a
Department of Chemical and Biomolecular Engineering, University of California at Los Angeles, Los Angeles (UCLA), California 90095, USA
b
California NanoSystems Institute, University of California at Los Angeles, Los Angeles (UCLA), California 90095, USA
c
Department of Mechanical and Aerospace Engineering, University of California at Los Angeles, Los Angeles (UCLA), California 90095, USA
E-mail: guzhen@mit.edu; yitang@ucla.edu
d
Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles (UCLA), California 90095, USA