1. Department of Applied Physics; Aalto University, School of Science; Puumiehenkuja 2 FI-02150 Espoo Finland
2. Department of Chemistry, Nanoscience Center; University of Jyvaskyla; P. O. Box 35 FI-40014 University of Jyvaskyla Finland
3. Central China Normal University; College of Chemistry; 152 Luoyu Road, Wuhan 430079 Hubei China