A small-molecule organic ferroelectric with piezoelectric voltage coefficient larger than that of lead zirconate titanate and polyvinylidene difluoride.

Piezoelectric materials that generate electricity when deforming are ideal for many implantable medical sensing devices. In modern piezoelectric materials, inorganic ceramics and polymers are two important branches, represented by lead zirconate titanate (PZT) and polyvinylidene difluoride (PVDF). However, PVDF is a nondegradable plastic with poor crystallinity and a large coercive field, and PZT suffers from high sintering temperature and toxic heavy element. Here, we successfully design a metal-free small-molecule ferroelectric, 3,3-difluorocyclobutanammonium hydrochloride ((3,3-DFCBA)Cl), which has high piezoelectric voltage coefficients g 33 (437.2 × 10 -3 V m N -1 ) and g 31 (586.2 × 10 -3 V m N -1 ), a large electrostriction coefficient Q 33 (about 4.29 m 4 C -2 ) and low acoustic impedance z 0 (2.25 × 10 6 kg s -1 m -2 ), significantly outperforming PZT ( g 33 = 34 × 10 -3 V m N -1 and z 0 = 2.54 × 10 7 kg s -1 m -2 ) and PVDF ( g 33 = 286.7 × 10 -3 V m N -1 , g 31 = 185.9 × 10 -3 V m N -1 , Q 33 = 1.3 m 4 C -2 , and z 0 = 3.69 × 10 6 kg s -1 m -2 ). Such a low acoustic impedance matches that of the body (1.38-1.99 × 10 6 kg s -1 m -2 ) reasonably well, making it attractive as next-generation biocompatible piezoelectric devices for health monitoring and "disposable" invasive medical ultrasound imaging.