1. Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542, National University of Singapore Nanoscience and Nanotechnology Initiative (NUSNNI), 2 Science Drive 3, Singapore 117542, Division of Bioengineering, National University of Singapore, Blk E3A, 9 Engineering Drive 1, Singapore 117576, and Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1 Singapore 117576