Force spectroscopy on single xanthan molecules

Received: 3 July 1998 / Published online: 10 February 1999     

Bonds and Fermi surfaces studied by photoelectron spectroscopy

Photoelectron spectroscopy with synchrotron radiation employing high energy and angular resolutions is a very efficient tool for experimental investigations of the electronic structure of solids and their surfaces. In addition to standard band-mapping applications, photoemission intensity and line-shape analyses provide valuable information about wave functions, bonds and interactions of a many-electron system. In this report we choose covalent semiconductor surfaces as well as metallic clean and nanostructured surfaces of layered materials to serve as model systems for assessing the spatial origin of photoelectrons and the three-dimensional shape of Fermi surfaces. Received: 11 July 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Photo-ionization efficiency curves of alkali nanoclusters in a beam and determination of metal work functions

We produced free beams of cold nanometer-sized particles of lithium, sodium and potassium and measured the three corresponding photo-ionization yield curves. Quadratic (Fowler) plots, originally developed for bulk surfaces, were found to provide a good fit to the threshold shape and were used to obtain the particle ionization potentials. The latter match precisely the bulk work functions cited in the literature, suggesting that photo-ionization of free nanoclusters may form a useful complement to traditional photoelectron studies of surfaces. Within 0.25–1 eV above the threshold, the ionization efficiency begins to drop. This effect, which has parallels in bulk-surface and small-cluster photoemission, is presently not well understood but may be related to an interplay between electron emission and collective surface plasma excitations. Received: 21 June 2001 / Revised version: 7 July 2001 / Published online: 10 October 2001     

Surface magnetism: From the spin-resolved density of states to magnetic domain imaging on the nanometer scale

Making use of the information depth of only a few layers for electrons with low kinetic energies, photoemission experiments, with an additional measurement of the electron spin polarization or with excitation using circularly polarized radiation, allow one to determine whether adatoms change the magnetic properties at the surface and whether they “feel” the magnetism of the underlying substrate. This is investigated for oxygen adsorbed on Fe (110) and Co (0001). Enhanced surface sensitivity can be gained in tunneling experiments. The technique of spin-polarized metastable de-excitation spectroscopy thus enables one to determine the spin-resolved density of states in the near-surface region. By making the highly lateral resolving scanning tunneling microscope sensitive to the electron spin polarization it becomes possible to image magnetic domains on the nanometer scale; this is demonstrated for Gd (0001) surfaces. Received: 28 April 2000 / Accepted: 4 September 2000 / Published online: 7 March 2001     

Strain relaxation at cleaved surfaces studied by atomic force microscopy

Atomic force microscopy (AFM) in air is used to study the (110) cleaved surface of strained (100) In_xGa_1-xAs/ InP heterostructures for different compositions and thicknesses of the ternary compound layers. We find that the elastic strain relaxation induces a surface undulation of a few ? amplitude, even for very small misfits, provided the layers are thick enough. Using finite-element calculations of the strain relaxation near the cleaved edge, we reproduce quantitatively the AFM observations for compressive- as well as for tensile-strained layers with an accuracy better than 0.1 nm. This demonstrates the ability of AFM to quantify strain distributions by making use of surface profile measurements. Received: 9 November 1998 / Accepted: 11 March 1999 / Published online: 7 July 1999     

Dynamics of gold cluster systems

x surface of Si(111) wafers was studied as a function of annealing time at relatively low annealing temperatures (50 and 100 °C) by application of scanning force microscopy. A fast increase in cluster diameter and height as a function of annealing time was detected. After more than 3 h of annealing depletion zones and nucleation exclusion zones were observed. Received: 13 August 1998 / Published online: 10 February 1999     

Transfer of Au Cusp-clusters on Si(111) 7×7 surfaces from a pure Au tip of a scanning tunneling microscope

Received: 16 August 1996/Accepted: 6 March 1997     

Study of Fe/Mo multilayer systems by SFM and AES

Received: 13 August 1998 / Published online: 10 February 1999     

Evaluation of structural and adhesive properties of nylon 6 and PTFE alignment films by means of atomic force microscopy

