1. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
2. College of Electron and Information Engineering, Hebei University, Baoding 071002, China
3. NUSNNI-Nanocore, National University of Singapore, Singapore 117411, Singapore
4. Department of Electrical and Computer Science Engineering, National University of Singapore, Singapore 117583, Singapore
5. Department of Physics, National University of Singapore, Singapore 117542, Singapore
6. Integrative Science and Engineering, National University of Singapore, Singapore 119077, Singapore