1. Department of Molecular and Cellular Biology, Harvard University, 52 Oxford Street, Cambridge, Massachusetts 02138,United States
2. Center for Nanoscale Systems, Harvard University, 11 Oxford Street, Cambridge, Massachusetts 02138, United States
3. Department of Chemistry, University of California, Berkeley, California 94720, United States
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States
6. Kavli Energy NanoScience Institute, Berkeley, California 94720, United States