1. School
of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China
2. Institute of Photonics, University of Eastern Finland (Joensuu Campus), P.O. Box 111, FI-80101Joensuu, Finland
3. Laboratory of Inorganic
Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014, Helsinki, Finland