Pyrovoltage in ytterbium bisphthalocyanine Langmuir-Blodgett thin films

Abstract

In investigating the electrical and gas sensing properties of Langmuir-Blodgett (LB) thin films of ytterbium bisphthalocyanine, Yb(pc)(15c), and thermally cycling samples, it has been observed that these samples establish a pyrovoltage. This is similar to an effect observed previously in heat-treated polymeric iron monophthalocyanine [1], although there the effect was measured as a pyrocurrent rather than voltage. A more significant difference is that in that case the sample was in pellet form. The effect reported here was not observed in sublimed films of ytterbium bisphthalocyanine, suggesting that the regularity of structure as produced by LB deposition is important. The ytterbium bisphthalocyanine was prepared and LB films deposited as reported previously [2]. Films used in this investigation were made up of 25 layers of molecules deposited at a surface pressure of 25 mNm -1 with a concentration of 10-4M Cd 2+ in the water subphase (pH 6.5). Deposition was onto glass slides that had an indium-tin oxide (ITO) coating on one side and sublimed interdigitated gold electrodes on the other. During dipping the ITO surface was protected by a second glass slide and deposition occurred onto the side having the gold electrodes. Deposition was Z-type, with the substrate being passed through clean water surface in one compartment and withdrawn through the monolayer in the other compartment, such that deposition commenced at the upper end of the slide and continued downwards. Any film deposited on the edges of the slide was carefully removed. The two gold electrodes consisted of 10 interdigitated fingers each 5 mm long, 0.5 mm wide, 0.5 mm apart and orientated parallel to the shorter side of the slide, such that electrical voltage was measured along the dipping direction (Fig. 1). The ITO layer