Dynamics and stability of nanostructures on metal surfaces

Ultrathin metal films consisting of two-dimensional clusters are typically unstable: the cluster ensemble has the tendency to reduce its total free energy via Ostwald ripening or dynamic coalescence of mobile clusters. In this paper we give an overview of recent model experiments addressing these coarsening mechanisms. The experiments have been performed using STM on ensembles consisting of adatom or vacancy clusters with typical diameters in the nanometer range on fcc(111)-metal surfaces. Agreement with and deviations from conventional theories are discussed. Received: 29 March 1999 / Accepted: 17 August 1999 / Published online: 30 September 1999     

Real time chemical dynamics at surfaces

It is a major goal in surface science to make movies of molecules on surfaces, in which the reaction of the molecules on the surface can be followed on a femtosecond time scale, with sub-nanometer resolution. By moving the actors (the molecules) to precisely determined positions on the stage (the surface) at some well-defined moment in time, and subsequently making a space- and time-resolved documentary of what happens next, we would be able to understand the reactive interactions between molecules on surfaces in the greatest possible detail. This would enable us to set the stage and bring together the actors in such a way as to produce the chemical outcomes our society needs, by improving existing catalysts and designing novel catalysts, and by engineering novel reactions on surfaces. Any future director of such movies needs to know which techniques (i.e., which theoretical and experimental methods) hold promise for movie making, what has been done with these techniques, and what can be done with appropriate extensions. The methods we discuss are: (i) the time-dependent wave packet method, which is a theoretical method for simulating molecule–surface reactions with sub-nanometer resolution on a femtosecond time scale, (ii) molecular beam experiments, which allow detailed investigation of the molecule–surface interaction at a molecular level, and (iii) time-resolved laser pump–probe experiments, which allow reactions to be studied with femtosecond resolution. In particular, we discuss (i) theoretical studies of the dissociation reaction of hydrogen on metal surfaces, the reactive system presently understood at the greatest level of detail, (ii) the reactive and non-reactive scattering of heavy diatomics (NO,CO) from metal surfaces, and (iii) the competition between reaction of coadsorbed CO with O and desorption of CO, again on a metal surface. We examine possibilities to extend these methods to make movies at the desired level of detail. We also discuss which reactions are likely to provide good material for plots of movies that will be exciting for future generations of surface scientists.     

Single-pulse time- and fluence-resolved optical measurements at femtosecond excited surfaces

We describe a new technique for optical pump–probe measurements at femtosecond excited surfaces. By combining time-resolved microscopy with cylindrical focusing of the pump, complete mapping of the time and fluence dependence of laser-induced optical changes becomes possible in a single-pulse experiment. Received: 16 August 1999 / Accepted: 17 August 1999 / Published online: 30 September 1999     

III-V compounds for solar cell applications

Research activities in the field of III-V solar cells are reviewed. III-V compound semiconductors are used for space solar cells, concentrator solar cells, and in thermophotovoltaic generators. The epitaxial growth of ternary and quaternary material by MOVPE and LPE allows us to realize various band gaps. Multi-junction solar cells with different band gaps are necessary to obtain efficiencies larger than 30%. Recent results of the III-V solar cell research at the Fraunhofer ISE are presented. A mechanically stacked GaAs/GaSb tandem concentrator solar cell achieved an efficiency of 31.1% under 100×AM1.5d. An efficiency of 23% for a two-terminal concentrator module (486 cm²) with Fresnel lenses has been measured under realistic outdoor conditions. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.?Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T _l=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T _c=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below T _c as well as the two-dimensional decay above T _c.?In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers. Received: 25 March 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

Off-Bragg-angle light diffraction and structure of dynamic interband photorefractive gratings

Steady-state and time-resolved off-Bragg-angle diffraction experiments are used to determine the structure and the dynamics of photorefractive gratings induced by interband photoexcitation. In potassium niobate, we identify in such gratings basically a two-layer structure. Close to the surface, we find a space-charge electric field generated by a charge modulation stored directly in the bands. This grating component is typically 50 μm thick, the amplitude of the refractive index modulation is larger than 10^-4, and the response time is a few μs for resonant intensities of 100 mW cm^-2. This component is also robust under non-resonant illumination. Deeper in the crystal, a second holographic layer extends over a few hundreds of μm, its amplitude is smaller, and its slower response time is in the ms range. The mutual phase shift between the grating components is also determined. Received: 23 November 1998 / Revised version: 14 January 1999 / Published online: 12 April 1999     

Application of atomic force microscopy to microbial surfaces: from reconstituted cell surface layers to living cells.

