1. a Department of Physics , Taras Shevchenko Kyiv National University , Kyiv, Ukraine
2. b Department of Biophysics , Taras Shevchenko Kyiv National University , Kyiv, Ukraine
3. c Institute of Physics, Technical University of Ilmenau , Ilmenau, Germany
4. d Groupe Surfaces-Interfaces, Institut de Physique et Chimie des Matériaux , Strasbourg, France
5. e Davey Laboratory, Penn State University, University Park , PA, USA