1. Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University; Changsha 410083 China
2. School of Material Science and Engineering, Guilin University of Electronic Technology; Guilin 541004 China
3. Department of Physics and Astronomy, University of Rochester; Rochester NY 14627 USA
4. Center for Excellence in Nanoscience, Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology; Beijing 100190 China