1. Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Luoyu Road 1037 Wuhan 430074 China
2. Department of Chemistry The Hong Kong University of Science and Technology Clear Water Bay Kowloon 999077 Hong Kong China
3. Department of Materials Science and Engineering University of Washington Seattle WA 98195 USA
4. Department of Materials Science and Engineering City University of Hong Kong Kowloon 999077 Hong Kong