1. Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, University of Akron, Akron, Ohio 44325, United States
2. Department of Electrical and Computer Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States
3. State Key Laboratory of Luminescence Physics and Chemistry, South China University of Technology, Wushan Road, Tianhe District, Guangzhou, 510641, P. R. China