1. Key Laboratory of Advanced Materials (MOE) and Beijing National Center for Electron Microscopy; Tsinghua University; Beijing 100084 China
2. Institute of Physics; Chinese Academy of Sciences; Beijing 100084 China
3. Institute of Microelectronics; Tsinghua University; Beijing 100084 China
4. Division of Physical Science and Engineering; King Abdullah University of Science and Technology (KAUST); Thuwal 239955 Kingdom of Saudi Arabia