1. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication
2. CAS Center for Excellence in Nanoscience
3. National Center for Nanoscience and Technology
4. Beijing 100190
5. P. R. China
6. Division of Physics and Applied Physics
7. School of Physical and Mathematical Sciences
8. Nanyang Technological University
9. Singapore 637371
10. Singapore
11. Department of Physics
12. Beihang University
13. Beijing 100083
14. China