*
Corresponding authors
a
Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA
b
Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, USA
c
Center of Integrated Nanomechanical Systems, University of California at Berkeley, Berkeley, California 94720, USA
d
Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
E-mail:
azettl@berkeley.edu
e
Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, USA
f
Physical and Life Science Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA
g
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
h
Department of Polymer Science & Engineering, Conte Polymer Research Center, University of Massachusetts, Amherst, Massachusetts 01003, USA
i
Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA