1. National Center for Electron Microscopy in Beijing; School of Materials Science and Engineering; The State Key Laboratory of New Ceramics and Fine Processing; Key Laboratory of Advanced Materials (MOE); Tsinghua University; Beijing 100084 P. R. China
2. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing China
3. Center for Nano and Micro Mechanics; Tsinghua University; Beijing 100084 China
4. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
5. Institute of Microelectronics; Tsinghua University; Beijing 100084 China
6. Key Laboratory for Micro-Nano Physics and Technology of Hunan Province; Hunan University; Changsha Hunan 410082 China