The application of atomic force microscopy (AFM) to probe the ultrastructure and physical properties of microbial cell surfaces is reviewed. The unique capabilities of AFM can be summarized as follows: imaging surface topography with (sub)nanometer lateral resolution; examining biological specimens under physiological conditions; measuring local properties and interaction forces. AFM is being used increasingly for: (i) visualizing the surface ultrastructure of microbial cell surface layers, including bacterial S-layers, purple membranes, porin OmpF crystals and fungal rodlet layers; (ii) monitoring conformational changes of individual membrane proteins; (iii) examining the morphology of bacterial biofilms, (iv) revealing the nanoscale structure of living microbial cells, including fungi, yeasts and bacteria, (v) mapping interaction forces at microbial surfaces, such as van der Waals and electrostatic forces, solvation forces, and steric/bridging forces; and (vi) probing the local mechanical properties of cell surface layers and of single cells.     

Time-resolved two photon photoemission electron microscopy

Femtosecond, time-resolved two photon photoemission has been used to map the dynamics of photo-excited electrons at a structured metal/semiconductor surface. A photoemission microscope was employed as a spatially resolving electron detector. This novel setup has the potential to visualize variations of hot electron lifetimes in the femtosecond regime on heterogeneous sample surfaces and nanostructures. Received: 22 October 2001 / Revised version: 10 January 2002 / Published online: 7 February 2002     

The electronic structure and the band gap of nano-sized Si particles: competition between quantum confinement and surface reconstruction

The electronic structure and especially the band gap of Si_n clusters (n=3–45 atoms) is studied by photoelectron spectroscopy. Contrary to expectations of quantum confinement, almost all clusters studied here have a band gap smaller than that of crystalline Si or even display a continuous (metallic) density of states. We attribute this to covalent bond formation analogous to the reconstructions observed on single-crystal surfaces. Additionally, for Si₃₀ and Si₃₃ a gap size of 0.6 eV (0.4 eV) is observed, supporting the prediction of stable, spherically symmetric structures of these particular clusters. Received: 18 November 1999 / Accepted: 24 November 1999 / Published online: 5 April 2000     

Modeling self-organization of nano-size vacancy clusters in stochastic systems subjected to irradiation

We study the self-organization of vacancy clusters in irradiated materials under reactor and accelerator conditions. Using a continuum stochastic model we take into account dynamics of point defects and their sinks with elastic interactions of vacancies. Dynamics of vacancy clusters formation is studied analytically and numerically. We have shown a difference in patterning dynamics at irradiation under reactor and accelerator conditions. The external noise influence related to fluctuation in a defect production rate is studied in detail. Applying our approach to pure nickel irradiated under different conditions we have shown that vacancy clusters having a linear size ～eq 6 nm can arrange in a statistical periodic structure with nano-meter range. We have found that the linear size of vacancy clusters at accelerator conditions decreases down to 20%, whereas a period of vacancy clusters reduces to 6.5%.     

Excimer laser-induced ablation of clean and CO-covered polycrystalline copper surfaces: Generation and characterization of high energy copper species

Copper atoms, clusters and ions were generated by ablation of clean and CO-covered polycrystalline copper surfaces under UHV conditions, using a high peak power, pulsed excimer laser as the energy source. The species in the vapor phase were characterized by time-resolved mass spectrometry and optical detection of laser induced fluorescence. The laser power density threshold of vaporization for the clean copper (300(30) MW/cm²) was significantly lower than the threshold in the case of CO-covered copper surfaces (> 400 MW/cm²).     

The platelet integrin alpha(IIb) beta(3) imaged by atomic force microscopy on model surfaces

