1. Materials Department University of California Santa Barbara CA 93106 USA
2. Electrical and Computer Engineering Department University of California Santa Barbara CA 93106 USA
3. Department of Chemistry and Biochemistry University of California Santa Barbara CA 93106 USA
4. Laboratoire de Physique de la Matière Condensée CNRS-Ecole Polytechnique IP Paris 91128 Palaiseau Cedex France