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P Oelhafen

Publications and source records attributed to P Oelhafen.

13 recordsLinked to original sources
PubMed2005

Gold cluster formation on a fullerene surface.

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 C60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C60 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 C60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C60 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.

Journal Article↗
PubMed2005

Alloy formation of supported gold nanoparticles at their transition from clusters to solids: does size matter?

Gold nanoclusters of a size approaching the molecular limit (<3 nm) were prepared on Si substrates in order to study alloy formation on the nanometer scale. For this purpose, indium atoms are deposited on top of the gold particles at room temperature and the formation of AuIn(2) is studied by x-ray photoelectron spectroscopy in situ. It is observed that the alloy formation takes place independent of whether the particles electronically are in an insulating molecular or in a metallic state. Most important, however, closed packed full-shell clusters containing 55 Au atoms are found to exhibit an outstanding stability against alloying despite a large negative heat of formation of the bulk Au-In system. Thus, Au(55) clusters may play a significant role in the design of nanoscaled devices where chemical inertness is of crucial importance.

Journal Article↗
PubMed2002

Oxidation-resistant gold-55 clusters.

Gold nanoparticles ranging in diameter from 1 to 8 nanometers were prepared on top of silicon wafers in order to study the size dependence of their oxidation behavior when exposed to atomic oxygen. X-ray photoelectron spectroscopy showed a maximum oxidation resistance for "magic-number" clusters containing 55 gold atoms. This inertness is not related to electron confinement leading to a size-induced metal-to-insulator transition, but rather seems to be linked to the closed-shell structure of such magic clusters. The result additionally suggests that gold-55 clusters may act as especially effective oxidation catalysts, such as for oxidizing carbon monoxide.

Journal Article↗
PubMed2001

Chemically induced metal-to-insulator transition in Au55 clusters: effect of stabilizing ligands on the electronic properties of nanoparticles.

The cluster compound Au55(PPh3)12Cl6 has been reanalyzed by photoelectron spectroscopy giving direct evidence for a nonmetallic behavior of the individual Au clusters as long as their ligand shell remains intact. The exposure to x-rays during the measurements is found to partly decompose the shell by removal of the chlorine atoms, resulting in a metallic behavior of the clusters as demonstrated by a steplike intensity at the Fermi energy. These observations resolve a long-standing controversy about the metallic behavior of ligated Au clusters emphasizing, in addition, the influence of the local environment on the electronic properties of nanoscaled materials.

Journal Article↗
PubMed1993

Three-dimensional aspects of caloric nystagmus in humans: I. The influence of increased gravitoinertial force.

The influence of increased gravitoinertial force on the horizontal vertical and torsional components of caloric nystagmus response was examined. Video-oculographic (VOG) recordings were made on a group of 10 human subjects so that all three components of eye movement could be evaluated. The caloric nystagmus response at all tested g-levels included nystagmus components around all three rotation axes. Over the tested range of 1.0 g to 3.0 g, the results demonstrate that nystagmus intensity does not increase linearly with effective gravitoinertial force but appears to saturate at levels of 2.0 g and beyond. It is proposed that the vertical and torsional nystagmus components were elicited both by caloric stimulation to the (anterior) vertical canals and direct thermal mediation to the otolithic sensory cells. Vertical LZ-nystagmus response was also observed during centrifuge runs (previous to caloric irrigation) at all g-levels and in all subjects. The caloric-induced vertical nystagmus response was also clearly recognisable. The observation of a reduction and inversion of nystagmus intensity during g-transitions agrees with earlier findings and is attributable to the stimulation to the canals during centrifuge acceleration, respectively deceleration.

Caloric Tests↗