Regular arrays of highly ordered ferroelectric polymer nanostructures for non-volatile low-voltage memories

Scaled down glass transition temperature in confined polymer nanofibers

Arrays of polymer nanostructures have been widely used in many novel devices and nanofabrication methods. The glass transition temperature, which is a key parameter influencing the long-term stability of polymer nanostructures, has not yet been systematically studied and well understood. Here we study this technological and fundamental issue with polymers of different values of molar mass M confined in nanocylinders of a varying diameter D. The glass transition temperature Tg loses its dependence on the molar mass for D ≲ 100 nm, a range in which the relative depression of Tg varies as D(-0.44). For higher cylinder diameters, Tg progressively recovers its dependence on the molar mass. This is quantitatively reproduced by a model based on an equilibrium interfacial excess of free volume, which needs to be created unless provided by the chain ends. Our findings suggest that the structural perturbations during nanofabrication may strongly affect the long-term stability of arrays of polymer nanostructures.

Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance

The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg(0)) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg(0) vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.

Graphene quantum dots as a highly efficient solution-processed charge trapping medium for organic nano-floating gate memory

A highly efficient solution-processible charge trapping medium is a prerequisite to developing high-performance organic nano-floating gate memory (NFGM) devices. Although several candidates for the charge trapping layer have been proposed for organic memory, a method for significantly increasing the density of stored charges in nanoscale layers remains a considerable challenge. Here, solution-processible graphene quantum dots (GQDs) were prepared by a modified thermal plasma jet method; the GQDs were mostly composed of carbon without any serious oxidation, which was confirmed by x-ray photoelectron spectroscopy. These GQDs have multiple energy levels because of their size distribution, and they can be effectively utilized as charge trapping media for organic NFGM applications. The NFGM device exhibited excellent reversible switching characteristics, with an on/off current ratio greater than 10(6), a stable retention time of 10(4) s and reliable cycling endurance over 100 cycles. In particular, we estimated that the GQDs layer trapped ∼7.2 × 10(12) cm(-2) charges per unit area, which is a much higher density than those of other solution-processible nanomaterials, suggesting that the GQDs layer holds promise as a highly efficient nanoscale charge trapping material.

Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment

A method to produce highly efficient and long-range vertical charge transport is demonstrated in an undoped polythiophene thin film, with average mobilities above 3.1 cm(2) V(-1) s(-1) . These record high mobilities are achieved by controlled orientation of the polymer crystallites enabling the most efficient and fastest charge transport along the chain backbones and across multiple chains. The significant increase in mobility shown here may present a new route to producing faster and more efficient optoelectronic devices based on organic materials.

Organic ferroelectric/semiconducting nanowire hybrid layer for memory storage

Ferroelectric materials are important components of sensors, actuators and non-volatile memories. However, possible device configurations are limited due to the need to provide screening charges to ferroelectric interfaces to avoid depolarization. Here we show that, by alternating ferroelectric and semiconducting nanowires over an insulating substrate, the ferroelectric dipole moment can be stabilized by injected free charge carriers accumulating laterally in the neighboring semiconducting nanowires. This lateral electrostatic coupling between ferroelectric and semiconducting nanowires offers new opportunities to design new device architectures. As an example, we demonstrate the fabrication of an elementary non-volatile memory device in a transistor-like configuration, of which the source-drain current exhibits a typical hysteretic behavior with respect to the poling voltage. The potential for size reduction intrinsic to the nanostructured hybrid layer offers opportunities for the development of strongly miniaturized ferroelectric and piezoelectric devices.

Multi-floor cascading ferroelectric nanostructures: multiple data writing-based multi-level non-volatile memory devices

Multiple data writing-based multi-level non-volatile memory has gained strong attention for next-generation memory devices to quickly accommodate an extremely large number of data bits because it is capable of storing multiple data bits in a single memory cell at once. However, all previously reported devices have failed to store a large number of data bits due to the macroscale cell size and have not allowed fast access to the stored data due to slow single data writing. Here, we introduce a novel three-dimensional multi-floor cascading polymeric ferroelectric nanostructure, successfully operating as an individual cell. In one cell, each floor has its own piezoresponse and the piezoresponse of one floor can be modulated by the bias voltage applied to the other floor, which means simultaneously written data bits in both floors can be identified. This could achieve multi-level memory through a multiple data writing process.

