1. Department
of Electrical and Computer Engineering, Micro and Nanotechnology Laboratory,
Materials Research Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
2. NUS
Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456
3. Department
of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583
4. Institute
of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634
5. International
Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Fukuoka 819-0395, Japan