Browsing by Author "Buttner, M."

We studied the interaction of adsorbed thiol molecules with gold nanoparticles as a function of the mean particle size. The results obtained from MXPS (monochromated X-Ray Photoelectron Spectroscopy) measurements showed that attachment of the thiol sulfur headgroup onto the cluster surface leads to a positive binding energy shift in the Au 4f core-level. The absence of line width broadening upon adsorption indicates that these changes affect the whole particle and not only the particle surface, where the actual Au-S bond is located. The positive binding energy shift depends on the cluster size and increases with decreasing diameter. A maximum shift of 0.41 eV could be measured for the smallest particles (similar to 1 nm). The valence band exhibited positive binding energy shifts similar to the Au 4f core-levels, but smaller in absolute values. Changes in the valence band shape were interpreted as re-hybridization of Au 5d electrons due to the creation of Au-S bonds. Furthermore, we observed a disappearance of the Fermi edge upon thiol adsorption, which we attribute to a sulfur-induced metal-insulator-transition of the gold cluster. (c) 2005 Elsevier B.V. All rights reserved.

The growth of Au clusters on a fullerene thin film was investigated by in situ photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as molecular templates and serve to create a cluster superstructure. To gain insight into the fullerene-Au interaction two types of experiments were performed: (i) the deposition of Au on a fullerene surface, and (ii) the deposition of fullerenes on a Au surface. In both experiments an island growth mode is observed. The deposition of submonolayer amounts of C-60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C-60 monolayer. In addition a constant band bending in the fullerene film is detected, but the UPS valence-band spectra show that there is no charge transfer from the Au to the C-60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C-60 surface, the analysis of the Au core level does not reveal a specific bonding or nucleation site for Au atoms and clusters. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene molecules. The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to 150 atoms for the deposition of a nominal coverage of 1.5 nm. These data are derived from an analysis of the d-band splitting and the Au 4f core-level shift due to delayed photohole relaxation. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temperatures of 650 degrees C. For temperatures up to 450 degrees C a continuous growth of the clusters is detected, which is accompanied by a slight drop in Au concentration in the range of XPS for annealing temperatures higher than 350 degrees C. This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. The film shows a very good thermal stability and even after annealing at 650 degrees C there is still a fullerene film detectable in the photoelectron spectroscopy spectra. (c) 2005 American Institute of Physics.

The feasibility of endohedral iron doping of C-60 is studied using a mass selected ion beam to create an Fe+ beam with variable energy (60-380 eV, 1.2x10(-3) C) which is directed at a C-60 thin film. The surface is characterized by x-ray photoelectron spectroscopy, and oxidation of the samples is used to discriminate between Fe@C-60 and carbides. The fullerene cages are damaged during the irradiation, and participate in direct Fe-C bond formation. The reaction with oxygen indicates the absence of sizable amount of Fe@C-60. Comparison with reference samples composed of iron and damaged fullerene layers (by Ar+ irradiation) supports this interpretation. The experiment serves as a model study for the investigation of different metal-fullerene combinations. (C) 2004 American Institute of Physics.

We have studied the growth and the properties of (t)a-C:F films prepared by the deposition of mass separated C-12(+) and F-19(+) ions as a function of the F concentration. The films are always strongly F deficient due to the formation of volatile F-2 and CFx molecules during the deposition process. A maximum F content of about 25 at. % is obtained for an ion charge ratio of C+:F+ = 1:1. The observed mechanical, optical, electrical, and structural properties as well as the thermal stability of the films are strongly influenced by the F content. A three step progression of the film structure is evident for increasing F concentration: the amorphous three-dimensional network of tetrahedrally bonded carbon atoms of pure carbon films (ta-C) with diamondlike properties is doped for very low F concentrations (ta-C:F). A further increase of the F content results first in transformation to a graphitelike amorphous structure (a-C:F) before the deposited films become porous and to a polymerlike one for the highest F content. (C) 2001 American Institute of Physics.

Boron carbide thin films were grown by mass selected ion beam deposition using low energy B-11(+) and C-12(+) ions at room temperature. The amorphous films exhibit any desired stoichiometry controlled by the ion charge ratio B+/C+. Films with a stoichiometry of B4C showed the optimal combination of a high mechanical strength and a low electrical resistivity for the coating of atomic force microscopy (AFM) silicon cantilevers. The properties of such AFM tips were evaluated and simultaneous topography and Kelvin mode AFM measurements with high lateral resolution were performed on the systems (i) Au nanoparticles on a p-WS2 surface and (ii) conducting/superconducting YBa2Cu3O7-x. (C) 2001 American Institute of Physics.