Fabrication of ferroelectric polymer nanostructures on flexible substrates by soft-mold reverse nanoimprint lithography

Conventional nanoimprint lithography with expensive rigid molds is used to pattern ferroelectric polymer nanostructures on hard substrate for use in, e.g., organic electronics. The main innovation here is the use of inexpensive soft polycarbonate molds derived from recordable DVDs and reverse nanoimprint lithography at low pressure, which is compatible with flexible substrates. This approach was implemented to produce regular stripe arrays with a spacing of 700 nm from vinylidene fluoride co trifluoroethylene ferroelectric copolymer on flexible polyethylene terephthalate substrates. The nanostructures have very stable and switchable piezoelectric response and good crystallinity, and are highly promising for use in organic electronics enhanced or complemented by the unique properties of the ferroelectric polymer, such as bistable polarization, piezoelectric response, pyroelectric response, or electrocaloric function. The soft-mold reverse nanoimprint lithography also leaves little or no residual layer, affording good isolation of the nanostructures. This approach reduces the cost and facilitates large-area, high-throughput production of isolated functional polymer nanostructures on flexible substrates for the increasing application of ferroelectric polymers in flexible electronics.

Highly oriented poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films with improved ferroelectricity

Fabrication of poly(vinylidene fluoride) copolymer monolayer (3.5 nm thick) was succeeded, exhibiting superior ferroelectricity and potential applications as non-volatile memories.

Low-voltage electro-optical memory device based on NiO nanorods dispersed in a ferroelectric liquid crystal

A low-power nonvolatile memory device is fabricated by dispersing nickel oxide nanorods (nNiO) into a ferroelectric liquid crystal (FLC) host. The dipolar nNiO adsorbed ions in the FLC and thereby reduced the screening effect, which resulted in the enhanced memory behavior.

Two-Step Polarization Switching in Ferroelectric Polymers

The correlation between hierarchical structures and polarization switching in ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene) has been probed by combining transmission electron microscopy studies with piezoresponse force microscopy observations. Differences are demonstrated between homogeneous and anisotropic thin films with well-defined lamellar orientation, with the later exhibiting quadrangular domain shape and double hysteresis. We propose that the polarization switching within lamella is dominated by domain wall flow motion, while the amorphous components between lamellae impede full polarization switching. The coupling between lamellae is controlled by a creep process. These results and interpretations explain well the seemingly contradicting polarization reversal dynamics reported and offer opportunities to change domain reversal speed by making ferroelectric polymer nanostructures.

The Fabrication of Ordered Bulk Heterojunction Solar Cell by Nanoimprinting Lithography Method Using Patterned Silk Fibroin Mold at Room Temperature

The performance of organic solar cell is greatly determined by the nanoscale heterojunction morphology, and finding a practical method to achieve advantageous nanostructure remains a challenge. We demonstrate here that ordered bulk heterojunction (OBHJ) solar cell can be fabricated assisted by a simple, cost-effective nanoimprinting lithography method using patterned silk fibroin film mold at room temperature. The P3HT nanogratings were achieved by nanoimprinting lithography (NIL) process, and phenyl-C61-butyric acid methyl ester (PCBM) was spin-coated on the top of P3HT nanogratings. The conducting capacity of P3HT nanograting film has little difference compared with the unimprinted film in the vertical direction, due to the same edge-on chain alignment. However, it can be found that the fabrication of OBHJ nanostructure using room temperature NIL technique with patterned silk fibroin mold is able to promote optical absorption, interfacial area, and bicontinuous pathway. Therefore, the ordered heterojunction morphology plays an important part in improving device performance due to efficient exciton diffusion, dissociation, and reducing charge recombination rate.

Direct Writing of Three-Dimensional Macroporous Photonic Crystals on Pressure-Responsive Shape Memory Polymers

Here we report a single-step direct writing technology for making three-dimensional (3D) macroporous photonic crystal patterns on a new type of pressure-responsive shape memory polymer (SMP). This approach integrates two disparate fields that do not typically intersect: the well-established templating nanofabrication and shape memory materials. Periodic arrays of polymer macropores templated from self-assembled colloidal crystals are squeezed into disordered arrays in an unusual shape memory "cold" programming process. The recovery of the original macroporous photonic crystal lattices can be triggered by direct writing at ambient conditions using both macroscopic and nanoscopic tools, like a pencil or a nanoindenter. Interestingly, this shape memory disorder-order transition is reversible and the photonic crystal patterns can be erased and regenerated hundreds of times, promising the making of reconfigurable/rewritable nanooptical devices. Quantitative insights into the shape memory recovery of collapsed macropores induced by the lateral shear stresses in direct writing are gained through fundamental investigations on important process parameters, including the tip material, the critical pressure and writing speed for triggering the recovery of the deformed macropores, and the minimal feature size that can be directly written on the SMP membranes. Besides straightforward applications in photonic crystal devices, these smart mechanochromic SMPs that are sensitive to various mechanical stresses could render important technological applications ranging from chromogenic stress and impact sensors to rewritable high-density optical data storage media.

Organic Electronic Memory Devices

With the rapid development of the electronics industry in recent years, information technology devices, such as personal computers, mobile phones, digital cameras and media players, have become an essential part of our daily life. From both the technological and economic points of view, the development of novel information storage materials and devices has become an emergent issue facing the electronics industry. Due to the advantages of good scalability, flexibility, low cost, ease of processing, 3D-stacking capability and high capacity for data storage, organic-based electrical memory devices have been promising alternatives or supplementary devices to conventional inorganic semiconductor-based memory technology. The basic concepts and historical development of electronic memory devices are first presented. The following section introduces the structures and switching mechanisms of organic electronic memory devices classified as transistors, capacitors and resistors. Subsequently, the progress in the field of organic-based memory materials and devices is systematically summarized and discussed. Finally, the challenges posed to the development of novel organic electronic memory devices are summarized.

Adsorbing a PVDF polymer via noncovalent interactions to effectively tune the electronic and magnetic properties of zigzag SiC nanoribbons

On the basis of first-principle computations, we first propose a simple and effective strategy through surface-adsorbing a poly(vinylidene fluoride) (PVDF) polymer via noncovalent interactions to tune the electronic and magnetic behaviors of zigzag SiC nanoribbons (zSiCNRs). It is revealed that depositing the strong electron-withdrawing PVDF polymer with a permanent dipole moment can induce the evident change of the electrostatic potential in the substrate zSiCNRs, like applying an electric field. As a result, this kind of noncovalent surface-modification by a polymer can break the magnetic degeneracy of zSiCNRs independent of the adsorption type and position, and sole ferromagnetic metallicity and even antiferromagnetic half-metallicity can be achieved. Moreover, all PVDF-modified zSiCNR systems can exhibit considerable adsorption energies in the range of -0.436 to -1.315 eV, indicating that these joint systems possess high structural stabilities. These intriguing findings will be advantageous for promoting excellent SiC-based nanomaterials in the applications of spintronics and multifunctional nanodevices in the near future.

Laser Fabrication of Polymer Ferroelectric Nanostructures for Nonvolatile Organic Memory Devices

Polymer ferroelectric laser-induced periodic surface structures (LIPSS) have been prepared on ferroelectric thin films of a poly(vinylidene fluoride-trifluoroethylene) copolymer. Although this copolymer does not absorb light at the laser wavelength, LIPSS on the copolymer can be obtained by forming a bilayer with other light-absorbing polymers. The ferroelectric nature of the structured bilayer was proven by piezoresponse force microscopy measurements. Ferroelectric hysteresis was found on both the bilayer and the laser-structured bilayer. We show that it is possible to write ferroelectric information at the nanoscale. The laser-structured ferroelectric bilayer showed an increase in the information storage density of an order of magnitude, in comparison to the original bilayer.

Stability of Grafted Polymer Nanoscale Films toward Gamma Irradiation

The present article focuses on the influence of gamma irradiation on nanoscale polymer grafted films and explores avenues for improvements in their stability toward the ionizing radiation. In terms of applications, we concentrate on enrichment polymer layers (EPLs), which are polymer thin films employed in sensor devices for the detection of chemical and biological substances. Specifically, we have studied the influence of gamma irradiation on nanoscale poly(glycidyl methacrylate) (PGMA) grafted EPL films. First, it was determined that a significant level of cross-linking was caused by irradiation in pure PGMA films. The cross-linking is accompanied by the formation of conjugated ester, carbon double bonds, hydroxyl groups, ketone carbonyls, and the elimination of epoxy groups as determined by FTIR. Polystyrene, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, dimethylphenylsilanol, BaF2, and gold nanoparticles were incorporated into the films and were found to mitigate different aspects of the radiation damage.

Interfacing Solution-Grown C60 and (3-Pyrrolinium)(CdCl3 ) Single Crystals for High-Mobility Transistor-Based Memory Devices

Aligned ferroelectric single crystals of (3-pyrrolinium)(CdCl₃ ) can be prepared from solution on top of aligned semiconducting C₆₀ single crystals using an orthogonal solvent. Memory devices based on these ferroelectric/semiconductor bilayered heterojunctions exhibit much larger hysteresis compared with that of only C₆₀ single crystals. More importantly, the introduction of the ferroelectric layer induces the memory window without dramatically reducing the charge mobility.

Vertically aligned P(VDF-TrFE) core-shell structures on flexible pillar arrays

PVDF and P(VDF-TrFE) nano- and micro- structures have been widely used due to their potential applications in several fields, including sensors, actuators, vital sign transducers, and energy harvesters. In this study, we developed vertically aligned P(VDF-TrFE) core-shell structures using high modulus polyurethane acrylate (PUA) pillars as the support structure to maintain the structural integrity. In addition, we were able to improve the piezoelectric effect by 1.85 times from 40 ± 2 to 74 ± 2 pm/V when compared to the thin film counterpart, which contributes to the more efficient current generation under a given stress, by making an effective use of the P(VDF-TrFE) thin top layer as well as the side walls. We attribute the enhancement of piezoelectric effects to the contributions from the shell component and the strain confinement effect, which was supported by our modeling results. We envision that these organic-based P(VDF-TrFE) core-shell structures will be used widely as 3D sensors and power generators because they are optimized for current generations by utilizing all surface areas, including the side walls of core-shell structures.

An organic approach for nanostructured multiferroics

Multiferroics are materials that simultaneously exhibit more than one ferroic order parameters, such as ferroelectricity, ferroelasticity and ferromagnetism. Recently, multiferroicity has received a significant revival of interest due to the colossal magnetoelectric coupling effect for the development of nano-ferronics. In this mini-review, we focus on a recent study of ferroelectricity, magnetism and magnetoelectric coupling within the newly discovered organic charge-transfer complexes. A systemic understanding of the origin of organic ferroelectricity and magnetism is provided. Furthermore, based on the recent mechanism of the magnetoelectric coupling effect: spin-ordering-induced electric polarization and ferroelectricity-induced spin alignment, we further present the recent progress in organic charge-transfer multiferroics and metal-organic framework multiferroics. The coexistence of polarization and magnetism at room temperature of organic charge-transfer complexes will be critical for the development of all-organic multiferroics.

Nanoconfinement induced crystal orientation and large piezoelectric coefficient in vertically aligned P(VDF-TrFE) nanotube array

Vertically aligned piezoelectric P(VDF-TrFE) nanotube array comprising nanotubes embedded in anodized alumina membrane matrix without entanglement has been fabricated. It is found that the crystallographic polar axes of the P(VDF-TrFE) nanotubes are oriented along the nanotubes long axes. Such a desired crystal orientation is due to the kinetic selection mechanism for lamellae growth confined in the nanopores. The preferred crystal orientation in nanotubes leads to huge piezoelectric coefficients of the P(VDF-TrFE). The piezoelectric strain and voltage coefficients of P(VDF-TrFE) nanotube array are observed to be 1.97 and 3.40 times of those for conventional spin coated film. Such a significant performance enhancement is attributed to the well-controlled polarization orientation, the elimination of the substrate constraint, and the low dielectric constant of the nanotube array. The P(VDF-TrFE) nanotube array exhibiting the unique structure and outstanding piezoelectric performance is promising for wide applications, including various electrical devices and electromechanical sensors and transducers.

Systematic Control of the Nanostructure of Semiconducting-Ferroelectric Polymer Composites in Thin Film Memory Devices

In polymer-based ferroelectric diodes, films are composed of a semiconducting polymer and a ferroelectric polymer blend sandwiched between two metal electrodes. In these thin films, the ferroelectric phase serves as the memory retention medium while the semiconducting phase serves as the pathway to read-out the memory in a nondestructive manner. As such, having distinct phases for the semiconducting and ferroelectric phases have proven critical to device performance. In order to evaluate this crucial structure-property relationship, we have fabricated ordered ferroelectric devices (OFeDs) through common lithographic techniques to establish systematically the impact of nanoscale structure on the macroscopic performance. In particular, we demonstrate that there is an optimal domain size (∼400 nm) for the interpenetrating networks, and we show that the ordered device, with semiconducting domains that span the entire length of the active layer film, provides a significant increase in the ON/OFF ratio relative to the blended film fabricated using standard solution blending and spin-coating techniques. This improved performance occurs due to a combination of the ordered nanostructure and the nature of the ferroelectric-semiconductor interface. As this is the first demonstration of macroscopic OFeDs, this work helps to elucidate the underlying physics of the device operation and establishes a new archetype in the design of polymer-based, nonvolatile memory devices.

Tailoring surface phase transition and magnetic behaviors in BiFeO3 via doping engineering

The charge-spin interactions in multiferroic materials (e.g., BiFeO₃) have attracted enormous attention due to their high potential for next generation information electronics. However, the weak and deficient manipulation of charge-spin coupling notoriously limits their commercial applications. To tailor the spontaneous charge and the spin orientation synergistically in BiFeO₃ (BFO), in this report, the 3d element of Mn doping engineering is employed and unveils the variation of surface phase transition and magnetic behaviors by introducing chemical strain. The spontaneous ferroelectric response and the corresponding domain structures, magnetic behaviors and spin dynamics in Mn-doped BFO ceramics have been investigated systematically. Both the surface phase transition and magnetization were enhanced in BFO via Mn doping. The interaction between the spontaneous polarization charge and magnetic spin reorientation in Mn-doped BFO are discussed in detail. Moreover, our extensive electron paramagnetic resonance (EPR) results demonstrate that the 3d dopant plays a paramount role in the surface phase transition, which provides an alternative route to tune the charge-spin interactions in multiferroic materials.

Solvent-dependent properties of poly(vinylidene fluoride) monolayers at the air-water interface

The present work addresses the solvent-dependent properties of Langmuir films of poly(vinylidene fluoride) (PVDF) and amphiphilic poly(N-dodecylacrylamide) (pDDA) at different mixing ratios. After introducing pDDA nanosheets, PVDF Langmuir films obtain a tremendously enhanced modulus as well as high transfer ratios using the vertical dipping method caused by the support of the pDDA two-dimensional hydrogen bonding network. Brewster angle microscopy (BAM) was used to investigate PVDF monolayers at the air-water interface in situ. Spreading from different solvents, the PVDF molecules take completely different aggregation states at the air-water interface. The PVDF molecules aggregate to become large domains when spread from N-methyl-2-pyrrolidone (NMP). However, the volatile and low-polarity methylethyl ketone (MEK) made the PVDF molecules more dispersive on the water surface. This study also discovers a versatile crystallization control of PVDF homopolymer from complete β phase (NMP) to complete α phase (MEK) at the air-water interface, thereby eliciting useful information for further manipulation of film morphologies and film applications.

Enhancement of local piezoresponse in polymer ferroelectrics via nanoscale control of microstructure

Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators, and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over a microscale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals via sliding diffusion and crystal alignment along the scan axis under high mechanical stress. We believe that this technique can be useful for local enhancement of piezoresponse in ferroelectric polymer thin films.

Polymer melt flow through nanochannels: from theory and fabrication to application

This short review presents the theory, fabrication, and application of polymer melts through nanochannels.

The induction of poly(vinylidene fluoride) electroactive phase by modified anodic aluminum oxide template nanopore surface

The polar phase of PVDF nanowires was improved significantly with the modified AAO templates using a solution wetting method.

Asymmetric polarization and hysteresis behaviour in ferroelectric P(VDF–TrFE) (76 : 24) copolymer thin films spatially resolved via LIMM

The role of charges for achieving and stabilizing the ferroelectric polarization in PVDF–TrFE (76 : 24) is revisited using advanced photothermal methods.

High-symmetry polarization domains in low-symmetry ferroelectrics

We present experimental evidence for polygonal domain faceting in the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) films with the lower orthorhombic crystallographic symmetry. It is proposed that this effect can arise from purely electrostatic depolarizing forces. We show that, in contrast to magnetic bubble shape domains, where such type of deformation instability has a predominantly elliptical character, the emergence of more symmetrical circular harmonics is favored in ferroelectrics with high dielectric constants.

Piezoelectric nanoparticle-polymer composite foams

Piezoelectric polymer composite foams are synthesized using different sugar-templating strategies. By incorporating sugar grains directly into polydimethylsiloxane mixtures containing barium titanate nanoparticles and carbon nanotubes, followed by removal of the sugar after polymer curing, highly compliant materials with excellent piezoelectric properties can be fabricated. Porosities and elasticity are tuned by simply adjusting the sugar/polymer mass ratio which gave an upper bound on the porosity of 73% and a lower bound on the elastic coefficient of 32 kPa. The electrical performance of the foams showed a direct relationship between porosity and the piezoelectric outputs, giving piezoelectric coefficient values of ∼112 pC/N and a power output of ∼18 mW/cm3 under a load of 10 N for the highest porosity samples. These novel materials should find exciting use in a variety of applications including energy scavenging platforms, biosensors, and acoustic actuators.

Ordered arrays of a defect-modified ferroelectric polymer for non-volatile memory with minimized energy consumption

Ferroelectric polymers are among the most promising materials for flexible electronic devices. Highly ordered arrays of the defect-modified ferroelectric polymer P(VDF-TrFE-CFE) (poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)) are fabricated by nanoimprint lithography for nonvolatile memory application. The defective CFE units reduce the coercive field to one-fifth of that of the un-modified P(VDF-TrFE), which can help minimize the energy consumption and extend the lifespan of the device. The nanoimprint process leads to preferable orientation of polymer chains and delicately controlled distribution of the defects, and thus a bi-stable polarization that makes the memory nonvolatile, as revealed by the pulsed polarization experiment.

3D optical printing of piezoelectric nanoparticle-polymer composite materials

Here we demonstrate that efficient piezoelectric nanoparticle-polymer composite materials can be optically printed into three-dimensional (3D) microstructures using digital projection printing. Piezoelectric polymers were fabricated by incorporating barium titanate (BaTiO3, BTO) nanoparticles into photoliable polymer solutions such as polyethylene glycol diacrylate and exposing to digital optical masks that could be dynamically altered to generate user-defined 3D microstructures. To enhance the mechanical-to-electrical conversion efficiency of the composites, the BTO nanoparticles were chemically modified with acrylate surface groups, which formed direct covalent linkages with the polymer matrix under light exposure. The composites with a 10% mass loading of the chemically modified BTO nanoparticles showed piezoelectric coefficients (d(33)) of ∼ 40 pC/N, which were over 10 times larger than composites synthesized with unmodified BTO nanoparticles and over 2 times larger than composites containing unmodified BTO nanoparticles and carbon nanotubes to boost mechanical stress transfer efficiencies. These results not only provide a tool for fabricating 3D piezoelectric polymers but lay the groundwork for creating highly efficient piezoelectric polymer materials via nanointerfacial tuning.

