1. State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.
2. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China.
3. Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China.
4. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China.
5. State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China.