1. Key Lab 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 Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization; The University of Toledo; Toledo OH 43606 USA
3. Department of Physics; The Chinese University of Hong Kong; New Territories Hong Kong 999077 P. R. China