1. Department of ChemistryCollege of Sciences Nanjing Agricultural University Nanjing 210095 Jiangsu P. R. China
2. Department of Materials Science and Engineering City University of Hong Kong 83 Tat Chee Avenue Hong Kong SAR 999077 P. R. China
3. Center of Super-Diamond and Advanced Films (COSDAF) City University of Hong Kong Hong Kong SAR 999077 P. R. China