1. Research Laboratory of Functional Materials Technologies, Faculty of Materials Science and Applied Chemistry, Riga Technical University, Paula Valdena 3/7, 1048 Riga, Latvia
2. Institute of Solid State Physics, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia
3. MIND-IN2UB, Department of Electronic and Biomedical Engineering, Universitat de Barcelona, 08028 Barcelona, Spain