1. Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
2. Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom
3. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore
4. College of Physics and Microelectronics, Hunan University, Changsha 410082, China