1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore
2. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
3. College of Science, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
4. Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong, SAR 999077, P. R. China
5. Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong, SAR 999077, P. R. China