1. Ordered Matter Science Research Center, Southeast University, Nanjing, Jiangsu 211189, P. R. China.
2. Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA.
3. Department of Physics, Southeast University, Nanjing, Jiangsu 211189, P. R. China.
4. College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China.
5. Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China.
6. Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, Guangdong 518055, P. R. China.
7. Department of Mechanical Engineering, University of Washington, Seattle, WA 98195-2600, USA.