1. Shenzhen Geim Graphene Research CenterDivision of Energy and EnvironmentGraduate School at ShenzhenTsinghua University Shenzhen 518055 P. R. China
2. Department of PhysicsSouthern University of Science and Technology Shenzhen 518055 P. R. China
3. School of Chemical and Biomedical EngineeringNanyang Technological University 639798 Singapore Singapore