1Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Department of Applied Physics, School of Science, Tianjin University, Tianjin 300072, China
2Nanofabrication Laboratory, CAS Key Laboratory for Nanophotonic Materials and Devices, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China
3Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China