1. Department of Applied Physics; The Hong Kong Polytechnic University; Hung Hom, Kowloon Hong Kong P. R. China
2. Institute of Functional Nano and Soft Materials (FUNSOM); Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology; Soochow University; Suzhou 215123 P. R. China
3. Center of Super-Diamond and Advanced Film (COSDAF) and Department of Physics and Materials Science; City University of Hong Kong; Hong Kong P. R. China
4. Department of Materials Science and Engineering; Monash University; Clayton VIC 3800 Australia
5. Department of Electronic and Information Engineering; The Hong Kong Polytechnic University; Hung Hom, Kowloon Hong Kong P. R. China