Pyroelectricity in langmuir-blodgett films

Abstract

The fabrication of pyroelectric devices using the Langmuir-Blodgett (LB) technique is described. Studies of a wide range of materials are reported; however, the thesis concentrates on electrical and structural investigations of two specific alternate layer films: 22-tricosenoic acid/l-docosylamine and 22-tricosenoic acid/4-octadecylaniline. The latter system possesses a pyroelectric coefficient of 0.65 nCcm(^-2)K(^-1), representing the largest reported value, to date, for an LB film. The pyroelectric figure of merit (p/e(^1)(_T)) of such films is approximately 0.22 nCcm(^-2)K(^-1), which is comparable with the values for commercially available materials. The difference in pyroelectric coefficient of the two types of alternate layer film is attributed to differences in inter-layer bonding, as revealed by infrared spectroscopy. The dependence of the pyroelectric coefficients on parameters such as film thickness, substrate thickness and temperature is investigated. Structural studies, performed using electron and X-ray diffraction techniques, are also described. These provide information on the orientation of the molecules relative to the substrate and on the d-spacing of the LB films. It is shown that the substrate has a deleterious effect on the responsivity of LB film devices, and studies of films deposited onto different substrate materials indicate that there is a significant piezoelectric ally induced secondary effect contributing to the overall pyroelectric coefficient. This secondary effect is small at low temperatures, but becomes dominant at around 250 K. The results of thermally stimulated discharge experiments indicate that both free charges and dipolar groups are incorporated in the films during deposition, and become tightly bound within the polar structure.