Ch. Ziegler

Bioelectronic noses: A status report Part I

The present state of the art to record or mimic electronically the human senses of olfaction and taste is characterized. In this part I, an introduction to our present understanding in the development of electronic and bioelectronic noses is given. Finally the natural olfactory system is described in detail.

Characterization and optimization of microelectrode arrays for in vivo nerve signal recording and stimulation

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 omega and 10 k omega. To attain low impedances for all electrodes on the INTER device, platinum from H2PtCl6 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 microns. 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 microC/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.

A new cantilever system for gas and liquid sensing

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.

Electronic transitions in α‐oligothiophene thin films. Comparison of ultraviolet/visible absorption spectroscopy and high resolution electron energy loss spectroscopy investigations

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 Huckel 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‐qu...

Interaction of alkali metals with perylene-3,4,9,10- tetracarboxylic–dianhydride thin films

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.

Gas phase and bulk ultraviolet photoemission spectroscopy of 3,4,9,10-perylene-tetracarboxylic dianhydride, 1,4,5,8-naphthalene-tetracarboxylic dianhydride, and 1,8-naphthalene-dicarboxylic anhydride.

The pi-conjugated organic molecules 3,4,9,10-perylene-tetracarboxylic dianhydride, 1,4,5,8-naphthalene-tetracarboxylic dianhydride, and 1,8-naphthalene-dicarboxylic anhydride were investigated via gas phase and bulk ultraviolet photoemission spectroscopy and compared to density functional theory calculations. Values for final state effects such as intermolecular polarization were determined and the differing features in the spectra interpreted as a consequence of interactions in the thin films. Additionally, the highest occupied molecular orbitals of the molecules clearly show distinctive peaks originating from vibrational excitations, leading to results for Franck-Condon factors.

Alkali metals in perylene-3,4,9,10-tetracarboxylicdianhydride thin films

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.

Characterization and simulation of organic adsorbates on the Si(100)(2 × 1)-surface using photoelectron spectroscopy and quantum mechanical calculations

Abstract The chemical reactivity of the clean Si(100)(2 × 1)-surface was studied by means of photoelectron spectroscopy and quantum chemical calculations. Assuming an analogous reactivity of the Si(100)(2 × 1)-surface and disuene molecular compounds, new concepts for a defined coupling of organic molecules to silicon surfaces were tested by applying well-known disuene reactions to the Si(100)(2 × 1)-surface.

Doping and stability of ultrapure α-oligothiophene thin films

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 FeCl 3 . 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 FeCl 3 is not stable as a dopant in the presence of water and oxygen.

HREELS and UV/VIS spectroscopic studies on the electronic structure of oligothiophene thin films

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 Huckel-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.