1. Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, PR 00931, USA
2. Department of Physics, University of Puerto Rico, Río Piedras Campus, San Juan, PR 00931, USA
3. Pontificia Universidad Católica Madre y Maestra, P.O. Box 2748, Santo Domingo, Dominican Republic
4. Department of Chemistry, University of Puerto Rico, Río Piedras Campus, San Juan, PR 00931, USA