1. Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln, Theodore Jorgensen Hall, 855 North 16th Street, Lincoln Nebraska 68588-0299, United States
2. Department of Physics and Electronics, University of Puerto Rico—Humacao, 100 Road 908 CUH Station, Humacao, Puerto Rico 00791, and the Institute for Functional Nanomaterials, University of Puerto Rico, Facundo Bueso Building, Rio Piedras, Puerto Rico 00931