1. Institute of Fundamental and Frontier Science University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. Department of Electronic Materials and Engineering Research School of Physics and Engineering The Australian National University Canberra ACT 0200 Australia
3. Institut National de la Recherche Scientifique – Énergie Materiaux Télécommunications Varennes J3X 1S2 Québec Canada
4. School of Information and Communication Engineering University of Electronic Science and Technology of China Chengdu 611731 P. R. China
5. Center for Nanoscale Materials Argonne National Laboratory Lemont IL 60439 USA
6. Department of Physics and Astronomy Ohio University Athens OH 45701 USA