1. California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States
2. Department of Chemistry & Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States
3. Department of Materials Science & Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States
4. Center for Memory and Recording Research, University of California, San Diego, La Jolla, California 92093, United States