions were painted onto the respective substrates, dried in air and then calcined at 720 K in air for 1 h; the resulting films have a whitish appearance and are several μm thick. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. X-ray diffraction (XRD) and cross-section transmission electron microscopy (TEM) data demonstrated that two of the films are solely composed of the anatase phase whereas in the third also the brookite phase of TiO2 might be present. The average crystallite size was derived from the width of the XRD diffraction peaks and was found to agree within ∼5% with the nominal size. The elemental composition and the amount of impurities in the different films was investigated by secondary-ion mass spectrometry. Whereas the Ti/O concentration is constant throughout all films, the presence of contaminants is substrate-dependent: while in films deposited on silicon or sapphire the atomic fraction of impurities (most prominently Na and K) is in the range of ∼10−4, for glass substrates an about 100 times higher level of those species is observed, probably the result of outdiffusion during the calcination step or during the reduction of the samples.A detailed study of the influence of the sample temperature T and the ambient oxygen pressure p(O2) on the DC electrical conductivity σ of the films was performed. For all films, a power-law dependence of σ on p(O2), σ∝p(O2)−n, was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=470 K, n amounts to 1.31, 1.15 and 0.56 for the 6-, 12-, and 20-nm films, respectively. For reduced films, an exponential dependence of σ on T−1 was determined, yielding activation energies EA with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes. Impedance spectroscopy was utilized to determine the frequency-dependent complex resistance of the various films. In all cases, a pure RC behavior was found: whereas the magnitude of R strongly depends on the crystallite size, the sample temperature and the ambient oxygen pressure, C exhibits little variations and falls in the range ∼18 to ∼25 pF. Using thermal desorption mass spectrometry, distinct desorption peaks for O2 species were observed which shift to higher temperature values with increasing heating rate. An activation energy for that desorption process of ∼0.9 eV was obtained thereof for a 6-nm nanocrystalline film.
  • 2004
    • Yong Men, Hubert Gnaser, Ralf Zapf, Volker Hessel, Christiane Ziegler, Gunther Kolb
    • Steam reforming of methanol over Cu/CeO2/γ-Al2O3 catalysts in a microchannel reactor
    • Applied Catalysis A: General, Volume 277, Issues 1–2, Pages 83–90 (2004)
    • Abstract >>
    • The activity behavior of steam reforming of methanol was studied over Cu/CeO2/γ-Al2O3 catalysts with different compositions in a parallelized 10-channel microreactor. It was found that the catalytic performance is strongly influenced by the catalyst compositions, i.e. copper and ceria loading. Cu/CeO2/γ-Al2O3 catalyst thin films before and after use in the microreactor were extensively investigated by means of surface analysis techniques like X-ray photoelectron spectroscopy, energy dispersive analysis of X-rays, and thermal desorption spectrometry. The parallel screening result is tentatively discussed on the basis of the surface analysis characterization results and previous proposals. The rationalized correlation of the catalytic activity dependence with the characteristics is in line with a reaction mechanism, which supposes that the copper/ceria boundary is the active site for steam reforming of methanol and the oxygen reverse spillover from ceria to copper is involved in the catalysis cycle.
    • Yong Men, Hubert Gnaser, Ralf Zapf, Volker Hessel, Christiane Ziegler
    • Parallel screening of Cu/CeO2/γ-Al2O3 catalysts for steam reforming of methanol in a 10-channel micro-structured reactor
    • Catalysis Communications, Volume 5, Issue 11, Pages 671–675 (2004)
    • Abstract >>
    • Steam reforming of methanol (SRM) was investigated over Cu/CeO2/γ-Al2O3 catalysts with different compositions in a parallelized 10-channel micro-structured reactor. The catalytic activity was found to be strongly dependent on the copper loading. The parallel screening result was tentatively discussed with surface analysis characterization results and previous proposals. A reaction mechanism is proposed to rationalize the catalytic activity data and characteristics of the catalysts, which supposes that the copper/ceria interfacial area (partially oxidized copper nanoparticle and defective ceria) is the active site for SRM. The oxygen reverse spillover from ceria to copper is suggested to be involved in the catalysis cycle.
    • F. Heer, W. Franks, A. Blau, S. Taschini, C. Ziegler, A. Hierlemann, H. Baltes
    • CMOS microelectrode array for the monitoring of electrogenic cells
    • Biosensors and Bioelectronics, Volume 20, Issue 2, Pages 358–366 (2004)
    • Abstract >>
    • Signal degradation and an array size dictated by the number of available interconnects are the two main limitations inherent to standalone microelectrode arrays (MEAs). A new biochip consisting of an array of microelectrodes with fully-integrated analog and digital circuitry realized in an industrial CMOS process addresses these issues. The device is capable of on-chip signal filtering for improved signal-to-noise ratio (SNR), on-chip analog and digital conversion, and multiplexing, thereby facilitating simultaneous stimulation and recording of electrogenic cell activity. The designed electrode pitch of 250 μm significantly limits the space available for circuitry: a repeated unit of circuitry associated with each electrode comprises a stimulation buffer and a bandpass filter for readout. The bandpass filter has corner frequencies of 100 Hz and 50 kHz, and a gain of 1000. Stimulation voltages are generated from an 8-bit digital signal and converted to an analog signal at a frequency of 120 kHz. Functionality of the read-out circuitry is demonstrated by the measurement of cardiomyocyte activity. The microelectrode is realized in a shifted design for flexibility and biocompatibility. Several microelectrode materials (platinum, platinum black and titanium nitride) have been electrically characterized. An equivalent circuit model, where each parameter represents a macroscopic physical quantity contributing to the interface impedance, has been successfully fitted to experimental results.
    • Bernd Huber, Hubert Gnaser, Christiane Ziegler
    • Electrical properties of nanocrystalline anatase TiO2 thin films with different crystallite size
    • Surface Science, Volumes 566–568, Part 1, Pages 419–424 (2004)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films with crystallites of three different sizes (approximately 6, 12, and 18 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. A detailed study of the influence of sample temperature T and the ambient oxygen pressure p(O2) on the electrical conductivity σ of the films was performed. For all films a power-law dependence of σ on p(O2), σ∝p(O2)n, was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=200 °C, n amounts to −1.31, −1.15 and −0.56 for the 6-, 12-, and 18-nm films, respectively. For reduced films, an exponential dependence of σ on T−1 was determined, yielding activation energies EA with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes.
    • Hubert Gnaser, Wolfgang Bock, Elisabeth Rowlett, Yong Men, Christiane Ziegler, Ralf Zapf, Volker Hessel
    • Secondary-ion mass spectrometry (SIMS) analysis of catalyst coatings used in microreactors
    • Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Volumes 219–220, Pages 880–885 (2004)
    • Abstract >>
    • Secondary-ion mass spectrometry (SIMS) was used for the characterization of the catalytically active parts of a microreactor, both before and after being used in the reactor. The catalyst coatings for the microreactor were prepared in a standard way: nanoporous washcoats were deposited on the microstructures by a particle route and the active components were applied by wet impregnation using aqueous solutions of metal nitrates. On two specific catalyst systems employed for methanol steam reforming (CuO/Cr2O3/γ-Al2O3 and CuO/CeO2/γ-Al2O3) the distributions of the relevant elements in these films were analyzed, both in depth and laterally, by SIMS. Depth profiling demonstrates a rather homogeneous distribution of the active components throughout the Al2O3 layers, whereas some inhomogeneities were observed in lateral elemental mappings. The compositional modifications induced in these films by their use in the microreactor will also be discussed.
    • Y. Men, H. Gnaser, R. Zapf, V. Hessel, C. Ziegler, G. Kolb
    • Steam reforming of methanol over Cu/CeO2/γ-Al2O3 catalysts in a microchannel reactor
    • Appl. Catal. A 277, 83 (2004)
    • Abstract >>
    • The activity behavior of steam reforming of methanol was studied over Cu/CeO2/γ-Al2O3 catalysts with different compositions in a parallelized 10-channel microreactor. It was found that the catalytic performance is strongly influenced by the catalyst compositions, i.e. copper and ceria loading. Cu/CeO2/γ-Al2O3 catalyst thin films before and after use in the microreactor were extensively investigated by means of surface analysis techniques like X-ray photoelectron spectroscopy, energy dispersive analysis of X-rays, and thermal desorption spectrometry. The parallel screening result is tentatively discussed on the basis of the surface analysis characterization results and previous proposals. The rationalized correlation of the catalytic activity dependence with the characteristics is in line with a reaction mechanism, which supposes that the copper/ceria boundary is the active site for steam reforming of methanol and the oxygen reverse spillover from ceria to copper is involved in the catalysis cycle
    • Y. Men, H. Gnaser, R. Zapf, V. Hessel, C. Ziegler
    • Parallel screening of Cu/CeO2/γ-Al2O3 catalysts for steam reforming of methanol in a 10-channel micro-structured reactor
    • Catal. Comm. 5, 671 (2004)
    • Abstract >>
    • Steam reforming of methanol (SRM) was investigated over Cu/CeO2/γ-Al2O3 catalysts with different compositions in a parallelized 10-channel micro-structured reactor. The catalytic activity was found to be strongly dependent on the copper loading. The parallel screening result was tentatively discussed with surface analysis characterization results and previous proposals. A reaction mechanism is proposed to rationalize the catalytic activity data and characteristics of the catalysts, which supposes that the copper/ceria interfacial area (partially oxidized copper nanoparticle and defective ceria) is the active site for SRM. The oxygen reverse spillover from ceria to copper is suggested to be involved in the catalysis cycle.
