1. Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong, SAR 999077, P. R. China
2. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, SAR 999077, P. R. China
3. Department of Chemistry & Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong, SAR 999077, P. R. China