1. Department of Materials Science, Department of Electrical Engineering and Computer Sciences, and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Centre for Nanoscale Science and Technology (CNST), Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China