1. Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava 811 02, Slovakia
2. Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava 841 04, Slovakia
3. Department of Physics and Energy, University of Limerick, Limerick V94 T9PX, Ireland
4. Energy, Materials and Surface Science Institute, University of Limerick, Limerick V94 T9PX, Ireland