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
||Analyse und Steuerung ultraschneller
||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
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. Wintterlins 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,
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
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
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.
||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. |
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
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