1. Max Planck Institute for Solid State Research; Heisenbergstraße 1 70569 Stuttgart Germany
2. Department of Mechanical Engineering and Materials Science; Duke University; Durham NC 27708 USA
3. Photonics and Optoelectronics Group; Department of Physics and Center for Nanoscience (CeNS); Ludwig-Maximilians-Universität (LMU); Amalienstraße 54 80799 Munich Germany
4. Nanosystems Initiative Munich (NIM); Schellingstraße 4 80799 Munich Germany
5. Department of Chemistry; University of Munich; Butenandtstraße 5-13 81377 Munich Germany