1. School of Materials Science and Engineering; University of Science and Technology Beijing; Beijing 100083 China
2. National Center for Electron Microscopy in Beijing; School of Materials Science and Engineering; Tsinghua University; Beijing 100084 China
3. Institute of Physics; Academia Sinica; Taipei 11529 Taiwan
4. Department of Physics; National Chiao Tung University; Hsinchu 30010 Taiwan
5. Department of Materials Science and Engineering; National Chiao Tung University; Material and Chemical Research Laboratories; Industrial Technology Research Institute; Hsinchu 30010 Taiwan