1. Department
of Physics and Astronomy and Wright Center for Photovoltaics Innovation
and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States
2. Department
of Mechanical Engineering and Materials Science, and Department of
Chemistry, Duke University, Durham, North Carolina 27708, United States
3. School
of Physics and Technology, Center for Electron Microscopy, MOE Key
Laboratory of Artificial Micro- and Nano-structures, and Institute
for Advanced Studies, Wuhan University, Wuhan 430072, China