1. College of Chemistry and Materials Science; The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials; Center for Nano Science and Technology; Anhui Normal University; No.1 Beijing East Road Wuhu 241000 P. R. China
2. Nano and Molecular Systems Research Unit; University of Oulu; P.O. Box 3000 90014 Oulu Finland
3. Department of Physics; University of Helsinki; P.O. Box 64 00014 Helsinki Finland