1. Shenzhen Geim Graphene CenterTsinghua–Berkeley Shenzhen InstituteTsinghua University Shenzhen Guangdong 518055 P. R. China
2. Department of PhysicsSouth University of Science and Technology of China Shenzhen Guangdong 518055 P. R. China
3. Center of Electron Microscopy and State Key Laboratory of Silicon MaterialsSchool of Materials Science and EngineeringZhejiang University Hangzhou Zhejiang 310027 P. R. China
4. Shenyang National Laboratory for Materials SciencesInstitute of Metal ResearchChinese Academy of Sciences Shenyang Liaoning 110016 P. R. China