1. School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798
2. Institute of Material Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602
3. Department of Applied Physics, College of Science, Tianjin University, Tianjin 300072, China
4. School of Physical and Mathematical Sciences, Nanyang Technological University, Nanyang Avenue, Singapore 637371
5. Department of Electrical and Electronics Engineering, Department of Physics, UNAM, and National Institute of Materials Science and Nanotechnology, Bilkent University, Bilkent, Ankara, Turkey 06800