1. Department of Physics and Institute for Functional Nanomaterials, P.O. Box 70377, University of Puerto Rico, San Juan, Puerto Rico 00936-8377, USA
2. Molecular Science Research Center, University of Puerto Rico, 1390 Ponce de Leon Ave, San Juan, Puerto Rico 00926, USA
3. Centre for Nanostructured Media, School of Maths and Physics, Queen's University Belfast, University Road, Belfast, BT7 1NN, Northern Ireland, United Kingdom