Self-organization is a wide-spread phenomenon in nature and manifests itself in that an ordered state is formed that is far from the thermodynamic equilibrium and often shows complex spatio-temporal dynamics. The infinite diversity of patterns in physical, chemical and biological systems can be classified with modern concepts of nonlinear dynamics and a kind of soft-control of their spatio-temporal evolvement.
In complex systems, self-organizing structures arise as a consequence of the underlying nonlinear dynamics. In the early days, research was concentrated on low-dimensional systems. Mostly nontrivial behavior in the time domain was considered, such as oscillations and deterministic chaos. Over the past decades, research efforts were increasingly focused on pattern formation in high-dimensional, spatially extended systems. Besides the classical hydrodynamic instabilities, systems of the reaction-diffusion type and complex networks are among the most widely studied examples which can be found in physics, chemistry, biology, ecology and other fields. Our research on complex systems therefore covers a broad range of topics, ranging from the observation and control of self-organizing patterns in reaction diffusion systems and corrosion dynamics to protein folding, oscillator networks, and enzyme dynamics.
In the Surface Imaging Group, experimental and theoretical research on critical nonÂequilibrium phenomena during the onset of pitting corrosion on stainless steel has been continued and extended to other important systems. Newly developed in-situ methods have been applied to the corrosion of aluminum and the etching and pore formation of n-InP. An interesting feature of the latter system is, that the process can be controlled by light which allows for patterning of the material. Patterned porous semiconductor structures are of great importance for the design of photonic devices and sensors.
The research on the catalytic oxidation of carbon monoxide on Pt(110) single crystal surfaces has been continued. Since a few years attention has shifted from analyzing and understanding the spontaneously emerging spatiotemporal patterns to actively influencing and manipulating the system dynamics. The experimental setup has been extended such that patterns can be simultaneously manipulated globally as well as locally. Global system parameters can be modulated by real time control of partial pressures in the reactor while local control is achieved by means of focussed laser light. Recent results include the observation of frequency locked patterns with subharmonic entrainment at high forcing frequencies. The combination of global and local manipulation allowed for observation of so-called 2p phase kinks that exist at the boundaries of cluster patterns. These observations were possible because of crucial improvements of our experimental setups, like the development of a new reflection anisotropy microscope (RAM) and the construction of novel gas driving compressor that increased the accessible forcing frequency range drastically. (Rotermund)
Several new research directions, related to various aspects of nanobiology, were explored. Attention was focused on the operation of single-molecule protein machines and on the collective behaviour in large populations of interacting molecular machines. Molecular synchronization waves in the arrays of interacting allosteric enzymes were predicted. In cooperation with the Department of Computational Molecular Biology of the Max Planck Institute for Molecular Genetics in Berlin, properties of complex biochemical networks were analyzed. Nonequilibrium pattern formation in active Langmuir monolayers including chiral molecules has been considered; similar phenomena in biomembranes with active molecular inclusions are being investigated.
Additional research funding has been provided by DFG in the framework of Sfb 555 Complex Nonlinear Processes, from the European Union through the Marie Curie Research Training Network PATTERNS: Unifying Principles in Nonequilibrium Pattern Formation, and from the Volkswagen Foundation within its program Complex Networks as a Cross-Disciplinary Phenomenon.
Prof. A. S. Mikhailov has received an invitation from the Solvay Foundation to the International Solvay Chair in Chemistry in Belgium take in 2009. This invitation has been accepted. (Mikhailov)
Further experimental studies of electrochemical pattern formation have been extended by lowering the conductivity of the electrolyte substantially. In formic acid oxidation on a Pt ring, decreasing conductivity leads to a richer variety of patterns, including (besides standing waves and rotating pulses which already exist for high conductivity) cluster formation, pulse reflection and propagation failure. The pattern selection seems to depend on the type of electrochemical oscillator used (NDR or HNDR), a systematic theoretical investigation using reaction-migration equations is planned.
Application of ideas from algebraic geometry (graph theory and ideal theory) to chemical reaction dynamics not only gives theoretical insight into discrete-algebraic aspects of bifurations, but also allows analytical solutions of the stationary states of kinetic models and the stability problem. The set of roots can be obtained by an intersection of the convex flux cone and a deformed toric variety obtained from the Grbner basis of the reaction binomials. In contrast to earlier approaches involving convex geometry only, this intersection can be mapped back into standard kinetic parameters, allowing the determination multistability and the regions where local bifurcations occur.
Recently, a new project involving discrete mathematics has been started in cooperation with the group of P. Plath (Institut für Angewandte und Physikalische Chemie, Universität Bremen). The dynamics of foam decay is characterised and modelled using order theory (in particular classical and weak majorisation). (Eiswirth)
2. Raman spectroscopy
In science and thechnology there is a strong demand to characterize even smaller entities and structures. A recently developed approach, tip-enhanced Raman spectroscopy (TERS) is very promising in this respect, it is a variant of surface enhanced Raman spectroscopy (SERS) but avoids its limitations. Both spectroscopies are based on the optical excitation of localized surface plasmons in small metal structures. In the case of TERS, surface plasmons are excited at the tip only, or between the Au (or Ag) tip of a scanning probe microscope and any (smooth) surface, whereby a very strong local field enhancement is achieved in the gap. A Au-tip / Au-substrate configuration proved to support particularly strong field enhancements. This approach has a great potential for application in research and technology as it provides a vibrational spectroscopy with very high sensitivity and nanometer resolution.
TERS spectra from various adsorbed organic and inorganic molecules were recorded. With a sharp tip of ~20 nm tip radius and the dye malachite green isothiocyanate, single-molecule sensitivity has been achieved: less than 5 molecules underneath the tip can be seen by this technique, nevertheless providing clear fingerprint spectra. For molecules not in optical resonance with the exciting laser, such as adenine, the detection sensitivity is currently about 130 molecules. The high spatial resolution is evident from two facts: upon retracting the tip by 10 nm from the Au(111) surface, (i) the TERS intensity drops rapidly to 5% of its initial level and (ii) a blue shift of the gap resonance mode is observed, because the short range near-field coupling between tip and metal ceases.
The TERS approach has been transferred to UHV, by developing a third-generation TERS apparatus. The necessary optics, including a high numerical aperture parabolic mirror, are completely included into the UHV chamber with the parabolic mirror integrated between the STM scanner and the sample. Optical fibers deliver the incident laser beam and the inelastically scattered light to the sample and spectrograph, respectively. First results are very promising: The sharp focus (λ/2) supports a high contrast between TERS and usual Raman scattering, leading - for example - to a 4000-fold increase of the Raman signal upon tip approach for the dye brilliant cresyl blue. The underlying enhancement of Raman scattering is about 106. Raman mapping is also possible, e.g. the recording of an optical image with a resolution better than 50 nm, which is much below the Abbe limit of ~λ/2.
Parallel to the UHV TERS, a setup for electrochemical TERS (EC-TERS) has been built. It uses a parabolic mirror riding on a glass plate, a configuration that serves simultaneously as an electrochemical cell and as an optical device for focusing and collecting light. This sophisticated and challenging approach is currently in the test phase.
Thus, a powerful tool for vibrational spectroscopy is being developed that provides chemical and topographic information with spatial resolution in the nanometer regime and is applicable in UHV, gaseous and liquid phases. (Pettinger)