1. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, United States
2. Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
3. Ames National Laboratory, U.S. Department of Energy, Ames, Iowa 50011, United States
4. Department of Physics, Xiamen University, Xiamen 361005, China
5. Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States