1. Department of Materials and Energy Science and Engineering, The Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha, Tanzania
2. Department of Physics, Mkwawa University College of Education, University of Dar es Salaam, P.O. Box 2513, Iringa, Tanzania
3. Department of Chemistry, South Eastern Kenya University, P.O. Box 170-90200, Kitui, Kenya