1. Department of Chemistry, University of California, Berkeley 94720, United States
2. Department of Physics, University of Cambridge, Cambridge CB3 0HE, U.K.
3. Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
4. Kavli Energy NanoSciences Institute, University of California, Berkeley 94720, United States
5. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley 94720, United States
6. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley 94720, United States