1. Faculty of Physics University of Vienna 1090 Vienna Austria
2. State Key Laboratory of Optoelectronic Materials and Technologies Nanotechnology Research Center School of Materials Science and Engineering Sun Yat-sen University Guangzhou 510275 P. R. China
3. School of Physical Science and Technology ShanghaiTech University Shanghai 201210 P. R. China
4. Central Facility for Electron Microscopy Electron Microscopy Group of Materials Science Ulm University 89081 Ulm Germany
5. Nanomaterials Research Institute National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Ibaraki 305-8565 Japan