1. Yunnan Key Laboratory for Micro/Nano Materials & Technology, National Center for International Research on Photoelectric and Energy Materials, School of Materials and Energy, Yunnan University, Kunming 650091, P. R. China
2. Advanced Analysis and Measurement Center of Yunnan University, Kunming 650091, P. R. China
3. Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States