Room temperature ferroelectricity in continuous croconic acid thin films

Abstract

Ferroelectricity at room temperature has been demonstrated in nanometer-thin quasi 2D croconic acid thin films, by the polarization hysteresis loop measurements in macroscopic capacitor geometry, along with observation and manipulation of the nanoscale domain structure by piezoresponse force microscopy. The fabrication of continuous thin films of the hydrogen-bonded croconic acid was achieved by the suppression of the thermal decomposition using low evaporation temperatures in high vacuum, combined with growth conditions far from thermal equilibrium. For nominal coverages ≥20 nm, quasi 2D and polycrystalline films, with an average grain size of 50-100 nm and 3.5 nm roughness, can be obtained. Spontaneous ferroelectric domain structures of the thin films have been observed and appear to correlate with the grain patterns. The application of this solvent-free growth protocol may be a key to the development of flexible organic ferroelectric thin films for electronic applications.Recent reports of room temperature ferroelectricity in croconic acid, 1 related oxocarbons, 2 benzimidazoles 3 , and related hydrogen-bonded proton transfer systems 2 are currently accelerating the emergence of molecular ferroelectrics (MFE) as viable materials alternatives to inorganic ferroelectrics, such as the prototypical barium titanate, BaTiO3. Importantly, croconic acid (CA), C5O5H2, exhibits room temperature polarization of the order of 10 -20 C/cm 2 in bulk crystals, which is comparable to that of BaTiO3, while at the same time the polarization switching fields are of practical order of magnitude. 3 The ferroelectric behavior of proton transfer organics like CA emerges from the resonance-assisted hydrogen bonding, specifically from the strong coupling between the protons in the hydrogen bonds and the π-electron system of the molecules that give them their dipole moments. 4MFEs have distinctive advantages over complex oxides and may replace oxides in some applications, with benefits in terms of flexibility, scalability, and sustainability. [5][6][7] The potential of MFEs to become viable material alternatives to inorganic ferroelectrics hinges on the availability of strategies to fabricate thin films with defined structure and morphology on a large scale, which at the same time preserve their ferroelectric properties. Vapor deposition, especially chemical vapor deposition polymerization, has been a method of choice for the fabrication of thin films of numerous organic polymers, including the popular ferroelectric polyvinylidene fluoride, PVDF. 8,9 However, previous work has ruled out, the possibility to utilize thermal evaporation growth techniques for CA because the decomposition temperature is lower than the melting point (177 °C). Another challenge is that the film growth tends to be three-dimensional, due to the weak interaction between the MFEs and most non-reactive inorganic substrates. Matrix-assisted pulsed laser deposition was performed as an alternative strategy for croconic acid thin films (100-200 nm thick), but without...

Nonvolatile voltage controlled molecular spin state switching

“…4(c). The resistance switching is reproducible, as indicated, and follows the expected ferroelectric switching of croconic acid, 20 where the coercive voltages seen here (6-9 V) are found to be similar to previous results (about 4 V). 21 The voltage-controlled switching of the spin state cannot be followed with X-ray absorption measurements, because the synchrotron beam spot size is 180 lm  80 lm while the gap between electrodes is 260 nm.…”

“…19 Here, in this work, we exploit the spin state switching of [Fe{H 2 B(pz) 2 } 2 (bipy)] 14,19 in combination with several molecular ferroelectrics, including a variation of ferroelectric polyvinylidene-fluoride 19 (here, polyvinylidene fluoride hexafluoropropylene or PVDF-HFP) as well as croconic acid (4,5-dihydroxycyclopentenetrione, i.e., C 5 H 2 O 5 ). 20 These molecular thin film combinations provide a demonstration that voltage mediated interface interactions can be used to achieve the desired nonvolatile room-temperature switching of the spin state, as indicated in Fig. 1.…”

A Molecular Polycrystalline Ferroelectric with Record‐High Phase Transition Temperature

“…This is because the efficiency of polarization switching in the polycrystalline state relies on the available ferroelectric axes, the maximum remanent polarization ( P r ) max could reach 0.25 P s and 0.9 P s (spontaneous polarization of the single‐crystal state) in the uniaxial and multiaxial cases, respectively . For example, the P r of uniaxial croconic acid film reported by Jiang et al (≈0.4 µC cm −2 ) is only 0.02 times that of the single crystal (≈20 µC cm −2 ) . If molecular ferroelectrics are able to be multiaxial and facilitate practical utilizations in polycrystalline state, just like the inorganic ceramic ferroelectrics, a class of organic/molecular polycrystalline ferroelectrics can therefore be defined.…”

A brief review of ferroelectric control of magnetoresistance in organic spin valves

“…The recent efforts of exploiting ferroelectric control of MR in organic spin valves show encouraging indication of effects on the energy alignment and on the interfacial crystal structures. However, since the exact mechanism of spin transport through organic spin valves is still not fully understood, more work on similar devices with other ferroelectric materials [95][96][97] and more characterization on the interfacial crystal structures and on the interfacial electronic structures are needed to elucidate the ferroelectric effect as well as other concomitant effects such as the change of magnetism of the electrodes [98][99][100] and change of transport mechanism [101][102][103]. At the same time, as the field of organic spintronics keeps growing, the spin polarization at the FM/organic interfaces (spinterface) will remain a focus.…”