1. DSI/CSIR, Centre for Nanostructures and Advanced Materials, Council for Scientific Industrial Research, Pretoria 0001, South Africa
2. Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein ZA9300, South Africa
3. Department of Physics, University of Limpopo, Private Bag X1106, Sovenga 0727, South Africa
4. National Metrology Institute of South Africa (NMISA), CSIR Campus, Building 5, Meiring Naude Road, Brummeria, Pretoria 0182, South Africa
5. Analytical Services Division, Mintek, 200 Malibongwe Drive, Randburg 2194, South Africa