1. Shenzhen Key Laboratory of Flexible Memory Materials and Devices, College of Electronic Science and Technology, Shenzhen University, Guangdong 518060, China
2. Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, United States
3. Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
4. Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong