* 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