1. Key Laboratory of Luminescence and Optical Information; Ministry of Education; Beijing Jiaotong University; Beijing 100044 P. R. China
2. Department of Materials Science and Engineering; College of Engineering; Key Laboratory of Polymer Chemistry and Physics of Ministry of Education; Peking University; Beijing 100871 P. R. China
3. College of Material Science and Technology; Guangxi Key Laboratory of Information Materials; Guilin University of Electronic Technology; Guilin 541004 Guangxi P. R. China