1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
3. School of Physics and Technology, Center for Electron Microscopy and MOE Key Laboratory of Artificial Micro- and Nano-Structures, Wuhan University, Wuhan 430072, China
4. Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798