1. Institute of Materials and Surface Engineering, Faculty of Natural Sciences and Technology, Riga Technical University, Paula Valdena 3, 1048 Riga, Latvia
2. Laboratory of Sensor Technologies, Institute of Physics, University of Tartu, W. Ostwald St. 1, 50411 Tartu, Estonia
3. Institute of Solid State Physics, University of Latvia, LV-1063 Riga, Latvia