1. Department of Physics and Department of Chemistry, Hong Kong University of Science and Technology, Hong Kong, China
2. HKUST Fok Ying Tung Research Institute, Guangzhou, China
3. Materials Science Division and Institute for Molecular Engineering, Argonne National Laboratory, Argonne, Illinois 60439, USA
4. Institute for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, USA
5. Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, USA