1. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore
2. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602 Singapore
3. Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China