1 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China
2 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
3 School of Physics and Technology, Center for Nanoscience and Nanotechnology, Wuhan University, Wuhan 430072, China
4 Department of Mechanical Engineering, University of North Texas, Denton, Texas 76207, USA
5 Department of Physics and Astronomy and Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701, USA