1. Key Laboratory of Advanced Technique and Preparation for Renewable Energy Materials, Ministry of Education, Yunnan Normal University, Kunming 650500, P. R. China
2. Department of Electrical and Computer Engineering and Center for Optoelectronics, University of North Carolina at Charlotte, 9201 University City Blvd, Charlotte, North Carolina 28223, United States
3. Southwest United Graduate School, Kunming 650092, China