Affiliation: | a Materials Physics Research Institute, School of Physics, University of the Witwatersrand, Johannesburg, South Africa b Raman and Luminescence Laboratory, University of the Witwatersrand, Johannesburg, South Africa c DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, South Africa d Electron Microscope Unit, University of the Witwatersrand, Johannesburg, South Africa |