1. Department of Physical Chemistry, University of Chemical Technology and Metallurgy, 8 Kliment Ohridski Boulevard, BG-1756 Sofia, Bulgaria
2. Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, BG-1784 Sofia, Bulgaria
3. School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London, E1 4NS, U.K.