The platelet membrane receptor alpha(IIb) beta(3) binds to adsorbed protein ligands including fibrinogen, von Willebrand factor and fibronectin, and is critically important in mediating platelet adhesion to damaged subendothelium and to synthetic biomaterial surfaces. This receptor is a member of the integrin family, a highly prevalent class of heterodimeric molecules consisting of a single alpha and beta subunit. In an ongoing effort to understand the mechanisms underlying platelet adhesion events, high-resolution atomic force microscopy (AFM) under dynamic conditions was used to obtain images of alpha(IIb) beta(3) molecules as well as aggregates of the protein. Images of integrin molecules were obtained by tapping mode AFM under aqueous buffer conditions following adsorption on a series of ultrasmooth model surfaces. On a model hydrophobic surface, detergents stabilizing the protein in solution competed for surface adsorption sites. When this detergent was removed from the system, the protein was predominantly seen as aggregates with head groups pointing outward. A limited number of individual integrin molecules were observed, and were found to have dimensions consistent with those reported previously by electron microscopy studies. Integrin molecules showed weak adhesion to the two hydrophilic surfaces used in the study, although formation of a lipid bilayer around surface-adsorbed molecules improved the resolution. At longer time periods, the integrin molecules embedded in this lipid bilayer exhibited sufficient mobility to form molecular aggregates. The structural measurements described in this study not only reveal three-dimensional features of the molecule, they represent an important step towards dynamic adsorption experiments and visualizing the integrin interacting with surface-adsorbed proteins as in biomaterial-induced thrombogenesis.     

Wave packet dynamics in different electronic states investigated by femtosecond time-resolved four-wave-mixing spectroscopy

This paper reviews results on wave packet dynamics investigated by means of femtosecond time-resolved four-wave-mixing (FWM) spectroscopy. First, it is shown that by making use of the various degrees of freedom which are offered by this technique information about molecular dynamics on different potential-energy surfaces can be accessed and separated from each other. By varying the timing, polarization, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dynamics are coherently excited and probed by the nonlinear spectroscopy. As a model system we use iodine in the gas phase. These techniques are then applied to more-complex molecules (gas phase: benzene, toluene, a binary mixture of benzene and toluene; solid state: polymers of diacetylene matrix-isolated in single crystals of monomer molecules). Here, ground-state dynamics are investigated first without any involvement of electronically excited states and then in electronic resonance to an absorption transition in the investigated molecules. Signal modulations result which are due to wave packet motion as well as polarization beats between modes in different molecules. Phase and intensity changes yield information about intramolecular vibrational energy redistribution, population decay (T₁), phase relaxation (T₂), and coherence times. Received: 12 October 1999 / Published online: 13 July 2000     

Evanescent wave scattering by aggregates of clusters – application to optical near-field microscopy

Extended Mie-theory is used to investigate scattering and extinction of evanescent waves by aggregates of clusters. In an application to apertureless near-field optical microscopy involving total internal reflection at the surface substrate–air, the variation of the scattered power is calculated when a silicon particle is scanned across single clusters or aggregates of clusters in the evanescent field. Metallic, dielectric, and semiconducting particles are taken into consideration, and the dependence on sizes, materials, and the wavelength is discussed. Received: 27 August 1999 / Revised version: 8 November 1999 / Published online: 1 March 2000     

Analysis of semiconductor surface phonons by Raman spectroscopy

Only recently Raman spectroscopy (RS) has advanced into the study of surface phonons from clean and adsorbate-covered semiconductor surfaces. RS allows the determination of eigenfrequencies as well as symmetry selection rules of surface phonons, by k-conservation limited to the Brillouin zone-center, and offers a significantly higher spectral resolution than standard surface science techniques such as high-resolution electron energy loss spectroscopy. Moreover, surface electronic states become accessible via electron–phonon coupling. In this article the fundamentals of Raman scattering from surface phonons are discussed and its potential illustrated by considering two examples, namely Sb-monolayer-terminated and clean InP(110) surfaces. Both are very well understood with respect to their atomic and electronic structure and thus may be regarded as model systems for heteroterminated and clean semiconductor surfaces. In both cases, localized surface phonons as well as surface resonances are detected by Raman spectroscopy. The experimental results are compared with surface modes predicted by theoretical calculations. On InP(110), due to the high spectral resolution of Raman spectroscopy, several surface modes predicted by theory can be experimentally verified. Surface electronic transitions are detected by changing the energy of the exciting laser light indicating resonances in the RS cross section. Received: 7 April 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Structure and topography modifications of austenitic steel surfaces after friction in sliding contact

