1. Department of Physics, North Dakota State University, Fargo, North Dakota 5810, United States
2. Materials and Nanotechnology Program, North Dakota State University, Fargo, North Dakota 58108, United States
3. Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States
4. Department of Physics and Astronomy, University of California, Irvine, California 92697, United States