1. Key Laboratory for Ultrafine Materials of Ministry of Education
2. School of Materials Science and Engineering
3. East China University of Science & Technology
4. Shanghai 200237, China
5. Shanghai Nanotechnology Promotion Center
6. Shanghai Institute of Micro-system and information Technology
7. Chinese Academy of Sciences
8. Shanghai 200050, China
9. Department of Physics
10. East China Normal University
11. Shanghai 200062, China