1. Department of Physics and Astronomy, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701, United States, and
2. Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Apartado Postal 2681, Ensenada, Baja California, 22800, México
3. Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California, United States, and Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina, United States