1. College of Chemistry and Materials Engineering Zhejiang A&F University Hangzhou 311300 P. R. China
2. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China
3. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences Beijing 100190 P. R. China
4. Department of Electrical and Computer Engineering National University of Singapore Singapore 117583 Singapore
5. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology Wuhan 430070 P. R. China
6. Department of Materials Science and Engineering National University of Singapore Singapore 117574 Singapore