1. Center for Nanoscale Characterization & Devices (CNCD), Wuhan National Laboratory for Optoelectronics (WNLO) and School of Physics, Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, People's Republic of China
2. Laboratory of Advanced Materials, Department of Materials Science, Fudan University, Shanghai 200438, China