High pyroelectric sensitivity in alternate layer Langmuir-Blodgett superlattices

Abstract

A family of polysiloxanes which have been designed for the purpose of achieving high pyroelectric activity in Langmuir-Blodgett (LB) film form is reported. The alternate layer LB deposition process has been used in which each siloxane monolayer (S) is co-deposited with a monomeric eicosylamine monolayer (E) to form alternate layer multilayer assemblies of the form SESESE… This approach ensures a macroscopic polarization within the LB film, which is an essential criterion for observation of the pyroelectric effect. Pyroelectric devices have been fabricated using aluminium electrodes and their pyroelectric sensitivity has been measured using a quasi-static technique. This collaborative study has involved investigating the effect on the pyroelectric response of (i) varying the degree of substitution on the polysiloxane backbone with aliphatic pendant side-chains, (ii) varying the length of the aliphatic pendant side-chains and (iii) incorporating polar aromatic pendant side-chains. This iterative strategy has led to an improvement in the pyroelectric coefficient from around 0.8 μC m −2 K −1 for an aliphatic substituted homopolymer to 4.6 μC m −2 K −1 for a copolysiloxane substituted with an aromatic pendant side-chain. The effects of LB film thickness (number of deposited monolayers) and the electrode overlap area have also been investigated and have brought about a further dramatic increase in activity to around 13 μC m −2 K −1. This is the highest pyroelectric coefficient ever reported for an alternate layer LB film assembly. It is believed that the activity of these thin films arises predominantly from the primary pyroelectric effect since an investigation into the response from two substrates possessing very different thermal expansion coefficients yielded almost identical behaviour, suggesting that the secondary piezoelectric component is small.