1. State Key Laboratory for Mesoscopic Physics School of Physics Collaborative Innovation Center of Quantum Matter Peking University Beijing 100871 P. R. China
2. Center for Nanochemistry Beijing Science and Engineering Center for Nanocarbons College of Chemistry and Molecular Engineering Peking University Beijing 100871 P. R. China
3. Department of Physics University of California at Berkeley, Advanced Light Source Division and Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley CA 94720 USA