1. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602
2. Singapore Synchrotron Light Source (SSLS), National University of Singapore, 5 Research Link, Singapore 117603
3. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576
4. Department of Physics, Imperial College London, London SW7 2AZ, U.K.