1. State Key Laboratory of Photon-Technology in Western China Energy, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon Technology, School of Physics, Northwest University, Xi’an 710069, P. R. China
2. Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China
3. Department of Physics, Xiamen University, Xiamen 361005, P. R. China
4. Shaanxi Joint Key Laboratory of Graphene, School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710126, P. R. China