1. Center
for Nanoscale Characterization and Devices (CNCD), Wuhan National
Laboratory for Optoelectronics (WNLO) and School of Physics, Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, P.R. China
2. School
of Material Science and Engineering, Guangxi Nonferrous Metals Mineral
and Materials, Collaborative Innovation Center, Guilin University of Technology, Jian’gan Road 12, Guilin, Guangxi 541004, P.R. China