1. Key Laboratory for Microstructural Material Physics of Hebei ProvinceSchool of ScienceYanshan University Qinhuangdao 066004 P. R. China
2. Tianjin Key Laboratory of Molecular Optoelectronic ScienceDepartment of Physics and Department of ChemistrySchool of ScienceTianjin University Tianjin 300072 P. R. China