1. Department of Materials Science and EngineeringUniversity of California Berkeley CA 94720 USA
2. Division of Materials SciencesLawrence Berkeley National Laboratory Berkeley CA 94720 USA
3. Department of Electrical and Computer EngineeringNational University of Singapore Singapore 117583 Singapore
4. Tsinghua‐Berkeley Shenzhen InstituteTsinghua University Shenzhen 518055 China
5. NSF Nanoscale Science and Engineering Center (NSEC)University of California Berkeley CA 94720 USA
6. University of Hong Kong Hong Kong China
7. Department of Mechanical EngineeringUniversity of California Berkeley CA 94720 USA