[1] UNESCO-UNISA, Africa Department of Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria, South Africa
[2] Nanosciences African Network (NANOAFNET), IThemba LABS, National Research Foundation, 1 Old Faure road, Somerset West 7129, PO Box 722, Western Cape, South Africa
[3] Physics Dept., University of South Africa, Muckleneuk ridge, PO Box 392, Pretoria, South Africa