1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore
2. Faculty of Science, Beijing University of Technology, 100124 Beijing, China
3. School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798, Singapore
4. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 639798, Singapore
5. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore, 117551, Singapore