1. Materials
Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
2. Department
of Physics, Friedrich-Alexander University of Erlangen-Nürnberg, Erwin-Rommel-Strasse 1, 91058 Erlangen, Germany
3. Department
of Chemistry, University of California, Berkeley, California 94720, United States
4. Department
of Materials Science and Engineering, University of California, Berkeley, California 94720, United States