1. Center for Nanoscale Characterization and Devices; Wuhan National Laboratory for Optoelectronics; Huazhong University of Science and Technology; Wuhan 430074 China
2. Department of Physics; Arizona State University; Tempe Arizona 85287-1504 USA
3. School of Electrical, Computer, and Energy Engineering; Arizona State University; Tempe Arizona 85287-5706 USA
4. Department of Physics and Astronomy; The University of Toledo; Toledo Ohio 43606 USA
5. School of Physics; Huazhong University of Science and Technology; Wuhan 430074 China
6. School of Physics and Technology; Center for Electron Microscopy and MOE Key Laboratory of Artificial Micro- and Nano-Structures; Wuhan University; Wuhan 430072 China