1. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583, Singapore
2. School of Physics and Electronics, Central South University, Changsha, 410083, China
3. WinTech Nano-Technology Services Pte Ltd. 10 Science Park Road #03-26, The Alpha, Science Park II, Singapore117684
4. Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom