1. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
2. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
3. School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China
4. Facility for Analysis Characterisation Testing and Simulation (FACTS), Nanyang Technological University, Singapore 639798, Singapore