1. Division
of Chemistry and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637371
2. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, 08-03, Singapore 138634
3. Department
of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China
4. School
of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, Shaanxi, China