1. Department of Materials Science and Engineering; College of Engineering; Peking University; Beijing 100871 P. R. China
2. Center for Nanochemistry (CNC); Beijing National Laboratory for Molecular Sciences; College of Chemistry and Molecular Engineering; Academy for Advanced Interdisciplinary Studies; Peking University; Beijing 100871 P. R. China
3. State Key Laboratory of Optoelectronic Materials and Technologies and Guangdong Province Key Laboratory of Display Material and Technology; School of Physics and Engineering; Sun Yat-sen University; Guangzhou 510275 P. R. China
4. State Key Laboratory for Mesoscopic Physics; Department of Physics; Peking University; Beijing 100871 P. R. China
5. National Center for Nanoscience and Technology; Beijing 100190 P. R. China