1. Department of Physics, University of Wyoming, Laramie, Wyoming 82071, United States
2. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
3. Nanoscale Device Characterization Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
4. Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, United States
5. Department of Physics and Astronomy, Howard University, Washington, D.C. 20059, United States