J.W. Schultze

Electron and proton conducting polymers: recent developments and prospects

Abstract The most important topics of the rapidly developing field of conducting polymers are surveyed. Particular emphasis is laid on the problems of synthesis, structure, thermodynamics and kinetic behaviour of these systems. The relevant experiences, existing models and theories are outlined. Abundant examples of the growing applications are also discussed.

Stability, reactivity and breakdown of passive films. Problems of recent and future research

Abstract Due to a large variety of properties, passive films and their stability play an important role in recent and future research. At first, the nature of passive films is discussed for the steady state. New techniques allow the detailed determination of film thickness, stoichiometry, microstructure, and electronic properties. The distinction of these properties allows a rough classification of oxide films and their properties. In the second part, various processes, such as growth, reduction, dissolution, and modification are discussed. These overall processes depend on the rate of ion and electron transfer reactions (ITR, ETR) in a complex manner. Examples are given for some special reactions. Applications are discussed for surface protection, micro and nano systems and electrocatalysis.

Nucleation, growth and branching of polyaniline from microelectrode experiments

Abstract The kinetics of electrochemical formation of polyaniline were studied under potentiostatic conditions on gold and platinum microelectrodes. Two different stages of growth were observed, the first corresponding to the formation of a compact layer with average thickness of about 200 nm, the second corresponding to the growth of a branched structure. The initial parts of the current transients obtained at two different aniline concentrations were interpreted by means of the theory for progressive and instantaneous nucleation and growth. The study of the potential dependence at the very initial stage of film formation has shown that nucleation is not likely to be an electrochemical step in the overall polymerization process. A detailed investigation of the growth process alone yielded different apparent transfer coefficients corresponding to the two different stages of growth.

Capacitive microsensors for biochemical sensing based on porous silicon technology

Abstract Porous EIS (electrolyte–insulator–semiconductor) structures of n-Si/SiO2/Si3N4 with a mean pore diameter of about 1 μm and a mean pore depth of about 2 μm have been realized for capacitive pH sensors. For the fabrication of the porous microsensors (down to “spot” sizes of 10 μm×10 μm), the n-doped silicon substrates have been photolithographically patterned by means of mask-matching technique using polyimide as a passivation material. The average pH sensitivity of the porous pH microsensor amounts about 56 mV/pH in the concentration range between pH 4 and pH 8. In order to prepare porous EIS biosensors the enzyme penicillinase has been adsorptively immobilized inside the porous structure. In the case of the porous biosensors an average penicillin sensitivity of about 90 mV/mM in the concentration range from 0.01 to 1 mM exists. Microreference electrodes, also prepared by the same porous silicon technology as for the pH- and biosensors, show a potential stability of more than 1 week in the long term.

Miniaturization of potentiometric sensors using porous silicon microtechnology

A new capacitive field-effect microsensor based on a porous EIS (electrolyte-insulator-semiconductor) structure is presented. The porous silicon sensor was prepared using standard techniques of semiconductor processing. A well-defined macroporous layer was formed on silicon by electrochemical etching and a SiO2Si3N4 sandwich was deposited as insulating and pH-sensitive layer. The porous sensor exhibits a high, near-Nernstian pH sensitivity of about 54 mV per decade in the concentration range from pH 4 to pH 8, similar to a planar non-porous EIS structure with the same layer sequence. The enlargement of the active sensor area (surface) due to the porous structure increases the measured capacitance and thus allows a scaling down of the sensor. The preparation of biosensors based on the same structure is demonstrated by immobilization of the enzyme penicillinase as biosensitive component.

Microscopic investigations of electrochemical machining of Fe in NaNO3

Abstract Electrochemical micro machining (ECMM) is a promising process in microsystem technologies. The work-pieces of steel and other metals are structured in neutral NaNO 3 solution by anodic dissolution at large current densities (about 100 A/cm 2 ) and high electrolyte flow rates (some m/s). Accordingly, an identification of the processes, the structure and the current distribution at the interface of the work-piece is critical. Since available simulation software for ECMM neglects details of the work-piece—electrolyte interface and needs detailed information on rate determining processes, two new strategies are presented. In a first approach, the scanning droplet cell was modified to enable such large current densities and flow rates in a three-electrode arrangement under potentiostatic conditions. A new flow-through concept was developed to realize flow and to suppress effects of side reactions like oxygen bubble formation and precipitation of products. A simultaneous product analysis yields the Fe 2+ /Fe 3+ ratio. The second approach concerns (super-) saturated solutions of Fe(NO 3 ) 3 and Fe(NO 3 ) 2 , which we expect at the metal surface. These liquids are meta-stable without crystallization for days and form highly viscous honey-like surface layers, which represent the transition from solutions to melts. Conductivities and viscosities are measured to determine diffusion coefficients under these conditions to get parameters for modeling of the process in future finite element simulations. On the basis of our experiments, a two-layer interface is assumed: a modified solid passive film and a viscous adherent layer of supersaturated iron nitrates.

