1. Department of Electrical and Computer Engineering, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA.
2. Materials Science and Engineering, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA.
3. Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA.
4. Center for Memory and Recording Research, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA.