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. University of Chinese Academy of Sciences, Beijing 100049, P.R. China
3. State Key Laboratory for Mesoscopic Physics and Department of Physics, Collaborative Innovation Center of Quantum Matter Nano-optoelectronics Frontier Center of Ministry of Education, Peking University, Beijing 100871, P.R. China
4. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China