¹ Shaanxi Joint Laboratory of Graphene, State Key Laboratory 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, China

² National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China

³ School of Electronic Engineering, Xi’an University of Posts and Telecommunications, Xi’an 710121, China

⁴ e-mail: xfwang@nju.edu.cn

⁵ e-mail: rzy@nwu.edu.cn