1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China
2. State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China
3. LCP, Institute of Applied Physics and Computational Mathematics, Beijing 100088, P. R. China
4. Key Laboratory for Advanced Materials Processing Technology, Ministry of Education of China, State Key Laboratory of Tribology, Department of Mechanical Engineering and Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China