1. Department
of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China
2. Division
of Photonics, CAS Key Laboratory of Standardization and Measurement
for Nanotechnology, CAS Center of Excellence for Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China
3. State
Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China
4. State
Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, People’s Republic of China
5. Division
of Physics and Applied Physics, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637371