1. Institute for Energy Efficiency, University of California Santa Barbara, Santa Barbara, California 93117, USA
2. Center for Opto-electronic Materials and Devices, Korea Institute of Science and Technology, Seoul, 02792, South Korea
3. IBM Research, Albany, New York 12203, USA
4. Department of Electrical and Computer Engineering, University of Texas Austin, Austin, Texas 78758, USA
5. Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
6. Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA