1. Center
for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons,
Beijing National Laboratory for Molecular Sciences, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, P. R. China
2. State Key Laboratory of Low Dimensional
Quantum Physics, Collaborative Innovation Center of Quantum Matter
and Department of Physics, Tsinghua University, Beijing 100084, China
3. Academy for Advanced Interdisciplinary
Studies, Peking University, Beijing 100871, China
4. Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, United States
5. Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1
3PU, U.K.