1. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, China
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
3. Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
4. Center for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore