1. Department of Physics, University of California, Berkeley, California 94720, United States
2. Department of Chemistry, University of California, Berkeley, California 94720, United States
3. Center for Nanometer-Scale Science and Advanced Materials, NANOSAM, Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, PL 30-348 Kraków, Poland
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