Equilibrium fluctuations of islands of adsorbed O atoms on Ru(0001) were investigated by scanning tunneling microscopy (STM), density functional theory calculations (DFT) and Monte Carlo (MC) simulations. Very ramified (2x2)-O islands were observed by high-speed STM that point to complex interactions between the O atoms. The DFT calculations show that, in addition to pairwise attractive interactions between third-nearest neighbors, a repulsive three-body interaction exists between these. MC simulations that include three-body interactions reproduce the observed ordering behavior.

Tip-enhanced Raman spectroscopy (TERS) was investigated with malachite green isothiocyanate adsorbed at an Au(111) surface. TERS is based on the excitation of localized surface plasmons in the tip apex, producing strongly enhanced electromagnetic fields. The key conditions for giant TERS are side-illumination of the tip, well-prepared single-crystalline surfaces and sharp, smooth gold tips. A TERS enhancement of about 6 X 106 has been observed for dye molecules adsorbed at the Au(111) substrate in a region of about 50 nm diameter beneath the tip. This corresponds to a 2500-fold increase in the light intensity at the Au(111)/air interface, which in addition causes fast but local bleaching of the dye. This bleaching behavior was analyzed in detail, giving direct insight into the strength and size of the enhanced field. In addition, the bleaching constant was higher for MGITC in an unperturbed environment than for MGITC in an environment that had been substantially bleached. The MGITC spectra were also different for these two cases. Copyright (c) 2005 John Wiley & Sons, Ltd.

Angewandte Chemie International EditionVolume 44, Issue 1 p. 139-142 Communication Tip-Enhanced Raman Spectroscopy of Benzenethiol Adsorbed on Au and Pt Single-Crystal Surfaces† Bin Ren Prof. Dr., Bin Ren Prof. Dr. bren@xmu.edu.cn Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, 361005 Xiamen, China, Fax: (+86) 592-208-5349Search for more papers by this authorGennaro Picardi Dr., Gennaro Picardi Dr. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this authorBruno Pettinger Dr., Bruno Pettinger Dr. pettinger@fhi-berlin.mpg.de Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this authorRolf Schuster Dr., Rolf Schuster Dr. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this authorGerhard Ertl Prof. Dr., Gerhard Ertl Prof. Dr. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this author Bin Ren Prof. Dr., Bin Ren Prof. Dr. bren@xmu.edu.cn Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, 361005 Xiamen, China, Fax: (+86) 592-208-5349Search for more papers by this authorGennaro Picardi Dr., Gennaro Picardi Dr. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this authorBruno Pettinger Dr., Bruno Pettinger Dr. pettinger@fhi-berlin.mpg.de Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this authorRolf Schuster Dr., Rolf Schuster Dr. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this authorGerhard Ertl Prof. Dr., Gerhard Ertl Prof. Dr. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Plank-Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany, Fax: (+49) 30-8413-5106Search for more papers by this author First published: 15 December 2004 https://doi.org/10.1002/anie.200460656Citations: 147 † B.R. thanks the Alexander von Humboldt Foundation for a fellowship, the National Science Foundation of China (grant no. 90206039), and the 973 project (2001CB610506). G.P. thanks the Max Planck Society for a scholarship. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract A tip for surface scientists: Tip-enhanced Raman spectroscopy (TERS) was used to record surface Raman spectra of benzenethiol and pyridine-4-thiol adsorbed on Au and Pt single-crystal surfaces (see picture). The benzenethiol spectra on the two surfaces are distinctly different. This result illustrates the power of TERS as a diagnostic tool with high spatial resolution for surface studies. Citing Literature Volume44, Issue1December 17, 2004Pages 139-142 RelatedInformation

Tip-enhanced Raman spectroscopy (TERS) is based on the optical excitation of localized surface plasmons in the tip-substrate cavity, which provides a large but local field enhancement near the tip apex. We report on TERS with smooth single crystalline surfaces as substrates. The adsorbates were CN- ions at Au(111) and malachite green isothiocyanate (MGITC) molecules at Au(111) and Pt(110) using either Au or Ir tips. The data analysis yields Raman enhancements of about 4 x 10(5) for CN- and up to 10(6) for MGITC at Au(111) with a Au tip, probing an area of less than 100 nm radius.

A GaAs surface layer of 10 nm thickness was grown on the cleaved edge of an In0.1Al0.9As/Al0.33Ga0.67As multilayer in order to induce a lateral periodic strain modulation. We apply surface sensitive grazing incidence x-ray diffraction to distinguish between compositional/morphological and purely strain induced modulations. The experimentally determined strain profile is confirmed by finite-element model calculations. The GaAs layer is found to be purely strain modulated with an average lattice parameter change of (0.8±0.1)% with respect to relaxed GaAs.

The structure formation upon spinodal decomposition of a two-dimensional model system, a Au adatom gas on a Au(111) surface, was observed in situ by scanning tunneling microscopy (STM). A thermodynamically unstable state was prepared by applying microsecond voltage pulses to the STM tip in an electrochemical system, causing the random dissolution of Au atoms from the uppermost monolayer. Interconnected, labyrinthine island patterns were formed at Au coverages between 0.4 and 0.9 monolayer with dominating length scales lambda(m) of the order of a few nanometers.

Abstract Application of ultrashort voltage pulses to a tiny tool electrode under suitable electrochemical conditions enables precise three-dimensional machining of stainless steel. In order to reach submicrometer precision and high processing speed, the formation of a passive layer on the workpiece surface during the machining process has to be prevented by proper choice of the electrolyte. Mixtures of concentrated hydrofluoric and hydrochloric acid are well suited in this respect and allow the automated machining of complicated three-dimensional microelements. The dependence of the machining precision on pulse duration and pulse amplitude was investigated in detail.

Combined surface- and tip-enhanced Raman spectroscopy (SERS and TERS, respectively) experiments for CN− adsorbed at Au-surfaces are presented. STM tip-induced frequency shifts for the CN-stretch vibration are observed. This effect is attributed to a distinct weighting of the individual contributions to TERS, depending on the particular field distribution, which peaks underneath the tip apex and drops quickly beyond this region.

The strong electromagnetic field enhancement, occurring at illuminated metal structures with sub-wavelength dimensions, is exploited to develop Raman spectroscopy with exceedingly high sensitivity and lateral resolution, possibly down to single molecule detection. After reviewing Surface Enhanced Raman Spectroscopy (SERS) on single crystalline surfaces and colloids we present recent results on the Tip Enhanced Raman Spectroscopy (TERS) approach, where a metal tip is used as an external enhancing unit. In this way the electromagnetic and chemical surface enhancement are physically separated: the former is confined to the tip, the latter to the metal-adsorbate system. So far, TERS has been reported only for a few molecules exhibiting large Raman cross sections, such as sulphur or dye molecules. Here, we present combined SERS and TERS studies for the CN- ions and TERS for Brilliant Cresyl Blue adsorbed at smooth thin gold films.

An electrochemical pulse technique enables the fabrication of three-dimensional microelements from stainless steel. The method is based on the application of ultrashort (nanosecond) voltage pulses, whereupon electrochemical reactions are locally confined with submicrometer precision. Employing properly shaped tool electrodes enables the machining of freestanding cantilevers or microstructures directly to a metal sheet. Due to gentle removal of the material, the grain structure of the material is revealed without any chemical or mechanical modifications. This is demonstrated by measuring the vibration frequency of a cantilever, and agrees well with the value derived from the bulk material properties.

The surface enhancement of the Raman scattering (SERS) is based on two major mechanisms. First, the optical resonance of the incident light with the metallic surface leads to the excitation of localized surface plasmons and the enhancement of the local electromagnetic field. Second, optical resonances of the energy levels between the adsorbate orbitals and the metal electrons may result in a resonant charge transfer, which causes the resonance Raman or charge transfer enhancement. For dyes at aggregated colloids, the total differential cross sections can reach values of about (d sigma /d Omega) similar to 10(-16) cm(2) sr(-1), which are comparable with cross sections for the fluorescence of laser dyes in solution. In contrast, Raman spectroscopy at surfaces that do not support surface plasmon excitation (smooth surfaces or transition metal surfaces) is based solely on chemical or resonance Raman enhancement, yielding only weak signals. After reviewing recent results on SERS, we present an approach that combines the Raman spectroscopy at smooth surfaces with the local electromagnetic field enhancement by an optically active Ag STM tip. The high local enhancement of the Raman scattering cross section in the vicinity of the tip opens promising avenues towards single molecule Raman spectroscopy.

The formation and phase transitions of underpotentially deposited (root 3x root 3)R30 degrees and (1x1) Cu adlayers on Au(111) were studied by in-situ STM. Upon a negative potential sweep from the region of a disordered lattice gas into the stability region of the (root 3x root 3)R30 degrees phase, this structure is formed via fast homogeneous nucleation and subsequent lateral growth, resulting in a domain wall network which coarsens on a time scale of several minutes. The subsequent transition into a (1x1) phase upon further decreasing the potential occurs via instantaneous nucleation at the prevailing domain walls of the (root 3x root 3)R30 degrees and subsequent one-dimensional growth. Hence, this phase transition depends crucially on the history of the sample, i.e., the density of morphological defects in the (root 3x root 3)R30 degrees adlayer. On the other hand, the mechanism of the reverse transition from the (1x1) to the (root 3x root 3)R30 degrees phase is determined by the magnitude of the potential step. Small potential steps lead to heterogeneous nucleation at step edges, whereas for large potential steps homogeneous nucleation and growth predominate.

