Application of nanostructured film systems for recording and storing hidden information

Abstract

This paper examines Ag-Pd thick films on a ceramic substrate. After local modification of a part of the film, the authors observed a change in the thermo-EMF values. The modification of the Ag-Pd film surface was performed in hydrogen, namely in atomic and molecular hydrogen. For atomic hydrogen modification the films were subjected to cathodic hydrogenation in an electrolyte solution (H 2 SO 4 ) for 1 minute. Modification in molecular gaseous hydrogen was performed at the temperature higher than 60°C. In process of the modification, palladium oxide, which is present in the film, was reduced to a metal. The authors demonstrated that the changes in the chemical composition of the film resulted in the decrease of its total electric resistance from 60 to 2 Ω. Under the effect of laser irradiation the initial and modified films demonstrated different values of the surface thermo-EMF. A negative charge was formed on the part of the Ag-Pd film surface, which can be explained by the phonon drag effect and the diffusion of electrons to the area with lower temperature. Targeted laser action makes possible local modification of separate film areas. It was shown that in case of the modification by the targeted action onto the film surface a sharp change of an electric signal -from 50 to 250 µV -occurred on the boundary between the modified and non-modified areas. The effect observed can be used to obtain a hidden image on the film surface. The authors managed to successively record, read out, and restore the recorded image. Hence, the results of this research demonstrate the possibility of obtaining hidden information carriers based on the film coatings studied.