Fang Yi,a Huaying Ren,ab Jingyuan Shan,ab Xiao Sun,ab Di Wei*c and Zhongfan Liu*ac
a 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
b Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China
c Beijing Graphene Institute, Beijing 100094, P. R. China