1. Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA.
2. Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA.
3. Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA.
4. Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, 04103 Leipzig, Germany.
5. Department of Physics & Earth Science, Jacobs University Bremen, 28759 Bremen, Germany.
6. School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907, USA.