1. Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA
2. Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA
3. Department of Physics, South Dakota State University, Brookings, South Dakota 57007, USA
4. Department of Physics, University of Northern Iowa, Cedar Falls, Iowa 50614, USA
5. Department of Chemistry and Biochemistry, University of Northern Iowa, Cedar Falls, Iowa 50614, USA