1. School of Physics and Electronics, Hunan University, Changsha 410082, China
2. Physics Department, College of Science, Central South University of Forestry and Technology, Changsha 410004, China
3. School of Physics and Electronic Information, Luoyang Normal University, Luoyang, Henan 471022, P. R. China
4. Fujian Strait Research Institute of Industrial Graphene Technologies, Jinjiang 362200, China
5. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China