1. School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
2. Hubei Yangtze Memory Laboratories, Wuhan 430205, China
3. Hubei Key Laboratory of Micro-Nanoelectronic Materials and Devices, School of Microelectronics, Hubei University, Wuhan, 430062, P. R. China
4. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore