1. Department of Physics, University of California, Berkeley, CA 94720, USA.
2. Department of Chemistry, Columbia University, New York, NY 10027, USA.
3. Department of Physics, Columbia University, New York, NY 10027, USA.
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.