Résumé des résultats (Abstract)
(Anglais)
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Our role in the project is the functionalization of Carbon Nanotubes (CNT) by hydrogen plasma and the investigation of the electronic properties of the modified nanostructures.
In the first research period we investigated the interaction of atomic hydrogen on different sp2-bonded Carbon materials (Graphite, C60 and CNT). The investigations have shown that the adsorption energy barrier for bond formation decreases with the local curvature of the sp2-network. This behavior can be attributed to the higher sp3-character of convex sp2-structures, which is in agreement with the DFT calculation (X. Sha and B. Jackson, Surf. Sci. 496, 318 (2002', showing a lowering of the energy barrier for a carbon atom that raised above the plane defined by its nearest neighbors. Our result show that the adsorption energy barrier increases with increasing H-coverage, leading to saturation coverage which depend on the form of hydrogen (i.e. atomic or ionic) used for the treatment. Accordingly, we found the highest H-coverage (8 = 0.7) on the C60 multilayer using low energy hydrogen ions. The experiments indicate a lower estimation of the critical radius of curvature of r cr -0.6 nm for the chemisorption of hydrogen on sp2-bonded carbon nanostructures.
Publication:
Hydrogen adsorption on sp2-bonded carbon: Influence of the local curvature
P. Ruffieux, o. Groning, M Bielmann. P. Mauron, L. Schlapbach, and P. Groning, Physical Review B (2002) in press
In the second research period we investigated the influence of chemisorbed hydrogen on the local electronic properties of graphite. In conclusion we found a marked redistribution of the local density of states (LDOS) in the vicinity of point defects on graphite surfaces. The point defects, consisting of hydrogen chemisorbed sites and atomic vacancies, break the lattice symmetry and act as scatteres for the delocalized electrons. The resulting standing waves are observed as a -f3 -superstructure in the STM tunnel current image, which can be directly related to the point-like contour of the Fermi surface of graphite. For isolated defects, the typical range of the charge redistribution id of ~6 nm. Our findings show that the origin of the electronic superstructure in the LDOS is large momentum scattering of the pi-electrons at the point defects. For samples with high defect densities, the interference of two or more standing waves results in a variety of patterns in the LDOS, which depends on the relative phase between the individual standing waves. The experimentally observed patterns in the LDOS could be reproduced by a simulation where the electron wave-functions are described as a superposition of plane waves having vectors that correspond to the comers of the first Brillouin zone which make up the Fermi surface. Beside the influence on the transport properties due to scattering at the point defects, a modulation of the local reactivity for chemically adsorbing atoms has to be expected due to the marked redistribution of the LDOS in the vicinity of the defects.
Publication:
Charge-density oscillation on graphite induced by interference of electron waves
P. Ruffieux, o. Groning, M Bielmann, L. Schlapbach, and P. Groning, to be published in Physical Review Letters
In addition to the scientific works we organized within the FUNCARS project a 3-day lecture course on scanning probe microscopy. The course was attended by 17 participants from all laboratories involved in the project.
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