1. Centre for Nanostructures and Advanced Materials Council for Scientific and Industrial Research Meiring Naude Rd, Brummeria Pretoria 0184 South Africa
2. Molecular Sciences Institute School of Chemistry University of the Witwatersrand Private Bag 3 2050 Johannesburg South Africa
3. DSI/NRF Centre of Excellence in Strong Materials University of the Witwatersrand Private Bag 3 2050 Johannesburg South Africa
4. Microscopy and Microanalysis Unit University of the Witwatersrand Private Bag 3 2050 Johannesburg South Africa