1. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P.R. China
2. Department of Physics, Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education, Wuhan University, Wuhan 430072, P.R. China
3. Research Institute of Natural Gas Technology, PetroChina Southwest Oil and Gasfield Company, Chengdu 610213, Sichuan, China
4. Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610014, P.R. China
5. Department of Physics and School of Engineering, Brown University, Providence, Rhode Island 02912, United States