    • B. Huber, H. Gnaser, C. Ziegler
    • Electrical properties of nanocrystalline anatase TiO2 thin films with different crystallite size
    • Surf. Sci. 566-568, 419 (2004)
    • Abstract >>
    • Nanocrystalline anatase TiO2 films with crystallites of three different sizes (approximately 6, 12, and 18 nm) were deposited on various substrates (glass, sapphire, and silicon) using colloidal suspensions. These films were thoroughly characterized with respect to their surface morphology, crystal structure, phase homogeneity, elemental composition, and the presence of impurities. A detailed study of the influence of sample temperature T and the ambient oxygen pressure p(O2) on the electrical conductivity σ of the films was performed. For all films a power-law dependence of σ on p(O2), σ∝p(O2)n, was observed. The values of the exponent n were found to exhibit a distinct dependence both on the crystallite size and on the specimen temperature; furthermore, an influence due to the presence of doping species was noted. For nanocrystalline films on sapphire substrates and for T=200 °C, n amounts to −1.31, −1.15 and −0.56 for the 6-, 12-, and 18-nm films, respectively. For reduced films, an exponential dependence of σ on T−1 was determined, yielding activation energies EA with values of 0.34, 0.38, and 0.51 eV for films with those crystallite sizes.
    • H. Gnaser, W. Bock, E. Rowlett, Y. Men, C. Ziegler, R. Zapf, V. Hessel
    • Secondary-ion mass spectrometry (SIMS) analysis of catalyst coatings used in microreactors
    • Nucl. Instrum. Methods B 219-220, 880; (Erratum) B 226, 468 (2004)
    • Abstract >>
    • Secondary-ion mass spectrometry (SIMS) was used for the characterization of the catalytically active parts of a microreactor, both before and after being used in the reactor. The catalyst coatings for the microreactor were prepared in a standard way: nanoporous washcoats were deposited on the microstructures by a particle route and the active components were applied by wet impregnation using aqueous solutions of metal nitrates. On two specific catalyst systems employed for methanol steam reforming (CuO/Cr2O3/γ-Al2O3 and CuO/CeO2/γ-Al2O3) the distributions of the relevant elements in these films were analyzed, both in depth and laterally, by SIMS. Depth profiling demonstrates a rather homogeneous distribution of the active components throughout the Al2O3 layers, whereas some inhomogeneities were observed in lateral elemental mappings. The compositional modifications induced in these films by their use in the microreactor will also be discussed.
  • 2003
    • A. Vidic, D. Then, Ch. Ziegler
    • A new cantilever system for gas and liquid sensing
    • Ultramicroscopy, Volume 97, Issues 1–4, Pages 407–416 (2003)
    • Abstract >>
    • A novel setup for gas and liquid sensing was developed and tested. It is based on both detection of frequency shift and of bending of micro-cantilevers to measure mass changes as well as viscosity changes. To drive the cantilevers new electrostatic and magnetic actuations were invented with a closed feed-back loop which forces the cantilever to oscillate always at its resonance frequency. The oscillation is detected via the beam-deflection technique. By measuring the DC signal of the photodiode the static bending of the cantilever can be monitored simultaneously. The closed feed-back loop propagates a very stable oscillation at the resonance frequency and gives a strong increase in the quality factor compared to a system without such feed-back loop. Furthermore, it is possible to operate this cantilever transducer system in liquids.These cantilever sensors hence, show the potential for use in easy-to-use and highly sensitive sensor systems for gas and liquid phase chemical and biochemical sensing.
    • Y. Men, H. Gnaser, C. Ziegler
    • Adsorption/desorption studies on nanocrystalline alumina surfaces
    • Anal. Bioanal. Chem. 375, 912 (2003)
    • Abstract >>
    • Two kinds of nanocrystalline alumina powders, boehmite (gamma-AlOOH, particle size d approximately 10 nm, BET surface area A(BET) = 180-200 m(2) g(-1)) and corundum (alpha-Al(2)O(3), d approximately 400 nm, A(BET) = 7 m(2) g(-1)) were used for comparative investigation by thermogravimetry (TG). The remarkable difference in the dehydration profiles between the two samples gives evidence for a distinct difference in their structures. In the following pyridine adsorption/desorption experiment, gamma-alumina was found to possess much more (20 times) and much stronger acidic sites than corundum. The activation energy of pyridine desorption was obtained from the respective minima in the first derivative of the TG-curves (DTG) at various heating rates (1-20 K min(-1)); the activation energy for pyridine desorption is smaller for gamma-alumina (61.5 kJ mol(-1)) than for corundum (78.8 kJ mol(-1)). Furthermore, the adsorption of water, carbon tetrachloride, and hexane on those alumina specimens provides evidence for the highly hydrophilic nature of their surfaces. The shift of T(max) to higher temperatures upon desorption of water was ascribed to the different adsorption coverage and the different energy required for removal of adsorbed water molecules.
    • B. Huber, H. Gnaser, C. Ziegler
    • Characterization of nanocrystalline anatase TiO2 thin films
    • , Anal. Bioanal. Chem. 375, 917 (2003)
    • Abstract >>
    • Nanoporous thin films were deposited onto glass substrates by painting with a solution of nanocrystalline anatase TiO2 particles (with a size of either 6 nm or 16 nm) suspended in an organic solvent. Upon drying in air for about 1 day, the films were tempered at 450 °C in air for 1 h. This procedure results in stoichiometric TiO2 films with a thickness of several µm and a milky whitish appearance. Scanning force microscopy of the surface revealed that the nanoparticles of the films agglomerated into structures with lateral dimensions of some 100 nm. Transmission electron microscopy was utilized to investigate the structural arrangement of the crystallites in the films. High-resolution electron diffraction and X-ray diffraction analyses demonstrated, furthermore, that the material consists exclusively of a single TiO2 phase, namely anatase, and that the films do not exhibit any preferential texture. The elemental stoichiometry and the possible presence of impurities were monitored throughout the films by means of secondary-ion mass spectrometry depth profiling. Electrical measurements have been carried out as a function of both the sample temperature T and the ambient oxygen partial pressure p(O2). From these data the electrical conductivity σ of the porous films was determined in dependence of those parameters
    • Yong Men, Hubert Gnaser, Christiane Ziegler
    • Adsorption/desorption studies on nanocrystalline alumina surfaces
    • Analytical and Bioanalytical Chemistry, Volume 375, Number 7, 912-916 (2003)
    • Abstract >>
    • Two kinds of nanocrystalline alumina powders, boehmite (n-AlOOH, particle size d , 10 nm, BET surface area ABET = 180-200 m2 gm1) and corundum (f-Al2O3, d , 400 nm, ABET = 7 m2 gm1) were used for comparative investigation by thermogravimetry (TG). The remarkable difference in the dehydration profiles between the two samples gives evidence for a distinct difference in their structures. In the following pyridine adsorption/desorption experiment, n-alumina was found to possess much more (20 times) and much stronger acidic sites than corundum. The activation energy of pyridine desorption was obtained from the respective minima in the first derivative of the TG-curves (DTG) at various heating rates (1-20 K minm1); the activation energy for pyridine desorption is smaller for n-alumina (61.5 kJ molm1) than for corundum (78.8 kJ molm1). Furthermore, the adsorption of water, carbon tetrachloride, and hexane on those alumina specimens provides evidence for the highly hydrophilic nature of their surfaces. The shift of Tmax to higher temperatures upon desorption of water was ascribed to the different adsorption coverage and the different energy required for removal of adsorbed water molecules.
