1. Materials and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2. Department of Physics, University of California Berkeley, Berkeley, California 94720, USA
3. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
4. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
5. Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, USA
6. Energy Science Institute, Yale University, New Haven, Connecticut 06520, USA
7. Thomas J. Watson Laboratory of Applied Physics, Caltech, Pasadena, California 91125, USA
8. Stanford Institute for Materials and Energy Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
9. Kavli Energy Nanosciences Institute at Berkeley, Berkeley, California 94720, USA