1. Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, United States
2. Department of Electrical and Computer Engineering, University of Illinois Urbana−Champaign, Champaign, Illinois 61801, United States
3. Institute for Energy Efficiency, University of California, Santa Barbara, Santa Barbara, California 93106, United States
4. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan