1 Department of Electricity, Solid-State Physics and Biophysics, Faculty of Physics, University of Bucharest, 405 Atomistilor Street, 077125 Magurele, Romania.
2 General Chemistry Department, Faculty of Chemical Engineering and Biotechnology, University "Politehnica" of Bucharest, 1-7 Polizu Street, 011061 Bucharest, Romania.
3 National Institute of Materials Physics, Atomistilor 405A, 077125 Magurele, Romania.