1. Key Laboratory of Cluster Science of Ministry of Education Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials School of Chemistry and Chemical Engineering Beijing Institute of Technology Beijing 100081 P. R. China
2. School of Material Science and Engineering Beijing Institute of Technology Beijing 100081 P. R. China
3. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 P. R. China