1. California NanoSystems Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA.
2. Department of Chemistry and Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA.
3. Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA.
4. Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA 90095, USA.