1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, USA
4. Sekisui Chemical Co. LTD, Shimamoto, Mishima, Osaka 618-0021, Japan
5. Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, USA
6. Kavli Energy NanoScience Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA