1. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore
2. Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, 138634, Singapore
3. Department of Physics, College of Science, Sultan Qaboos University, P.O. Box 36, PC 123, Muscat, Oman