    • M. Mondon, S. Berger, C. Ziegler
    • Scanning-force techniques to monitor time-dependent changes in topography and adhesion force of proteins on surfaces
    • Analytical and Bioanalytical Chemistry, Volume 375, Number 7, 849-855 (2003)
    • Abstract >>
    • Scanning-force microscopy (SFM) investigations were conducted to probe the influences of the interactions of proteins with surfaces relevant in medicine. These interactions are an important feature in the area of biofilm formation. The adsorption of proteins leads to changes in topography, which was monitored for the build up of protein layers of hen egg-white lysozyme and bovine serum albumin (BSA) on mica in real time in phosphate-buffered aqueous solution over a time period of 10 min. Phase imaging was additionally applied to compare material contrasts and to evaluate this method for further application in this field. The adhesion forces that develop on a time scale below 20 s between a protein-modified SFM tip and titanium surfaces (TiO2, TiAl6V4 and TiAl6Nb7) were investigated. The influences of the parameters loading force and interaction time between the protein and the surface were monitored as well as the influence of protein structure. The interaction time dependency of the adhesion force could be described with a kinetic model of two consecutive first-order reactions. For the maximal adhesion force a correlation to the ratio of the amino acids cysteine, proline and glycine has been proposed.
    • Bernd Huber, Hubert Gnaser, Christiane Ziegler
    • Characterization of nanocrystalline anatase TiO2 thin films
    • Analytical and Bioanalytical Chemistry, Volume 375, Number 7, 917-923 (2003)
    • Abstract >>
    • Nanoporous thin films were deposited onto glass substrates by painting with a solution of nanocrystalline anatase TiO2 particles (with a size of either 6 nm or 16 nm) suspended in an organic solvent. Upon drying in air for about 1 day, the films were tempered at 450 °C in air for 1 h. This procedure results in stoichiometric TiO2 films with a thickness of several µm and a milky whitish appearance. Scanning force microscopy of the surface revealed that the nanoparticles of the films agglomerated into structures with lateral dimensions of some 100 nm. Transmission electron microscopy was utilized to investigate the structural arrangement of the crystallites in the films. High-resolution electron diffraction and X-ray diffraction analyses demonstrated, furthermore, that the material consists exclusively of a single TiO2 phase, namely anatase, and that the films do not exhibit any preferential texture. The elemental stoichiometry and the possible presence of impurities were monitored throughout the films by means of secondary-ion mass spectrometry depth profiling. Electrical measurements have been carried out as a function of both the sample temperature T and the ambient oxygen partial pressure p(O2). From these data the electrical conductivity † of the porous films was determined in dependence of those parameters.
    • Matthias Mondon, Christiane Ziegler
    • (a) Changes in Water Contact Angles During the First Phase of Setting of Dental Impression Materials
    • The International Journal of Prosthodontics, Volume 16 , Issue 1, Pages 49 - 53 (2003)
    • Abstract >>
    • PURPOSE:The purpose of this investigation was to examine the changes in wettability of dental impression materials during setting. This study compared the properties of the initial water contact of two different dental impression materials and their subsequent development during polymerization.MATERIALS AND METHODS:Two dental impression materials (Impregum Penta Soft and Aquasil) with different chemical compositions (polyether and polyvinyl siloxane, respectively) were investigated with respect to their changing wetting properties by time-resolved static contact angle measurements. Ten sets of measurements each were taken over a period of 400 seconds with 150 points of data each; the first pictures were used for further characterization of the initial interaction.RESULTS:With 73 degrees, Impregum Penta Soft exhibited a significantly lower contact angle, which stayed lower during the process of setting, compared to the silicone-based material. The initial interaction of the droplet showed a repulsive interaction of Aquasil with the water droplet.CONCLUSION:Impregum Penta Soft showed a more hydrophilic behavior during the process of setting compared to Aquasil and can therefore be expected to exhibit better flow properties. The method of time-resolved static contact angle measurements is a well-suited analytic instrument to monitor temporally changing wetting phenomena.
    • Matthias Mondon, Christiane Ziegler
    • (b) Cambiamenti negli angoli di contatto dell‘acqua durante la prima fase d’indurimento dei materiali per impronte dentali
    • La Rivista Internazionale di Odontoiatria Protesica 16, 49-53 (2003)
    • Abstract >>
    • Scopo:lo scopo di questo studio era esaminare i cambiamenti di umettabilità dei materiali per impronte dentali durante l'indurimento. Questo studio confronto le proprietà del contatto iniziale con l'acqua di due diversi materiali per impronte dentali e il loro successivo sviluppo durante la polimerizzazione.Materiali e metodi:si studiarono due materiali per impronte dentali tlmpregum Penta Soft e Aquasill con diverse composizioni chimiche (polietere e polivinilsilossano rispettivamente,- rispetto alle loro proprietà di umettabilità mutevoli tramite misurazioni dell'angolo di contatto statico risolto nel tempo. Si fecero dieci serie di misurazioni ciascuno in un periodo di 400 secondi con 150 punti di dati ciascuno; le prime immagini si usarono per l'ulteriore caratterizzazione dell'interazione iniziale.Risultati:con 73 gradi, tmpregum Penta Soft mostrava un angolo di contatto significativamente inferiore, che restava più basso durante il processo di indurimento. rispetto al materiale a base di silicone. L'interazione iniziale della goccioiina mostrava un'interazione repulsiva di Aquasil con la gocciolina d'acqua.Conclusione:lmpregum Penta Sott mostrava un comportamento più idrofilo durante il processo di indurimento rispetto ad Aquasil e ci si può quindi aspettare che presenti proprietà di flusso migliori. ll metodo delle misurazioni dell'angolo di contatto risolto nel tempo è uno strumento analitico ben adatto a monitorare fenomeni di umettamento che cambiano nel tempo.
    • H. Stadler, M. Mondon, C. Ziegler
    • Protein adsorption on surfaces: dynamic contact-angle (DCA) and quartz-crystal microbalance (QCM) measurements
    • Analytical and Bioanalytical Chemistry, Volume 375, Number 1, 53-61 (2003)
    • Abstract >>
    • Adsorption of the protein bovine serum albumin (BSA) on gold has been tested at various concentrations in aqueous solution by dynamic contact-angle analysis (DCA) and quartz-crystal microbalance (QCM) measurements. With the Wilhelmy plate technique advancing and receding contact angles and the corresponding hysteresis were measured and correlated with the hydrophilicity and the homogeneity of the surface. With electrical admittance measurements of a gold-coated piezoelectrical quartz crystal, layer mass and viscoelastic contributions to the resonator's frequency shift during adsorption could be separated. A correlation was found between the adsorbed mass and the homogeneity and hydrophilicity of the adsorbed film.
  • 2002
    • F. Rupp, D. Axmann, Ch. Ziegler, J. Geis-Gerstorfer
    • Adsorption/desorption phenomena on pure and Teflon® AF-coated titania surfaces studied by dynamic contact angle analysis
    • Journal of Biomedical Materials Research, Volume 62, Issue 4, pages 567–578 (2002)
    • Abstract >>
    • As a result of inflammatory processes, plaque formation on dental titanium implants often leads to clinically pathogenic situations. This special biofilm formation on (bio)materials in contact with saliva is initiated by ionic and protein interactions. In this interfacial process, albumin becomes a main constituent of dental pellicle. Interfacial reactions change the surface characteristics. They determine the following steps of macromolecular adsorption and bacterial adhesion. This work focuses on the dynamic contact angle analysis (DCA), which is a tool for online measurements of dynamic changes of wettability without disturbing the interface during detection. Repeatability of the DCA method has been assessed according to the Bland and Altman method. The kinetics and equilibrium data of shifts in the wetting tension hysteresis indicate ionic influences at the titanium/bovine serum albumin (BSA) interface: the Ca-mediated increase of the BSA adsorption on titanium and the adsorption maximum at the isoelectric point (IEP) of BSA. Ti was surface modified by Teflon® AF polymeric coatings. The result of the assessment gives reason to consider Teflon® AF as a reference material for DCA repeatability studies. This surface modification caused drastic changes in the dynamic interfacial reactions. Shifts in the wetting tensions during DCA adsorption-desorption experiments clearly demonstrated the partially irreversible adsorption of BSA on Teflon® AF. In contrast, reversible adsorption behavior was detected on pure Ti surfaces. These findings strengthen the hypothesis that the analysis of dynamic changes in wetting tension and wetting tension hysteresis is a sensitive analytical method for the detection of dynamic interfacial changes at biomaterial/biosystem interfaces during the initial steps of biofilm formation.
    • Christopher Fietzek, Michael Seiler, Bernd Görlach, Peter Schütz, Udo Weimar, Michael Hanack, Christiane Ziegler, Helmut Bertagnolli
    • Reversible intercalation of volatile amines into stacks of soluble phthalocyanines
    • J. Mater. Chem.,12, 2305-2311 (2002)
    • Abstract >>
    • Different alkyl amines were intercalated into stacks of tetraalkenylphthalocyaninatozinc by exposing the solids to amine vapor. Since the intercalation is reversible, quantitative and kinetic aspects of the sorption were investigated using quartz crystal microbalances (QCMs) and thermal desorption mass spectrometry (TDS). Two different types of sorption were found and these were interpreted as interaction with aliphatic sidechains and preferential intercalation. At higher vapor concentration most of the interstitial spaces are occupied, suggesting a 1 ∶ 1 host–guest stoichiometry. Structure elucidation of an intercalated n-butylamine was carried out by EXAFS transmission measurements. Intercalation was found to take place while maintaining the columnar arrangement and without significant expansion of the spacing. Additional QCM measurements revealed less favorable sorption of sterically demanding amines which can be explained by morphological discrimination.
