Ph.D. Work

The Pyroelectric Effect in Langmuir-Blodgett Films Containing Linear Polysiloxanes

A material which possesses a spontaneous temperature-dependent electric polarisation is known as a pyroelectric. The pyroelectric effect is observed only in materials whose polar entities are arranged in a non-centrosymmetric organisation. Practical pyroelectric devices generate an electrical current whenever their ambient temperature changes. Organic thin films are interesting candidates for pyroelectric detectors due to their ease of processing compared with ceramics and single crystals and due to the way in which their functionality, at the molecular and macroscopic level, can be controlled.

This thesis describes the pyroelectric effect within a family of polysiloxanes which are each alternately co-deposited with a long chain amine. Alternate layer deposition facilitates the formation of a macroscopically polar assembly of molecules which requires no post-deposition electrical poling, unlike spin-coated films and ceramics. The LB technique also provides the opportunity to study the thickness dependence of phenomena owing to the precise control over the number of monolayers deposited to form a multilayer assembly.

This study has revealed for the first time that there exists an optimum thickness effect in the pyroelectric activity of alternate layer LB films and that the measured coefficient is sensitive to the overlap area of the metal electrodes which form part of the pyroelectric devices. Furthermore, the incorporation of divalent metal cations within the polar LB films has enhanced the pyroelectric activity further, leading to some of the highest pyroelectric coefficients ever reported for pyroelectric LB assemblies ( ~ 15 uC m-2 K-1). This research has also shown how molecular engineering of polysiloxane molecules has led to the evolution of high pyroelectric activity through iterative material/device-design and characterisation procedures.