1. IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA
2. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA
3. Institute of Physical Chemistry, University of Hamburg, D‐20146 Hamburg, Germany
4. Max Planck Institute of Colloids and Interfaces, D‐14424 Potsdam, Germany
5. Institute of Chemistry, University of Osnabrück, D‐49076 Osnabrück, Germany
6. Photonics and Optoelectronics Group, Physics Department & CeNS, Ludwig‐Maximilians‐Universtität München, D‐80799 Munich, Germany