1. Department of Nanoengineering, University of California-San Diego, La Jolla, California 92093, USA
2. Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA
3. Center for Memory and Recording Research, University of California San Diego, La Jolla, CA 92093-0401, USA
4. Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, USA