1. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
3. School of Materials Science and Engineering, University of New South Wales, NSW 2052, Australia
4. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore