1. Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
2. University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
4. Department of Physics and Astronomy, Ultrafast Photophysics of Quantum Devices Laboratory, Clemson University, Clemson, South Carolina 29634, United States
5. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States