1. Institute of Optoelectronics & Nanomaterials, MIIT Key Laboratory of Advanced Display Material and Devices, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2. Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
3. College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China