1. Wuhan National Laboratory for Optoelectronics (WNLO), and College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China
2. Department of Physics, Jinan University, Guangzhou 510632, China
3. Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, China
4. Department of Materials Science and Engineering, National Tsinghua University, Hsinchu 30013, Taiwan
5. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States