1. Department of Applied Physics, Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China
2. State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices, Centre for Physical Mechanics and Biophysics, School of Physics, Sun Yat-sen University, Guangzhou 510275, China
3. Electron Imaging and Spectroscopy Tools, P.O.
Box 506, Sans Souci, NSW 2219, Australia
4. Physics, School of Science, RMIT University, Melbourne, Victoria 3001, Australia