1. Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education College of Chemistry Xiangtan University Xiangtan 411105 P. R. China
2. Laboratory of Molecular Materials and Devices Department of Materials Science Fudan University Shanghai 200438 P. R. China
3. Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education School of Physics and Electronics Hunan University Changsha 410082 P. R. China