1. Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, P. R. China
2. Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P. R. China
3. Graduate School of Chinese Academy of Sciences, Beijing 100080, P. R. China
4. Nano-Organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, P. R. China