1. J. Heyrovský Institute of Physical Chemistry, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic, Laboratory of Photonics and Interfaces, Swiss Federal Institute of Technology, EPFL, Ecublens, CH-1015, Lausanne, Switzerland, and Laboratory of Crystallography, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zürich, Switzerland