1. LEX Laboratory, Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
2. Graphene Laboratory, Center for Solid State Physics and New Materials, Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia
3. Institute of Physics, Montanuniversität Leoben, Franz Josef Strasse 18, Leoben 8700, Austria