1. Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark
2. Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
3. Technische Universität München, Electrical Engineering and Information Technology, Arcisstr. 21, 80333 München, Germany
4. Consiglio Nazionale delle Ricerche, ISMN, Via Salaria Km 29.6, 00017 Monterotondo, Rome, Italy