1. Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore
2. Department of Physics and Astronomy, Queen Mary University of London, London E1 4NS, United Kingdom
3. Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore
4. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 50 Nanyang Avenue 639798, Singapore