1. DST/CSIR National Centre for Nanostructured Materials, Council for Scientific Industrial Research, Pretoria 0001, South Africa
2. Sensor Lab, Chemistry Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
3. Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein ZA9300, South Africa
4. Department of Physics, Sefako Makgatho Health Sciences University, Ga-Rankuwa, South Africa
5. Council for Scientific Industrial Research, Materials Science and Manufacturing, Advanced Materials and Engineering, Meiring Naude Road, P. O. Box 395, Pretoria, South Africa
6. Department of Chemistry, University of Limpopo, Private Bag X1106, Sovenga, 0727, South Africa
7. Mintek, Analytical Service Division, 200 Malibongwe Drive, Randburg 2194, South Africa
8. Electron Microscopy Unit, University of the Western Cape, Bellville, 7535, South Africa