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