1. Department of Physics National University of Defense Technology Changsha 410073 P. R. China
2. Guangxi Key Laboratory of Automatic Detecting Technology and Instrument Guilin University of Electronic Technology Guilin 541004 P. R. China
3. Shaanxi Joint Lab of Graphene International Collaborative Center on Photoelectric Technology and Nano Functional Materials Institute of Photonics and Photon‐Technology Northwest University Xi'an 710069 P. R. China
4. School of Science Xi'an University of Posts & Telecommunications Xi'an 710121 P. R. China