1. Department of Physics, University of South Africa (UNISA), PO Box 1709, Florida, South Africa
2. DST/CSIR National Centre for Nanostructured Material, Council for Scientific and Industrial Research (CSIR), PO Box 0001, Pretoria, South Africa
3. Department of Civil and Chemical Engineering, University of South Africa (UNISA), PO Box 1709, Florida, South Africa