Symposium EL02—Molecular and Organic Ferro- and Piezoelectrics—Science and Applications

Electrocaloric effect (ECE) is the ability of a dielectric to change its temperature and entropy as an electric field is applied and released. ECE provides an effective means to realize solid-state cooling that is environmentally benign and potentially highly energy efficient. During past several years, we have been investigating approaches in developing ferroelectrics which can generate giant electrocaloric effect over a broad temperature range. This talk will present these progress, especially, on how to generate a large ECE under low electric fields.

We investigate and discuss the pyroelectric response of poled and unpoled thin films of the ferroelectric co-polymer P(VDF-TrFE) at a composition 70/30 mol%. Having the films prepared in a low humidity environment reduces the dark current yielding polarization versus temperature hysteresis loops in unprecedented resolution. The loops are recorded between the glass transition temperature and the melting temperature. Poled and unpoled films are presented to exhibit clear first order Curie transitions. In poled samples we monitor a current in close vicinity to the Curie transition, which is assigned to detrapped charges released from the crystal-amorphous interface.

In the later sections of this work we show, that below the Curie transition, the pyroelectric response in poled films can be divided in reversible and irreversible contributions. The reversible contributions are a result of the thermal expansion of the non-ferroelectric amorphous phase in association with rigid ferroelectric crystallites. The irreversible contributions can be assigned to imprinting. Imprinting is an aging mechanism: Stray fields generated by the ferroelectric polarization in the ferroelectric crystallites, force the dipoles outside the ferroelectric crystallites to reorient anti-parallel to the ferroelectric polarization vector. According to that, the irreversible pyroelectric response is a current caused by dipole reorientation processes in the polymer due to internal fields. A consecutive annealing procedure below the Curie transition is used to experimentally separate the reversible from the irreversible pyroelectric response. Then, with the aid of numerical calculations, we model the current flow due to irreversible pyroelectricity. We show that above a certain temperature (Ta=45°C), the relaxation processes associated to imprinting / irreversible pyroelectricity stop being Arrhenius-like and start following a compensation law. We think that these relaxation processes take place at the surface of ferroelectric crystallites, in the so called defect-ferroelectric (DFE) phase. The DFE phase is postulated to be located at the boundary crystal-amorphous, so to say enveloping the ferroelectric crystallites. The conjecture that the relaxation processes associated to imprinting / irreversible pyroelectricity are located in the DFE phase is founded on two facts: First, the temperature Ta coincides with the onset-temperature at which the cell parameters of the DFE phase start to continuously expand with increasing temperature due to inclusion of defects. Besides, the DFE phase with fast changing cell parameters seems suitable to host non-Arrhenius-like relaxation processes. Second, both, the compensation law and the DFE phase depend substantially on the Curie Temperature. We conclude with the hypothesis, that there is an interphase at the crystal-amorphous interface, which hosts relaxation processes associated to imprinting.

New non-toxic biocompatible materials with piezoelectric and pyroelectric properties are developed for biomedical applications. Lead-free pyroelectric thin films are promising candidates for CMOS compatible and environmental-friendly miniaturized infrared sensors. Piezo responsivity of pyroelectric materials due to mechanical clamping makes the electrical response to transient thermal changes depended of geometrical properties of the pyroelectric material and its mounting. In this study, we report on the characterization of micro structured pyroelectric thin films. By measurement of the transverse effective piezoelectric coefficient, pseudo pyroelectrical contribution due to piezoelectric clamping can be separated from real pyroelectricity. Our novel IR-laser-based measurement setup is designed to record frequency dependent electrical response to spot wise intensity modulated IR irradiation of the sample surface and provides 2D responsivity maps. 3D information of the pyroelectric properties is extracted by LIMM (Laser Intensity Modulation Method). Therefore, the modulation of the IR source is swept up to high-frequencies while recording the sample’s electrical response (including absolute value and phase angle) by a lock-in amplifier. Thermal distribution inside the pyroelectric material is simulated to extract the material’s polarization distribution out of the LIMM data. These results are complemented by microscopic techniques for fundamental thermal analysis below the microscale. Our novel laser-based measurement setup is an essential step forward for precise material characterization with high spatial resolution for research and can be easily adapted for quality control on industrial scales to enable pyroelectric IR sensor arrays for mass-manufacturing.

Vibrational energy harvesting using piezoelectric materials has been an emerging topic of a renewable energy technology. The technology has a possibility to establish a self-powering device for IoT system that do not depend on conventional power supplies such as batteries, which can deliver electric power only for limited-duration operations. Therefore, the technology is expected to apply to novel sensors for health care and intelligent buildings [1]. The inorganic piezoelectric materials such as Pb(Zr, Ti)O₃ have been a promising material which shows highly efficient energy conversion owing to their high electromechanical coupling coefficients. From another point of view, power generation using piezoelectric polymers such as polyvinylidene fluoride (PVDF) are also attractive because of their prospective features of the robustness, flexibility and large scale production. Note that piezoelectric polymers are expected to generate high electric power under large deformation. For example, we designed the power generating systems which enable PVDF to induce large strain by applying a lever structure. The power generating floor generated 7 mW by human stamping [2], which is relatively large output using a piezoelectric material.
However, the variety of the piezoelectric polymer is still limited. Thus, we have focused on developing the novel piezoelectric polymers which are more flexible, easy to install everywhere and eco-friendly. Firstly, we report on the self-polarization phenomenon observed in melt-crystallization process of VDF copolymer thin films. Ferroelectric polarization was spontaneously formed when Pt was used as the bottom electrode [3]. Thus, the poling process can be omitted by controlling the crystallization process , which has the advantage to avoid the breakdown. In addition, we also report on the recent progress of VDCN/VAc copolymer. VDCN/VAc is known as an amorphous polymer with great dielectric property and relatively large piezoelectric property. However, the mechanism of the piezoelectricity has not been understood yet. We have revealed that VDCN/VAc has nanoscale domains observed by scanning probe microscopy. Based on controlling the hierarchical structure of the polymer, we challenge to improve the energy conversion efficiency and formability for the fabricating process of the vibration energy harvesting devices.

References
[1] T. Ito, M. Hasegawa, and T. Nakajima, IEEE newsletter 2017, Internet of Things for Buildings that Make Life Safe and Secure.
[2] T. Nakajima et al., Jpn J Appl Phys., 50, 09ND14 (2011).
[3] Y. Nakagawa et al., Jpn J Appl Phys., 55 10TA12 (2016).

Piezoresponse Force Microscopy (PFM) has been used to investigate the switching behavior in ultrathin films of PVDF-TrFE. The mechanism of polarization reversal has been investigated via sub-10 nm real space imaging of domain pattern evolution under an applied electric field. The domain switching behavior revealed in PVDF-TrFE nanomesas is drastically different from that observed in inorganic solid-state crystalline ferroelectrics. The nanoscale features of the switching process include remote domain nucleation and spatially nonuniform wall velocity. Local switching spectroscopy and domain dynamics studies relate the observed switching features to a random-bond type disorder associated with defects in conformation and molecular packing. The PFM approach has been also used for investigation of the static and dynamic behavior of ferroelectric domains in diisopropylammonium bromide (DIPA-B) microcrystals. It is shown that stable head-to-head or tail-to-tail domain configurations can be realized in DIPA-B. In-plane alignment of polarization allows investigation of the forward domain growth along the polar axis - a process that is difficult to attain in conventional ferroelectric switching studies. Using this sample geometry, we have directly measured anisotropy of the domain wall velocity for the forward and lateral growth and demonstrate the effect of the forming charged domain walls on morphology of the growing domains.

The bulk polarizability of self-assembled anti-ferroelectric materials is central to the energy storage capabilities of these molecular solids. This polarizability critically depends on poorly understood electric fields on length scales smaller than constituent molecular units. In this study we use vibrational spectroscopy of single crystals of the anti-ferroelectric material 2-trifluoromethylbenzimidazole (TFMBI) to directly analyze these fields and how they change with temperature. Experimentally, we find cooling the material causes red-shifts of Raman-active peaks near 3100 cm^-1 and blue-shifts of peaks between 900 cm^-1 and 1600 cm^-1. Density functional theory calculations provide assignments of these reporter vibrations. By comparing solid-state spectra to those acquired from molecules dissolved in different solvents, we are able characterize the direction and magnitude of changes to microscopic electric fields in TFMBI. We discuss how to relate the measured changes in the microscopic electrostatic structure of the material to temperature-dependent changes to its macroscopic dielectric properties. These results indicate vibrational spectroscopy may be a critical tool in assessing the microscopic electric fields central to the application of self-assembled materials in energy storage applications.

Electrically active materials, such as Poly(vinylidenefluoride) (PVDF) and its copolymers, have the ability to harvest mechanical energy through mechanical vibrations and structural flexing.
The ferroelectric phase of PVDF consist of long chains of the dipolar VDF (–CF₂-CH₂–) monomer that crystallize in a quasi-hexagonal close-packed “β-phase” structure with the molecular dipoles aligned perpendicular to the chain axis, producing a switchable spontaneous polarization and consequently, a significant piezoelectric response. Due to the inherent nanoscale dependency of PVDF’s molecular structure, quantitative methods for nanoscale molecular structure and mechanical analysis are limited. In this presentation, we demonstrate the methods for investigating the local mechanical properties quantitatively. These properties include the elastic modulus and damping which are measured using contact resonance atomic force microscopy (CR-AFM). In addition, we have used nanoscale infrared (nanoIR) spectroscopy with chemical mapping to probe the correlation between localized molecular structure and orientation on the nanoscale. The thin PVDF nanomesa studied here were made from a copolymer of 70% vinylidine fluoride and 30% trifluoroethylene, P(VDF-TrFe) using the horizontal Langmuir-Blodgett transfer technique. This nanomesa film was formed on a substrate of a p-type silicon wafer by a drying and fine annealing procedure. For CR-AFM, the height image, frequency map, and Q factor map are collected, which allows for the investigation of the local mechanical properties associated with the in plane and out of plane structure. In general, higher frequency value means stiffer and higher Q value reflects less energy dissipation. The orientation of PVDF’s molecular structure dependency was investigated by rotation of the sample stage. The distribution of the polymer chain axes was determined from the asymmetric frequency mapping -- which is attributed to the local stiffness of in-plane structure. The results demonstrate that CR-AFM can aid in the understanding of the fabrication process, to achieve high crystallinity and orientation with good mechanical properties. The nanoIR mapping data were used to determine the crystalline orientation of an individual nanocrystal. By fixing the wavelength at 1078 cm^-1 the C-C antisymmetric stretching absorption band can be mapped for the nanomesa crystal. In this way, we identified higher signals when the IR laser was aligned with the C-C-C chain and lower signals were found when the IR laser was not aligned. A similar approach was used to determine molecular orientation, where the IR laser direction is changed to investigate the orientation dependency within the nanomesa structure -- which is consistent with the stage rotation done for the CR-AFM measurements. Through combination of CR-AFM and nanoIR, we are able to correlate the molecular structure and localized mechanical properties, to optimize the electroactive polymer organization. This approach can guide the design and fabrication of high-performance, flexible, energy-harvesting devices. [Research supported by the Nebraska Center for Energy Sciences Research].

Combining semiconducting and ferroelectric functions in a given architecture has been a long-time goal in the electronics community. Various approaches have been used^1,2 to blend semiconductor and ferroelectric polymers for micro- and opto-electronic applications. Here we discuss model systems based on the archetypal semiconducting polymer poly(3-hexylthiophene) (P3HT) blended with the ferroelectric commodity polymer poly(vinylidene difluoride) (PVDF), to provide insights on the effect of the local environment and phase morphology, manipulated by the addition of PVDF, on the optoelectronic properties of P3HT. Unlike the phase separated structure generally reported in these blends,^1,3 we present signs of intermixing, with varying P3HT domain purity and crystallinity as a function of PVDF. We show strong influence of these structural effects, along with the blending induced effects of the ferroelectric β phase of PVDF, on the photo-physical processes of the semiconductor. We illustrate variations in the photoluminescence and charge dynamics of photo-induced absorption with blend compositions, suggesting localized enhancements of the polar, dielectric environment in the blend. Finally, a correlation between structure and relevant dielectric and semiconducting features is demonstrated, opening pathways to design new multicomponent material systems with multifunctional architectures for optoelectronic applications.

References
[1] K. Asadi et. al., Nature Materials, 2008, 7
[2] K. S. Nalwa et. al., Energy Environ. Sci., 2012, 5, 7042
[3] G. M. Su et. al., Macromol., 2015, 48, 5861

Ferroelectric molecular compounds present great advantages for application in electronics because they combine the high polarization values, comparable with those of inorganic materials, with the flexibility and low cost properties of organic ones. However, some limitations to their applicability are related to the high crystallinity required for deploying the ferroelectricity. Highly ordered ferroelectric patterned thin films of diisopropylammonium bromide have been successfully fabricated by lithographically controlled wetting technique. Confinement favors the self organization of ferroelectric crystals, avoiding the formation of polymorphs and promoting long range orientation of crystallographic axes. Patterned structures present high stability and polarization can be switched to be arranged in stable domain pattern for application in devices. The versatility of this methods allows to extend the results to the class of molecular ferrolectric materials.

Deuteration can be used as an effective probe for dynamic and structural properties of ferroelectric materials, especially those based on hydrogen-bond ordering, such as potassium dihydrogen phosphate (KDP) [1]. The diisopropylammonium halide (DIPA-X, X = Cl, Br, I) system is a ferroelectric material of interest due to its high room-temperature polarization, low coercive field, and low material cost [2]. The three DIPA halides all share similar structures and ferroelectric properties. In particular, on heating, the ferroelectric P2₁ phase gains a mirror plane to become the paraelectric P2₁/m phase. While the DIPA halides aren’t strictly hydrogen-bond-ordered ferroelectrics, they do contain NH–X hydrogen bonds, which form a continuous chain along the polarization axis. This hydrogen-bond chain plays an important role in crystal structure and may also influence the properties of the ferroelectric phase. The purpose of this report, therefore, is to address the role of deuteration on the properties of DIPA-X ferroelectric materials. We found that deuteration of the amine hydrogens in DIPA-X is easily carried out by recrystallization from heavy water. Like KDP, DIPA-Br exhibits a negligible change in lattice spacing on deuteration. Deuteration also lowered the temperature of the peak low-frequency dielectric constant associated with the ferroelectric-to-paraelectric transition by approximately 0.5 K in DIPA-Br. Further study with differential scanning calorimetry (DSC) revealed deuteration lowered the ferroelectric-paraelectric transition temperature and decreased the associated temperature hysteresis for all three DIPA halides. DIPA-I exhibited the largest shift with a decrease in transition temperature of 2.9 ± 0.4 degrees to its phase transition temperature, and DIPA-Cl the lowest with decrease of 0.5 ± 0.1 degrees. The temperature shift is downward, rather than upward, as is observed with in most hydrogen-ordered ferroelectrics. For example, deuteration of KDP increases the transition temperature by 91 K [1]. The enthalpy of the transition, which ranged from 3.6 ± 0.1 J/kg for DIPA-Br to 4.2 ± 1 J/kg for DIPA-I, exhibited negligible change on deuteration.

This work was supported by the National Science Foundation (NSF) through the Nebraska Materials Research Science and Engineering Center (MRSEC) (DMR-1420645).

[1] R. J. Nelmes et al., Journal of Physics C: Solid State Physics 15, 59 (1982).
[2] D. W. Fu et al., Science 339, 425-428 (2013)

The diisopropylammonium halide (DIPA-X, X = Cl, Br, I) system is a ferroelectric material of interest due to its high room-temperature polarization, low coercive field, and low material cost [1]. The three halides have similar structures and phase diagrams [1], that are characterized by two distinct crystal structures at room temperature, a ferroelectric P2₁monoclinic structure, and a non-polar P2₁2₁2₁orthorhombic structure. Above the ferroelectric-to-paraelectric transition temperature (426 K in DIPA-Br) there is a single paraelectric phase with monoclinic P2₁/m structure. Heating crystals with the non-polar phase crystals of DIPA-Br above 421 K converts them to the ferroelectric phase. Although this transition has been characterized as irreversible [1], this could be because the non-polar phase is metastable in DIPA-Br and DIPA-Cl but it appears to be the stable phase in DIPA-I [2]. The purpose of this report, therefore, is to address whether or not the transition between the non-polar and ferroelectric phases is continuous. The DIPA-Br crystals were grown from solution in the non-polar phase [1] and transformed to the ferroelectric phase by annealing above 421 K. The crystals were characterized at room temperature, both before and after annealing, by microscopy to identify the facet morphology [3]and by x-ray diffraction to determine the crystal structure and crystallographic orientations [1]. The as-grown crystals exhibited morphology and crystal structure consistent with the non-polar phase, with well-shaped facets, notably rectangular facets associated with the (001) crystal planes and sets of hexagonal facets that belong to the (011) and (101) crystal planes. After annealing into the ferroelectric phase, we found that the square facets again corresponded to the (001) crystal planes, while the hexagonal (011) facets from the nonpolar phase correspond to the (111) crystal planes the ferroelectric phase. Therefore, the crystals transformed from the non-polar phase to the ferroelectric phase occurs without changing morphology or exhibiting significant internal stress, suggesting that the transition is continuous. This hypothesis is further supported by the relatively low enthalpy of the transition (1.3 J/g), and the small (1%) change in density [1]. The elastic modulus of approximately 10 GPa is the same in both phases (<10%) change in the elastic modulus, which is approximately 10 GPa for both phases, although the loss modulus of the ferroelectric phase, at 1 GPa, is approximately twice that of the non-polar phase. This work was supported by the National Science Foundation (NSF) through the Nebraska Materials Research Science and Engineering Center (MRSEC) (DMR-1420645) and by the Nebraska Center for Energy Sciences Research.
[1] D. W. Fu et al., Science 339, 425 (2013).
[2] A. Piecha-Bisiorek et al., Inorg. Chem. Frontiers4, 553 (2017).
[3] C. F. Macrae et al.,J. Appl. Crystallography39, 453 (2006).

Creating new tissues and organs and promoting the regeneration of damaged or diseased tissues and organs are the objectives of tissue engineering and/or regenerative medicine. In spite of the complexity of regenerating tissues major advances have been achieved.
The potential of piezoelectricity-stimulated cell migration and growth unveils appealing opportunities in tissue engineering, as the use of devices allowing surface charges to be controlled in real time by simply applying a mechanical stimulus.
Poly (l-lactic) acid (PLLA) is an FDA approved biodegradable and piezoelectric polymer that has been object of extensive research, namely as a scaffolding material and drug delivery system. Here, we exploit the feasibility of using electrical polarization in PLLA coatings and aligned nanofibers and investigate the biological outcome of these electrically-induced poled PLLA platforms on neuronal relevant in vitro models. In addition we have carried out a metabolomics study of the effects of PLLA (either non-poled or negatively poled) on the metabolism of osteoblastic HOb cells, aiming at unveiling the metabolic cell adaptations to PLLA and its poled state. The presence of PLLA seems to trigger cell catabolism and anti-oxidative protective mechanisms in HOb cells, while directing them towards cellular growth. A qualitative tendency for negatively-poled PLLA to be more effective in sustaining cell growth than non-poled PLLA was suggested. Finally, the concept of a biological BioMEMs platform composed of piezoelectric PLLA and 316L stainless steel is proposed as an alternative to be used in the implant technology.

Molecular ferroelectrics with cheap, light-weight, flexible, wearable, and environmentally benign characteristics have been an attractive research topic in the past decades due to the potential applications in data storages, piezoelectric sensors, actuators, and nonlinear optical devices. Their intrinsic structure-property flexibility attracted particular interest for molecular design towards new materials with good ferroelectric performance at room temperature. Nevertheless, developing a reliable method to form polar structures and further tune the ferroelectricity is an imminent ongoing challenge.
In this talk, we present our recent achievements on molecular ferroelectrics in perovskite structures with new design strategies of selection of molecular building blocks and several new molecular ferroelectrics with exciting ferroelctric, optical and piezoelectric properties.

References:
Advanced Materials, 2019, DOI: 10.1002/adma.201901843.
Angew. Chem. Int. Ed, 2019, DOI: 10.1002/anie.201904305.
Advanced Materials, 2017. 29(29): 1700831.
Advanced Materials, 2016. 28(13): 2579.
Science, 2017. 357(6348): 306-309.
Science, 2018. 361(6398): 151-155.

Visualization of human senses has been of great interest for developing an emerging interactive display that can artificially stimulate synesthesia with numerous unprecedented applications. Especially, visualization of various daily sound and music, which are much more complicated than human touch, in a form of flexible thin film devices can be a great challenge. In this presentation, we first present flexible artificial synesthesia devices that visualize continuous and complicated sounds. The device is made of a thin composite film of a piezoelectric polymer for sound generation and inorganic electroluminescence (EL) microparticles for direct visualization of input sound signals. Field-induced alternating current (AC) EL of the microparticles in the device depends upon the source sound wave, making their EL synchronized with sound arising from the piezoelectric actuation. Our device showed extreme mechanical tolerance that can be repeatedly folded and crumpled with visible sound, allowing a variety of unexplored applications including synchronous sound-lightings and wearable, on-body sound-vision systems to facilitate emotional interaction of human being with sound in a human-friendly form. We also demonstrate that piezo and ferroelectric sound is capable of sensing, monitoring, and storing the information of various liquids. Our platform utilises sound arising from liquid-interactive ferroelectric actuation, which is dependent upon the polarity of the liquid. Liquid-interactive ferroelectric sound (LIFS) is successfully developed when a liquid droplet is placed on a ferroelectric polymer layer across two in-plane electrodes underneath the ferroelectric layer under an external in-plane AC field. An AC field built up vertically between one of the electrodes and the liquid results in ferroelectric vibration of the device, leading to sound. The sound pressure level (SPL) of the device depends upon the polarity of the liquid, allowing for facile liquid sensing and identification. More importantly, as the SPL arising from LIFS of a liquid is correlated with non-volatile remnant polarisation of the ferroelectric layer, the information of a liquid is readily stored and retrieved even after the liquid is removed, resulting in a sensing memory of the liquid. By programming different LIFSs on a thin film pad, we can also identify the 2D position of a liquid droplet on the pad. Moreover, our device successfully monitored the flow of a human serum liquid passing through channels with their diameters ranging from millimetres to tens of microns and determined its velocity. Mechanically flexible and optically transparent tube-type LIFS AC devices allow for in situ analysis of the flow of a liquid in terms of SPL.

Piezoelectric Rubbers prepared with mixing piezoelectric-ceramic particles and an elastic material have potential applications such as sensor and actuator due to the piezoelectricity and flexibility. In our previous study, in order to enhance the piezoelectricity, aligned-type piezoelectric rubbers in which piezoelectric particles were aligned in the direction parallel to the thickness by an electric field were fabricated. Since the piezoelectric rubbers previously fabricated had thinner shapes and large spring constants, it has not had the flexibility sufficient to use as vibration-isolating materials.
In this study, in order to lower the spring constants, the aligned-type piezoelectric rubbers with a thickness of more than 5 mm were prepared. For the thinner conventional aligned-type piezoelectric rubbers, piezoelectric particles precipitated in a molding were easily aligned by an electric field due to the thin shapes. On the other hand, it was difficult to align piezoelectric particles only by an electric field in the case of thicker rubbers because the piezoelectric particles used were submillimeter in size and they tended to precipitate to the bottom of the molding by gravity. Hence, in order to prepare the thicker aligned-type piezoelectric rubbers, a novel process in which the molding was inverted before applying an electric field was introduced. In the process, a precipitation layer of piezoelectric particles was set to the top of the molding before applying an electric field. The piezoelectric particles gradually precipitated in an uncured rubber medium by gravity, which supported to align piezoelectric particles by an electric filed.
To confirm the formation of particle alignments, numerical simulations on the process in which the molding was inverted before applying an electric field were carried out. As the results of the simulation study, piezoelectric particles were aligned from the top to the bottom in the molding even if the piezoelectric rubbers had more than 5 mm of thickness. Thus, the numerical simulations clarified that the particle alignments were affected by particle position before applying an electric field. Furthermore, in the experimental study, considering the case that piezoelectric particles comparably fast precipitated after inverting the molding due to the low viscosity of a medium, the viscosity of a medium was controlled by changing the period from starting to heat an uncured medium to inverting a molding and applying an electric field, which led to adjusting the speed of particle precipitation. Based on the actual experiments, the aligned-type piezoelectric rubbers with thicknesses of 5 mm and 10 mm were successfully fabricated for the piezoelectric particle volume of 10–30%.
The piezoelectric performance of the piezoelectric rubbers obtained in this study was comparable to previous ones. For the piezoelectric rubbers with a thickness of 10 mm, the spring constants as small as approximately 1/30 of the conventional piezoelectric rubbers were obtained. The thicker aligned-type piezoelectric rubbers newly fabricated have potential uses as anti-vibrating and sensing materials.

Traditional microphone units focus on human perceptible sound waves (between 20 Hz and 20 kHz). Acoustic development also includes the ultrasound region, compromising the frequencies above the detection limit of the human ear (from 20 kHz to several GHz). These sound waves can be used for a wide variety of applications, including medical imaging, therapeutic treatment, non-destructive testing, fingerprint detection, gesture recognition and mid-air haptic feedback. Nowadays, the ultrasonic transducer and sensor technology is largely based on rigid bulk piezoelectric ceramic materials, such as lead zirconate titanate or barium titanate. Classical techniques to grow such ceramic materials require high process temperature and dedicated substrates. Although this is a mature technology for discrete passive components, offering a reasonably wide bandwidth and sensitivity, it is not amenable to machine large two-dimensional (2D) transducer arrays. Monolithic integration with other electronic components such as signal processing electronics is hardly possible. For this, a display like process technology is required.
During this presentation, I will give an overview of the recent developments at imec to use polymer based piezoelectric materials processed on glass substrates for large arrays of ultrasound transducers. Several challenges specific to organic material processing are tackled or under investigation: uniformity over larger substrate size, photo-lithographic based patterning, integration of the ultrasound elements with thin-film circuitry. Larger arrays of up to 64x64 elements are developed, which can be used to either emit ultrasound or to detect reflected acoustic waves. Using beam-forming techniques, the acoustic pressure can be focused on a single point in 3D space, which enables future application such as mid-air haptic feedback or local wound healing.

Ferroelectric vinylidene fluoride (VDF)-based polymers are overviewed with special interest on the chain dynamics governing phase transition and polarization switching phenomena. Their ferroelectricity originates in the large dipole moment of CH₂CF₂ unit. All-trans conformation and parallel packing induce alignment of all molecular dipoles to generate spontaneous polarization P_s that is switchable by action of high external electric field. The copolymer with trifluoroethylene (TrFE) containing 50-80mo% VDF exhibits Curie point T_c associated with transition into the paraelectric phase. As the VDF content exceeds 80 mol%, T_c is overtaken by melting. For TrFE-rich copolymers, the paraelectrc phase is preceded by the anti-ferroelectric phase consisting of randomly packed all-trans chains. Thus the phase behavior in VDF-based polymers is related to sucessive loss of intermolecular, intramolecular and crystalline dipolar order.
. In the molten phase, VDF-based polymers assume a random mixture of three conformational states (TT, TG, TG’) and gain the conformational entropy of Rln3. That in the paraelectric phase is calculated to be Rln1.6. These values are consistent with the observed heat of transition at T_m. and T_c. Chain dynamics comprising such conformational disorder can be characterized by dielectric spectroscopy. One particular relaxation process has been found to exist near 1MHz at room temperature and to move toward higher frequency side to reach 1GHz on melting. Although the relaxation time shows down near T_c in accord with dielectric anomaly, essentially the same chain dynamics govern the dielectric relaxation process. Below room temperature, the relaxation spectra split into two components. One is of VTF-type characteristic of segmental motion and the other is of Arrhenius type referred to a local mode. The latter is important because it lies on a linear extension to the molten phase with activation energy of 50kJ/mol. This is a clear demonstration of an elementary molecular motion governing the ferroelectric, paraelectric and molten phases.
The time domain measurements of polarization reversal using a step-wise electric field reveal another chain dynamics associated with 180-degree rotation of all-trans molecules about their chain axis. We have consistently examined the switching characteristics of VDF-based polymers from highly crystalline-thick to ultra-thin films. One of the common features is that the switching time obeys an exponential law that is transferred to a power law above 300MV/m. The fastest switching time was found to be limited to 1 ns at 800 MV/m. The time-resolved PFM confirmed that polarization reversal progressed via. nucleation-growth mechanism in respective crystalline grains filling VDF/TrFE thin film.
The effect of high electric field in VDF-based polymers is not limited to polarization switching. It has been already established that extremely high fields lead to conversion among crystalline polymorphs of PVDF from nonpolar form II to ferroelectric form I via form IV and III. Another example is related to the observation that the VDF/TrFE films as prepared exhibit much reduction of dielectric constant after poling. This result is consistent with the reduction of lattice spacing and crystalline disorder. In the presence of dc field, T_c moves to higher temperature side. Disorder induced by bulky TrFE in VDF sequence is a key factor for the appearance of T_c. Even if fully poled, there remains disorder inherent to ferroelectric instability. In VDF-based polymers, the elementary process observed dielectrically from molten phase to glassy state provides further information about such disorder.

Significantly enhanced electromechanical responses are inherent to the piezoelectric materials at morphotropic phase boundary (MPB). However, this well-established physical concept has never been realized in organic materials. This work demonstrates the stereochemically-induced MPB in the ferroelectric copolymers. We reveal that compositionally tailored tacticity can lead to intramolecular order-disorder evolution in the crystalline phase and thus an intermediate MPB region allowing for simultaneous occurrence of competing ferroelectric and relaxor properties. Our first-principles calculations confirm the crucial role of chain tacticity in driving the formation of MPB via structural competition between trans-planar and 3/1-helical phases. The copolymer with the morphotropic composition exhibits significantly enhanced piezoelectric responses, approximately two times the state-of-the-art piezoelectric polymers. This finding (Y. Liu et al, Nature 562, 96 (2018)) not only enables the investigation of the mechanisms of MPB from a completely new perspective but also offers unprecedented opportunities for the development of scalable high-performance piezoelectric polymers that are promising for flexible, wearable and biocompatible applications.

The strong dipole moment of polar organic semiconductor materials like tris-(8-hydroxyquinolate)aluminum (Alq3), if not oriented perfectly isotropic, will lead to the buildup of a giant surface potential (GSP), and thus to a macroscopic dielectric polarization of the organic film. Despite this being a known fact [1], the implications of such high potentials on charge transport and injection within and into an organic layer stack have only been studied recently. For example, we could explain and reproduce in silico the dependence of electron current in organic light emitting diodes (OLEDs) depending on the sign of the GSP of the electron transport layer (ETL)[2].
In this contribution, we will discuss the influence of the GSP of the hole transport layer (HTL) on hole injection into organic layers.
Unfortunately, most hole conducting materials are either non-polar or show an isotropic orientation, hence we resort to the new concept of dipolar doping to tune the GSP of a hole conducting layer.
Therefor, we chose the prototypical organic materials N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) as host and Alq3 as dopant and varied the doping ratios to tune the GSP in the HTL of a prototypical OLED.
The mixtures are investigated in single-layer, monopolar devices with only the HTL as well as bilayer OLEDs with Alq3-doped NPB as HTL and neat Alq3 as ETL, respectively. The latter are treated as metal insulator semiconductor (MIS) structures following and applying our recently published method of charge extraction by linearly increasing voltage (CELIV) on polar OLEDs[3, 4].
Characterization is then done electrically using current-voltage characteristics, impedance spectroscopy, CELIV and time of flight, as well as optically with ultraviolet photoelectron spectroscopy.
For all device types, we find an optimum in device performance and carrier injection for moderate doping concentrations of about 5%. By comparing all different methods with a focus on charge injection barriers, we reveal a complex relationship of carrier transport, substrate workfunction, modified injection and the effect of polarization, effectively manipulating charge carrier injection across the metal-organic interface and transport in the device.

[1] Noguchi, Y., Brütting, W., & Ishii, H. (2019). Spontaneous orientation polarization in organic light-emitting diodes. Japanese Journal of Applied Physics, 58(SF), SF0801. https://doi.org/10.7567/1347-4065/ab0de8

[2] Altazin, S., Züfle, S., Knapp, E., Kirsch, C., Schmidt, T. D., Jäger, L., … Ruhstaller, B. (2016). Simulation of OLEDs with a polar electron transport layer. Organic Electronics, 39, 244–249. https://doi.org/10.1016/j.orgel.2016.10.014

[3] Züfle, S., Altazin, S., Hofmann, A., Jäger, L., Neukom, M. T., Schmidt, T. D., … Ruhstaller, B. (2017). The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes. Journal of Applied Physics, 121(17). https://doi.org/10.1063/1.498290

[4] Züfle, S., Altazin, S., Hofmann, A., Jäger, L., Neukom, M. T., Brütting, W., & Ruhstaller, B. (2017). Determination of charge transport activation energy and injection barrier in organic semiconductor devices. Journal of Applied Physics, 122(11). https://doi.org/10.1063/1.4992041

Combining semiconductors and ferroelectrics is a promising strategy to realize novel non-volatile organic memories. While this has been demonstrated by blending ferroelectric and semiconducting polymers, we show that the same functionality can be incorporated in a single supramolecular material, solving amongst others phase separation and scaling issues. This unprecedented class of materials has a disc-like semiconducting core to which dipolar side groups are attached, making the materials both semiconducting as well as ferroelectric.

The ferroelectric and semiconducting properties are found to couple: the conductivity switches between a high and a low state when the ferroelectric polarization reverses. We show that there are two different switching mechanisms at play: bulk conductivity switching or injection barrier modulation. Depending on the material and the energy level alignment with the contacts, one of the two mechanisms will be dominant. Both mechanisms are rectifying, allowing for implementation in high-density, low-cost crossbar memory arrays.

Injection barrier modulation is the working mechanism behind the ferroelectric/semiconductor blends, and functions through a lowering of the injection barrier by the ferroelectric dipoles at the interface. The conductivity will switch from the low to the high state upon polarization reversal. This mechanism will be dominant when there is an injection barrier present.

In contrast, the bulk conductivity switching mechanism occurs when the energy levels of the material and the contact are well-aligned. Here the conductivity will switch from the high to the low state upon polarization reversal. Using a modified Marcus hopping model, we show that the asymmetric potential caused by the ferroelectric dipoles is responsible for the conductivity switching. Fitting the JV-curves to a combination of Ohmic and Space-Charge Limited current provides further proof for the conductivity switching being a bulk effect. This bulk conductivity switching is unique as in general conductivity switching mechanisms are caused by interface effects.

We have thus introduced a new class of materials that are both semiconducting and ferroelectric. The interplay of these two properties leads to rectifying and switchable conductivity. This makes these materials an excellent candidate for printable non-volatile memories.

P(VDF-TrFE) based thin-film transistors and diodes have emerged as a promising storage technology for low-cost microelectronic applications on substrates such as plastic and paper. Items where today integration of a silicon chip is not economical, such as toys, cards, labels, badges, value paper, medical disposables, could be imagined to be equipped with electronics – with embedded memory functionality.

In this presentation, some of the higher-order integration aspects are discussed, as in almost all foreseeable applications, multiple memory devices are needed to store a multiple number of bits. An attractive architecture for exploiting the strengths of ferroelectric diodes for memory and logic is the cross-point array, without the need for transistors or other electronic components. In the context of memory, the memory cells are defined by patterning the top electrode into columns and the bottom electrodes into rows. The memory state is changed by addressing the corresponding row and column and applying a bias that will provide the desired ferroelectric polarization. To program all memory elements, each row is sequentially addressed. Large-size memory arrays – up to 1 kbit – will be demonstrated.

Other achievements reported here include multi-bit memory elements, via innovative schemes in fabrication as well as electric driving schemes; integration with relatively complex thin-film transistor addressing circuitry; and finally the combination with OLED pixels, resulting in non-volatile pixels.

Ferroelectric memories are endowed with high data storage density by nanostructure designing, while the robustness is also impaired. For organic ferroelectrics favored by flexible memories, low Curie transition temperature limits their thermal stability. Herein, we demonstrate a ferroelectric random access memory (FeRAM) based on an array of P(VDF-TrFE) lamellae by self-assembly. Written data shows enhanced thermal endurance up to 90 °C and undergoes 12 thermal cycles between 30 and 80 °C with little volatilization. The promoted thermal stability is attributed to pinning effect at interfaces between grain boundaries and lamellae, where charged domain walls and charged defects are coupled. These results provide a novel strategy for improving robustness of organic flexible FeRAMs, and reveal an attracting coupling effect between different phases of ferroelectric polymer.

The proton-transfer-type organic ferroelectrics are a novel class of ferroelectric materials composed of π-conjugated molecules that are linked by hydrogen bonding. The spontaneous polarization originates from the cooperative transfer of protons within the crystals as well as from the electric dipoles of π-conjugated organic molecules [1]. Many of the materials present room-temperature ferroelectricity with a much lower coercive field (10 – 20 kV/cm) than that of ferroelectric polymers (> 500 kV/cm). The soluble, lead-free, and rare-metals-free characteristics of the organic ferroelectrics allow us to expect that they could become useful components for printed electronics devices operating at low voltages [2]. Nonetheless, the appearance and dynamics of ferroelectric domains and domain walls in such a weak lattice-polarization coupling system are still unclear, although they primarily determine the ferroelectric switching characteristics.
Here we show that the variation in electro-optic response of π-conjugated organic molecules enables visualization of the ferroelectric domains and domain walls in organic ferroelectric thin films [3]. For this purpose, we adopt difference optical image sensing between the forward and reverse field applications with use of CMOS image sensor. We successfully demonstrate that highly sensitive and rapid visualization becomes possible by this technique which we call “ferroelectrics field modulation imaging (FFMI).” Based on the FFMI, we show that the unique optical-probe nature reveals the existence of two types of domain walls showing different three-dimensional orientations within the ferroelectric single-crystal films of Hdppz-Hca: one type of domain wall is roughly perpendicular to the film plane, whereas the other is considerably tilted from the normal to the plane. We consider that both of the domain walls are neutral ones that are stabilized to generate substantial neutrality by being directed parallel to the direction of spontaneous polarization. We discuss and suggest that the FFMI technique opens a route for exploring the three-dimensional topological nature of domain walls in ferroelectric materials.
[1] S. Horiuchi et al. Nat. Commun. 8, 14426 (2017).
[2] Y. Noda et al. Adv. Mater. 27, 6475 (2015).
[3] Y. Uemura et al. Phys. Rev. Applied 11, 014046 (2019).

Electrostrictive polymers with large deformation and low hysteresis are attractive for sensing, actuation, and energy-harvesting. To understand the origin for large electrostriction observed in electroactive polymers, we chose to study a variety of semicrystalline polymers, including poly(vinylidene-co-fluoride) [P(VDF-TrFE)] random copolymer, poly(VDF-co-TrFE-co-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] random terpolymer, even-numbered nylons (e.g., nylon-12 and nylon-6), poly(ether-amide) multiblock copolymers (PEBAX), and thermoplastic polyurethanes (TPUs). It is observed that uniaxiallyl stretched P(VDF-TrFE)-based polymers exhibit positive longitudinal strain upon electroactuation, whereas uniaxiallyl stretched polyamide-based polymers exhibit negative longitudinal strain. From these observations, the large electrostriction in electroactive polymers can be attributed to nanoactuation of the crystalline phases. In P(VDF-TrFE)-based polymers, nanoactuation is realized by the twisted-to-all trans conformation transformation. In polyamide-based polymers, nanoactuation is realized by the trans-to-twisted conformation transformation. This understanding will help us design new electroactive polymers for large electrostriction in practical applications.

Electrospun polyvinylidene fluoride (PVDF) has been widely investigated for energy harvesting and sensing applications due to its high sensitivity and flexibility. The electrical response of electrospun PVDF is largely attributed to the piezoelectricity of the polymer when poled and stretched to promote the β-phase. In this phase, dipolar charges are generated in the material when a force is applied due to the separation of hydrogen and fluorine atoms. However, when an air gap is present between the PVDF nanofibers and an electrode, the material can demonstrate an electrostatic response in addition to the piezoelectric response. This electrostatic response is created by surface and space charges that are trapped in the material during electrospinning, as well as triboelectric charges that are transferred between the electrode and material when in contact. The individual contributions of dipolar, trapped, and triboelectric charges in PVDF with an air gap present between the electrode and material remain unclear. In this work, each of these charge contributions is quantified and determined as a function of distance between the electrode and material, electrode material type, and duration, frequency, and load amplitude of the applied force. It is demonstrated that under certain conditions the piezoelectric response of electrospun PVDF is overwhelmed by the electrostatic response caused by trapped and triboelectric charges.

Silk obtained from the formation of the cocoon during the larval stage of the Bombyx mori moth has been used in the textile industry for more than 5,000 years. This natural resource has found a wide range of applications, e.g. textiles, air cleaning filters, drug delivery, and tissue engineering [1, 2].
Silk fibroin (SF) is a copolymer made from heavy, light chains and a glycoprotein (P25) with a molecular weight of approximately 350, 26 and 36 kDa [1, 3, 4], respectively, linked together by a disulfide bridge [3]. While the heavy chain is predominantly made of alternating hydrophobic oligopeptides, the light chains are mainly hydrophilic and contain a high concentration of glutamic and aspartic acid residues [1, 2].
SF is known to have five different molecular structures: the metastable α-helix, also known as silk I; the trans-planar beta-sheets parallel to the fiber axis (silk II) with alternate amino acids chains projected to opposite sides of the sheet; turns and bends, beta strands and the polyglycine II, where the amino acids adopt a right-handed helix with three amino acids per turn [5].
SF presents a crystalline dimorphism, where the silk II crystals have a monoclinic unit cell [6, 7], while silk I crystallizes in an orthorhombic one [6]. The lack of center of symmetry on both crystalline units suggests that silk fibroin is piezoelectric, independently of the crystalline polymorphism present in the protein. Furthermore, the first quantitative measurements of silk piezoelectricity were made by Fukada [7] and reported a piezoelectric response of ≈1 pC.N^-1, similar to the piezoelectric constant of quartz crystal (≈2 pC.N^-1) [1, 7].
Here we will present our last findings in the understanding of the origin of silk piezoelectric properties. Silk fibroin processed by electrospinning has a high degree of orientation and is readily electrically poled. The electroactive properties of the protein were assessed by piezo-force microscopy (PFM) and correlated with its crystalline structure. A simple and cost-effective structure was fabricated to demonstrate the remarkable electroactive properties of this protein for energy scavenging and harvesting. The wearable nano-harvester device shows potential to be used in real-time monitoring of human physiological signals. Moreover, this work opens up new opportunities to explore silk fibroin materials as self-powered materials for biomedical and tissue engineering applications.

Acknowledgments
This research was undertaken on the Small/wide-angle X-ray scattering beamline at the Australian Synchrotron under the project number M12973 and M13374.

References
1. Yucel, T., Cebe, P. and Kaplan, D. L, Structural Origins of Silk Piezoelectricity. Advanced Functional Materials, 2011. 21(4): p. 779-785.
2. Rockwood, D.N., R.C. Preda, T. Yücel, X. Wang, M.L. Lovett, and D.L. Kaplan, Materials fabrication from Bombyx mori silk fibroin. Nature Protocols, 2011. 6: p. 1612.
3. Inoue, S., K. Tanaka, F. Arisaka, S. Kimura, K. Ohtomo, and S. Mizuno, Silk Fibroin of Bombyx mori Is Secreted, Assembling a High Molecular Mass Elementary Unit Consisting of H-chain, L-chain, and P25, with a 6:6:1 Molar Ratio. Journal of Biological Chemistry, 2000. 275(51): p. 40517-40528.
4. Tanaka, K., S. Inoue, and S. Mizuno, Hydrophobic interaction of P25, containing Asn-linked oligosaccharide chains, with the H-L complex of silk fibroin produced by Bombyx mori. Insect Biochemistry and Molecular Biology, 1999. 29(3): p. 269-276.
5. Sutherland, T.D., J.H. Young, S. Weisman, C.Y. Hayashi, and D.J. Merritt, Insect Silk: One Name, Many Materials. Annual Review of Entomology, 2010. 55(1): p. 171-188.
6. Lotz, B. and F. Colonna Cesari, The chemical structure and the crystalline structures of Bombyx mori silk fibroin. Biochimie, 1979. 61(2): p. 205-214.
7. Fukada, E., On the Piezoelectric Effect of Silk Fibers. Journal of the Physical Society of Japan, 1956. 11(12): p. 1301A-1301A.

Electrospinning is a versatile processing technology that offers a promising route to achieve large quantities of oriented nanostructured piezo- and ferroelectric materials. The processing conditions during electrospinning open the opportunity to impact on material morphology and to improve electromechanical material properties. As a consequence electrospun piezo- and ferroelectric materials have been employed in ferroelectric memory, strain and pressure sensors as well as energy harvesters. Despite this interest, the detailed mechanism of how nano-confinement and high electric fields during electrospinning impact on material properties is yet to be discovered.
Here we prepare PVDF-TRFE nano-fibres at different electrospinning conditions. We compare macroscopic electromechanical response of fibre mats with microscopic analysis performed by PFM switching spectroscopy and electrostatic force microscopy on single fibres.¹ Our results show that while electrospinning favors the formation of the piezoelectric b-phase in the polymer, the electric field is not strong enough to achieve piezelectric domain polarization. Instead we observe accumulation of space charge in the polymer that explains the more electret like macroscopic electromechanical response.

1. Zaccaria, M. et al. Electret Behavior of Electrospun PVdF-based Polymers. IEEE Conf. Electr. Insul. Dielectr. Phenom. 137–140 (2016).

Poly(vinylidene fluoride) (PVDF)-based dielectric materials are prospective candidates for electric high power density energy storage applications because of their ferroelectric nature, high dielectric breakdown strength and superior processability. However, obtaining a polar phase with relaxor-like behavior in PVDF, as required for high energy storage density, is a major challenge. To date, this has been achieved using complex and expensive synthesis of copolymers and terpolymers or via irradiation with high-energy electron-beam or γ-ray radiations. Herein, a facile process of pressing-and-folding (P&F) is proposed to produce ferroelectric β-PVDF with relaxor-like behavior, without the need of any hazardous gases, solvents, electrical or chemical treatments. This simple and scalable processing route generates unprecedentedly high β-phase content (~98%) and relaxor-like behavior in PVDF with high molecular weights (M_w) (>534 kg mol^-1). An ultra-high energy density (35 J cm^-3) with a high efficiency (74%) is achieved in a P&F PVDF (M_w: 670-700 kg mol^-1), which is the highest value ever reported for a polymer-based dielectric capacitor.

Integration of piezoelectric materials into flexible electronics systems can enable enhanced functionalities in sensing and actuation. An example of this is the use of ferroelectric polymers to form flexible audio speakers. This talk will discuss the development of a flexible hybrid electronics (FHE) technology platform that combines printed interconnect, printed passive components, and discrete silicon chips with a bimorph PVDF-based speaker. The bimorph configuration is shown to produce increased sound pressure levels (SPL) over a single-layer device. The audio system has been demonstrated using two flexible circuits that include ink-jet-printed resistors developed for long-term reliability. One circuit amplifies the output of a portable music player and feeds it to the speaker. The second is a record-and-playback circuit designed around a dedicated audio integrated circuit. In both cases, the audio is clearly intelligible.

The introduction of IoT (Internet of Things) has created an unprecedented demand toward the development of self-powered and high-performance sensors, among which piezoelectric materials are of great use in various important fields due to their unique properties such as high sensitivity, mechanical stability, energy harvesting and fast response time. Despite recent advances, the current state-of-the art solutions are still primarily based on brittle ceramic materials and hence cannot be easily incorporated into existing flexible electronic devices. In addition, the large-area processing of the modern advanced materials still poses a challenge.

Here, we report the roll-to-roll production of dual-functional high-performance piezoelectric pressure sensors and loudspeakers via electric field alignment of PZT (Lead Zirconate Titanate) and Graphene Nanoplatelets (GNPs) in a silicone-based polymer matrix. The alignment of particles in the thickness direction (“Z” direction) has not only shown to dramatically increase the piezoelectric performance, but also improves the transparency of the system by creating depletion zones where light can travel with less scattering. The ultra-sensitivity of the system has been demonstrated by the detection of a small bird feather (1.4 mg) dropped from a certain distance. The material can also generate up to 8.2 V signal when the pressure is applied by the touch of a fingertip.

Apart from its outstanding sensing capabilities and response time, the exact same system can also be used as a transparent, flexible and thin film loudspeaker when a voltage is applied across the material. To better understand the material’s performance and capabilities, sound pressure level (SPL) of the loudspeaker has been characterized across the entire range of human hearing (20 Hz-20 kHz) along with the sound directivity measurements at certain angles in an anechoic chamber. The potential applications of this system can range from built-in speakers (e.g., TV, computer or mobile phone screens) to active noise cancellation systems.

Finally, a large area sample (10 ft long and 6-inch-wide) has been produced to demonstrate the scalability of the manufacturing process via a novel custom-built 44 ft long roll-to-roll production line which was also designed by our group.

Artificial touch and pressure sensing have become a critical technology. New generation neural implants with multifunctional, high resolutions minimally invasive brain-machine interfacing techniques made it possible to control external devices by assigning them to corresponding cortical networks in the brain. Researchers have shown that it is also feasible to interface a touch-sensitive pressure sensor with somatosensory cortex to transfer the touch feeling to the human brain. Such superior multi-disciplined studies converging both medical and engineering skills are the next milestones of technology shortly. State of the art technological requirements drives engineers to create robotic arms, skin-like functioning electronics and versatile sensors necessary to cure the disabilities and deformations in animals and human. Although researchers have shown a great success for mimicking the motion systems, they have yet to show the same performance to explore sensor tools entirely.
Here, we developed piezoelectric fiber-based, large area, transparent, modular, and flexible devices for small pressure sensing and cardiac activity recording. Nano-structured piezoelectric fibers were produced using a two-step thermal size reduction (TSR) technique. In TSR, we start with designing a macroscopic preform which works as a template for the resulting fibers. Our initial preform contains a poly(vinylidene fluoride trifluoroethylene) (PVDF-TrFE) slab made from powders using a hot press at ~20 Barr pressure and above 90°C. This preform was embedded in polysulfone (PSU) protective cladding and drawn in fiber tower. Resulting fibers were between 10 µm to 1 mm. We stacked and stretched the first step fiber in a second PSU preform in order to obtain < 400 nm structure. TSR method simultaneously orients PVDF-TrFE molecules along the fiber length and creates dipoles perpendicular to the molecular chains. Piezoelectric characterizations have shown that the d33 constant of these fibers is exceptional (> 300 pm/V). This performance metrics have proved to be enhancing the performance of large area piezoelectric devices. We demonstrated a proof principle energy harvesting device, larger area electronic skin, an arterial pressure measurement system and an anthropometric artificial hand mimicking the human neural pressure measurement mechanism. We expect this work will find a broader range of application including smart and nano-textile, structural health monitoring, implantable high precision sensors.

Ferroelectric polymers from the PVDF-family have proven to be multifunctional and self-sustaining materials with a broad deployment in printed and flexible electronics. They can be used in large and flexible form factors for detecting mechanical excitations such as pressure variations, force touch and impact, for sensing human-body radiation and proximity, as vibration sensors for structure-borne sound detection and acoustics, as stretchable vital parameter sensors for movement, heart and respiratory rate monitoring, as well as piezoelectric energy harvesting elements, just to name a few [1].
Our multimodal ferroelectric polymer sensors are entirely fabricated by screen printing which is one of the most common techniques used in printed electronics, since it is highly tolerant to the type and form factor of substrates, the rheology of ink materials, provides sufficient alignment accuracy for multilayer printing and can be done in a sheet-to-sheet or roll-to-roll schemes.
By optimizing the design, the printing and annealing process as well as the poling conditions and the source material, functional sensors with a yield of more than 98% with less than ± 5% deviation in the performance (e.g. remanent polarization) were demonstrated. It is also interesting to note that extended aging tests under definite climate and shock conditions revealed more than 98% preservation of the remnant polarization for high molecular weight PVDF-TrFE polymers. The sensors come in two versions; (i) either they have a sandwich-type structure of three layers that are printed onto a flexible or stretchable substrate (e. g. plastic films, paper and textiles) and response accurately, fast and reproducibly to pressure and temperature changes over large dynamic ranges or (ii) a two layer structure that is predominantly sensitive to lateral strain and vibrations and provides accurate direction sensitivity.

First a short overview of applications of printed PVDF-TrFE sensors will be given covering flexible 3D user interfaces [2,3], large-area force, impact and proximity sensors, ultrathin object-integrated microphones as well as medical patches.
One focus will be the realization of a concept for direction-sensitive and flexible strain sensors based on the ferroelectric copolymer P(VDF:TrFE) with a single layer of interdigitated embedded electrodes. The process employed for producing the embedded electrodes is based on a combination of (self-aligned) photolithography and electroforming or, alternatively, on capillary force driven filling of microfluidic channels hot embossed into P(VDF:TrFE) with conductive ink.
Finally, a novel printable nanocomposite material, which allows reducing the cross-sensitivity between the pyro- and piezoelectric sensing modes, will be presented. This material is composed of inorganic ferroelectric nanoparticles blended in a ferroelectric polymer matrix. By exploiting the fact that the piezoelectric coefficient in inorganic ceramics has an opposite sign to that one of the ferroelectric polymer either the piezo- or the pyroelectric activity can be suppressed by independently defining the poling direction of particles and matrix in a clever poling procedure [4].

[1] B. Stadlober, M. Zirkl, M. Irimia-Vladu, Chem. Soc. Rev.48, 1787 (2019)
[2] M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, P. Bodö, P. Andersson Ersman, D. Platt, S. Bauer, G. Domann, and B. Stadlober, Adv. Mat. 23, 2069 (2011)
[3] C. Rendl, D. Kim, P. Parzer, S. Fanello, M. Zirkl, G. Scheipl, M. Haller, S. Izadi, FlexCase: Enhancing Mobile Interaction with a Flexible Sensing and Display Cover, Proceedings of the 2016 CHI Conference on Human Factors in Computing Systems, San Jose, California, USA, pp. 5138-5150, doi: 10.1145/2858036.2858314 (2016)
[4] I. Graz, M. Krause, S. Bauer-Gogonea, S. Bauer, S. P. Lacour, B. Ploss, M. Zirkl, B. Stadlober, and S. Wagner, J. Appl. Phys. 106, p. 034503 (2009)

A flexible Piezoelectric nanogenerators(PNGs) based on polymer-ceramic composite materials are promising candidate for energy resources to provide power sources for portable and personal electronics because of their inherent capability of combining the favorable properties of both ceramic and polymer materials. In this work, we report high-performance flexible piezoelectric nanogenerators (PNGs) based on a composite thin film, comprising of amine-functionalized PZT nanoparticles (NPs) and a thermoplastic triblock copolymer grafted with maleic anhydride. A high-performance flexible PNG device based on a chemically strengthened system compose aminefunctionalized PZT NPs (PZT-NH₂ NPs) with a thermoplastic triblock copolymer without additional dispersion enhancers. The generated output voltage and current reached 65 V and 1.6 mA, respectively, sufficient to drive commercial LEDs. The PZT-NH₂ NP-based flexible PNG exhibited remarkable stability without degradation over 5000 cycles of mechanical deformation. By using a full-wave bridge rectifier circuit, the alternating output energy generated from the PNG device was stored in capacitors and subsequently used to power a commercial LED device. Our composite system successfully overcomes the dispersion stability-related restrictions of previous nanogenerators, and also our proposed approach is an unprecedented improvement in the composite-based PNG research for self-powered flexible energy source to realize the wearable electronics and sensors in ubiquitous wireless communication.

High dielectric and low energy loss polymers are of great interest for electrical components and energy storage devices, as they enable high power density and energy density while maintaining dielectric strength. While much of the work in this area has focused on ever increasingly complex donor and acceptor units in the backbone of polymers, this work incorporates the conjugated species, with highly polarizable end groups, as the polymers’ side chains. Synthesis of a flexible polymer with precisely sequenced phenylene vinylene pendant groups provides improved control over the side chains’ alignment, and the local dipole moment, while poling. Strategic control of the side chains’ sequence produces a material with tunable dielectric behavior and capacitance.

Simultaneous application of uniaxial stretching and electrical poling made the electrospinning process as a suitable method for preparing poly(vinylidene fluoride) nanofibers (PVDF NFs) containing β-crystalline phase. To facilitate the rapid and large scale preparations, flow rate (FLr) of electrospinning must be optimized. In this regard, the efforts made in the literature were lacking in terms of systematic evaluation of crystalline composition, mechanical properties, and the assessing the role of residual solvents during annealing. In the present investigation, an attempt has been made to systematically evaluate the FLr on these properties. A 15% PVDF solution was used for this study. A 70:30 (v/v%) mixture of N, N’-dimethylformamide and acetone mixture was used as the solvent. First, at the FLr of 5 ml/h and the nozzle voltage of +20 kV, the voltage applied to collecting roller (CR) was optimized to control the loss of fibers due to web drifting. Application of -20 KV to CR was fully controlled the web drifting and favored the collection of nearly all fibers ejected from the nozzle. Primarily, NFs were prepared at the FLr of 2.5, 5.0, 7.5 and 10 ml/h. Field-emission scanning electron microscopic studies confirmed that the NFs prepared at 2.5 ml and above had the average diameter of 466 and 225 nm, respectively. The composition of crystalline phases were evaluated using the Fourier-transform infrared (FTIR) spectroscopy. The integral area under the characteristics peaks of α (975 cm^-1), β (1275 cm^-1) and γ (1275 cm^-1) were taken for consideration. The as prepared NFs, showed ~ 75% of β-phase, the rest 25% was shared by α- and γ-phases. Upon annealing at 150 °C, the NFs prepared at 2 ml/h found to be the best and contained 90% β-, 2.5% α- and 7.5% γ-phases. Differential scanning calorimetry, and X-ray diffraction studies were proved the composition of crystalline phases identified in FTIR studies. As observed in thermogravimetric studies, in as prepared NFs, the peak degradation temperature and residue content were higher for NFs prepared at 2.5 ml/h (466 °C and 32.5%) and the annealed NFs did not show any drastic change. Further, NFs will be prepared up to the maximum achievable flow rate (40 ml/h), the mechanical and piezoelectric characteristics of representative samples will be evaluated.

We optimized the module structure based on the lead-free piezoceramic nanofibers to improve flexible energy harvesting characteristics. Flexible lead-free piezoceramic nanofibers were prepared by the electrospinning process, and then flexible energy harvester modules based on piezoelectricity were fabricated through WIP process. The shapes, intervals and widths of electrodes in the flexible piezoelectric single modules were controlled, and the energy harvesting characteristics of the modules were investigated according to the oscillation frequency and the bending motion. Furthermore, the displacement and vibrational mode shapes of the flexible modules were analyzed using a laser scanning vibrometer. The flexible piezoelectric dual modules were fabricated according to x- and z-axis array arrangements of the single modules with the optimized electrode structures, and were connected to serial and parallel methods. The energy harvesting properties of the optimized dual modules were measured at various running speeds, after directly attached to the shoes. These results confirmed that the flexible piezoelectric modules can be applied as a sub-power source for wearable devices.

Wearable skin electronics is a next generation product that monitor biological signals from the human to the electronic devices. Numerical technological advances have been reported for thermal, strain and chemical sensors on the skin and for neuron-stimulated soft electronics on the brain. Even we could have a lots of wearable devices on the human skin, the final question is “the power supplies from where? on the skin”. To overcome this issue, we have developed two types of wireless power generation devices: one is enzymatic power generation from biofuels, especially glucose in sweat or blood, and the other is wireless power transfer system between an eyeglass/contact lens. To generate a power on the skin, we used two types of enzymes as biocatalysts and then developed enzyme/CNT hybrid electrodes: bioanode for glucose oxidation and biocathode for oxygen reduction. In contrast, the wireless power transfer system is based on two types of inductance(L)-capacitance(C) resonant circuits at the resonant frequency of 13.56 MHz. In the both cases, we demonstrated a LED lighting on the skin or on the eye surface.

Organic ferroelectrics can find application in a broad range of fields. They might, for instance, solve the ‘missing memory’ problem in printed electronics. However, a full understanding of their switching kinetics on all length and time scales is still lacking. A variety of computational models have previously been used to tackle this problem and to study different aspects of both inorganic and organic ferroelectrics. Unfortunately, these are usually restricted to idealized morphologies or short time scales.
In contrast, we have developed an electrostatic kinetic Monte Carlo model that can reproduce the static and kinetic ferroelectric properties for realistic 3D morphologies on experimental time scales. We apply this model to the prototype small molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA).
We simulate hysteresis loops, depolarization curves, and switching transients, and find good agreement with experiments. The simulations provide molecular-scale insight in the thermally activated nucleation-limited switching behavior that was also observed in experiments.
Specifically, we investigate the effect of structural disorder on the ferroelectric properties. When the disorder in the system is increased, the retention time decreases dramatically, while the coercive field remains mostly unchanged. This is due to different nucleation mechanisms: field-driven switching nucleation happens at the contacts, while for spontaneous depolarization nucleation occurs at disorder-induced defects. The switching transients become highly dispersive in the presence of disorder. We find that especially the presence of chiral defects has a retarding effect on the switching kinetics, suggesting chiral ferroelectrics might provide sub-µs switching times.
Our results thus provide a detailed insight in (the limiting factors in) polarization switching processes on experimental length and time scales. Furthermore, we have shown that reducing the disorder can lead to an organic ferroelectric with moderate coercive field, long retention, and fast switching.

With an increasing demand for cheap, efficient, lightweight and bendable electronics, a matching memory technology is required. Ferroelectricity-based memory devices assure non-volatility and low energy consumption, while organic materials, potentially used for this purpose, guarantee easy processability, low cost and advantageous physical properties including flexibility. A recently discovered class of organic self-assembling supramolecular ferroelectrics has emerged as a promising candidate for memory applications [1], [2], yet it lacked interest from a device perspective due to typically high operation temperature, modest remnant polarization and/or poor polarization retention. Here, we show that rational design and optimization of the molecular structure leads to advanced supramolecular materials that not only demonstrate excellent ferroelectric properties but also semiconductivity, opening new application possibilities arising from the hybridization of both phenomena [3].

The synthesized molecules are based on a C3 symmetry core-shell template, comprising three equally important structural units: a planar π-stacking core, to which three strong dipolar amide groups (O=C-NH) are attached, that are followed by flexible solubilizing alkyl (-CH₂-)_n chains. While the conjugated cores and the hydrogen-bonding amide moieties are responsible for the axial self-assembly and stabilization of the columnar structure, the mesogenic sidechains separate the columns and provide sufficient molecular mobility to flip the columnar macrodipole, i.e. facilitate ferroelectric switching. Consequently, a charge transport channel is formed at the centre of the column, which is surrounded by directional and switchable polarization field.

To obtain structure-property relations we tested molecules with varied tail substitutions as well as different molecular cores, both experimentally and in computer simulations [4], [5]. We find that, and rationalize why, all key properties of the ferroelectric – remnant polarization, coercive field, operational temperature range, polarization switching time and data retention – depend strongly on these molecular modifications. As a result, ferroelectric and ferro-semiconductive materials with high remnant polarization (~60 mC/m²), low electrical fatigue, temperature stability surpassing P(VDF:TrFE) and polarization retention exceeding the 10-year requirement for commercial memory devices even at high temperatures, are unearthed. The conclusions of this structural optimization study provide guidelines for further advancement in molecular engineering of self-assembling multifunctional materials.

[1] A. V. Gorbunov et al., “Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity,” Sci. Adv., vol. 3, no. 9, p. e1701017, Sep. 2017.
[2] A. V. Gorbunov et al., “Switchable Charge Injection Barrier in an Organic Supramolecular Semiconductor,” ACS Appl. Mater. Interfaces, vol. 8, no. 24, pp. 15535–15542, Jun. 2016.
[3] N. M. Casellas et al., “Resistive switching in an organic supramolecular semiconducting ferroelectric,” Chem. Commun., 2019.
[4] I. Urbanaviciute et al., “Tuning the Ferroelectric Properties of Trialkylbenzene-1,3,5-tricarboxamide (BTA),” Adv. Electron. Mater., vol. 3, no. 7, p. 1600530, 2017.
[5] I. Urbanaviciute et al., “Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric,” Phys. Chem. Chem. Phys., vol. 21, no. 4, pp. 2069–2079, 2019.

Neuromorphic computing has attracted much attention due to its power-efficient data processing. To realize efficient neuromorphic hardware systems, the development of reliable and robust synaptic devices is essential. However, the limited performance of the synaptic devices remains as a challenge for realization of neuromorphic hardware systems. Ferroelectric materials have a great potential to meet the requirements for synaptic devices. Here, we report analog memory characteristics in the ferroelectric thin-film transistor (FeTFT) based on oxide semiconductor and ferroelectric hafnium oxide layer [1]. The conductance of the oxide semiconductor channel is modulated by controlling the polarization of the ferroelectric hafnium oxide layer. Conductance modulation characteristics of FeTFT are used to demonstrate potentiation and depression characteristics. The desired synaptic device characteristics, such as linear weight update, small variation, and multiple states are achieved in FeTFTs. In addition, the possibility of FeTFT as a synaptic device in neural network is evaluated by artificial neural network simulation. In simulations, the neuromorphic system based on FeTFTs achieves 91% recognition accuracy for handwritten digits. This work can offer a possibility for FeTFT as a synaptic device to be used in neuromorphic hardware systems. In this presentation, analog memory characteristics of FeTFT for neuromorphic applications will be presented in detail.
[1] M.-K. Kim and J.-S. Lee, Nano Letters 19, 2044-2050 (2019).

The class of active polymers is gaining an increasing attention since it combines the appealing mechanical properties of polymers of strength and toughness to weight ratio with additional functionalities such electrical, magnetic, or luminescence. For example, a subset of electroactive polymers exhibit electromechanical coupling, where an application of electric field results in a mechanical deformation and vice versa. The latter coupling situates this class of materials for sensing and actuation applications. Despite the appealing attributes and potential application, characterization protocols are limited to the macroscale when it comes to the concurrent interrelationship between mechanical and electrical energies. Terahertz Time Domain Spectroscopy (THz-TDS), on the other hands, is capable of elucidating the simultaneous intrinsic electromechanical coupling since the energy below the activation energies of polymers that are also transparent to terahertz waves. In other words, the propagation of terahertz waves becomes intimate with the polymer continue, hence the reported frequency spectrum inherit the intermolecular vibrations resulting from either applying electrical field or mechanical stresses. In this presentation, the electromechanical response of standalone, ultrathin Poly (vinylidene fluoride-co-trifluoroethylene) film fabricated using spin coating technique. The frequency spectra of propagating terahertz waves are compared before, during, and after the application of varying electric field and mechanical stresses.

Ferroelectric polymer poly(vinylidene fluoride) (PVDF) and PVDF based polymer nanocomposites have received booming interest for film capacitor applications due to their relatively high permittivity at approximate. However, the addition of highly polarized fillers with a high proportion into PVDF leads to poor film processability and high energy loss, which significantly destory the structural integrity of the film and limits the pratical application of film capacitors. Herein, we prepared a unique core-shell structured GO@BaTiO₃ nanoparticle by a simple and continuous process, and then obtained the nanoparticle filled PVDF films (GO@BTO/PVDF), which exhibited excellent energy storage performance. BaTiO₃ nanoparticles were successfully wrapped in crumpled graphene oxide(GO), and the core-shell structured GO@BTO nanoparticles were well dispersed in PVDF film, which was demonstrated by transition electron microscopy(TEM) and scanning electron microscopy(SEM). Compared with PVDF films, the GO@BTO/PVDF films with a ultralow filler content of 0.8wt% show a high discharged energy density at 225 MV/ m, which is 40% higher than PVDF films, as well as a 15% enhancement of discharged efficiency at 225 MV/m. These superior performance is resulted from the interfacial engineering based on GO. GO is capable to reduce the huge difference between polymer and ceramic particles, making the film have less defective and present higher discharged efficiency. Moreover, the ultralow content of fillers can greatly improve the film processability, maintaining the structural integrity and performance stability of the films during stretching. This work provides an effecient strategy to enhance the energy conversion efficiency.

Triboelectric nanogenerators (TENG) are appealing mechanical energy harvesting devices that could power small portable devices and detectors.^[1] They can also act as self-powered sensors for mechanical displacement, chemical, acoustic sensing, and biomedical monitoring. The working principles of TENG devices are based on friction-related electrification – triboelectrification. Some of the recent works report that TENG output can be improved by using polarized ferroelectric films as the contacting layers.^[2] In the present work, porous polyvinylidene fluoride (PVDF) and BaTiO₃ nanocomposite films (BaTiO₃ content in a range from 0 to 35 vol%) have been prepared using spin-coating and consecutive immersion-precipitation. A clear correlation between the piezoelectric response of inversely polarized ferroelectric PVDF/BaTiO₃ nanocomposite films and the performance of the TENG device based on these films is demonstrated. Nanocomposite films with the optimal composition (25 vol% BaTiO₃) show a 48 pC N⁻¹ piezoelectric coefficient, considerably higher than usually reported for such composites, and in TENG mode a record-high 2.7 kV open-circuit voltage is achieved. This observation is explained by magnified electrostatic induction that is driven by the piezoelectric charges and ferroelectric properties of these films. However, our results also indicate that piezoelectric charges cannot be solely responsible for great TENG performance; thus, a “double capacitor model” has been introduced. The mechanism involves interaction between two charged ferroelectric layers during contact−separation and contacted inversely polarized layers can be considered as capacitors connected in series. Air gap formation during separation rapidly decreases the total capacitance while the potential difference increases. The induced charge redistribution in the external circuit is registered as a current. Predictions of our model also hold true when experimentally obtained charge density (6.55 nC cm⁻²) values are compared with ones obtained from model calculations (6.60 nC cm⁻²). Furthermore, findings uncover the potential for vast improvement in the field of nanogenerators for mechanical energy harvesting as a significantly better piezoelectric performance of flexible nanogenerators has been reported elsewhere. As triboelectric research continues to thrive and expand, we anticipate that our work will help the TENG community to further improve the performance of ferroelectric-based TENG devices.

[1] Z.L. Wang, ACS Nano, 2013, 7, 9533-9557.
[2] A. Šutka, K. Mālnieks, A. Linarts, M. Timusk, V. Jurkāns, I. Gornevs, J. Blums, A. Berzina, U. Joost, M. Knite, Energy Environ. Sci. 2018, 11, 1437-1443.

The contact electrification of polymer interfaces opens the possibility to harvest energy from triboelectric (nano)generators (TEG). The electron transfer between contacted-separated surfaces has been considered as the main electrification mechanism for polymers in TEG. The electron transfer mechanism requires to contact chemically different materials, as well as to increase specific contact area, which has been accomplished via nano-structuring. Here we show that contact electrification can be controlled by intramolecular forces in polymer bulk and adhesive forces at contact interface. Results also confirm covalent bond breaking as a mechanism for contact electrification of polymer insulators. By taking in account findings mentioned above, we produced a TEG device using soft thermoplastic styrene-ethylene-co-butylene-styrene (SEBS) block copolymer, which had shown the highest surface charging. We modified the surface of SEBS to make it more adhesive by treating the surface with hydrogen peroxide solution. As a result, we observed increase in separation stress by 38% from 2.97 N/cm² to 4.11 N/cm². The increased adhesion of peroxide treated SEBS surfaces can be attributed to formation of quasi-free ends of macromolecular chains. As expected, for modified SEBS surface charge increased 3.14 times from 0.92 nC/cm² to 2.89 nC/cm².

To study the applications for ferroelectric non-volatile memory and ferroelectric memristor, etc, post-annealing process dependent electric the properties of (Hf, Zr)O2 thin films by RF sputtering deposition method were investigated. The bottom electrode was TiN thin film to produce stress effect on the formation of orthorhombic phase and top electrode was Pt thin film by DC sputtering deposition. Post-annealing process, for example , post-annealing temperature, annealing gas, annealing time, etc, was varied along with the same other deposition conditions, for example, sputtering power, target to substrate distance. The structural and electric properties of the above thin films were investigated. As a result, it is confirmed that the electric properties of the (Hf, Zr)O2 thin films depend on the post-annealing process which affects structural properties of the thin films, such as, structural phase, ratio of the constituents, etc.

The nanocomposite of ZnO nanorods and ZnO vertically aligned nanorods with polyvinyledene fluoride (PVDF) are prepared via a solution casting method to make flexible and cost-effective piezoelectric nanogenerator. The structural and surface morphological studies of nanocomposite films have been carried out by X-ray diffraction (XRD) and Scanning electron microscopy (SEM) respectively. The crystalline polar β-phase of nanocomposite films has been evaluated by Fourier transform infrared spectroscopy (FTIR). The results showed enhancement in this phase from 53% in PVDF to 80.6% in PVDF-ZnO vertically aligned nanorods (VANRs) nanocomposite films, respectively. The electric polarization studies showed the maximum value of remnant polarization P_r is 0.2188 μC/cm² for PVDF-ZnO VANRs nanocomposite film, measured by Polarization-electric field (P-E) hysteresis loop. The value of output voltage for PVDF-ZnO VANRs is found to be 46.64 V which is more than seven times in comparison to PVDF-ZnO nanorods (NRs) based nanocomposite film generated by repeated human finger imparting on these nanocomposite films. The value of power density obtained for PVDF-ZnO VANRs is 45.87 μW/cm² at the load resistance of 15 MΩ, which can be used to drive energy to small-scale electronic appliances.

Polymeric piezoelectric nanogenerators (PNGs) have emerged as a suitable candidate to harvest waste mechanical energy to power up portable electronic devices. Smart textiles, with piezoelectric functionalities integrated into the fabrics, have been envisioned. In this regards, numerous piezoelectric (nano-) generators based on PVDF or P(VDF-TrFE) nanofibers have been reported.
However, the piezoelectric polymer (nano-)generators have typically shown low output energy densities; A the common issue hindering their application.
Introduction of porosity into the piezoelectric polymer has been proposed to increase the voltage output of PNGs. However, designing a process that allows the introduction of pores in polymer fibers with a typical diameter of the order 100 nm, would be a breakthrough in the field of PNGs. In this contribution, we discuss an elegant approach to tailor porosity in electrospun P(VDF-TrFE) nanofibers. The approach is based on the thermodynamics of polymer solutions, and solvent/non-solvent interactions with the polymer. We calculated the ternary phase diagram of P(VDF-TrFE)/ non-solvent (water)/solvent, and experimentally verified it. Based on the phase diagram, a conscious amount of water is intentionally added into the P(VDF-TrFE) solution to induce porosity in the fiber. PNGs based on the porous electrospun P(VDF-TrFE) nanofibers show systematic increases of the output voltage with porosity. The output power increased from 0.1 mW/cm³ for PNGs with zero porosity to 7 mW/cm³ for PNGs with 50% porosity. Dielectric spectroscopy of the nanofibers attributes the enhanced output to the reduced dielectric permittivity of the fibers and that the voltage generation in the porous fibers is of the same origin as in neat piezoelectric P(VDF-TrFE) films and is due to the relaxation of segments within the restricted amorphous phase.

Trialkylbenzene-1,3,5-tricarboxamide (BTA) is an archetypical amide-based liquid crystalline ferroelectric. It is a C₃ symmetry discotic molecule with a benzene core, to which three amide groups, having a dipole moment of around 3 Debye each, are attached. These amides form intermolecular N H–O hydrogen bonds that induce molecular self-assembly into columns with a triple-helix H-bonded network. Microdipoles within this system add up to a net axial macrodipole in a cooperative manner. The π-π stacking of the benzene cores and packing of the tails further stabilize the molecular arrangement, which allows for unhampered dipole-flipping. When flexible mesogenic peripheral tails are attached, BTA enters a well-ordered liquid-crystalline columnar hexagonal phase, which facilitates rotation of the macrodipole and thus ferroelectric behavior. The length and the nature (linear or branched) of the tails can play a crucial role in the data retention as well as in the growth of the coercive field.
Molecular simulations combining Density Functional Theory (DFT) and Molecular Dynamics (MD) are a precious tool to understand and rationalize the behavior of BTA liquid crystalline ferroelectric at the molecular level. We were able to decipher the detailed nature of the switching of the amide groups under an electric field and how those changes between the different possible conformations influence the helicity of the stack. [1] Moreover, the influence of the side chains were investigated. We showed that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/ inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs in comparison with linear-tailed BTAs. [2] Finally, we were able to demonstrate a pronounced negative piezoelectric effect in the family of BTA molecules, which can be tuned by mesogenic tail substitution and structural disorder. [3]

[1] Tim D. Cornelissen, Michal Biler, Indre Urbanaviciute, Patrick Norman, Mathieu Linares, Martijn Kemerink, Phys. Chem. Chem. Phys., 2019, 21, 1375-1383

[2] Indre Urbanaviciute, Subham Bhattacharjee, Michal Biler, Jody A. M. Lugger, Tim D. Cornelissen, Patrick Norman, Mathieu Linares, Rint P. Sijbesma, Martijn Kemerink, Phys. Chem. Chem. Phys., 2019, 21, 2069-2079

[3] Indre Urbanaviciute, Xiao Meng, Michal Biler, Yingfen Wei, Tim D. Cornelissen, Subham Bhattacharjee, Mathieu Linares, Martijn Kemerink, Mater. Horiz., 2019, DOI: 10.1039/C9MH00094A

Doping of molecular crystals is a promising avenue to design of functional materials. Stereospecific occlusion of guest molecules in host crystals reduces their symmetry by creation of polar domains, engendering macroscopic polarization and thereby inducing pyroelectricity and piezoelectricity. We have investigated the possibilities of stereospecific doping of non-polar molecular crystals comprising of molecules with a large dipole moment, in particular, crystals of amino acids: (i) asparagine monohydrate(ASN) doped with aspartic acid (ASP) and (ii) glycine (GLY) doped with L-alanine (ALA), L-threonine(THR), L-serine (SER), L-allo-threonine (ATH), L-phenylalanine (PAL), L-glutamic acid (GLU), and L-tyrosine (TYR).
We show that the macroscopic polarity in such crystals has two contributions: a) the polarity arising from the difference between the dipole moment of the guest molecule with respect to the dipole moment of the host molecule; and b) the polarity induced by distortion of the local environment pushing the host molecules out of perfect mutual compensation, leading for the case of zwitterionic amino acids (dipole moment of ≈14 D), to particularly large polarization.
For the case of ASN-ASP, the concentration dependence of the pyroelectric coefficient shows three distinctively different regions. Below 8 mol%, the pyroelectric coefficient, a, increases linearly with ASP concentration, C , indicating that the dopant molecules do not interact and their polarity is additive. This allows to introduce a pyroelectric coefficient per molecule a_m=d(a)/d(C)/(r×q) = 0.005 pm/K, where r is molecular density (cm^-3) and is the elemental charge. Between 8 and 12 mol% ASP, there is a sharp increase in the pyroelectric coefficient reaching at its peak 0.18 nC/(cm²×K) and a_m=0.016 pm/K, indicating strong interactions between the polar domains. Above 12 mol% ASP, the pyroelectric coefficient decreases indicating anti-polar domain arrangement. The XRD data on macroscopic symmetry support the data on the pyroelectric coefficient.
For the case of doped GLY, although dopant solubility is limited to <1 mol%, the pyroelectric coefficient reaches a=0.02 nC/(cm²×K), leading to a_m =0.001-0.005 pm/K. Two substantially different types of temperature dependence of the pyroelectric coefficient were observed. For ALA, THR, ATH and PAL-doping, the pyroelectric coefficient remains negative with respect to the crystal growth direction in the range 0-100 °C. For SER, GLU and TYR-doping the pyroelectric coefficient changes from positive at 0 °C to negative at 100 °C. So far, this is the only example, not associated with a phase transition, when a pyroelectric coefficient crosses a zero point. We attribute this behavior to the different temperature dependence of the guest and host polarization.
Our experiments demonstrate that stereospecific doping of centrosymmetric crystals comprising polar molecules is a viable route to design functional polar materials.

We present examples of computational approaches for studying organic ferro-, antiferro- and piezoelectric materials.
The first example is a study about the origin of exotic ferroelectricity in tetrathiafulvalene-p-chloranil (TTF-CA). As reported by Kobayashi et al. [1], its polarization (6.3 μC/cm²) is more than 20 times larger than that expected by the point charge model (PCM), and their directions are opposite. TTF-CA is in strong contrast to typical inorganic perovskite ferroelectrics. Recently, we proposed a procedure to analyze the polarization mechanism within the framework of maximally-localized Wannier orbitals and applied it to TTF-CA as well as to perovskite transition metal oxides (BaTiO₃ and PbTiO₃) [2,3]. For TTF-CA, only 2 bands below the band gap are responsible for the emergence of polarization, while, for BaTiO₃ and PbTiO₃, even relatively-deep states show significant contributions. The polarization has been decomposed into three contributions: (1) the PCM, (2) the local polarization (LP) and (3) the electron flow (EF) from cell to cell. It has been revealed that the EF term amounts to more than 100% for TTF-CA, 68% for BaTiO₃, and 56% for PbTiO₃ of the total polarization. As for the LP term, it is nearly zero for TTF-CA or BaTiO₃ while 12 % for PbTiO₃. The valence top is derived from the anion (oxygen) sates for BaTiO₃ and PbTiO₃ while it is from the cation (TTF) states for TTF-CA. Consequently, the EF term has the same sign as the PCM term in BaTiO₃ and PbTiO₃, whereas the EF term has the opposite sign to the PCM term in TTF-CA. For TTF-CA, as mentioned above, the predominant contribution to the polarization is from the EF term. This is the reason for the opposite polarization direction to that expected by the PCM in TTF-CA. Thus, the distinct polarization mechanism for TTF-CA has been figured out.
The second example is a simulation of the antiferroelectric-to-ferroelectric transition in squaric acid (SQA). Recently, in SQA, we observed experimentally, the transition from an antiferroelectric phase to a ferroelectric phase in the presence of a strong electric field [4]. Furthermore, we proposed two possible ferroelectric phases (FE-α and FE-β) with their molecular arrangements and space groups. The experimentally observed ferroelectric phase is thought to be the FE-α phase. On the other hand, the FE-β phase could not be practically achieved because of the dielectric breakdown of crystals. We simulate the antiferroelectric-to-ferroelectric transition [5] by computationally applying a static electric field [6]. Depending on the direction of the electric field, two different metastable ferroelectric (and piezoelectric) phases have been found. One of them corresponds to the experimentally confirmed FE-α phase, whereas the other corresponds to the FE-β phase, which has not yet been experimentally confirmed. The structural details of these phases have been determined as a function of the electric field. The spontaneous polarization values of the phases are 14.5 and 20.5 μC/cm², respectively, and are relatively high among those of the existing organic ferroelectrics. Significant converse piezoelectric effects are observed for both the phases (∼10 pm/V).
Some other examples (e.g. ferroelectrics based on proton tautomerism such as croconic acid and benzimidazoles [7]) will also be presented.
[1] K. Kobayashi, S. Horiuchi, R. Kumai, F. Kagawa, Y. Murakami and Y. Tokura, Phys. Rev. Lett. 108, 237601 (2012).
[2] S. Ishibashi and K. Terakura, J. Phys. Soc. Jpn. 83, 073702 (2014).
[3] K. Terakura and S. Ishibashi, Phys. Rev. B 91, 195120 (2015).
[4] S. Horiuchi, R. Kumai and S. Ishibashi, Chem. Sci. 9, 425 (2018).
[5] S. Ishibashi, S. Horiuchi and R. Kumai, Phys. Rev. B 97, 184102 (2018).
[6] I. Souza, J. Íñiguez and D. Vanderbilt, Phys. Rev. Lett. 89, 117602 (2002).
[7] S. Horiuchi, K. Kobayashi, R. Kumai and S. Ishibashi, Nat. Commun. 8, 14426 (2017).

Fluorinated electroactive polymers (FEPs) are materials of increasingly high interest due to their ferroelectric and relaxor-ferroelectric properties and find application in electronic devices such as sensors, actuators, organic field-effect transistors (OFETs) and electrocaloric devices. Before devices that use those materials find wider commercial applications, patterning issues need to be solved. Limited compatibility of those polymers with photolithography, which is the method of choice for large throughput electronics production, prevents their wider commercial realization. In this work we present a method to make those fluoropolymers cross-linkable upon UV irradiation by grafting photo-initiator groups directly on the fluoropolymer’s backbone. Such photo cross-linkable materials can be used as negative photoresists in conventional photolithographic patterning process for the fabrication of electronics. Main challenges of this approach, include the poor chemical reactivity of fluoropolymers as well as the negative effect that modification can have on the crystallinity and thus on the electroactive properties of the polymer.

ACKNOWLEDGEMENTS:
This work was performed with the concourse of Konstantinos Kallitsis ,Thibaut Soulestin, Eric Cloutet,,, Cyril Brocho, Fabrice Domingues Dos Santos . Nouvelle Aquitaine Region is thanked for PhD funding to Konstantinos Kallitsis. We are also thankful to ELORPrintTec ANR-10-EQPX-28-01 and to LCPO/Arkema/ANR INDUSTRIAL CHAIR “HOMERIC” ANR-13-CHIN-0002-01.

A boron nitride nanotube (BNNT), an electrically-insulating counterpart of a carbon nanotube (CNT), is a great nanofiller candidate for ferroelectric polymeric nanocomposites due to BNNT’s exceptional properties in piezoelectricity, thermal stability, mechanical strength, and radiation shielding capability. Fast and high-yield polar crystallization of polyvinylidene fluoride (PVDF) was accomplished by incorporating BNNTs as a nucleating agent. As-fabricated BNNT-PVDF nanocomposites were characterized by Fourier-transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (XRD), and differential scanning calorimetry (DSC) to identify and quantify chain conformations. Besides, piezoelectric coefficients, d₃₃, of BNNT-PVDF nanocomposites were measured and compared with that of a mechanically drawn β phase polar PVDF. The polar transformation due to BNNT incorporation has a great advantage over conventional β or γ phase transformation methods that require (1) mechanical drawing often resulting in defects or (2) high-temperature annealing for an extended time. In addition to the fast polar crystallization, BNNTs had a role of reinforcement of the polymer matrix. The improved Young’s modulus and electromechanical coupling coefficient of BNNT-PVDF nanocomposites indicated potential applications in energy harvesting under harsh environments such as large deformation, wide temperature cycles, and high radiation.

The characteristics of high dielectric constant in poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer has attracted research attentions. However, previous literatures have shown that the dielectric constant would be decreased with film thickness reduced in metal-organic ferroelectric-metal device [1]. Furthermore, the metal-organic ferroelectric-semiconductor (MFS) device also has this phenomenon [2,3]. This phenomenon is caused by the capacitance of low-k interfacial layer (between P(VDF-TrFE)-metal or silicon) in series with dielectric capacitance. The molecular chains are constrained by the restrictive force from the substrate surface. It results in the low-k interfacial layer between the ferroelectric and semiconductor. The characteristics of semiconductors are more complex than metals. Therefore, it is necessary to explore whether there have the other causes that may cause the interfacial layer between P(VDF-TrFE) and silicon heterojunction.
In this work, we investigate the interface trap charges induced P(VDF-TrFE) dielectric characteristics which is study by a MFS device structure. To explore this effect, different interface trap density at P(VDF-TrFE)-silicon interface is generated. The generated interface trap density can be measured by a conductance technique at room temperature. The interface trap density of the two sample is approximately 4.1×10¹¹ – 2.2×10¹² and 1.3×10¹² – 2.6×10¹² eV^-1 cm^-2, respectively. The experiment results show that the lower interface trap density of MFS capacitor has lower interfacial layer effect. It is mean that the interface dipoles switching is influenced by the interface trap charge, which will lead to more significant interfacial layer effects. On top of that, we propose a mechanism to explain how the interface trap charge influences the interface dipoles switching.

References:
[1] H. Xu, “Dielectric properties and ferroelectric behavior of poly(vinylidene fluoride-trifluoroethylene) 50/50 copolymer ultrathin films”, J. Appl. Poly. Scie., vol. 80, pp. 2259–2266, 2001.
[2] P.H. Chen, C.T. Lin, “Effects of silicon interface and frequency dependence in solution-processed high-K poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) dielectric characteristics”, Thin Soild Films, vol. 628, pp. 75–80, Apr. 2017.
[3] K. Henkel, I. Lazareva, D. Mandal, I. Paloumpa, K. Müller, Y. Koval, P. Müller, and D. Schmeißer, “Electrical investigations on metal/ferroelectric/insulator/semi-conductor structures using poly[vinylidene fluoride trifluoroethylene] as ferroelectric layer for organic nonvolatile memory applications”, J. Vac. Sci. Technol. B, vol. 27, pp. 504–507, Jan./Feb. 2009

Parallel information processing, energy efficiency and unsupervised learning make the human brain a model computing system for unstructured data handling. Oxide memristors, from filamentary to metal-insulator transition devices have been shown to emulate synaptic and neuronal functionalities in artificial neuromorphic circuits. However, challenges like non-linear conductance update, cycle-to-cycle and device-to-device variability and leakage current related issues in a dense crossbar structure are still not resolved. For neurons, often a circuit with multiple active or passive components are required which can complicate circuit designs in a large-scale neural network. Earlier, we found out that solution-processable ferroelectric tunnel junctions (FTJs) with P(VDF-TrFE) copolymer barriers on semiconducting bottom electrode can show analog memristive behavior with a broad range of accessible conductance states and low energy dissipation of 100 fJ for the onset of depression and 1 pJ for the onset of potentiation by resetting small tunneling currents on nanosecond timescales. Key synaptic functions like programmable synaptic weight, long- and short-term potentiation and depression, paired-pulse facilitation and depression, and Hebbian and anti-Hebbian learning through spike shape and timing dependent plasticity can be demonstrated. ¹ In our current work, we demonstrate that by manipulating the carrier concentration of the semiconducting bottom electrode in our FTJs, it is possible to control the dynamics of ferroelectric domain rotations and their subsequent relaxation in a way so that from synaptic devices they can act similar to neuronal devices.² These results offer a promising outlook for the FTJ memristors on semiconducting bottom electrodes as both synapse and neuron devices in artificial neural networks by controlling carrier doping concentrations only.

[1] S. Majumdar, H. Tan, Q. Qin and S. van Dijken. Adv. Electron. Mater. (2019). https://doi.org/10.1002/aelm.201800795
[2] S. Majumdar, H. Tan, I. Pande and S. van Dijken. APL Mater. (Under review).

Electroactive polymer and polymer composites offer large application potential in areas such as sensors and actuators, energy generation and storage, filtration membranes and biomedical applications, among others. Many of the applications rely both in the ability to proper tailor polymer properties, including microstructure, crystallinity and phase, and/or in the suitable choice of micro and nanofillers.
In this talk the main properties and characteristics of piezoelectric and magnetoelectric polymer will be presented. In particular, the main aspects for properly process materials with tailored shape and response as well the reasons and strategies for the development of specific magnetoelectric composites will be shown. Finally, some of the most interesting applications will be discussed, in which specific shape and/or materials response are particularly relevant. Finally, the main challenges and future directions in this research field will be outlined.
Acknowledgements
The WEARPLEX project is funded under H2020-EU.2.1.1 program, grant agreement number 825339. Basque Government (under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06).. Spanish Ministry of Economy and Competitiveness: (MAT2016-76039-C4-3-R Fundação para a Ciência e Tecnologia: UID/FIS/04650/2019, POCI-01-0145-FEDER-028157, POCI-01-0145-FEDER-028159 and POCI-01-0145-FEDER-028237.

