1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore
2. Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, 138634 Singapore
3. Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue Cambridge, Cambridgeshire CB3 0HE, United Kingdom
4. School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Ave, 639798 Singapore
5. Institute of Sustainability for Chemicals, Energy and Environment (ISCE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, 627833 Singapore
6. School of Chemistry, Chemical Engineering and Biotechnology, Nanyang Technological University, 21 Nanyang Link, 637371 Singapore