1. Department of Physics, University of California, Berkeley, CA 94720, USA.
2. Department of Physics, Columbia University, New York, NY 10027, USA.
3. Department of Chemistry, University of California, Berkeley, CA 94720, USA.
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA.
6. Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.