1. Center of Materials Science and Optoelectronics Engineering & College of Materials Science and Opto-Electronic TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 China
2. Sino-Danish CollegeUniversity of Chinese Academy of Sciences Beijing 100190 China
3. Department of Chemistry and BioscienceAalborg University Aalborg DK-9220 Denmark
4. Sino-Danish Center for Education and Research Aarhus DK-8000 Denmark