1. Department of Physics Center for Optoelectronics Engineering Research Yunnan University Kunming 650091 P. R. China
2. Department of Materials Science and Engineering University of Toronto Toronto M5G 3E4 Canada
3. Department of Chemistry University of Toronto Toronto Ontario M5S 3H6 Canada
4. Department of Electrical and Computer Engineering University of Toronto Toronto Ontario M5S 3G8 Canada
5. School of Microelectronics Southern University of Science and Technology Shenzhen 518055 P. R. China