  • 2001
    • Axel Blau, Christiane Ziegler
    • Prototype of a novel autonomous perfusion chamber for long-term culturing and in situ investigation of various cell types
    • Journal of Biochemical and Biophysical Methods, Volume 50, Issue 1, Pages 15–27 (2001)
    • Abstract >>
    • In the context of a neurobionic approach to chemical analysis and sensorics, this article depicts the development of a miniaturized autonomous perfusion chamber setup for the growth and the electrical as well as optical investigation of (neural) cell cultures in vitro. We suggest an autonomous, modular, temperature-controlled, transparent, and sealed perfusion cell culture housing adaptable to various mounts, sizes and different needs. The design includes the electronics of a temperature and medium supply control unit. The setup combines the possibility of uninterrupted cell culturing with simultaneous microscopic and analytical investigation of variable amounts of cells or organs of human, animal, or plant origin under sterile conditions on different substrates without the need of an external incubator or a sterile working environment. Its use is demonstrated exemplarily with neuronal cultures from embryonic chicken that were cultured in a prototype system for 3 weeks. It turned out that cell survival in such a chamber was prolonged with timed medium flow rather than continuous perfusion.
    • Hartmut Stadler, Matthias Mondon, Jesper Wallentin, Yuekan Jiaoz, Christiane Ziegler, Stanislav Gorbz
    • Viscosity force of the fly's pad secretion measured by atomic force microscopy
    • BIONA-report 15, 340-344 (2001)
    • Abstract >>
    • Flies are able to attach to both rough and smooth surfaces. This is attributed to several attachment mechanisms. A mechanical attachment to surface irregularities can be achieved by two claws located at the end of each foot Deformable adhesive pad covered by setae and supplemented by epidermal secretion supports the fly‘s attachment to smooth surfaces (Walker at al., 1985). The latter mechanism is interesting from scientific and technical points of view. The details of the attachment process are not yet well understood and an insight into the process might help in the future to mimic the natural system in technical applications.ln the present work, the adhesive secretion was investigated. An exact function of the liquid, which is essential for the lty's attachment to smooth surfaces, is still unknown. A combination of different liquid properties, such as viscosity, surface tension, wettability and lubrication ability might play a role in attachment. A high viscosity or capillary force exerted by the liquid might be able to attract the fly's foot to the surface.Another possible function of the secretion is the enlargement of the contact area between foot and surface when the liquid gets into surface microstruclures. These properties are difficult to examine because the volume of the secretion is an order of magnitude too small for conventional analytical techniques.We set up a kind of model of the fly's single seta consisting of a scanning force microscopy cantilever. The dimensions of the lip located at the end of the cantilever are in the range of a few micrometers. This is close to the size of the setae. in contrast to the natural system, our model was neither flexible nor of the same chemical composition. The secretion droplets of fly's footprints on the glass surface were used in experiments, The eemilever was dipped into the secretion perpendicular to the surface, and the forces acting on the cantilever were measured by a scanning force microscope (SFM). It was expected that the magnitude and the distance dependence of the forces affected by the type ef interaction would give us hints on the adhesion mechanism.In the present work we concentrated on viscous or capillary influences on the adhesion of the model seta to the glass surface covered by the secretion. If the interaction is of a viscous nature, the magnitude of forces depends on the velocity to separate the cantilever and me surface. A pure capillary force does not depend on the separation velocity.
    • A. Blau, C. Weinl, J. Mack, S. Kienle, G. Jung, Ch. Ziegler
    • Promotion of neural cell adhesion by electrochemically generated and functionalized polymer films
    • Journal of Neuroscience Methods, Volume 112, Issue 1, Pages 65–73 (2001)
    • Abstract >>
    • New strategies for spatially controllable cell adhesion have been developed for brain cells from embryonic chicken. They are based on electrochemically active phenol and pyrrole derivatives, and can be used for the selective coverage of electroconductive substrates. Besides mimicking standard laminin-related adhesion promoting mechanisms by means of an electroactive monomer-linked 18-peptide segment from laminin (SRARKQAASIKVAVSADR), electrochemically generated thin (6–30 nm) polymer films of 3-hydroxybenzyl-hydrazine (3HBH) and 2-(3-hydroxyphenyl)-ethanol (2(3HP)E) with and without mechanically entrapped or covalently linked d-lysine have proved to promote cell adhesion in serum-free medium on indium-doped tin oxide (ITO) substrates during the first 6 culturing days in vitro. The effectiveness of the peptide was strongly density-dependent. Unexpectedly, laminin itself or a combination of laminin and poly-d-lysine (PDL) did not promote cell adhesion and neuron differentiation in serum-free cultures on ITO. However, they worked perfectly well on regular polystyrene substrates in serum-free medium or on ITO when medium with serum was used. This finding might suggest that the adhesion efficiency of laminin does not depend only on the kind of medium supplement but also on the type of substrate. In contrast, the adhesion-promoting properties of ‘artificial’ polymeric films seemed to be based on a more direct cell–film interaction, with the film masking the substrate properties.
  • 2000
    • Christiane Ziegler, Annette Harsch, Wolfgang Göpel
    • Natural neural networks for quantitative sensing of neurochemicals: an artificial neural network analysis
    • Sensors and Actuators B: Chemical, Volume 65, Issues 1–3, Pages 160–162 (2000)
    • Abstract >>
    • In cell culture, mammalian neurons form fault tolerant, spontaneously active systems with great sensitivity to their chemical environment and generate response profiles that are often concentration and substance specific. In the experiments to be discussed, the response of spontaneously active murine spinal cord cultures coupled to an array of 64 transparent microelectrodes to the glycine receptor blocker strychnine was evaluated. Strychnine reliably generated increased multichannel bursting at 5–20 nM and regular, coordinated bursting above 5 μM. In comparison bicuculline was applied which shows a similar network response but at higher concentrations. By principal component analysis a good discrimination of signals produced by strychnine and by bicuculline was possible. By artificial neural network analysis a quantitative interpretation of the strychnine data was shown for the first time. In particular it could be demonstrated that for subsequent strychnine expositions a quantitative analysis of an unknown strychnine concentration is possible. These results indicate that cultured neuronal networks are practical systems that can be used as a biosensor system for the characterization of a great variety of neuroactive substances.
    • Eckhard Müller, Christiane Ziegler
    • Preparation of highly ordered quinquethiophene thin films on TiO2 substrates
    • Journal of Materials Chemistry, 10, 47-53 (2000)
    • Abstract >>
    • To understand the intrinsic electrical properties of conjugated polymers in general and of oligothiophenes in particular, ultra-pure and highly ordered thin films or single crystals are required. In this context, studies on thin films of quinquethiophene (α5T) prepared by evaporation in UHV were performed. As substrates insulating and conducting oxides such as different single crystal surfaces of TiO2 were chosen. The latter are particularly interesting because TiO2 can be doped to be n-conducting. These substrates may therefore be used for hybrid inorganic–organic thin film pn-diodes. Here, we report on SFM-measurements under ambient conditions, which show that the deposition of α5T from a Knudsen cell under UHV conditions leads to cluster growth on TiO2(011) and to ordered layers on undoped TiO2(001). On doped TiO2(001) the growth mode is largely influenced by the surface roughness, i.e. the presence of pinning centers. It could be shown that the α5T molecules on TiO2(001) are oriented with their long axis mainly perpendicular to the substrate plane and form two complete monolayers (Frank–Van der Merwe growth mode) before the growth mode changes to a simultaneous multilayer growth, in line with earlier experiments using other oxidic substrates.
  • 1998
    • Ralph-Michael Rummel, Christiane Ziegler
    • Room temperature adsorption of aniline (C6H5NH2) on Si(100)(2×1) observed with scanning tunneling microscopy
    • Surface Science, Volume 418, Issue 1, Pages 303–313 (1998)
    • Abstract >>
    • The chemisorption of aniline (C6H5NH2) on Si(100)(2×1) at room temperature has been studied for the first time with scanning tunneling microscopy (STM) and spectroscopy (STS). The STM images reveal that aniline adsorbs on Si(100)(2×1) and forms regions of moderately ordered p(2×2) or c(4×2) structures. The STS results show a quenching of the silicon surface states after adsorption and an increased energetic gap between occupied and unoccupied surface states if compared with the clean surface. The first occupied state coincides with the corresponding peak in the ultraviolet photoelectron spectrum.