Atomic force microscopy (AFM) has become a powerful technique for submicron investigation of surface properties. In this work we use the capability of this technique to investigate dielectric films used to align ferroelectric liquid crystals (FLC). In fact, the final performance of a surface stabilized FLC (SSFLC) flat panel display strongly depends on the alignment layer properties and quality. This work focuses on a comparison of two alignment films: the more conventional polyamide, nylon 6, and polytetrafluoroethilene (PTFE, commercially known as Teflon), only recently used as a new aligning material. A micromorphological characterization of the sample surfaces has been carried out in order to correlate structure with alignment properties of both polymer films. The results show varying roughness and periodicity wavelengths for the two alignment layers. These different properties can be related to different anchoring forces between aligning surfaces and FLC molecules and therefore to a different electrooptical response of SSFLC cells. In addition to the topographic characterization, AFM non-conventional measurements have been performed on alignment layers deposited on different transparent conductive oxides, such as indium tin oxide (ITO) and SnO₂, used to make electrodes in SSFLC displays. These measurements provide local information on the adhesive properties of the studied alignment materials as a function of substrate coating. These observations indicate less adhesion of PTFE with respect to nylon 6. Received: 16 April 2000 / Accepted: 16 April 2000 / Published online: 13 September 2000     

Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy

Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water. Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become unstable and start to coagulate. Received: 24 June 2002 / Revised version: 25 July 2002 / Published online: 25 October 2002 RID="*" ID="*"This work is part of the thesis of H. M?ltgen RID="**" ID="**"Corresponding author. Fax: +49-211/811-5195, E-mail: kleinermanns@uni-duesseldorf.de     

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Received: 6 November 1998     

Time-resolved coherent spectroscopy of surface states

Received: 4 December 1998     

Fabrication of large-scale ultra-smooth metal surfaces by a replica technique

The IR double-resonance techniques IR/R2PI (infrared/resonant 2-photon ionization), IR/PIRI (infrared-photo-induced Rydberg ionization) and IR-photodissociation spectroscopy are valuable tools to investigate structure, vibrations, and dynamical processes of neutral and ionic hydrogen-bonded clusters containing aromatic molecules. In this paper we report on the application of the IR double-resonance techniques to determine the NH and OH stretching vibrations of 4-aminophenol and 4-aminophenol(H₂O)₁, both in the neutral (S₀) and ionic (D₀) ground state. All vibrational frequencies obtained for 4-aminophenol and the cluster are compared with the values obtained from ab initio and DFT calculations. In the S₀ state, a trans-linear arrangement of 4-aminophenol(H₂O)₁ is obtained containing an O-H·· O hydrogen bond. In the D₀ state an overlay of two spectra can be observed resulting from the trans-linear structure and a second structure which contains a N-H·· O hydrogen bond. The observation of these two structures within the ion is an interesting example of a rearrangement reaction in the ionic state. Received: 3 November 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Monolayers of graphite rotated by a defined angle: hexagonal superstructures by STM

Received: 27 March 1998     

Photoemission and scanning tunneling experiments with small particles on solid surfaces

The fascination of research with nanometersized objects in contact with a macroscopic surface will be illustrated by two examples: mass-selected supported transition-metal clusters and C₆₀ molecules on metallic single-crystal substrates. The preparation, mass-selection and deposition of the small particles will be described in some detail. The main experimental techniques involved in the characterization of their electronic and structural properties are photoelectron spectroscopy and scanning tunneling methods. For the transition-metal clusters the evolution of the valence band with cluster size reveals a trend to metal formation. When the tip of a Scanning Tunneling Microscope (STM) is placed above individual C₆₀ molecules intense light emission is observed. The diameter of this emission spot is approximately 4 Å. This observation indicates the possibility of an optical spectroscopic analysis on the scale of individual molecules.     

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     

Field enhancement of optical radiation in the nearfield of scanning probe microscope tips

Received: 8 April 1997/Accepted: 23 June 1997     

Nonlinear surface photoeffect in multi-color laser field: effect of the finite width of the surface potential barrier

Received: 20 September 1998     

Surface-state linewidth from scanning tunnelling spectroscopy

The STM can be used to investigate lifetimes of hot holes in a surface state at low temperatures. We analysed dI/dV data from Ag(111) using detailed tunnelling calculations and a simple model and found an electron self-energy of Σ=4.9±0.6 meV. The corresponding lifetime τ= 67±8 fs is considerably higher than those determined by angle-resolved photoemission, although it remains below theoretical predictions. Spatially resolved dI/dV spectra reveal that the lifetime decreases drastically in proximity to defects such as surface steps. Received: 22 March 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Atomic force microscopy study of lymphoblastoid cells under 50-Hz 2-mT magnetic field irradiation

Received: 18 December 1997/Accepted: 5 January 1998