1. Center for Nanoscale Characterization & Devices (CNCD), School of Physics & Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, P. R. China
2. National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yu-Tian Road, Shanghai 200083, China
3. Materials Engineering & Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, Queensland 4072, Australia