Pyroelectric effect in a mixed polysiloxane/eicosylamine Langmuir–Blodgett thin film

Abstract

A pyroelectric material possessing a non-centrosymetric structure exhibits a temperature-dependent spontaneous electric polarisation, which is known as the pyroelectric effect. Organic thin film materials have been extensively studied as pyroelectric heat sensors because of their low cost, high quality and easy processing compared with ceramics and single crystals. Langmuir–Blodgett (LB) film technique is an elegant method to fabricate pyroelectric thin films because their thickness and molecular architecture can be precisely controlled. In this study, a mixed monolayer containing linear copolysiloxane molecules (A) with a controllable number of side groups has been deposited alternately with eicosylamine (B) using LB film deposition technique to form multilayer sequences possessing the non-centrosymmetric ABABA… architecture. The pyroelectric activity of these heterogeneous LB films has been assessed using a quasi-static measurement technique. The experimental result for a 13-layer LB film has shown that the experimental pyroelectric coefficient (5.6±0.01 μC m −2 K −1 at 30 °C) is higher than the theoretically calculated pyroelectric coefficient (4.7±0.01 μC m −2 K −1 at 30 °C). These results indicate that the mixing process leads to enhanced pyroelectric activity. Fourier transform infrared (FTIR) spectroscopy is employed to investigate the pyroelectric activity in LB film multilayer. IR spectra show a significant change in the region 1700–1300 cm −1 due to proton transfer between the acid and amine head groups. The most obvious change is that a band at 1408 cm −1 due to the RCOO − carboxylate ion and a band at 1565 cm −1 due to R′NH 3 + are observed.