1. Department
of Mechanical Engineering and Materials Science, Duke University, Box
90300 Hudson Hall, Durham, North Carolina 27708, United States
2. Department
of Chemistry, Duke University, Durham, North Carolina 27708, United States
3. Department
of Physics and Astronomy and Center for Photovoltaics Innovation and
Commercialization, The University of Toledo, Toledo, Ohio 43606, United States
4. Department
of Physics, Shanghai University, Shanghai 200444, China
5. Department
of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada