1. i -Lab, Suzhou Institute of Nano-Tech and Nano-Bionics; Chinese Academy of Sciences; 398 Ruoshui Road Suzhou 215123 Jiangsu P. R. China
2. Institute of Functional Nano and Soft Materials (FUNSOM); Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices; Soochow University; 199 Ren'ai Road Suzhou 215123 Jiangsu P. R. China
3. Department of Applied Physics; The Hong Kong Polytechnic University; Hong Kong P. R. China
4. School of Materials Science and Engineering; Nanchang University; Nanchang 330031 Jiangxi P. R. China
5. Department of Physics; Tsinghua University; Beijing 100084 P. R. China