1. Department of Physics, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
2. Laboratory for Nanodevices, National Center for Nanoscience and Technology, Beijing 100190, China
3. Key Laboratory of Standardization and Measurement for Nanotechnology, Chinese Academic of Sciences, Beijing 100190, China
4. School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore