1. Division of Nanophotonics; CAS Key Laboratory of Standardization and Measurement for Nanotechnology; CAS Center for Excellence in Nanoscience; National Center for Nanoscience and Technology; Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. School of Physics and Electronics; Shandong Normal University; Jinan 250014 P. R. China
4. Department of Materials Science and Engineering; College of Engineering; Peking University; Beijing 100871 P. R. China
5. Institute of Materials and Clean Energy; Shandong Normal University; Jinan 250014 P. R. China