1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
2. Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, Catalonia, Spain
3. Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Catalonia, Spain
4. Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310058, China
5. Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798