1 Low Energy Electronic Systems (LEES), Singapore-MIT Alliance for Research and Technology (SMART), Singapore 138602, Singapore
2 School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510006, China
3 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
4 Materials Research Laboratories, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
5 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
6 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore