1. Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
2. LOA, ENSTA Paris, Ecole Polytechnique, CNRS, Institut Polytechnique de Paris, 181 Chemin de la Hunière et des Joncherettes 91120 Palaiseau, France
3. Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
4. Department of Physics, University of Duisburg-Essen, Lotharstrasse 1, 47057 Duisburg, Germany
5. Center for Nanointegration (CENIDE), University of Duisburg-Essen, Carl-Benz-Str. 199, 47057 Duisburg, Germany