1. Department of Chemical Technologies, Faculty of Technology, University of Nis, Bulevar Oslobodjenja 124, 1600 Leskovac, Serbia
2. Department of Electricity, Solid-State Physics and Biophysics, Faculty of Physics, University of Bucharest, 405 Atomistilor Street, P.O. Box MG-11, 077125 Măgurele, Romania