1. NUSNNI-NanoCore, National University of Singapore, Singapore 117411
2. Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117576
3. Department of Physics, National University of Singapore, Singapore, 117542
4. Center for Ion Beam Applications (CIBA), Department of Physics, National University of Singapore, Singapore 117542
5. Singapore Synchrotron Light Source (SSLS), National University of Singapore, Singapore 117603
6. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
7. MESA+ Institute for Nanotechnology, University of Twente, Enschede 7500 AE, The Netherlands