1. National University of Singapore (Suzhou) Research Institute, 377 Linquan Street, Suzhou Industrial
Park, Jiangsu 215123, People’s Republic of China
2. Department
of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore
3. Center
for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
4. Department
of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore