1. Key Laboratory of Applied Surface and Colloid Chemistry Ministry of Education) School of Chemistry and Chemical Engineering Shaanxi Normal University Xi'an Shaanxi 710119 P. R. China
2. Fluorescence Research Group Singapore University of Technology and Design 8 Somapah Road Singapore 487372 Singapore
3. State Key Laboratory of Superhard Materials Jilin University Changchun Jilin 130012 P. R. China