1. Shenzhen Geim Graphene CenterTsinghua Shenzhen International Graduate SchoolTsinghua University Shenzhen 518055 P. R. China
2. Laboratory of Advanced MaterialsSchool of Materials Science and EngineeringTsinghua University Beijing 100084 P. R. China
3. School of Materials Science and EngineeringHenan University of Science and TechnologyCollaborative Innovation Center of Nonferrous Metals of Henan Province Luoyang 471023 P. R. China