Friction experiments between two austenitic stainless steel (AISI 304L) surfaces in sliding contact were carried out under very low loads in two liquid environments, namely demineralized water and methanol, in order to study the correlation between surface damage (wear and surface topography) and structural modifications (phase formation and microstructure). The particularity of our approach was to perform the tests under Hertzian pressures, which were several orders of magnitude lower than the elastic limit of stainless steel. The structural modifications produced during friction were analysed by X-ray diffraction and transmission electron microscopy and the surface topography was studied by scanning electron microscopy and three-dimensional (3D) profiling. Whatever the experimental conditions investigated, the morphology of the damage observed on both surfaces consisted of very fine, smooth and parallel grooves typical of an abrasive wear process of a ductile material caused by the ploughing action of hard particles. From the beginning of the tests, the transformation of austenite into martensite was observed in the superficial layers and the dominant presence of martensite was identified in the wear debris. These results suggest that, under our experimental conditions, abrasion is the dominant mechanism of material removal. Received: 12 March 2002 / Accepted: 3 May 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/4949-6692, E-mail: jean.paul.riviere@univ-poitiers.fr     

Clusters on surfaces: high-resolution spectroscopy at low temperatures

With controlled growth in nanometer-sized pits we produced silver and gold clusters on a graphite surface. We give a summary of the preparation method and discuss the scanning tunneling imaging and the crystalline orientation of the clusters. The electronic structure of the clusters was studied by an in-situ combination of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). For both techniques we obtained an energy resolution in the range of 10 meV employing low sample temperatures. Dynamic final-state effects together with averaging over a cluster-size distribution result in characteristic spectral shapes in UPS, which can be understood referring to STS data taken on individual clusters. Finally, directions for future experiments are pointed out. Received: 13 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Molecular dynamics simulation about porous thin-film growth in secondary deposition

The thin film growth has been confirmed to be assembled by an enormous number of clusters in experiments of CVD. Sequence of clusters’ depositions proceeds to form the thin film at short time as gas fluids through surface of substrate. In order to grow condensed thin film using series of cluster deposition, the effect of initial velocity, substrate temperature and density of clusters on property of deposited thin film, especially appearance of nanoscale pores inside thin film must be investigated. In this simulation, three different cluster sizes of 203, 653, 1563 atoms with different velocities (0, 10, 100, 1000 and 3000 m/s) were deposited on a Cu(0 0 1) substrate whose temperatures were set between 300 and 1000 K. Four clusters and one cluster were used in primary deposition and secondary deposition, respectively. We have clarified that adhesion between clusters and substrate is greatly influenced by initial velocity. As a result, the exfoliation pattern of deposited thin film is dependent on initial velocity and different between them. One borderline dividing whole region into porous region and nonporous region are obtained to show the effect of growth conditions on appearance of nanoscale pores inside thin film. Moreover, we have also shown that the likelihood of porous thin film is dependent on the point of impact of a cluster relative to previously deposited clusters.     

Physics of cell elasticity,shape and adhesion

We review recent theoretical work that analyzes experimental measurements of the shape, fluctuations and adhesion properties of biological cells. Particular emphasis is placed on the role of the cytoskeleton and cell elasticity and we contrast the shape and adhesion of elastic cells with fluid-filled vesicles. In red blood cells (RBC), the cytoskeleton consists of a two-dimensional network of spectrin proteins. Our analysis of the wavevector and frequency dependence of the fluctuation spectrum of RBC indicates that the spectrin network acts as a confining potential that reduces the fluctuations of the lipid bilayer membrane. However, since the cytoskeleton is only sparsely connected to the bilayer, one cannot regard the composite cytoskeleton–membrane as a polymerized object with a shear modulus. The sensitivity of RBC fluctuations and shapes to ATP concentration may reflect topological defects induced in the cytoskeleton network by ATP. The shapes of cells that adhere to a substrate are strongly determined by the cytoskeletal elasticity that can be varied experimentally by drugs that depolymerize the cytoskeleton. This leads to a tension-driven retraction of the cell body and a pearling instability of the resulting ray-like protrusions. Recent experiments have shown that adhering cells exert polarized forces on substrates. The interactions of such “force dipoles” in either bulk gels or on surfaces can be used to predict the nature of self-assembly of cell aggregates and may be important in the formation of artificial tissues. Finally, we note that cell adhesion strongly depends on the forces exerted on the adhesion sites by the tension of the cytoskeleton. The size and shape of the adhesion regions are strongly modified as the tension is varied and we present an elastic model that relates this tension to deformations that induce the recruitment of new molecules to the adhesion region. In all these examples, cell shape and adhesion differ from vesicle shape and adhesion due to the presence of the elastic cytoskeleton and to the fact that active processes (ATP, molecular motors) within the cell modify cytoskeletal elasticity and tension.