Phase transition in ferroelectric hemicyanine Langmuir-Blodgett multilayers

The temperature dependence of pyroelectric coefficient in LB films of hemicyanine dyes with different thickness has been investigated. It was found that, in the range from 20 to 45°C, the pyroelectric coefficient was not almost changed for the film thickness less than 32 monolayers. However, the one was enhanced obviously with temperature increasing for the film thickness larger than 32 monolayers. The facts indicated that there is a critical thickness in the ferroelectric LB multilayer films. For the LB films of 20 and 30 monolayers, we can observe two peaks in the temperature dependence of pyroelectric coefficient (at 9 and/or 12°C) under heating and/or cooling processes. But, the heat-hysteresis was not found hi 40 monolayers LB films. It is indicated that there are the heat-hysteresis and further confirmed the existence of phase transition in the LB films in the neighborhood of 10°C. It may be demonstrated primarily that the type of phase transition is the surface first-order ferroelectric-paraelectric phase transition by the pyroelectric coefficient measurement.