1. Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon 999077, Hong Kong, SAR, P. R. China
2. School of Energy and Environment, City University of Hong Kong, Tak Chee Avenue, Kowloon 999077, Hong Kong, SAR, P. R. China
3. School of Physics and Materials Science, Anhui University, Hefei 230601, China
4. Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K.