1. School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
3. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany
4. Department of Physics, Center for Ion Beam Application and Center for Electron Microscopy, Wuhan University, Wuhan 430072, China