1. Department of Physics, Institute of Applied Materials, SARChI Chair in Carbon Technology and Materials, University of Pretoria, Pretoria 0028, South Africa
2. Laboratoire de Photonique Quantique, d’Energie et de Nano-Fabrication, Faculté des Sciences et Techniques, Université Cheikh Anta Diop de Dakar (UCAD), B.P. 5005, Dakar-Fann Dakar, Senegal