1. Science and Technology on Microsystem Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People's Republic of China
2. Graduate University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
3. Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institute of Physical Science, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031, People's Republic of China