1. Physics Department, University of Hong Kong, Hong Kong SAR, P. R. China
2. State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310024, P. R. China
3. Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
4. Department of Aerospace and Mechanical Engineering, University of Arizona, Tucson, Arizona 85721, United States
5. Department of Electrical and Computer Engineering, University of Arizona, Tucson, Arizona 85721, United States