1 Department of Chemistry and the Key Laboratory of Atomic & Molecular Nanosciences (Ministry of Education, China), Tsinghua University, Beijing, 100084, P.R. China.
2 National Center for Nanoscience and Nanotechnology, Beijing, 100084, P.R. China, Fax: (+86) 10-6278-8765.
3 Department of Physics, Peking University, Beijing, 100084, P.R. China.