Abstract

Until now, the highest static pyroelectric co- efficient of Langmuir-Blodgett (LB) at room temperature among all the reported results^1,2 is 3 μC m⁻² K⁻¹. Comparing it with ferroelectric crystal and PVF₂ polymer, the value is too low to prevent the use of LB film as an infrared sensor. That the films were made from the azobenzene derivative of long fatty chains (AD) by the LB method ensures a stronger aggregative effect, and they are sensitive to temperature. We have formed polar LB film by using AD molecular. The static pyroelectric coefficient is 22 μC m⁻² K⁻¹ at room temperature, and the dynamic one is 23 μC m⁻² K⁻¹ at 4 Hz. The value is about ten times the reported one.³ X-ray grazing and UV-visible spectra related to temperature showed that the domain structure exists in the films because of the aggregation effect, and the structure is very sensitive to temperature. The UV-visible, x-ray, and FTIR spectra, which were measured at different temperatures, all showed that the domain will disaggregate at 70°, and the volume of the domain will expand when the temperature increases from 0° to 60°, This implies that the distance between two adjacent molecules increases. The results also showed that in the domain the interaction between two adjacent molecules depends strongly on temperature. FTIR linear dichroic spectra dependence on temperature showed that, when the temperature increases from 0° to 60°, the orientation angle between the film’s normal direction and the molecular long axis increases gradually from small to large; tire angle becomes a random angle at 70° Normal spontaneous polarization changes with change of the angle, therefore, pyroelectric current appears. According to these experimental results and the calculated aggregated structure by Kohn’s interaction theory of dipole-dipole, we suggest that the high pyroelectricity of the system is related to the temperature-dependent strong spontaneous polarization, which was related to the strong temperature-dependent domain structure.

© 1993 Optical Society of America