1. Institute of Solid Mechanics, Beihang University (BUAA), Beijing 100191, China e-mail:
2. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China e-mail:
3. Institute of Solid Mechanics, Beihang University (BUAA), Beijing 100191, China;
4. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208 e-mail: