[1] Univ Oulu, Nano & Mol Syst Res Unit, POB 3000, FI-90014 Oulu, Finland
[2] Jiangsu Normal Univ, Sch Phys & Elect Engn, Xuzhou 221116, Jiangsu, Peoples R China
[3] East China Univ Sci & Technol, Dept Phys, Shanghai 200237, Peoples R China
[4] Xiangtan Univ, Minist Educ, Key Lab Environm Friendly Chem & Applicat, Coll Chem, Xiangtan 411105, Peoples R China