Ordering of a system of particles into its thermodynamically stable state usually proceeds by thermally activated mass transport of its constituents. Particularly at low temperature, the activation barrier often hinders equilibration—this is what prevents a glass from crystallizing1 and a pile of sand from flattening under gravity. But if the driving force for mass transport (that is, the excesss energy of the system) is increased, the activation barrier can be overcome and structural changes are initiated2. Here we report the reordering of radiation-damaged protein crystals under conditions where transport is initiated by stress rather than by thermal activation. After accumulating a certain density of radiation-induced defects during observation by transmission electron microscopy, the distorted crystal recrystallizes. The reordering is induced by stress caused by the defects at temperatures that are low enough to suppress diffusive mass transport. We propose that this defect-induced reordering might be a general phenomenon.

The electrochemical deposition of Cu clusters in nanometer-sized holes on Au(111) was observed by in-situ scanning tunneling microscopy (STM). The holes were formed electrochemically by applying short negative voltage pulses to the STM tip prior to Cu deposition. The lateral extension of the clusters is confined by the width of the holes. Their equilibrium height depends on the applied overpotential rather than on the polarization time, reflecting a delicate energy balance between the electrochemical energy and the enlarged surface energy of the growing Cu cluster. We attributed this to the formation of steps at the size-confined Cu islands and derived a step formation energy of 0.4–0.5 eV/(step atom). This is in good agreement with expectations for the formation energy of kinky steps on metal surfaces.

The diffusive motion of O atoms adsorbed on Ru(0001) was observed on the atomic scale by scanning tunneling microscopy with temporal resolution of approximate to 10 ms at room temperature. From statistical analysis of the changes of the atom configurations in sequences of more than 1000 images, mean residence times of oxygen atoms in the neighborhood of other oxygen atoms were extracted. The residence times vary by more than 1 order of magnitude over distances of three lattice constants, indicating the necessity of including the lateral interaction in modeling collective phenomena such as surface diffusion or reactions.

Wave–particle duality, as manifest in the two-slit experiment, provides perhaps the most vivid illustration of Bohr's complementarity principle: wave-like behaviour (interference) occurs only when the different possible paths a particle can take are indistinguishable, even in principle1. The introduction of a which-path (welcher Weg) detector for determining the actual path taken by the particle inevitably involved coupling the particle to a measuring environment, which in turn results in dephasing (suppression of interference). In other words, simultaneous observations of wave and particle behaviour is prohibited. Such a manifestation of the complementarity principle was demonstrated recently using a pair of correlated photons, with measurement of one photon being used to determine the path taken by the other and so prevent single-photon interference2. Here we report the dephasing effects of a which-path detector on electrons traversing a double-path interferometer. We find that by varying the sensitivity of the detector we can affect the visibility of the oscillatory interference signal, thereby verifying the complementarity principle for fermions.

The lateral extension of electrochemically induced surface modifications is usually determined by the macroscopic size of the electrodes and the diffusion length of the reacting species. To overcome this constraint, we conducted an electrochemical reaction far from equilibrium. We applied short voltage pulses (less than or equal to 100 ns, up to +/-4 V) to a scanning tunneling microscope tip while imaging a Au(111) surface in concentrated electrolytes. They lead either to hole formation by anodic dissolution of the Au or to cathodic deposition of Cu islands (in the Cu2+ containing electrolyte), both of nanometer extension.

The co adsorption of N and O on Ru(0001) has been studied by scanning tunneling mwicroscopy; by this technique we can distinguish between the two atomic adsorbates. N and O form a dense intermixed 2 x 2 phase in equilibrium with a dilute lattice gas. The two-dimensional "vapor pressure" of N, i.e., its concentration in the lattice gas, has been determined for various ratios of N and O in the dense phase by adjusting the total coverage of N and O. The resulting vapor pressure curve indicates a positive enthalpy of mixing, due to relatively weak N-O interactions in the dense phase.

We have studied magnetoresistivity oscillations in rectangular lateral antidot superlattices both in low-temperature electron transport experiments as well as in a classical diffusion model. Within this model, the conductivity tensor is obtained from a numerical simulation of the spatial distribution of electrons diffusing in the antidot array. It is demonstrated that all the essential features of the measured magnetoconductivity can be reproduced within this classical model. Maps of the trajectories in real space give an intuitive understanding of the mechanism that causes the anisotropies in the conductivity.

We study dephasing of electrons induced by a which path detector and thus verify Bohr's complementarity principle for fermions. We utilize a double path interferometer with two slits, with one slit being replaced by a coherent quantum dot (QD). A short one dimensional channel, in the form of a quantum point contact (QPC), in close proximity to the QD, serves as a which path detector. We find that by varying the properties of the QPC detector we affect the visibility of the interference, inducing thus dephasing. We develop a simple model to explain the dephasing due to the nearby detector and find good agreement with the experiment.