1. Discipline of Physics, School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa
2. Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa
3. Energy Centre, Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa
4. Department of Electrical and Electronics Engineering, SRM Institute of Science and Technology, Kattankulathur 603203, India
5. Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, 1 Old Faure Road, Somerset West, 7129 P.O. Box 722, Somerset West, South Africa
6. School of Physics and Astronomy, Nottingham Nanotechnology and Nanoscience Center, University of Nottingham, Nottingham NG7 2RD, U.K.