1. Center
for Nanochemistry, Beijing National Laboratory for Molecular Sciences,
Key Laboratory for the Physics and Chemistry of Nanodevices, State
Key Laboratory for Structural Chemistry of Unstable and Stable Species,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China
2. Department
of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States