1. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication; CAS Center for Excellence in Nanoscience; National Center for Nanoscience and Technology; Beijing 100190 China
2. University of Chinese Academy of Science; Beijing 100049 China
3. Applied Mechanics Laboratory; Department of Engineering Mechanics and Center for Nano and Micro Mechanics; Tsinghua University; Beijing 100084 China
4. State Key Laboratory of Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 China
5. Department of Materials Science and NanoEngineering; Rice University; Houston TX 77005 USA