1. School of Chemistry Beihang University Beijing 100191 P. R. China
2. Key Laboratory for Physical Electronics and Devices of the Ministry of Education Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Photonic Technique for Information School of Electronics Science & Engineering Faculty of Electronic and Information Engineering Xi'an Jiaotong University Xi'an 710049 P. R. China
3. School of Physics State Key Laboratory of Crystal Materials Shandong University Jinan 250100 P. R. China
4. State Key Laboratory of Luminescent Materials and Devices South China University of Technology Guangzhou 510640 P. R. China