1. School of Materials Science and Engineering, Nanchang University, Nanchang 330031, P. R. China
2. Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China
3. Department of Physics, South University of Science and Technology of China, Shenzhen 518055, P. R. China
4. Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China
5. College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China