1. Department of Electrical and Computer Engineering and NUSNNI-NanoCore; National University of Singapore; 117576 Singapore
2. Department of Electrical and Computer Engineering; National University of Singapore; 117576 Singapore
3. Shanghai Key Laboratory of Special Artificial Macrostructure Materials and Technology; Institute of Advanced Study and School of Physics Science and Engineering; Tongji University; Shanghai 200092 China
4. Institute of Materials Research and Engineering; A-STAR; 138634 Singapore