1. CAS Key Laboratory of Standardization and Measurement for Nanotechnology
2. National Center for Nanoscience and Technology
3. Beijing 100190, P. R. China
4. Department of Electronic Engineering
5. National Taiwan University of Science and Technology
6. Taipei 106, Republic of China
7. State Key Laboratory of Superlattices and Microstructures
8. Institute of Semiconductors
9. Chinese Academy of Sciences
10. Beijing 100083, China
11. Center for Nanochemistry
12. College of Chemistry and Molecular Engineering
13. Peking University
14. Beijing 100871, China