1. School of Materials and Energy, Lanzhou University, Lanzhou 730000, P. R. China
2. State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics Chinese Academy of Sciences, Lanzhou 730000 China
3. Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555 Japan