1. IBS Center for Integrated Nanostructure Physics; Institute for Basic Science (IBS); Daejon 305-701 Republic of Korea
2. Department of Energy Science, Department of Physics; Sungkyunkwan University; Suwon 440-746 Republic of Korea
3. National Center for Nanoscience and Technology; Beijing 100190 PR China
4. School of Materials Science and Engineering; Harbin University of Science and Technology; Harbin 150040 PR China
5. Department of Energy Science; Sungkyunkwan University; Suwon 440-746 Korea
6. Institute of Physics; Chinese Academy of Sciences; Beijing 100190 PR China