    • J. Murr, Ch. Ziegler
    • Interaction of Na with sexithiophene thin films
    • Phys. Rev. B 57, 7299–7304 (1998)
    • Abstract >>
    • The effects of Na on the electronic structure and electrical properties of α-sexithiophene (α6T) are investigated with electron spectroscopic methods under UHV conditions. A shift of the valence-band edge away from the Fermi level, and the evolution of states in the gap, can be determined. In a classical band-structure picture this could be explained as an n-type doping process. However, all results point at a high localization of the negative charge at the sulfur atom which is reflected in a very low specific conductivity.
    • B. Reusch, J. Geis-Gerstorfer, C. Ziegler
    • Scanning probe microscopy on new dental alloys
    • Applied Physics A, Volume 66, Issue 1 Supplement, pp S805-S808 (1998)
    • Abstract >>
    • Surface analytical methods such as scanning force microscopy (SFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to determine the surface properties of amalgam substitutes as tooth filling materials. In particular the corrosion and the passivation behavior of new gallium restorative materials were studied. To give relevant practical data, the measurements were performed with and without the alloys being stored in artificial saliva to simulate physiological oral conditions.
    • C. Ziegler, B. Reusch und J. Geis-Gerstorfer
    • Interface analysis of new tooth filling materials
    • Fresenius' Journal of Analytical Chemistry, Volume 361, Numbers 6-7 , 547-553 (1998)
    • Abstract >>
    • Surface analytical methods such as scanning electron microscopy (SEM), scanning force microscopy (SFM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS) were used to study the surface properties of amalgam substitutes as tooth filling materials. In particular the corrosion resistance of new gallium restorative materials was determined. To give relevant practical data, the measurements were performed before and after storing the alloys in artificial saliva to simulate physiological oral conditions. Subsequently an analysis of the artificial saliva was carried out with atomic emission spectroscopy with inductively coupled plasma (ICP-AES). Selective corrosion of different phases and a high gallium release could be determined.
  • 1997
    • A. Blau, Ch. Ziegler, M. Heyer, F. Endres, G. Schwitzgebel, T. Matthies, T. Stieglitz, J.-U. Meyer, W. Göpel
    • Characterization and optimization of microelectrode arrays for in vivo nerve signal recording and stimulation
    • Biosensors and Bioelectronics, Volume 12, Issues 9–10, Pages 883–892 (1997)
    • Abstract >>
    • Revealing the complex signal-processing mechanisms and interconnection patterns of the nervous system has long been an intriguing puzzle. As a contribution to its understanding the optimization of the impedance behavior of implantable electrode arrays with via holes is discussed here. Peripheral axons will regenerate through these holes allowing for simultaneous nerve stimulation and signal recording. This approach is part of the ESPRIT project INTER and may eventually lead to devices driving sensory motor prosthesis with closed loop control.In the first set of experiments, micromachined platinum electrode arrays were prepared, characterized and optimized for nerve signal recording. The results of these studies are based on impedance spectroscopy and microscopic techniques. Equivalent circuits were modeled describing formally the electrical response behavior with ohmic resistances between 500 Ω and 10 kΩ. To attain low impedances for all electrodes on the INTER device, platinum from H2PCl6 was electrodeposited, and sputter technology as well as electrochemical deposition from H2IrCl6 solution were used to produce thin iridium films. For the former, a lift-off process was established at one of the institutes to generate electrode structures with a line width of 5 μm. As a result in all three cases the electrodes showed almost constant impedances over the entire frequency range (10 Hz–1 kHz), which is relevant for nerve signal recording.In the second set of experiments, electrodes were optimized to allow for nerve stimulation. For this purpose, the charge delivery capacity (CDC) had to be increased and the impedance had to be decreased. Iridium oxide is the material of choice, because its CDC is much higher than the CDC of platinum at 75 μC/cm2 (Ziaie et al., 1991, IEEE Sensors & Actuators Transducers, 6, 124–127). A significant increase of the electrochemically active surface of the electrode structures could be observed by measuring the surface roughness. In first experiments, an activated iridium oxide film was formed with cyclic voltammetry and was evaluated using scanning force microscopy and impedance spectroscopy. The evaluation of the cyclic voltammograms showed a CDC up to 400 mC/cm2 for sputter deposited and oxidatively treated iridium films.Further investigations are directed towards increasing the stability of the iridium oxide electrodes with regard to long-term implants. Parallel experiments aim at the controlled axon adhesion without changing the impedance behavior of the described electrodes.
    • B. Liedberg, Z. Yang, I. Engquist, M. Wirde, U. Gelius, R.-M. Rummel, Ch. Ziegler, W. Göpel, G. Götz, P. Bäuerle
    • Self-Assembly of α-Functionalized Terthiophenes on Gold
    • J. Phys. Chem. B, 101 (31), pp 5951–5962 (1997)
    • Abstract >>
    • α-Functionalized terthiophenes containing disulfide (−S−T3−H)2 and alkanethiol (HS−(CH2)11−T3−H) anchoring groups have been synthesized for direct immobilization onto gold. Monolayer structures of these compounds are prepared by spontaneous assembly from ethanol solutions on evaporated gold substrates and thoroughly characterized by ellipsometry, contact angle goniometry, infrared and X-ray photoelectron spectroscopy, and cyclic voltammetry. The two molecules coordinate to the gold substrate exclusively via the anchoring groups upon formation of gold−thiolate bonds. The kinetics of monolayer formation vary dramatically for the two compounds. The alkanethiol analogue assembles rapidly, within a few minutes, and forms a densely packed and highly organized monolayer, with the alkyl chains in an almost perfect all-trans conformation and the Cα−Cα axis of the α-T3 units tilted about 14° away from the surface normal. The assembly process is much slower for the disulfide, but an organized monolayer with an average α-T3 chain tilt of about 33° will eventually form when the assembly is allowed to equilibrate with a solution containing the disulfide for at least 1 day. Moreover, the two monolayer assemblies also display a remarkably different electrochemical behavior. The heterogeneous electron-transfer rate at the disulfide-covered gold substrate is almost indistinguishable from that at bare gold, suggesting that the assembly contains a large number of easily accessible defects. An alternative mechanism for explaining the large electron-transfer rate involving electronic coupling via the conjugated π-system of the α-T3 units is also proposed. The electrochemical response is significantly reduced for the HS−(CH2)11−T3−H assembly, but another type of defects, the so-called “shallow defects” originating from sparsely populated areas on the electrode surface, can be identified.
    • Annette Harsch, Christiane Zieglerb, Wolfgang Göpel
    • Strychnine analysis with neuronal networks in vitro: extracellular array recording of network responses
    • Biosensors and Bioelectronics, Volume 12, Issue 8, Pages 827–835 (1997)
    • Abstract >>
    • Neurons, by virtue of intrinsic electrophysiological mechanisms, represent transducers that report the dynamics of cell death, receptor–ligand interactions, alterations in metabolism and generic membrane perforation processes. In cell culture, mammalian neurons form fault-tolerant, spontaneously active systems with great sensitivity to their chemical environment and generate response profiles that are often concentration- and substance-specific. Changes in action potential patterns are usually detected before morphological changes and cell damage occur. This provides sensitivity and reversibility. Such biological systems may be used to screen rapidly for novel pharmacological substances, toxic agents and certain odorants.This paper reports on substance-dependent major changes in spontaneous native activity patterns by the synaptically active (glycine receptor blocker) strychnine. Via 64-channel array recordings of spontaneously active murine spinal cord cell cultures, increased multichannel bursting at 5–20 nM strychnine and regular, coordinated bursting above 5 μM could be reliably generated. By artificial neural network analysis a quantitative correlation of network signals and strychnine concentration could be evaluated for small concentrations of strychnine.The results indicate that cultured neuronal networks already represent reliable and practical systems which can be used for the detection of chemical substances and the characterization of their biological influences.
    • B. Reusch, Ch. Ziegler, T.G. Kwag, M.E. Kenney, W. Göpel
    • Thin film preparation of oligomeric Si-phthalocyanines: thermal stability and sublimation behavior
    • Thin Solid Films, Volume 299, Issues 1–2, Pages 63–66 (1997)
    • Abstract >>
    • The thermal stability of rod-like phthalocyanines of the general formula R3SiO(SiPcO)nSiR3 with n=1–4 and R≡C6H13 was studied by temperature dependent mass spectrometry and X-ray photoemission spectroscopy. The stability of the monomer, dimer, and trimer is high, whereas the tetramer is thermally unstable and therefore cannot be used for thin film preparation by sublimation.
