1. State Key Laboratory of Tribology Department of Mechanical Engineering Tsinghua University Beijing 100084 P. R. China
2. School of Engineering and Technology China University of Geosciences (Beijing) Beijing 100083 P. R. China
3. Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge MA 02139 USA
4. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083 P. R. China
5. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA