A pyroelectric thin film of oriented triglycine sulfate nano-crystals for thermal energy harvesting

Abstract

This study introduces a method for the fabrication of transparent and flexible pyroelectric thin films containing oriented triglycine sulfate (TGS) nanorods for thermal energy conversion or thermal sensing. For the fabrication of such thin films a highly nanoporous anodic aluminum oxide (AAO) template is mounted on top of a b-cut TGS seed crystal before filling the template’s nanopores with TGS solution. Crystal growth in the filled pores is done via the so-called temperature-lowering technique. Two principles are applied to grow oriented nanocrystals in the AAO template: first, the effect of different crystal growth rates in different directions of a nanopore’s confined space is exploited. Second, crystal growth at the bottom of the pores is initiated in a preferential direction by the TGS seed crystal. As confirmed by x-ray diffraction the crystal nanorods obtained are oriented in the desired spontaneous polarization vector of TGS (b-direction). Optical transparency of the filled AAO thin film is good when compared to a pure AAO film which indicates a reasonably high homogeneity and a reasonably low defect density of the TGS nanocrystals. Dielectric characterization at 1 kHz shows a relative permittivity similar to a bulk TGS crystal and a high resistivity of about 199 MΩ m. At its phase transition temperature (44 °C), the dielectric loss factor of the thin film is about 0.3. An average pyroelectric coefficient of 133 μC m−2 K−1 and a relative permittivity of 15.6 are calculated from measurement results at room temperature, taking the effect of the AAO thin film into account. With that, the pyroelectric voltage figure of merit (FOM) (pc/εrε0) and the energy harvesting FOM (pc2/εrε0) and are about 820 kV m−1 K−1 and 72 J m−3 K−2, respectively. In comparison to other approaches for growing thin films of oriented TGS crystals, the described method exhibits advantages concerning the growth of oriented nanocrystals, simplicity, fast growth, and an access to both sides of the nanocrystals.