1. Department of Physics and Siyuan Laboratory, Jinan University, Guangzhou, Guangdong 510632, China
2. Wuhan National Laboratory for Optoelectronics (WNLO), and College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China
3. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
4. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100085, China