1. National Science Foundation (NSF) Nanoscale Science and Engineering Center (NSEC), 5130 Etcheverry Hall, University of California, Berkeley, CA 94720–1740, USA.
2. Department of Chemistry, University of California, Berkeley, CA 94720, USA.
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cycletron Road, Berkeley, CA 94720, USA.