1. Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China
2. Institute of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, P. R. China
3. Hefei National Research Center for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China