1. School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
2. Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02138, USA.
3. Department of Chemistry and Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720, USA.
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. Kavli Energy NanoScience Institute, Berkeley, CA 94720, USA.