Harvesting of shear piezoelectricity in a molded multicomponent crystal disc

Biomolecular piezoelectrics, such as amino acids and peptides, exhibit significant shear piezoelectric responses in single crystal form. However, naturally occurring longitudinal piezoelectricity is rare and, when present, is dampened due to the multi-directional self-assembly in polycrystalline device layers. Here we utilise cocrystallisation to engineer a multicomponent crystalline salt hydrate of S[sbnd](+)[sbnd]Mandelic Acid and L[sbnd]Lysine, S-Mand•L-Lys•5H₂O (1). This material exhibits a predicted single crystal longitudinal piezoelectric response of d₃₃ = 3.5 pC/N. In polycrystalline form, 1 grows as an assembly of plates which increases the measured longitudinal piezoelectricity to 11 pC/N in its macroscopic solid-state. This is due to contributions from the shear piezoelectric response d₃₆ = 10.8 pC/N, originating from the presence of plates oriented at acute angles relative to the surface. The brittleness of the crystals (E = 37 GPa) is overcome by reinforcing the substrate-free piezoelectric disc with a thin polymer coating to prevent flaking. Structural analysis confirms that the triclinic structure of 1 gives rise to this increased response due to the relative orientations of individual crystallites. Confined crystallisation of this multi-component form with a plate-like morphology, results in macroscopic self-assembly of an amino acid cocrystal that allows for the harvesting of higher shear piezoelectricity, but in a facile longitudinal configuration.