1. Department of Chemistry and Biochemistry University of California, Los Angeles Los Angeles CA 90095‐1569 USA
2. Lake Shore Cryotronics Westerville OH 43082 USA
3. Department of Chemistry and Biochemistry University of California, San Diego La Jolla CA 92093 USA
4. Key Laboratory for Advanced Materials Center for Computational Chemistry and Research Institute of Industrial Catalysis School of Chemistry and Molecular Engineering East China University of Science and Technology Shanghai 200237 P. R. China
5. California NanoSystems Institute University of California, Los Angeles Los Angeles CA 90095‐1569 USA
6. Department of Materials Science and Engineering University of California, Los Angeles Los Angeles CA 90095‐1569 USA