1. State Key Laboratory of Optoelectronic Materials and Technologies and Guangdong Province Key Laboratory of Low-carbon Chemistry and Energy Conservation; School of Physics and Engineering; Sun Yat-sen University; Guangzhou 510275 P. R. China
2. Department of Physics; Xiangtan University; Xiangtan 411105 P. R. China
3. Fuels and Energy Technology Institute and Department of Chemical Engineering; Curtin University; Perth WA6102 Australia