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. Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610014, P. R. China
3. School of Engineering, Brown University, Providence, Rhode Island 02912, United States