1. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Republic of Singapore
2. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-03 Innovis, Singapore 138634, Republic of Singapore
3. School of Physical and Mathematical Sciences, 21 Nanyang Link, Nanyang Technological University, Singapore 637371, Republic of Singapore