1. Center for Nanochemistry, Beijing Science
and Engineering Centre for Nanocarbons, Beijing National Laboratory
for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
2. Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1
3PU, U.K.
3. Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, United States