1. National Laboratory of Solid State Microstructures and Department of Physics Nanjing University Nanjing 210093 P. R. China
2. College of Telecommunication and Information Engineering Nanjing University of Posts and Telecommunications Nanjing 210003 P. R. China
3. Collaborative Innovation Center of Advanced Microstructures Nanjing 210093 P. R. China
4. Department of Physics Tongji University Shanghai 200092 P. R. China
5. School of Physics and the Australian Institute for Nanoscale Science and Technology The University of Sydney NSW 2006 Australia
6. The Australian Centre for Microscopy and Microanalysis The University of Sydney NSW 2006 Australia
7. College of Sciences Northeastern University Shenyang 110819 P. R. China