1. Department Electrical and Computer Engineering, University of Illinois, Urbana, Illinois 61801, USA
2. Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA
3. International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
4. Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA