1. Key Laboratory of Artificial Micro‐ and Nano‐structures of Ministry of Education of China School of Physics and Technology Wuhan University Wuhan 430072 P. R. China
2. Department of Electronic and Information Engineering The Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong SAR China
3. Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization The University of Toledo Toledo OH 43606 USA