1. Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
4. Physics Department and Jiangsu Key Laboratory for Chemistry of Low Dimensional Materials, Huaiyin Normal University, Jiangsu 223300, China
5. Department of Physics and The Center for Physical Experiments, University of Science and Technology of China, Hefei, Anhui 230026, China
6. Institute of Experimental Physics (IEP UW), Warsaw University, Warsaw, Poland
7. Department of Postsecondary Teaching and Learning, University of Minnesota, Minneapolis, Minnesota 55455, USA