1. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue 639798 Singapore
2. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link 637371 Singapore
3. School of Mechanical & Aerospace Engineering; Nanyang Technological University; 50 Nanyang Avenue 639798 Singapore
4. Institute of Materials Research and Engineering; A*STAR (Agency for Science, Technology and Research); 2 Fusionopolis Way, Innovis #08-03 138634 Singapore
5. Faculty of Chemistry; Northeast Normal University; Changchun Jilin 130024 P. R. China