1. State Key Laboratory of Functional Materials for Informatics; Shanghai Institute of Microsystem and Information Technology; Chinese Academy of Sciences; 865 Changning Road Shanghai 200050 P. R. China
2. Department of Physics and State Key Laboratory of Surface Physics; Fudan University; Shanghai 200433 China
3. Center of Electron Microscopy and State Key Laboratory of Silicon Materials; School of Materials Science and Engineering; Zhejiang University; Hangzhou 310027 P. R. China
4. School of Physical Science and Technology; Shanghai Tech University; Shanghai 200031 P. R. China
5. Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices; Institute of Functional Nano and Soft Materials (FUNSOM); Soochow University; Suzhou 215123 P. R. China
6. Department of Materials Science and Engineering; Technion; Israel Institute of Technology; Haifa 3200003 Israel