Engineering crystal phase of Nylon-11 films for ferroelectric device and piezoelectric sensor

Among non-fluorinated ferroelectric polymers, odd number Nylons have recently attracted attention because of their solution-processibility and decent ferroelectric characteristics. Here, we demonstrate a ferroelectric and piezoelectric Nylon-11 δ` phase crystal film manufactured via a humidity modulated casting and subsequent melt-quenching process. The low relative humidity (RH) below 20% during the spin casting process effectively mitigates the formation of hygroscopic holes resulting in the formation of pin-hole free Nylon-11 thin films. In addition, the subsequent melt-quenching gives rise to the evolution of the ferroelectric δ` phase crystals, which is carefully confirmed by grazing incidence X-ray diffraction and infrared spectroscopy measurements. The resulting ferroelectric thin films show decent remnant polarization of ~4.6 μC/cm2 at a low operating voltage of ±30 V, suggesting the formation of high-quality Nylon-11 δ` phases thin film. Furthermore, the centrifugal casting at low RH facilitates the fabrication of freestanding and transparent 20-μm thick ferroelectric δ` phase film after the subsequent melt-quenching process. The resulting Nylon-11 film provides excellent piezoelectric performance upon external vertical pressure, which enables a proof-of-concept demonstration as a Morse code detector.