    • B. Reusch, J. Geis-Gerstorfer, C. Ziegler
    • Interface analysis of noble dental casting alloys
    • Fresenius' Journal of Analytical Chemistry, Volume 358, Issue 1-2, pp 64-66 (1997)
    • Abstract >>
    • Surface analytical methods such as scanning electron microscopy (SEM), electron dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS) were used to study the surface properties of several noble metal casting alloys which are used as core materials for fixed prosthodontics. In particular surface morphology, surface segregation, and oxide formation were studied as the key parameters influencing the adhesion properties between alloy and ceramic veneer.
  • 1996
    • Ch. Ziegler, D. Martin, W. Göpel
    • Thermal stability and sublimation behavior of (GaPcF)n, (AlPcF)n, and their 1:1 copolymer
    • Materials Research Bulletin, Volume 31, Issue 9, Pages 1149–1157 (1996)
    • Abstract >>
    • The polymeric fluorine-bridged phthalocyanines (GaPcF)n and (AlPcF)n, and their 1:1 copolymer were investigated by temperature dependent mass spectrometry to prove their thermal stability and their sublimation behavior. It is shown that all three materials can be sublimed reproducibly. This requires the removal of excess monomeric and dimeric species, which are present after the chemical synthesis of the polymers.
    • Ekkehard Lindner, Rudolf Schreiber, Theodor Schneller, Peter Wegner, Hermann A. Mayer, Wolfgang Göpel, Christiane Ziegler
    • Synthesis of Polysiloxane-Bound (Ether−phosphine)palladium Complexes. Stoichiometric and Catalytic Reactions in Interphases
    • Inorg. Chem., 35 (2), pp 514–525 (1996)
    • Abstract >>
    • The reaction of two equiv of the monomeric ether−phosphine O,P ligand (MeO)3Si(CH2)3(Ph)PCH2−Do [1a(T0), 1b(T0)] {Do = CH2OCH3 [1a(T0)], CHCH2CH2CH2O [1b(T0)]} with PdCl2(COD) yields the monomeric palladium(II) complexes Cl2Pd(PO)2 [2a(T0)2, 2b(T0)2]. The compounds 2a(T0)2 and 2b(T0)2 are sol−gel processed with variable amounts (y) of Si(OEt)4 (Q0) to give the polysiloxane-bound complexes 2a(Tn)2(Qk)y, 2b(Tn)2(Qk)y (Table 1) {PO = η1-P-coordinated ether−phosphine ligand; for Tn and Qk, y = number of condensed T type (three oxygen neighbors), Q type (four oxygen neighbors) silicon atoms; n and k = number of Si−O−Si bonds; n = 0−3; k = 0−4; 2a(Tn)2(Qk)y, 2b(Tn)2(Qk)y = {[M]-SiOn/2(OX)3-n}2[SiOk/2 (OX)4-k]y, [M] = (Cl2Pd)1/2(Ph)P(CH2Do)(CH2)3−, X = H, Me, Et}. The complexes 2b(Tn)2(Qk)y (y = 4, 12, 36) show high activity and selectivity in the hydrogenation of 1-hexyne and tolan. The dicationic complexes [Pd(PO)2][SbF6]2 [3a(T0)2, 3b(T0)2] are formed by reacting Cl2Pd(PO)2 with 2 equiv of a silver salt {PO = η2-OP-coordinated ether−phosphine ligand; 3a(T0)2, 3b(T0)2 = [M]−SiOMe3; [M] = {[Pd2+]1/2P(Ph)(CH2CH2OCH3)(CH2)3−}{SbF6} (a), {[Pd2+]1/2P(Ph)(CH2CHCH2CH2CH2O)(CH2)3−}{SbF6} (b)}. Their polysiloxane-bound congeners 3a(Tn)2, 3b(Tn)2 {[M]-SiOn/2(OX)3-n} are obtained if a volatile, reversible bound ligand like acetonitrile is employed during the sol−gel process. The bis(chelate)palladium(II) complexes 3a(Tn)2, 3b(Tn)2 are catalytic active in the solvent-free CO−ethene copolymerization, producing polyketones with chain lengths comparable to those obtained with chelating diphosphine ligands. The polysiloxane-bound palladium(0) complexes 5a(Tn)2(Qk)4, 5b(Tn)2(Qk)4 {[M]-SiOn/2(OX)3-n}2[SiOk/2(OX)4-k]4, [M] = [(dba)Pd]1/2P(Ph)(CH2Do)(CH2)3−} undergo an oxidative addition reaction with iodobenzene in an interphase with formation of the complexes PhPd(I)(PO)2·4SiO2 [6a(Tn)2(Qk)4, 6b(Tn)2(Qk)4] {[M]-SiOn/2(OX)3-n]2[SiOk/2(OX)4-k]4, [M] = [PhPd(I)]1/2P(Ph)(CH2Do)(CH2)3−}, which insert carbon monoxide into the palladium−aryl bond even in the solid state.
  • 1995
    • D. Oeter, C. Ziegler, W. Göpel
    • HREELS to identify electronic structures of organic thin films
    • Fresenius' Journal of Analytical Chemistry, Volume 353, Issue 3-4, pp 360-363 (1995)
    • Abstract >>
    • The electronic structure of α-oligothiophene (αnT) thin films has been investigated for increasing chain lengths of n= 4–8 thiophene units with high resolution electron energy loss spectroscopy (HREELS) in the specular reflection geometry at a primary energy of 15 eV. The great advantage of this technique in contrast to UV/VIS absorption spectroscopy results from the fact, that the impact scattering mechanism of HREELS makes it possible to also detect optically forbidden electronic transitions. On the other hand, the electrons used as probes in HREELS have a wavelength which is two orders of magnitudes smaller if compared to those of photons used in UV/VIS absorption spectroscopy. Therefore individual molecules are excited by HREELS independent from each other and hence the excitation of collective excitons is not possible. As a result, information about the orientation of the molecules cannot be achieved with HREELS, which, however, is possible in polarization-dependent UV/VIS spectroscopy.
    • A. W. Munz, Ch. Ziegler, W. Göpel
    • Atomically Resolved Scanning Tunneling Spectroscopy on Si(001)-( 2×1) Asymmetric Dimers
    • Phys. Rev. Lett., 74, 2244–2247 (1995)
    • Abstract >>
    • The tunneling spectrum of Si(001) reveals a significant difference in the electronic structure of the up and down atoms, which are separated by less than 0.25 nm. In particular, the peaks correlated with the unoccupied states at the conductance band edge could be determined in detail. The intensities of these peaks show periodic modulations coinciding with the periodicity of the (2×1) reconstruction of the asymmetric dimers along the [[(1̅ 10) direction. These results prove directly the theoretically predicted charge transfer from the down to the up atom.
    • H.-J. Egelhaaf, D. Oelkrug, D. Oeter, Ch. Ziegler, W. Göpel
    • HREELS and UV/VIS spectroscopic studies on the electronic structure of oligothiophene thin films
    • Journal of Molecular Structure, Volume 348, Pages 405–408 (1995)
    • Abstract >>
    • Vapour-deposited thin films of a series of α-oligothiophenes of varying chain-length are investigated by HREELS and polarized UV/VIS absorption spectroscopy. The observed loss and absorption bands are assigned to transitions between Hückel-type MOs. The assignment is, on one hand, based upon the dependence of the transition energies on the number of thiophene rings per molecule and on the polarization of the UV/VIS absorption bands. On the other hand, the different selection rules for photon and electron induced transitions can be employed for assignment, if the MOs are considered to form an energy band. The dependence of the spectral position of the UV/VIS absorption bands on film thickness can be related to the degree of molecular pin-cushion alignment on the basis of the Molecular Exciton model.
    • A. W. Munz, Ch. Ziegler, W. Göpel
    • Thermal etching of Si(001) — a STM study
    • Surface Science, Volume 325, Issues 1–2, Pages 177–184 (1995)
    • Abstract >>
    • The thermal etching of vicinal Si(001) in UHV was studied on an atomic scale with STM. Surfaces heated up to 1400 K show a statistical distribution of pyramidal structures, which are pinned by contaminations. At these surfaces, an anisotropic process leads to these tetragonal pyramids which are built up by double layer steps (DB) at the top and by single layer steps at the bottom, forming vicinal Si(11l) surfaces with decreasing l towards the top of the pyramid. From the concave profile of this surface, the stability of certain step edges can be characterized as a function of the tilt angle. Further heating to 1600 K leads to a complete disappearance of the contaminants and hence the pyramids. The well-known stepped Si(001) surface with single layer steps (SA + SB) appears with a high density of point defects. The c(4 × 2) reconstruction (covering up to 35% of the surface) can be resolved close to certain defects with a continuous transition to the p(2 × 1) reconstruction observed further away from the defects. A slight asymmetry within the c(4 × 2) reconstruction is found, appearing as a c(8 × 4) reconstruction of the surface.
  • 1994
    • Ch. Ziegler , Th. Schedel-Niedrig , G. Beamson , D. T. Clark , W. R. Salaneck , H. Sotobayashi , A. M. Bradshaw
    • X-Ray Absorption Study of Highly Oriented Poly(tetrafluoroethylene) Thin Films
    • Langmuir, 10 (12), pp 4399–4402 (1994)
    • Abstract >>
    • By use of highly oriented films, the polarization dependence of the C 1s and F 1s near-edge absorption spectra of poly(tetrafluoroethylene) has been measured. At least at the carbon K edge, good agreement is obtained with the predicted behavior based on a consideration of the one-dimensional band structure and the effect of symmetry breaking. The findings are also relevant for the assignment of the resonances in long chain alkanes, a subject which has recently been somewhat controversial.
    • D. Oetera, Ch. Ziegler, W. Göpel, H. Haarmann
    • Spectroscopic and electrical investigations of undoped and doped α-oligothiophene thin films
    • Conf. Proc. E-MRS Spring Meeting 1994, Straßburg (F); Synthetic Metals, Volume 67, Issues 1–3, Pages 267–271 (1994)
    • Abstract >>
    • Chemically pure α-oligothiophene (αnT, n = 4–8) were used in this study to prepare oligomeric thin film structures as model systems for polythiophene. The pure materials were characterized under UHV conditions with X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high resolution electron-energy-loss spectroscopy (HREELS). The change of the electronic structure with increasing number of rings could be evaluated quantitatively. Furthermore, the solid-state doping effects of the oligomers α5T and α7T were investigated in situ for systematically varied concentrations (0–70%) of FeCl3 as dopant. With increasing dopant concentration two additional loss structures were observed at energy losses below the bandgap transitions of the pure oligomers. Finally, four-point measurements were performed of co-sublimed thin films of p-doped α6T under UHV conditions. A conductivity of δ = 3·10−1 (Ω cm)−1 was determined for the p-doped material at T = 300 K.
    • O. Böhme, Ch. Ziegler, W. Göpel
    • Highly ordered ultra-thin α5-thiophene films on SiO2 and Si(100) + O(N2O)
    • Conf. Proc. E-MRS Spring Meeting 1994, Straßburg (F); Synthetic Metals, Volume 67, Issues 1–3, Pages 87–92 (1994)
    • Abstract >>
    • Highly ordered oligothiophene films with well-defined geometries are required in order to correlate unequivocally their structure with conductivity data. Of particular interest concerning suitable substrate materials for these films are microelectronic-compatible substrates based on Si(100), which either exhibit an insulating interlayer (SiO2) or allow for a direct electrical contact (Si(100) + O(N2O)) to ultra-thin (20–50 Å) organic films. In this context, we discuss the preparation and characterization of highly ordered, UHV-evaporated α5-thiophene (α5T) films on SiO2 and on oxygen-terminated Si(100) by presenting comparative results from atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS).
    • Oliver Böhme, Christiane Ziegler, Wolfgang Göpel
    • Oriented growth of quinquethiophene on SiO2—An atomic force microscopy study
    • Advanced Materials, Volume 6, Issue 7-8, pages 587–589 (1994)
    • Abstract >>
    • Highly ordered organic films must be prepared and characterized if the electrical properties of future organic-based electronic devices are to be improved, because it is only then that our understanding of intermolecular transport in polymer films can increase. An atomic force microscopy study is presented that shows that quinquethiophene can be physisorbed on Si02 to form highly ordered layers. A change from layer-by-layer to multilayer growth is reported to occur upon completion of the second monolayer.
    • D. Oeter, H.‐J. Egelhaaf, Ch. Ziegler, D. Oelkrug, W. Göpel
    • Electronic transitions in α‐oligothiophene thin films. Comparison of ultraviolet/visible absorption spectroscopy and high resolution electron energy loss spectroscopy investigations
    • J. Chem. Phys. 101, 6344 (1994)
    • Abstract >>
    • Vapor deposited thin films of a series of α‐oligothiophenes are investigated comparatively with polarized ultraviolet/visible absorption spectroscopy (UV/VIS) and by high resolution electron energy loss spectroscopy (HREELS) in specular reflection geometry. The complementary selection rules of these methods allow an assignment of the observed absorption and loss bands according to a Hückel molecular orbital model. By plotting the transition energies of corresponding bands of different members of the homologous series vs the reciprocal of the number of rings, the development of the one‐dimensional ‘‘π‐band‐structure’’ with an increasing number of rings could be followed. The extrapolation to infinite chain length leads to the electronic properties of an ideal (defect free) polythiophene. Furthermore, characteristic differences were observed in the results obtained from the two methods. The orientation of the molecules in thin films is only detectable with UV/VIS spectroscopy. It is most pronounced for α‐quinquethiophene. On the other hand, HREELS gives information about the position of optical parity forbidden electronic transitions.
    • P. Lemoine, K. Cazzini, I.T. McGovern, W.J. Blau, P. Bätz, C. Ziegler, W. Göpel
    • X-ray photoelectron spectroscopy of KrF laser-irradiated polycarbonate films
    • Chemical Physics Letters, Volume 220, Issues 3–5, Pages 177–180 (1994)
    • Abstract >>
    • Spun films of polycarbonate were photoablated in air at 248 nm. The etch and incubation data have a superlinear dependence on the laser fluence. The surface compositional changes were analysed by X-ray photoelectron spectroscopy (XPS). As evidenced by the decrease of the O/C ratio, photoablation is inferred as distinct from the low-intensity photooxidative degradation. A direct correlation is observed between the XPS and incubation data and the ablation seems to occur by chain scission at the CO bond, the fully incubated surface being an average of monomeric composition.
  • 1993
    • D. Oeter, Ch. Ziegler, W. Göpel
    • Doping and stability of ultrapure α-oligothiophene thin films
    • Conf. Proc. E-MRS Spring Meeting (1993); Synthetic Metals, Volume 61, Issues 1–2, Pages 147–150 (1993)
    • Abstract >>
    • A new method is presented for doping organic semiconductors. The organic materials were evaporated by the Knudsen method under UHV conditions and were subsequently investigated spectroscopically. As ‘model system’ we investigated α-sexithiophene (α6T) doped with FeCl3. Both compounds were co-sublimed under UHV conditions. The solid-state doping effects were studied in situ for systematically varied dopant concentrations with X-ray photoelectron, ultraviolet photoelectron and high-resolution electron-energy-loss spectroscopies. Up to a doping level of one effective charge per α6T, the observed changes in electronic states (energetic shifts of core level and valence electrons, additional states in the bandgap) can be explained by polaron formation. Subsequent gas exposure experiments indicate that FeCl3 is not stable as a dopant in the presence of water and oxygen.
    • J. Bufler, Ch. Ziegler, W. Göpel
    • Doping effects in thin films of radical phthalocyanines: UPS and conductivity studies
    • Synthetic Metals, Volume 61, Issues 1–2, Pages 127–131 and Conf. Proc. E-MRS Spring Meeting 1993, Straßburg (F) (1993)
    • Abstract >>
    • The influence of dopants on the electronic structure and the electrical properties of the radical phthalocyanines LuPc2 and LiPc are investigated. Oxygen and hydrogen act as irreversible ‘dopants’ which slightly decrease the conductivity of thin films. The influence of H2 and other dopants added by gas phase reactions (i.e., Cl2 and I2) or by electrochemical treatment on the electronic structure is studied by ultraviolet photoemission spectroscopy (UPS). Particular emphasis is placed on the determination of occupied and unoccupied electronic states around the Fermi level. Upon oxidation of LuPc2 and LiPc, the singlet electron in the semi-occupied HOMO (‘SOMO’) is removed. Upon reduction, a doubly occupied HOMO is found. Pronounced changes in the conductivity are found after heat treatment of films under UHV conditions. These changes correlate with sintering effects and variations in the grain structure at T ⩾ 450 K.
    • D. Martin, R. Elliot-Martin, Ch. Ziegler, M. Willis, W. Göpel
    • Conductivity measurements of (GaPcF)n and (AlPcF)n thin films: influence of gas exposure and structural disorder
    • Conf. Proc. E-MRS Spring Meeting (1993), Straßburg (F); Synthetic Metals, Volume 61, Issues 1–2, Pages 121–125 (1993)
    • Abstract >>
    • Conductivity measurements of ultrahigh vacuum-sublimed (MPcF)n thin films (M=Al, Ga; Pc = phthalocyanine) were performed under ultrahigh vacuum and well-defined gas conditions. The intrinsic dark conductivity of as-prepared (GaPcF)n thin films is remarkably high (4 × 10−6 S/cm at 300 K) if compared to pressed pellets (3 × 10−6 S/cm at 300 K). After heating to 485 K, the conductivity decreases to 2.4 × 10−6 S/cm. This is the result of changes in the geometric structure observed in scanning electron microscopy. In contrast to (GaPcF)nm, a very low dark conductivity (< 10−9 S/cm at 300 K) is observed for (AlPcF)n. Light illumination in the visible range and exposure to dry oxygen have only a small influence on the conductivity, whereas exposure to ambient air and to water in a nitrogen atmosphere increases the conductivity of (AlPcF)n remarkably (6 × 10−2 S/cm at 300 K for pH2O = 0.2 mbar).
    • P. Bodö, Ch. Ziegler, J. R. Rasmusson, W. R. Salaneck, D. T. Clark
    • Scanning tunneling microscopy study of oriented poly(tetrafluoroethylene) substrates
    • Synthetic Metals, Volume 55, Issue 1, Pages 329–334; Conf. Proc. ICSM '92, Göteborg (S) (1993)
    • Abstract >>
    • Highly oriented molecular structures of organic molecules are important in many applications of thin films and interfaces. Recently, it has been shown that poly(tetrafluoroethylene), or PTFE, can be deposited mechanically to form highly oriented thin films on glass substrates. Such oriented PTFE films can in rurn be used as substrates for growth of ordered films of organic molecules and polymers, e. g. conjugated polymers. Despite the fact PTFE is electrically insulating, we have been able to use scanning tunneling microscopy (STM) to examine details of oriented ultrathin PTFE films deposited on various substrates, in particularly Pt and highly ordered pyrolytic graphite (HOPG). Ordered structures originating from aligned chains were observed, and reproducible images were obtained at the submicron level. Nanometric molecular details are more difficult to resolve, but indicate interesting features. In particular, we have observed regions of parallel zig-zag shaped molecules, which are separated by approximately 6 Å.
    • D. Oeter, Ch. Ziegler, W. Göpel, H. Naarmann
    • Undoped and Doped Oligothiophenes: an In-situ XPS-, UPS-, and HREELS-Study
    • Berichte der Bunsengesellschaft für physikalische Chemie, Volume 97, Issue 3, pages 448–451 (1993)
    • Abstract >>
    • A new method for doping organic semiconductors is presented. The most common dopant for the widely used α-sexithiophene (α6T), FeCl3, was co-sublimed with the oligothiophene under UHV-conditions. The solid-state doping effects were studied for systematically varied dopant concentrations in-situ with XPS. UPS, and HREELS. Up to a doping level of one effective charge per α6T the results can be interpreted as polaron formation. At higher dopant concentrations polaron bands are most probably formed.
    • Th. Kugler, Ch. Ziegler, W. Göpel
    • Chemically modified semiconductor surfaces: 2,6-dimethylisocyanobenzene on Si(100)(2x1)
    • Journal of Molecular Structure, Volume 293, Pages 257–260 (1993)
    • Abstract >>
    • In view of developping controlled dense organic 2D-monolayers on silicon, we investigated the covalent binding of 2,6-dimethylisocyanobenzene to a clean Si(100)(2x1) single crystal surface using X-ray photoelectron spectroscopy (XPS). The experimental data on core-level binding energies are interpreted by using ab-initio calculations.
  • 1991
    • Ch. Ziegler, G. Frank, W. Göpel
    • Passivation Layers on YBa2Cu3O7-x Thin Films
    • Conf. Proc. ICAM 1991, Straßburg (F), "High Tc Superconductor Thin Films", L. Correra (Ed.) North Holland, Amsterdam 1992, p.707-713 (1991)
    • Ch. Ziegler, G. Frank, W. Göpel
    • Interface Properties of Thin YBa2Cu3O7–x Films
    • Berichte der Bunsengesellschaft für physikalische Chemie, Volume 95, Issue 11, pages 1404–1409 (1991)
    • Abstract >>
    • Structures and stability ranges of the YBa2Cu3O7–x interface were studied with AES, SIMS, XPS, and TEM. For silicon substrates, the formation of Ba silicates can be detected which even at T = 550 K leads to reduced Tc values and non-epitaxial growth of YBa2Cu3O7–x (YBCO). Yttria stabilized zirconia (YSZ) buffer layers between Si substrates and YBCO lead to epitaxial overlayer growth and improved superconducting properties whereas CaF2 is not suitable for this purpose.—Furthermore, SiO2, YSZ, and CaF2 were tested for their suitability as passivation layers against the influence of humidity. As-passivated superconducting films are stable up to 500 K in 100% humid air. Above this temperature the superconductor is reduced chemically. Non-passivated films, however, are stable only at temperatures below 450 K and are destroyed totally in humid air at higher temperatures.
    • G. Frank, Ch. Ziegler, W. Göpel
    • Surface composition of clean, epitaxial thin films of YBa2Cu3O7-x from quantitative x-ray photoemission spectroscopy analysis
    • Phys. Rev. B 43, 2828–2834 (1991)
    • Abstract >>
    • Angle-dependent x-ray photoemission spectra (XPS) of epitaxial thin-film YBa2Cu3O7-x show characteristic reversible and distinct changes with different surface treatments. These changes are analyzed by applying a new model for the quantitative XPS analysis, which takes into consideration the anisotropic structure of c-axis-oriented YBa2Cu3O7-x. The analysis makes it possible to differentiate between different elemental compositions of the outer layers in the surface unit cell. Evaluations of our experimental data indicate that only surfaces with a nominal BaO and CuO layer composition exist in the outermost atomic layer.
    • C. Ziegler, F. Baudenbacher, H. Karl, H. Kinder, W. Göpel
    • Interface analysis of the system Si/YBa2Cu3O7−x
    • Fresenius' Journal of Analytical Chemistry, Volume 341, Issue 3-4, pp 308-313 (1991)
    • Abstract >>
    • The influence of different preparation conditions and substrate surface orientations on the superconducting properties of thin YBa2Cu3O7−x (YBCO) films on silicon was studied. Comparative electrical and surface spectroscopic measurements were performed. SAM and SIMS depth profile analysis show an enrichment of barium at the interface between the superconductor and silicon for samples with Tc<76 K. Comparison with XPS data obtained for thin silicon films on YBCO indicates the formation of barium and yttrium silicates at the interface under these conditions.
  • 1990
    • Ch. Ziegler, G. Frank, W. Göpel
    • Photoemission study of the Ba core levels in YBa2Cu3O7-x
    • Zeitschrift für Physik B Condensed Matter, Volume 81, Issue 3, pp 349-353 (1990)
    • Abstract >>
    • Angle dependent X-ray photoemission spectra of epitaxial thin YBa2Cu3O7-x films show characteristic differences after various heat and oxygen pretreatments. The component with the lowest binding energy in Ba 3d 5/2 and 4d 5/2 peaks results from bulk Ba in YBa2Cu3O7-x . A component with 2 eV higher binding energy results from Ba in the surface unit cell with different bonding if compared with the bulk superconductor. This surface structure is observed for all YBa2Cu3O7-x surfaces which have been exposed to oxygen. It is not observed for UHV prepared surfaces after mechanical scraping or fracturing.
    • B. Dauth, P. Sen, B. Rupp, W. Gudat, Ch. Ziegler, W. Göpel
    • Non-superconducting and rendered superconducting YBa2Cu3O7−x . An in-situ photoemission study
    • Zeitschrift für Physik B Condensed Matter, Volume 79, Issue 2, pp 169-172 (1990)
    • Abstract >>
    • We present X-ray photoemission spectroscopy (XPS) results obtained on polycrystalline YBa2Cu3O7−x in the non-metallic (x>0.6) and metallic state (x<0.6) by an in-situ oxygen deloading/loading procedure, thus avoiding uncontrolled surface contaminations. The transition to the metallic (superconducting) state is characterized by corresponding changes in the O1s and Cu2p photoelectron lineshapes being in accordance with charge transfer from CuO2 planes to CuO chains. In particular, a peak at 531.1 eV binding energy is related to the presence of oxygen hole states (O−).
  • 1989
    • U. Straub, D. Krug, Ch. Ziegler, D. Schmeißer, W. Göpel
    • Ba2Cu2O5 as a stable prototype material for investigating the “Cu(III)-problem” in high-TC-superconductors
    • Materials Research Bulletin, Volume 24, Issue 6, June 1989, Pages 681-686 (1989)
    • Abstract >>
    • Ba2Cu2O5 was found to be a remarkably stable prototype material to be used as a Cu(III) reference in X-ray photoemission and Auger spectroscopy. These investigations are of particular importance in clarifying the influence of the copper valence state on the conduction mechanism of high-TC-superconductors. The valence state was determined in a comparison between iodometric measurements and results from atomic absorption spectroscopy as well as by EPR investigations. Stabilities were determined by effluent gas analysis and differential thermoanalysis spectra in a H2 flow and by thermogravimetry and differential thermogravimetry in a N2 flow. X-Ray photoemission and Auger results of Ba2Cu2O5 are compared with corresponding results from Cu, Cu2O, and CuO.