1. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
2. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research) 2 Fusionopolis Way Innovis #08-03, Singapore 138634
3. School of Physics and Electronics, Hunan University, Changsha 138634, China
4. Key Laboratory of Materials Processing and Mold, Ministry of Education, Zhengzhou University, Zhengzhou 450002, China