1. Department
of Physics and Astronomy, and Wright Center for Photovoltaic Innovation
and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States
2. 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
3. Department
of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States
4. Department
of Chemistry, Duke University, Durham, North Carolina 27708, United States