1. State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China
2. Eco-materials and Renewable Energy Research Center (ERERC), School of Physics, and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
3. Department of Chemistry, Faculty of Science, National University of Singapore (NUS) 3 Science Drive 3, Singapore 117543, Singapore