1. Department of Physics, University of California, Berkeley, Berkeley, California 94720, United States
2. Department
of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States
3. iNANO and
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus, Denmark
4. Materials Sciences
Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States