Solvent vapor annealing of ferroelectric P(VDF-TrFE) thin films

Ferroelectric polymers are a kind of promising materials for low-cost flexible memories. However, the relatively high thermal annealing temperature restricts the selection of some flexible polymer substrates. Here we report an alternative method to obtain ferroelectric poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films under low process temperatures. Spin-coated P(VDF-TrFE) thin films were solvent vapor processed at 30 °C for varied times. Structural analyses indicated that solvent vapor annealing induced crystallization to form a ferroelectric β phase, and electrical measurements from both macroscopic ferroelectric switching and microscopic vertical piezoresponse force microscopy further proved the films enduring solvent vapor annealing for suitable short times possessed good ferroelectric and piezoelectric properties. To illuminate the application of solvent vapor annealing on ferroelectric devices, we further fabricated ferroelectric capacitor memory devices with a structure of Al/P(VDF-TrFE)/Al2O3/p-Si/Al where the ferroelectric layer was solvent vapor annealed. Ferroelectric capacitors showed obvious bistable operation and comparable ON/OFF ratio and retention performance. Our work makes it possible to structure ferroelectric devices on flexible substrates that require low process temperatures.

Spontaneous high piezoelectricity in poly(vinylidene fluoride) nanoribbons produced by iterative thermal size reduction technique

We produced kilometer-long, endlessly parallel, spontaneously piezoelectric and thermally stable poly(vinylidene fluoride) (PVDF) micro- and nanoribbons using iterative size reduction technique based on thermal fiber drawing. Because of high stress and temperature used in thermal drawing process, we obtained spontaneously polar γ phase PVDF micro- and nanoribbons without electrical poling process. On the basis of X-ray diffraction (XRD) analysis, we observed that PVDF micro- and nanoribbons are thermally stable and conserve the polar γ phase even after being exposed to heat treatment above the melting point of PVDF. Phase transition mechanism is investigated and explained using ab initio calculations. We measured an average effective piezoelectric constant as -58.5 pm/V from a single PVDF nanoribbon using a piezo evaluation system along with an atomic force microscope. PVDF nanoribbons are promising structures for constructing devices such as highly efficient energy generators, large area pressure sensors, artificial muscle and skin, due to the unique geometry and extended lengths, high polar phase content, high thermal stability and high piezoelectric coefficient. We demonstrated two proof of principle devices for energy harvesting and sensing applications with a 60 V open circuit peak voltage and 10 μA peak short-circuit current output.

Laser-induced nondestructive patterning of a thin ferroelectric polymer film with controlled crystals using Ge8Sb2Te11 alloy layer for nonvolatile memory

We present a simple but robust nondestructive process for fabricating micropatterns of thin ferroelectric polymer films with controlled crystals. Our method is based on utilization of localized heat arising from thin Ge(8)Sb(2)Te(11) (GST) alloy layer upon exposure of 650 nm laser. The heat was generated on GST layer within a few hundred of nanosecond exposure and subsequently transferred to a thin poly(vinylidene fluoride-co-trifluoroethylene) film deposited on GST layer. By controlling exposure time and power of the scanned laser, ferroelectric patterns of one or two microns in size are fabricated with various shape. In the micropatterned regions, ferroelectric polymer crystals were efficiently controlled in both degree of the crystallinity and the molecular orientations. Nonvolatile memory devices with laser scanned ferroelectric polymer layers exhibited excellent device performance of large remnant polarization, ON/OFF current ratio and data retention. The results are comparable with devices containing ferroelectric films thermally annealed at least for 2 h, making our process extremely efficient for saving time. Furthermore, our approach can be conveniently combined with a number of other functional organic materials for the future electronic applications.