1. Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
2. Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117574, Singapore
3. Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
4. CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China
5. College of Science, China University of Petroleum, Qingdao 255680, China