Negative piezoelectric effect in an organic supramolecular ferroelectric

Abstract

The vast majority of ferroelectric materials demonstrate a positive piezoelectric effect. Theoretically, the negative piezoelectric coefficient d₃₃ could be found in certain classes of ferroelectrics, yet in practice, the number of materials showing linear longitudinal contraction with increasing applied field (d₃₃ < 0) is limited to few ferroelectric polymers. Here, we measure a pronounced negative piezoelectric effect in the family of organic ferroelectric small-molecular BTAs (trialkylbenzene-1,3,5-tricarboxamides), which can be tuned by mesogenic tail substitution and structural disorder. While the large- and small-signal strain in highly-ordered thin-film BTA capacitor devices are dominated by intrinsic contributions and originates from piezostriction, rising disorder introduces additional extrinsic factors that boost the large-signal d₃₃ up to −20 pm V⁻¹ in short-tailed molecules. Interestingly, homologues with longer mesogenic tails show a large-signal electromechanical response that is dominated by the quadratic Maxwell strain with significant mechanical softening upon polarization switching, whereas the small-signal strain remains piezostrictive. Molecular dynamics and DFT calculations both predict a positive d₃₃ for defect-free BTA stacks. Hence, the measured negative macroscopic d₃₃ is attributed to the presence of structural defects that enable the dimensional effect to dominate the piezoelectric response of BTA thin films.