1. Center for Nanoscale Characterization and Devices; Wuhan National Laboratory for Optoelectronics-School of Physics-School of Optical and Electronic Information; Huazhong University of Science and Technology; Luoyu Road 1037 Wuhan 430074 PR China
2. Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials; Faculty of Physics and Electronic Science; Hubei University; Wuhan 430062 PR China
3. Department of Physics; Florida International University; Miami FL 33199 USA
4. School of Physics and Technology; Center for Electron Microscopy and MOE Key Laboratory of Artificial Micro- and Nano-Structures; Wuhan University; Wuhan 430072 PR China