1. Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore
2. NUSNNI-Nanocore, National University of Singapore, 117411, Singapore
3. Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
4. Condensed Matter Physics & Materials Science Division, Brookhaven National Laboratory, Upton, New York, New York 11973, United States
5. Department of Physics, National University of Singapore, 117542, Singapore
6. Department of Electrical and Computer Engineering, National University of Singapore, 117583, Singapore
7. Department of Physics and Astronomy, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States