We have investigated the ferroelectric polarization switching properties of trialkylbenzene-1,3,5-tricarboxamide (BTA), which is a model system for a large class of novel organic ferroelectric materials. In the solid state BTAs form a liquid crystalline columnar hexagonal phase that provides long range order that was previously shown to give rise to hysteretic dipolar switching. In this work the nature of the polar switching process is investigated by a combination of dielectric relaxation spectroscopy, depth-resolved pyroelectric response measurements, and classical frequency- and time-dependent electrical switching. We show that BTAs, when brought in a homeotropically aligned hexagonal liquid crystalline phase, are truly ferroelectric. Analysis of the transient switching behavior suggests that the ferroelectric switching is limited by a highly dispersive nucleation process, giving rise to a wide distribution of switching times.
Combining dipolar and semiconducting functionality in a single molecule yields a ferroelectrically switchable conductivity.
ABSTRACT:A synthetic method for the end-functionalization of vinylidene fluoride oligomers (OVDF) via a radical reaction between terminal olefins and OVDF-I is described. The method shows a wide substrate scope and excellent conversions, and permits the preparation of different disc-shaped cores such as benzene-1,3,5-tricarboxamides (BTAs), perylenes bisimide (PBI) and phthalocyanines (Pc) bearing three to eight ferroelectric oligomers at their periphery. The formation, purity, OVDF conformation, and morphology of the final adducts has been assessed by a combination of techniques, such as NMR, size exclusion chromatography, (SEC), differential scanning calorimetry (DSC), polarized optical microscopy (POM) and atomic force microscopy (AFM). Finally, PBI-OVDF and Pc-OVDF materials show ferroelectric hysteresis behavior together with high remnant polarizations, with values of as high as Pr ~ 37 mC/m 2 for Pc-OVDF. This work demonstrates the potential of preparing a new set of ferroelectric materials by simply attaching OVDF oligomers to different small molecules. The use of carefully chosen small molecules paves the way to new functional materials in which ferroelectricity and electrical conductivity or light-harvesting properties coexist in a single compound. ■ INTRODUCTIONFerroelectric and piezoelectric materials play a vital role in modern technologies ranging from capacitors, hydrophones and actuators to frequency-controlled devices. 1 To further advance these technologies, access to cheaper and readily processable materials that show large ferroelectric and piezoelectric responses is highly desired. Organic materials are currently explored as they are potentially cheap, easily processable, biocompatible, and can be endowed with diverse and tunable functions. In addition, their mechanical flexibility is crucial for piezoelectric applications.Since the discovery of the first organic ferroelectric material in 1920 2 the observation of ferroelectric properties in organic materials has not been profuse. 3 In fact, most of the organic ferroelectric research is focused on polyvinylidene fluoride (PVDF). 4 PVDF displays a large remnant polarization, a short switching time, and an excellent thermal stability which makes it suitable for the fabrication of piezoelectric films. 5 The ferro-and piezoelectric properties of PVDF originate from the antiparallel intrachain arrangement of the alternated CH2 and CF2 segments in the zigzag all-anti conformation, the so-called β-form. 6 However, untreated PVDF thin films processed from the melt or from solution are not ferroelectric. 6b They possess a mixture of α, β, and γ conformations 4 and additional steps, such as mechanical stretching, 5b thermal annealing 7 and electrical poling 8 have to be performed in order to achieved the β-phase necessary to display ferro-and piezoelectric properties.More recently, vinylidene fluoride oligomers (OVDF) and poly(vinylidene fluoride-trifluoroethylene P(VDF-coTrFE), a copolymer based on PVDF, have been evaluated to enhance the format...
We disclose a supramolecular material which combines semiconducting and dipolar functionalities. The material consists of a discotic semiconducting carbonyl-bridged triarylamine core which is surrounded by three dipolar amide groups. In thin films, the material self-organizes in a hexagonal columnar fashion through π-stacking of the molecular core and hydrogen bonding between the amide groups. Alignment by an electrical field in a simple metal/semiconductor/metal geometry induces a polar order in the interface layers near the metal contacts that can be reversibly switched, while the bulk material remains non-polarized.On suitably chosen electrodes, the presence of an interfacial polarization field leads to a modulation of the barrier for charge injection into the semiconductor. Consequently, a reversible switching is possible between a high-resistance, injection-limited off-state and a low-resistance, space-charge-limited on-state. The resulting memory diode shows switchable rectification with on/off ratios of up to two orders of magnitude. This demonstrated multifunctionality of a single material is a promising concept towards possible application in lowcost, large-area, non-volatile organic memories.
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