1. Centre for Nanostructures and Advanced Materials (CeNAM), DSI-CSIR Nanotechnology Innovation Centre, Council for Scientific and Industrial Research, Pretoria 0001, South Africa
2. Department of Physics, University of the Free State, Bloemfontein ZA9300, South Africa
3. Advanced Materials Division, Mintek, DSI/Mintek Nanotechnology Innovation Centre (NIC), Private Bag
X3015, Randburg 2125, South Africa