1. Nano and Molecular Systems Research Unit, University of Oulu, P.O. Box 3000, FI-90014, Oulu, Finland
2. School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
3. Department of Physics, East China University of Science and Technology, Shanghai 200237, China
4. College of Chemistry, Key Lab of Environment Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, China