1. 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. College of Materials Science and Opto-Electronic Technology; University of Chinese Academy of Science; No. 19A Yuquan Road Beijing 100049 P. R. China
3. Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education; School of Physics & Microelectronics; University of Hunan; Changsha 410082 China