1. Ordered Matter Science Research Center, Southeast University, Nanjing 211189 (P. R. China)
2. Department of Mechanical Engineering, University of Washington, Seattle, WA 98195 (USA)
3. School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094 (P.R. China)
4. Department of Materials Science and Engineering, Nanjing University, Nanjing 210093 (P.R. China)
5. Faculty of Materials, Optoelectronics and Physics, Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Hunan 411105 (China)