Hydrogen Bonding-Assisted Complete Ferroelectric β-Phase Conversion in Poly(vinylidene fluoride) Thin Films: Exhibiting an Excellent Memory Window and Long Retention.

Abstract

Organic nonvolatile memory with low power consumption is a critical research demand for next-generation memory applications. Ferroelectric switching characteristics of poly(vinylidene fluoride) (PVDF) thin films modified with a trace amount of hydrated Cu salt (CuCl2·2H2O) are explored in the present study. Herein, a Cu salt-mediated PVDF (Cu/PVDF) thin film with preferential edge-on β-crystallites is fabricated through the orientation-controlled spin coating (OCSC) technique. This work proposes a convenient and effective approach to produce edge-on-oriented electroactive PVDF thin films with a high degree of polar β-phase, so as to realize the favorable switching under low operating voltages. Herein, chemically modified PVDF is anticipated to form a complex intermediate, which attains its stability by undergoing favorable hydrogen bonding that reorients the C-C structure of PVDF to obtain the β-conformation. Such information is verified by X-ray photoelectron spectroscopy (XPS). Grazing incidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that the Cu salt incorporated into the PVDF matrix favored the formation of the electroactive β-phase with edge-on crystallite lamellae. Consequently, the Cu/PVDF thin film demonstrates a good contrast between electric field-assisted written and erased data bits in the piezoresponse force microscopy (PFM) phase image. Furthermore, to obtain the ferroelectric memory window, a metal-ferroelectric-insulator-semiconductor (MFIS) diode with Cu/PVDF as a ferroelectric layer has been fabricated. The capacitance-voltage (C-V) characteristic of the MFIS diode exhibits a memory window of 12 V with a long-term retention behavior (∼longer than 7 days). In a nutshell, we tried to represent a clear understanding of the interfacial interactions of the Cu salt with PVDF, which favor the edge-on formation that results in the promising low-voltage ferroelectric switching and excellent retention response, where any additional electrical poling and/or external stretching is completely possible to be ruled out, thus offering a new prospect for the evolution of devices with long-lasting nonvolatile memories.