1. School of Electrical and Electronics Engineering, Nanyang Technological University, Block S2, Nanyang Avenue, Singapore 639798, Singapore, Advanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore, and School of Physical and Mathematical Sciences, Nanyang Technological University, 1 Nanyang Walk, Block 5, Singapore 637616, Singapore