1. S. Joint Lab of Graphene; State Key Lab Incubation Base of Photoelectric Technology and Functional Materials; International Collaborative Center on Photoelectric Technology and Nano Functional Materials; Institute of Photonics & Photon-Technology; Northwest University; Xi'an 710069 P. R. China
2. Guangxi Key Laboratory of Automatic Detecting Technology and Instruments; Guilin University of Electronic Technology; Guilin 541004 P. R. China