Fritz-Haber-Institut der Max-Planck-Gesellschaft  Department of Physical Chemistry
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Recent Developments in the
Department of Physical Chemistry (2003)

Director: G. Ertl

Since the last meeting of the Fachbeirat the following changes occurred among the staff scientists:

·  Dr. K. Doblhofer retired. 
·  Dr. W. Ekardt deceased. 
·  Dr. T. Hertel accepted an offer as Associated Professor at the Vanderbilt University, Nashville (USA) and will leave the institute in 2004. 
·  Prof. K. Jacobi retired officially but continues part-time until October 2004. 
·  Dr. K. Krischer became Professor of Physics (C3) at the Technische Universität München. 
·  Dr. H. Over became Professor of Physical Chemistry (C3) at the University of Giessen. 
·  Dr. H.H. Rotermund became Adjunct Professor of Physics at the Dalhousie University, Halifax (Canada). 
·  Dr. J. Wintterlin became Professor of Physical Chemistry (C3) at the University of München. 

Apart from the director of the department, several other staff scientists (J.F. Holzwarth, W. Schulze, M.S. Zei) will retire prior to the next meeting of the Fachbeirat in 2005.

 
 
Research:
 

Overview

 
Methods


Activity reports

2011 · 2009

2007 · 2005 · 2003

Despite this ongoing internal reorganization research in the department continues to focus on the dynamics and reactivity of solid surfaces in contact with gaseous and liquid phases, including carbon nanotubes, metal clusters, colloidal systems, electrochemical microstructuring, and in particular phenomena of nonlinear dynamics in complex systems.

As in previous years, members of the department were actively involved in the following DFG-Sonderforschungsbereiche: 
Sfb 296:  Wachstumskorrelierte Eigenschaften niederdimensionaler Halbleiterstrukturen; 
Sfb 450:  Analyse und Steuerung ultraschneller photoinduzierter Reaktionen; 
Sfb 555:  Komplexe nichtlineare Prozesse.

Apart from that there exist worldwide cooperations (in part associated with external fundings) with numerous other groups.

In the following the main research topics and a few highlights will be sketched.

1. Electronic excitations and ultrafast dynamics at surfaces

     Electronic excitations are the key for elementary processes in surface reactions as well as for a number of physical properties. While their lifetimes in confined systems (molecules) may become long enough to enable optical decay, this channel will be efficiently quenched as soon as the electronic structure becomes delocalized (i.e. band-like). The transition between molecular and metallic state with increasing number of atoms becomes evident in studies on the agglomeration of small metal clusters in noble gas matrices. As found previously, aggregation of Agn (or Cu) clusters with 3 < n < 20 may be associated with the emission of visible light which was identified with the optical decay of electronically excited monomers, dimers or trimers. The yield of chemiluminescence was nvestigated as a function of deposition rates of the metal and the noble gas, as well as the temperature of the matrix, and rather complex correlations were found. There are indications that this effect might be quite universal as suggested by recent observations with Mg.       Direct determination of the lifetimes of electronic excitations may be performed by time-resolved photoemission employing femtosecond laser techniques. In this context, previous studies with graphite and single-wall carbon nanotubes (SWNT) were extended to MgB2, a recently discovered strongly coupled phonon-mediated superconductor. These experiments enabled determination of the electron-phonon mass enhancement parameter , which for MgB2 was found to be more than three orders of magnitude larger than that for graphite or metallic SWNT and about one order of magnitude larger than in copper. Similarly, the dynamics of photoexcited holes in the surface state of Ag(111) and their coupling to phonons was investigated. 
     Rapid heating of the electron gas at a metal surface by absorption of an intense femtosecond laser pulse in the infrared may create a short-lived nonequilibrium situation initiating non-adiabatic surface reactions. The associative desorption of hydrogen from a Ru(0001) surface was investigated in this way. The molecules coming off the surface are ‘hot’ (i.e. > 1000 K) in all degrees of freedom, and their formation was identified as due to non-adiabatic coupling of the nuclear motion to transiently excited hot substrate electrons. A pronounced isotope effect in the yields of desorbing H2 and D2 is caused by different coupling times, which control the energy flow and give rise to a novel dynamic promoter effect. 
     If a STM tip of Ag or Au is approached to a surface, irradiation with visible light causes the excitation of localized surface plasmons within the gap of about 1 nm. For tip radii below 100 nm the enhanced electromagnetic field is confined to a surface area of less than 15 nm enabling localized Raman spectroscopy (tip enhanced Raman spectroscopy = TERS) with promising avenues towards single molecule spectroscopy. So far, this effect has been studied for dyes and CN ions adsorbed on Au (and Ag, Pt) films. In the STM mode of operation tunneling electrons may in turn excite optical resonances, and their spectral investigations reflect mode structure of the localized surface plasmons, depending on the local geometry

2. Structure and reactivity of surfaces

     Carbon monoxide oxidation on ruthenium served for long times as a striking example for the existence of a ‘pressure gap’ in heterogeneous catalysis: While Ru under UHV conditions is practically inert, it exhibits high catalytic activity if operated under atmospheric pressure conditions. The puzzle was solved by the discovery that a Ru(0001) surface transforms under high oxygen exposures (and at elevated temperatures) into the (110) surface of RuO2, whose structure and reactivity have in the meantime also been studied in detail by members of the Theory department. This transformation as well as the surface structures formed at high pressures may now be studied in situ by a newly constructed high pressure scanning tunneling microscope (STM), which may be operated at pressures up to 1 bar. After completion this instrument was successfully tested with the RuO2(110) system and will soon be transferred to J. Wintterlin’s laboratory at the University of München. 
     The reactivity of the RuO2(110) surface is studied, in addition, in detail particularly by vibrational spectroscopy (HRELS) in conjunction with mass spectroscopy. The role of the two oxygen surface species (O-cus and O-bridge) could be identified, and the kinetics of CO oxidation was analyzed and compared with high-pressure data. The interaction with H2 gives rise to a rich scenario of surface coordination chemistry, which was explored in collaboration with members of the Theory department. Current experiments concern interactions with NO, C2H4, NH3, etc. 
     Another technique for vibrational spectroscopy of adsorbates is offered by broadband sum-frequency spectroscopy (SFG), which, by applying femtosecond laser pulses, offers the additional possibility of extreme temporal resolution. The structure of adsorbed water was one of the topics studied in this way. 
     Studies on the adsorption of a variety of molecules on single-wall nanotubes in comparison with the properties of graphite surfaces provide information on fundamental questions concerning adsorbate–surface interactions and the surface properties of these systems. 
     Structures formed at electrode surfaces in contact with an electrolyte are not directly accessible to the standard surface science techniques. For this purpose an apparatus is used in which an electrochemical cell is attached through a transfer system to an UHV system containing facilities for LEED, RHEED and Auger electron spectroscopy. Current studies concern the characterization of modified Pt-Ru electrodes in connection with fuel cell reactions. 
     A quite different activity concentrates on the investigation of the aggregation processes of surface active compounds (surfactants) in aqueous solution. Small-angle neutron scattering, microcalorimetry, and spectroscopic techniques are the tools providing information on the molecular level. Stopped-flow and laser temperature jump methods yield kinetic data from nanoseconds to seconds

3. Nonlinear dynamics and complex surface reactions

     Activities in this area are embedded to a large extent in the Sonderforschungsbereich 555 “Komplexe nichtlineare Prozesse” and are characterized by a strong interplay between experiment and theory. Processes of spatio-temporal self-organization are studied in different areas, from single enzyme molecules and nanoscale structures to macroscopic concentration patterns in catalytic and electrochemical surface reactions. 
     CO oxidation on a Pt(110) surface has been further explored as a prototype system for complex surface reactions. While most observed features can be successfully modeled by deterministic equations based on experimental observations, certain features (such as the so-called ‘raindrop’ patterns) are likely to be stochastic in nature. A rescalable stochastic model was developed and successfully analyzed. Further attention was concentrated on the analysis of pacemaker structures. These are not only created by inhomogeneities of the oscillatory medium but may, under certain conditions, also develop spontaneously in uniform systems. 
     The control of spatiotemporal chaos by novel global feedback techniques leads to a wealth of new (periodic) structures and may be considered as a general strategy for pat-tern stabilization in complex systems. In experiments with an ultrathin Pt single crystal foil thermal effects come additionally into play. Periodic deformations as a consequence of the interplay between chemical reaction, evolution of heat and mechanical deformation could be observed and modeled. Local variation of the surface reactivity can be achieved by directing a laser spot onto a certain small area of the surface where thereby the temperature is increased and whose position can be controlled from outside. 
     Based on the analysis of experimental data provided by fluorescence correlation spectroscopy, kinetic models for single enzyme molecules as cyclic protein machines have been developed, and spontaneous emergence of collective synchronous activity in molecular networks formed by such interacting protein machines could be identified. 
     Theoretical studies on self-organization on nanoscale were performed for systems characterized by an interplay between chemical reactions and phase transitions. Two-component phase-separating reactive Langmuir monolayers exhibit the spontaneous development of traveling waves. With catalytic reactions involving adsorbate-driven surface reconstruction (such as CO oxidation on Pt(110)), on the other hand, localized reactive non-equilibrium nanostructures may emerge. 
     For three-dimensional chemical excitable media it was shown that the Winfree turbulence of scroll waves can be tamed – either induced or suppressed – by applying an appropriate weak periodic forcing. Since similar processes are considered to be responsible for the development of fibrillation in the cardiac tissue, the results of this study caused considerable interdisciplinary interest. 
     The theoretical understanding of spatio-temporal self-organization in electrode reactions has made significant progress in recent years, essentially through application of potential theory and derivation of reaction-migration equations describing the interplay between chemical reactions and electrical effects. Recently, particular emphasis was put on the study of edge effects at insulator–conductor interfaces. Edges may play an important role for the dynamics, in particular since the spatial (migrational) coupling diverges at the edges. Experiments were performed with formic acid oxidation at ring and ribbon electrodes, which clearly demonstrate the effect of geometry on pattern formation. 
     In another set of experiments small parts of a Pt ring electrode were covered with insulating layers. For conditions under which an unperturbed ring exhibits a pulse wave propagating with constant velocity, such pulses were still created, but were considerably accelerated near an insulating section across which they then jumped. This phenomenon is reminiscent of the saltatory conduction in myelinated axons (where the excitation jumps from one node of Ranvier to the next) and can readily be rationalized by the increase of coupling near the edges

