1. National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering; Nanjing University; Nanjing 210093 China
2. School of Materials Science and Engineering; Nanjing University of Science and Technology; Nanjing 210094 China
3. Department of Physical Chemistry; University of Science and Technology Beijing; Beijing 100083 China
4. Condensed Matter Science and Technology Institute, Harbin Institute of Technology; Harbin 150001 China