1. Electrical and Computer Engineering Department, University of California Santa Barbara, Santa Barbara, California 93106, USA
2. Material Department, University of California Santa Barbara, Santa Barbara, California 93106, USA
3. Institute for Energy Efficiency, University of California Santa Barbara, 2314 Phelps Hall, Santa Barbara, California 93106, USA
4. Sandia National Laboratories, Albuquerque, New Mexico 87185-1086, USA