Passivation and corrosion of microelectrode arrays

Application of silicon based microsensors in electrolyte solutions is hampered by insufficient barrier properties and poor corrosion resistance of common passivation layers used to protect the underlying conducting tracks and microelectronic structures. Therefore, the protectivity of various types of compatible passivation layers (organic polyimide and photoresist films, inorganic mono, duplex and triplex layers based on PECVD silicon oxide and silicon nitride) was investigated and improved on microelectrode arrays exposed to I M NaCI (pH 2 to 10) at 25°C. Duplex SiO 2 /Si 3 N 4 and oxide/nitride/oxide (ONO) triplex layers with optimised nitride PECVD process yielded the best barrier properties. Burying the conducting tracks in the thermal silicon oxide layer improves the performance significantly. Failures of the passivation layers, detected by leak current and layer resistance measurements with subsequent SEM investigation, result from cracking due to intrinsic and extrinsic (less important) mechanical stress, film defects (pinholes, particle inclusions), from chemical, physicochemical and electrochemical reactions (external, internal, sublayer corrosion) and from the combined action of mechanical stress and chemical interaction (stress corrosion cracking).

Effect of texture and formation rate on ionic and electronic properties of passive layers on Ti single crystals

Potentiodynamic (slow) and potentiostatic (fast) TiO2 layer formation on different oriented Ti single crystals of a coarse grain sample is compared. The crystal orientations were determined by the new AME-method (Anisotropy Micro Ellipsometry). Cyclovoltammograms, impedance, transient and electron transfer reaction (etr) measurements were carried out on each crystal applying the new nl photoresist droplet method. For slowly grown layers a clear texture dependence occurs. The microcoulometrically determined charge (layer thickness) increases and the defect state concentration decreases with decreasing package density of the substrate surface. Etr reveals both direct and resonance tunnelling. Texture dependence vanishes completely for fast formed layers as is proved by transient and impedance measurements. Due to the low defect state density etr shows direct tunnelling only. The influence of texture reappears for TiO2 layers formed potentiodynamically on top of the fast grown oxides.

Anodic polymerization of 3,4-ethylenedioxythiophene from aqueous microemulsions

Abstract The electrochemical polymerization of 3,4-ethylenedioxythiophene (EDT) was studied in microemulsions containing polyoxyethylene-10-laurylether as a non-ionic micellar surfactant. This allows to increase the solubility of the monomer in an aqueous solution. Thereby, the surfactant affects the transport of EDT. Capacity measurements and cyclic voltammetry on bare Pt show the influence of the surfactant by adsorption on the metal, too. An adsorption on the polyethylenedioxythiophene (PEDT) surface could not be proved. Combined charge transfer and diffusion limitations control the electropolymerization kinetics at low monomer and surfactant concentrations. Charge transfer becomes limiting at high EDT concentrations. Apparent diffusion coefficients and the charge transfer coefficient of the electrochemical reaction were obtained from RDE experiments. The redox activity, UV/VIS spectra and surface morphology (from SEM pictures) depend on the polymerization potential and the solution composition. Best films were obtained at low potentials in LiClO 4 . At high potentials overoxidation of the polymer takes place, but an incorporation of the surfactant could not be detected.

Electrochemical microsystem technologies : from fundamental research to technical systems

Abstract The interdisciplinary field of electrochemical microsystem technologies (EMST) has a large impact on electrochemistry, electrochemical engineering, microengineering, material science, chemical analysis and its applications in biology and medicine. EMST include electrochemical reactions applied in microsystem technologies (MST) as well as MST applied in electrochemistry. Fundamental research refers to the scaling-down of reactions taking place in the μm range. Control of electrochemical microreactions, their advantages and applications are discussed. The technical requirement of mass production presumes the scaling-up of the microreaction to a multifold macroscopic process. This is demonstrated for electrochemical processes, e.g. the LIGA process, as well as for MST for electrochemistry, e.g. the Foturan® technology. The final industrial process requires the realization of multistep processes which is illustrated with the phosphating process and the printed circuit board manufacturing as examples. Electrochemical microsystems are presented and classified by functionality and complexity.