Conductive metallic particulate inclusions in an electroactive polymer matrix composite have been widely studied for enhanced sensitivity applicable to sensors, energy harvesters, electromechanical actuators, and high charge storage capacitors. Moreover, if the metallic particulates exhibit magnetostrictive properties, the domain of applications expands due to the direct and converse magnetoelectric coupling between electric displacement and magnetization by strain-mediation. One such composite has been analytically postulated to have a giant magnetoelectric coupling, namely Terfenol-D magnetostrictive particles embedded into PVDF-TrFE electroactive matrix, but has been proven difficult to experimentally polarize due to electrical breakdown and percolation. This presentation will discuss one method to synthesize Terfenol-D/PVDF-TrFE composites which are capable to become polarized by direct contact electrodes. The experimental protocol and results to elucidate the magnetic, electric, and mechanical properties of the composite is also discussed. First, it was observed that the ferroelectric polarization was improved with a small weight fraction of Terfenol-D particles, but the polarization was hindered as the weight fraction of the particles increased. Second, the permittivity of the composite rapidly increased with the addition of more Terfenol-D particles, which was attributed to the reduction in polarization of the PVDF-TrFE matrix and discussed through a modified Maxwell-Wagner model developed herein. Third, a notable phase delay was observed in the piezoelectric strain in response to a high frequency electric field, such a delay was directly proportionally to the addition of Terfenol-D particles. Fourth, the addition of the stiff Terfenol-D particles were found to enhance the storage modulus of the composite near glass transition but had negligible effect at the crystallization temperature. A modified thermomechanical model was used to fit the experimental data, which provided insights into the interrelationship between thermal transition and mechanical performance. Finally, microscale characterization using scanning electron microscopy (SEM) and magnetic force microcopy (MFM) were conducted on the polarized composite. SEM micrographs provided evidence to the complete enclosure of the magnetostrictive particles into the electroactive matrix, a perquisite for enhanced strain-mediated magnetoelectric cooping. MFM scans of the composite indicate that the Terfenol-D magnetic particles retained magnetic properties from synthesis process and are indeed dispersed within the matrix.

The spin crossover (SCO) phenomenon, in 3d transition metal compounds, has potential applicability in molecular spintronic devices for "flexible" memory. Voltage-controlled isothermal changes in the electronic structure of the spin crossover molecular system have now been achieved for the Fe (II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)], where pz = tris(pyrazol-1-yl)- borohydride and bipy = 2,2’-bipyridine. This isothermal voltage-controlled switching is evident in thin film bilayer structures where the molecular spin crossover film is adjacent to a molecular ferroelectric thin film (the tested examples being polyvinylidene fluoride hexafluoropropylene or croconic acid (C₅H₂O₅)) [1]. These organic ferroelectric substrates appear to lock the spin crossover [Fe{H₂B(pz)₂}₂(bipy)] molecular complex largely in the low or high spin state depending on the direction of ferroelectric polarization [1,2]. This is clear evidence that a molecular multiferroic is possible. In both a planar two terminal diode structure and for a transistor structure, the voltage controlled isothermal reversible spin crossover switching of [Fe{H₂B(pz)₂}₂(bipy)]is accompanied by a conductance change, where higher conductance occurs at a high spin state while lower conductance is observed for a low spin state, and is seen to be nonvolatile, i.e. retained in the absence of an applied electric field. Nonvolatility arises from the interface between the SCO with nonconducting ferroelectrics which have a non-trivial coercive voltage and can be engineered to test and monitor the spin state of the film. The transistor structure fabricated with an interface between the SCO film and the ferroelectric polymer polyvinylidene fluoride hexafluoropropylene [1] is very different from other molecular spin transistors, a number of which have been fabricated, but are only functional at very low temperatures (i.e. below 10 K). The devices described in this talk are functional at room temperature and above.
Like other spin crossover molecular complexes, voltage control of the [Fe{H₂B(pz)₂}₂(bipy)]spin state is possible, but the voltage control of the spin state becomes volatile when the net electric field from the adjacent ferroelectric is absent. With the very small gap between electrodes (around 160 nm), the ferroelectric fails to lock the changing conductance state of [Fe{H₂B(pz)₂}₂(bipy)] at zero applied voltage. Yet the conductance change is seen to be about 4 orders of magnitude. This suggests a large on/off ratio is possible.
Because these molecular thin films are easily fabricated, and compatible with flexible substrates, there is the potential here for "inkjet printable" low cost molecular solid state memory from this next generation of molecular spintronic devices.

[1] G. Hao, A. Mosey, X. Jiang, A. J. Yost, K. R. Sapkota, G. T. Wang, X. Zhang, J. Zhang, A. T. N’Diaye, R. Cheng, X. Xu and P. A. Dowben, Appl. Phys. Lett. 114, 032901 (2019)

[2] X. Zhang, T. Palamarciuc, J.-F. Létard, P. Rosa, E. Lozada, F. Torres, L. G. Rosa, B. Doudin, and P. A. Dowben, Chem. Commun. 50, 2255 (2014)

Supramolecular soft materials that can respond to both electric and magnetic fields have great potential in many areas, including biosensing, information storage, organic electronics, soft robotics, and broadly in functions requiring transduction between different forms of energy. In supramolecular soft materials structure and function emerge from molecules designed to interact through noncovalent interactions. The frontiers of this area of materials science target the development of strategies to use these designed interactions across length and time scales to generate functional materials. The achievement of macroscopic polar order has been an important objective in order to create ferroelectric and piezoelectric molecular solids as well as ferroelectric liquid crystals. This lecture will discuss early strategies to generate polar supramolecular materials, and more recent efforts focused on the design of ferroelectric materials based on stacks of electron donor and electron acceptor molecules. These mixed donor-acceptor stacks have been found to generate ferroelectric crystals at room temperature through charge transfer when molecules interact through hydrogen bonding. The lecture will also discuss other emerging soft materials with capacity to respond to electric and magnetic fields.

Materials with a negative piezoelectric effect demonstrate atypical, inverse strain-field and polarization-stress characteristics. While mechanisms driving the conventional – positive – piezoelectricity are transparent and manifest in a plethora of organic and inorganic materials, the negative piezoelectricity requires special conditions and is only found in few unique systems, which P(VDF:TrFE) is the most renowned example of [1].

Here, we investigate the piezoelectric activity in a family of small-molecular organic ferroelectrics – the liquid-crystalline BTAs (benzene-1,3,5-tricarboxamides) [2]. Curiously, both the large- and small-signal piezoelectric responses, measured interferometrically on solution-processed thin-film capacitors of BTAs, reveal a pronounced negative piezoelectric effect with d₃₃ reaching values as high as −20 pm/V for short substituted homologues [3]. The measured ‘inverted’ piezoelectric butterfly hysteresis loops are of close-to-ideal form with a well-expressed negative linear slope.

The dipolar disc-like BTA molecules self-assemble into hydrogen-bonded axial aggregates resulting in columnar macrodipoles that pack tightly in a hexagonal lattice. Considering a single ideal molecular stack, a positive piezoelectric response is expected due to the applied field stretching the macrodipole, as confirmed by DFT and MD simulations. However, in real device conditions, the columnar-hexagonal ordering is interrupted by zones of higher disorder. The negative d₃₃ in such structures can therefore be explained by the so-called dimensional effect, which considers the ferroelectric layer as a set of rigid dipoles that are distributed in a deformable matrix. In BTAs this leads to a counterintuitive positive correlation between the large-signal piezoelectric constants and the structural disorder in the bulk, while the small-signal response remains unaffected. Comparison of the piezoelectric response under large- and small-signal conditions for different disorder further reveals that irreversible extrinsic polarization switching processes dominate the large-signal strain.

While the negative sign of the piezoelectric constants is determined by the dimensional effect, the linear strain-field trends in the short-tailed BTAs are caused by polarization-coupled electrostriction (or piezostriction). In longer-substituted BTAs quadratic stain-field relations can be observed due to the negative Maxwell strain starting to dominate because of their lower ferroelectric polarization and higher mechanical compliance. Strong modulation of the elastic constant upon polarization switching further leads to a unique amplification of the large-signal electromechanical response.

Due to their self-healing properties, flexibility and a large figure of merit for energy harvesting, BTAs may become relevant for a wide variety of applications like conformal and/or flexible sensors, energy harvesters and actuators for which rigid crystalline ferroelectrics are unsuited and can be outperformed.

[1] I. Katsouras et al., “The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride),” Nat. Mater., vol. 15, no. 1, pp. 78–84, Oct. 2016.
[2] I. Urbanaviciute et al., “Tuning the Ferroelectric Properties of Trialkylbenzene-1,3,5-tricarboxamide (BTA),” Adv. Electron. Mater., vol. 3, no. 7, p. 1600530, 2017.
[3] I. Urbanaviciute et al., “Negative piezoelectric effect in an organic supramolecular ferroelectric,” Mater. Horizons, 2019.

New polymer materials with high energy density while maintaining low loss at high applied electric fields are needed for increased energy and power density required in modern electronics. We propose a polarizable reactive molecular dynamics-based framework that offers near-quantum accuracy for studying various polymer systems within a small fraction of quantum-calculation time. We use this framework to demonstrate the essential role of morphological complexity that governs industrially relevant model polymer systems. Our framework supports performance of model polymer systems, which have been identified through rational co-design strategy based on advanced computational screening procedures. The proposed framework can augment screening procedures for identifying new polymer materials that are significantly better than the current state of the art.

Recent progresses in self-assembly and supramolecular chemistry have paved the way achieving control over the structure and function of organic and polymeric materials in an elegant and effective way. This bottom-up strategy, ubiquitous in biology, leads to dynamic and ordered nanostructures which function is directly related to the non-covalent interactions used to ensemble them. Therefore, a careful design of the building blocks and the resulting supramolecular interactions allows for the preparation of a great variety of macro- and nanoscopically ordered materials which combine different properties.¹
Organic non-volatile memory devices based on ferroelectricity represent a promising approach towards the development of a low-cost, efficient and mechanically robust bendable memory technology.² However, an important drawback of these materials is the utilization of insulating ferroelectrics, necessitating a destructive information read-out. In this context, the combination of ferroelectric and semiconducting properties in a functional material emerges as a valuable solution to this problem, resulting in a ferroelectric diode that retains the information when the power is turned off, and can be written, erased and non-destructively read. This bifunctionality was first achieved by blending distinct semiconducting and ferroelectric molecules in the bulk.³ Nonetheless, the possibility to combine these properties in a single molecular component is highly desirable since the interplay between the ferroelectric and the semiconducting nature would take place at the molecular level, diminishing phase segregation phenomena between dissimilar moieties.
Within this context, we present the synthesis of nanometer-sized self-assembled non-centrosymmetric architectures exhibiting intrinsic permanent polarization and, among others, ferroelectric properties or/and semiconducting properties.
These materials consist of disc-like organic molecules⁴ that (i) are semiconducting, (ii) possess dipolar side groups and (iii) can be organized into columnar morphologies that have sufficient long-range order to support both ferroelectric coupling between the dipolar groups and quasi-1D charge transport. In these materials the ferroelectric polarization is shown to couple strongly to the materials’ bulk conductivity, giving rise to switchable and rectifying current-voltage characteristics. The observed phenomenon is highly relevant for the fabrication of low-cost non-volatile rewritable memory devices.⁵ Moreover, making use of the previous design we envision the preparation of the next-generation of macro- and nanostructured organic photovoltaic devices (OPVs) using self-assembled donor-acceptor conjugates that demonstrate both a permanent dipole moment and light-harvesting properties. The use of non-centrosymmetric columnar aggregates in OPVs devices should promote fast photoinduced charge separation, as well as good electron transport and extraction features due to the extraordinary control over the internal structure and the presence of an intense permanent macroscopic dipole within the active layer.

References:

1) 1. T. Wöhrle, I. Wurzbach, J. Kirres, A. Kostidou, N. Kapernaum, J. Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giesselmann and S. Laschat, Chem. Rev., 2016, 116, 1139.
2) P. Heremans, G. H. Gelinck, R. Müller, K.-J. Baeg, D.-Y. Kim and Y.-Y. Noh, Chem. Mater., 2011, 23, 341.
3) K. Asadi, D. M. de Leeuw, B. de Boer and P. W. M. Blom, Nat. Mater., 2008, 7, 547.
4) M. García-Iglesias, B. F. M. Waal, A. V. Gorbunov, M. Kemerink, E. W. Meijer, J. Am. Chem. Soc., 2016, 138, 6217.
5) (a) A. V Gorbunov, M. García-Iglesias, J. Guilleme, T. D. Cornelissen, W. S. C. Roelofs, T. Torres, D. González-Rodríguez, E. W. Meijer, M. Kemerink, Sci. Adv., 2017, 3, e1701017. (b) N. M. Casellas, I. Urbanaviciute, T. D. Cornelissen, J. A. Berrocal, T. Torres, M. Kemerink, M. García-Iglesias, Chem. Commun., 2019, 10.1039/C9CC02466B.