1. Key Laboratory of Zhenjiang, Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China
2. Nanyang Environment and Water Research Institute (NEWRI), Interdisciplinary Graduate School (IGS), Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
3. School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China
4. Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, P. R. China