1. Hunan Province Key Laboratory of Materials Surface & Interface Science and Technology, College of Material Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, P. R. China
2. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
3. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China