1. Department of Physics, Graduate Center, City University of New York, New York, New York 10016, United States
2. Photonics Initiative, Advanced Science Research Center, City University of New York, New York, New York 10031, United States
3. Department of Chemistry, Columbia University, New York, New York 10027, United States
4. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
5. ARC Centre of Excellence in Exciton Science, School of Physics, UNSW Sydney, Sydney, 2052 NSW, Australia
6. Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, P. R. China