1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
2. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. Institute of Materials Research and Engineering, Agency for Science, Technologies and Research, Singapore 117602
4. Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798