1. Department of Applied Physics, Adama Science and Technology University, P.O. Box 1888, Adama, Ethiopia
2. Energy Centre, Smart Places, Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa
3. School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa
4. Department of Physics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia