1. Institute of Solid State and Materials Research, Helmholtzstrasse 20, D-01069 Dresden, J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague 8, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, and The Center for Nanoscale Science and Technology, Rice University, MS 60, 6100 Main St., Houston, Texas 77005-1892