1. Institute of Pure and Applied Chemistry, Universität Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany
2. Institut für Physik/ASP, Humboldt-Universität zu Berlin, Newtonstrasse 15, D-12489 Berlin, Germany
3. Mads Clausen Institute, NanoSYD, University of Southern Denmark, Dybbolsgade 2 - Alsion, DK 6400 Sønderborg, Denmark