1. School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China
2. Department of Physics, Southern University of Science and Technology, Shenzhen 518005, P. R. China
3. State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Weijin Road, CN-300072 Tianjin, P. R. China
4. School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States
5. Department of Quantum and Energy Materials, International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga s/n, Braga 4715-330, Portugal