4. Microstructuring of surfaces

     Deposition of material onto a surface usually leads to the formation of an overlayer via nucleation and growth. This strategy was applied to study the formation of InAs quantum dots (QD) on differently oriented GaAs single crystal surfaces by combining molecular beam epitaxy (MBE) with in situ scanning tunneling microscopy. The previous surprising finding that on GaAs(001) the InAs QD are mostly terminated by the open (137) surface and not by expected low-index (110) or (111) planes was further confirmed. From continuing studies with a large series of other GaAs surfaces several general conclusions could be drawn: 
·  The QD are fairly flat. This is achieved by terminating the QD with high-index surfaces if grown on low-index substrates and vice versa. The crucial aspect is the high stability of the high-index (137) and (2511) surfaces discovered in this group. 
·  The shape of the QD reflects the symmetry of the substrate. 
·  Nucleation is governed by kinetics while formation of the shape is near to thermodynamic equilibrium. 
·  The size distribution is bimodal: A small number of large QD presumably incorporate dislocations and are optically dead, while growth of the majority is limited by strain leading to quite uniform size. 
     If the (nominal) composition of a binary system is far inside the miscibility gap, another mechanism for phase separation will be operating if the change of the state occurs rapidly enough to suppress nucleation growth through surface diffusion. This spinodal decomposition has so far been observed by quenching binary alloys or polymeric melts, but was experimentally not accessible for epitaxial metallic monolayers. This demanding task could be solved with an electrochemical system for which very fast change of the state could be achieved. 
     By the application of a single microsecond voltage pulse to the tip of a STM, scanning a Au(111) surface, about half a monolayer of Au atoms was randomly removed from the topmost surface layers. Due to the fast change of the thermodynamic state of the surface system within microseconds, the remaining Au atoms ordered into labyrinthine island patterns, indicative of spinodal decay of an unstable two-dimensional adatom gas. This morphology contrasts the compact island patterns usually obtained by nucleation and growth processes, e.g., upon metal on metal deposition in UHV. The characteristic length scale of the labyrinthine patterns could be quantitatively explained within the theoretical framework of Cahn and Hilliard. Additionally the self-similar coarsening of the island structures was in situ followed by STM. 
     Ultrafast charging of an electrochemical double layer is also the principle underlying development of a technique for micromachining of metal and semiconductor surfaces, which was further improved and patented. In particular, the electrochemical behavior of passivating materials such as stainless steel upon application of short voltage pulses was studied in order to get further insights into the differences between conventional electrochemical machining methods and the novel technique. In addition, complex electrochemical reactions such as the electropolymerization of polypyrrol could be successfully localized by application of short voltage pulses. Further technical improvement could be achieved by shortening the voltage pulses down to 300 ps duration leading to spatial resolution of better than 50 nm. First attempts to parallelize the technique were undertaken: In collaboration with the University of Virginia, complex structures with elements as small as 100 nm could be imprinted onto a surface in a single step

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