1. State Key Lab of New Ceramics and Fine ProcessingSchool of Materials Science and EngineeringTsinghua University Beijing 100084 China
2. Shenzhen Geim Graphene CenterTsinghua‐Berkeley Shenzhen InstituteTsinghua University Shenzhen 518055 China
3. Department of Materials Science and EngineeringThe Pennsylvania State UniversityUniversity Park PA 16802 USA
4. Center for Flexible Electronics TechnologyTsinghua University Beijing 100084 China