1. CAS Key Laboratory of Standardization and Measurement for Nanotechnology; CAS Center for Excellence in Nanoscience; National Center for Nanoscience and Technology; Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. National Institute of Advanced Industrial Science and Technology (AIST); Tsukuba 305-8565 Japan
4. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication; CAS Center for Excellence in Nanoscience; National Centre for Nanoscience and Technology; Beijing 100190 P. R. China