1. Collaborative Innovation Center of Advanced Microstructures; National Laboratory of Solid State Microstructures; Nanjing University; Nanjing 210093 P. R. China
2. Department of Physics; Nanjing University; Nanjing 210093 P. R. China
3. School of Electronic Science and Engineering; Nanjing University; Nanjing 210093 P. R. China
4. Department of Electronics; University of York; York YO10 5DD UK
5. Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province; Yancheng Institute of Technology; Yancheng 224051 P. R. China
6. Diamond Light Source; Didcot OX11 0DE UK