1. School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China
2. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China
3. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
4. School of Physical Science and Technology, Sichuan University, Chengdu 610064, P. R. China
5. School of Materials Science and Energy Engineering, Foshan University, Foshan 528000, P. R. China