Space‐Charge‐Stabilized Ferroelectric Polarization in Self‐Oriented Croconic Acid Films

Abstract

Great progress has been made recently in molecular ferroelectrics with properties even comparable to those of inorganic ferroelectrics. However, it is difficult to develop basic thin films and devices for practical applications since most molecular ferroelectrics are uniaxial. The single polar axes of crystallites inside their films, if available, are usually oriented randomly. These can induce the components without contribution to ferroelectric polarization and a large depolarization electric field to suppress polarization. In this work, it is demonstrated that uniaxial croconic acid films in two‐terminal devices, deposited by thermal evaporation, can show effective ferroelectric polarization and nonvolatile memory switching behavior with small coercive fields of 11–30 kV cm−1. The polar c‐axes in thick crystalline films (>500 nm) are found to be self‐oriented nearly at a desired direction. With the assistance of trapped charges, stable ferroelectric polarization can be achieved, in spite of the existence of nonferroelectric components. These may pave a way to utilize uniaxial molecular ferroelectrics for various applications, such as gate dielectrics, electrets, and memory devices.

2D Organic Materials: Status and Challenges

“…In 2018, to prove that ferroelectric polarization can also achieve interface memory switching in uniaxial molecular ferroelectric materials, Hu et al used croconic acid crystallites to fabricate thin films for capacitive devices. [249] It can be found that the device exhibits an obvious ferro-polarized non-volatile memory switching behavior (Figure 30i,j). The hysteresis loops are unclamped at a voltage of 0 V, and the hysteresis loops of the first and third quadrants are mostly clockwise, which is a typical [246] Copyright 2019, Wiley-VCH.…”

Organic ferroelectric croconic acid: a concise survey from bulk single crystals to thin films

“…Likewise, Yuan et al 126 studied the temperature dependence of the nucleation rate of CA films with and without electric field-assisted growth and observed that the maxima of the grain density shifts in temperature when an electric field is applied during growth. Similarly, the void defects arising at the interface between CA films and substrates such as Si and SiO 2 have been studied by Peterson et al 127 Furthermore, Hu et al 70 have been able to grow micrometer thick CA films with crystallites uniaxially oriented, c -axes perpendicular to the film plane, without using any in situ electric field and have demonstrated stable ferroelectric properties as well as a non-volatile memory switching behavior stabilized by space charges.…”

“…Following the discovery of ferroelectricity in the crystal form of CA, several groups have started working on various aspects of the powder, [48][49][50] bulk single crystal, [51][52][53][54][55][56][57][58][59] self-assembled co-crystal, 60,61 composite film 62 and thin film [63][64][65][66][67][68][69][70] forms of the molecule. Most of the studies have focused on the ferroelectric and optical properties of the macroscopic crystals of CA.…”

A new organic–inorganic hybrid perovskite ferroelectric [ClCH2CH2N(CH3)3][PbBr3] and Its PVDF matrix-assisted highly-oriented flexible ferroelectric films

“…Limited by the uniaxial nature, the P s value of croconic acid thin films is only about 0.4 mC cm À2 , which is one-fiftieth of a single crystal sample. 36,37 In addition, the mechanical strength, hygroscopicity, and solubility are also obstacles to the practical application of molecular ferroelectrics. Therefore, finding proper protocols for achieving highly oriented ferroelectric films has become the hot issue in molecular ferroelectric chemistry.…”