Acentric Langmuir-Blodgett film assemblies
In recent years there has been considerable interest In the use of Langmuir-Blodgett (LB) films as the basis of active and passive thin film devices. One such application is in room temperature, pyroelectric thermal imaging systems. As the detectivity of these sensors is approximately inversely proportional to thickness, there may be significant advantage in exploiting the ability of the LB process to produce ultra-thin layers. Here we report the use of the LB technique to fabricate molecular assemblies with precisely defined symmetry properties. This has made possible the construction of ultra-thin organic films possessing the spontaneous polarisation necessary for the observation of the pyroelectric effect. Two distinct approaches are adopted to produce the required molecular packing arrangements. The first uses the ability of a novel alternate layer trough to assemble acentric ABAB structures. Various materials are studied for their suitability for alternate layer deposition. The co-deposition of fatty acids and amines produces alternate layer assemblies with a high degree of structural and electrical integrity. The second approach uses a novel polymeric LB film. Polybutadiene (PBD) is observed to deposit in highly asymmetric X-type layers if the transferred monolayer is exposed to u.v. radiation between each deposition step. The structural and electrical properties are characterised. The presence of the unique polar axis associated with pyroelectric materials is investigated using surface potential measurements. Dynamic and quasi-static detection techniques are employed to detect the pyroelectric activity within the assemblies. For the first time pyroelectric activity is observed in "thin" (11 layer) acid/amine assemblies. The pyroelectric coefficient is found to be thickness dependent, increasing for larger numbers of layers; the coefficient for a 99 layer sample is comparable to that of PVDF, a commercially available pyroelectric polymer. The incorporation of LB films into thin PVDF Metal Insulator Metal (MIM) structures is also investigated. The presence of a stable phthalocyanine LB film is shown to increase the electrical breakdown strength of the device. Finally the incorporation of highly polarisable sulphur substitutions into fatty acid LB films is reported. The permittivity is found to increase in proportion to the number of sulphur atoms introduced into the hydrocarbon chain.