Epitaxially Grown Ferroelectric PVDF-TrFE Film on Shape-Tailored Semiconducting Rubrene Single Crystal.

Epitaxial crystallization of thin poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) films is important for the full utilization of their ferroelectric properties. Epitaxy can offer a route for maximizing the degree of crystallinity with the effective orientation of the crystals with respect to the electric field. Despite various approaches for the epitaxial control of the crystalline structure of PVDF-TrFE, its epitaxy on a semiconductor is yet to be accomplished. Herein, the epitaxial growth of PVDF-TrFE crystals on a single-crystalline organic semiconductor rubrene grown via physical vapor deposition is presented. The epitaxy results in polymer crystals globally ordered with specific crystal orientations dictated by the epitaxial relation between the polymer and rubrene crystal. The lattice matching between the c-axis of PVDF-TrFE crystals and the (210) plane of orthorhombic rubrene crystals develops two degenerate crystal orientations of the PVDF-TrFE crystalline lamellae aligned nearly perpendicular to each other. Thin PVDF-TrFE films with epitaxially grown crystals are incorporated into metal/ferroelectric polymer/metal and metal/ferroelectric polymer/semiconductor/metal capacitors, which exhibit excellent nonvolatile polarization and capacitance behavior, respectively. Furthermore, combined with a printing technique for micropatterning rubrene single crystals, the epitaxy of a PVDF-TrFE film is formed selectively on the patterned rubrene with characteristic epitaxial crystal orientation over a large area.

Flexible Nonvolatile Transistor Memory with Solution-Processed Transition Metal Dichalcogenides.

Nonvolatile field-effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating-gate organic-FET memory with an all-in-one floating-gate/tunneling layer of the solution-processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS2 ) is efficiently liquid-exfoliated by amine-terminated polystyrene with a controlled amount of MoS2 nanosheets in an all-in-one floating-gate/tunneling layer, allowing for systematic investigation of concentration-dependent charge-trapping and detrapping properties of MoS2 nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 104 , a program/erase endurance cycle over 400 times, and data retention longer than 7 × 103 s. All-in-one floating-gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically-flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer-bending events over 500 times at the bending radius of 4.0 mm.

Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory.

Enhancing the device performance of organic memory devices while providing high optical transparency and mechanical flexibility requires an optimized combination of functional materials and smart device architecture design. However, it remains a great challenge to realize fully functional transparent and mechanically durable nonvolatile memory because of the limitations of conventional rigid, opaque metal electrodes. Here, we demonstrate ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer. The strong crystallographic interaction between PVDF-TrFE and graphene results in the orientation of the crystals with distinct symmetry, which is favorable for polarization switching upon the electric field. The epitaxial growth of PVDF-TrFE on a graphene layer thus provides excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices. Furthermore, a fully transparent and flexible array of ferroelectric field effect transistors was successfully realized by adopting transparent poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] semiconducting polymer.

Non-volatile ferroelectric memory with position-addressable polymer semiconducting nanowire.

One-dimensional nanowires (NWs) have been extensively examined for numerous potential nano-electronic device applications such as transistors, sensors, memories, and photodetectors. The ferroelectric-gate field effect transistors (Fe-FETs) with semiconducting NWs in particular in combination with ferroelectric polymers as gate insulating layers have attracted great attention because of their potential in high density memory integration. However, most of the devices still suffer from low yield of devices mainly due to the ill-control of the location of NWs on a substrate. NWs randomly deposited on a substrate from solution-dispersed droplet made it extremely difficult to fabricate arrays of NW Fe-FETs. Moreover, rigid inorganic NWs were rarely applicable for flexible non-volatile memories. Here, we present the NW Fe-FETs with position-addressable polymer semiconducting NWs. Polymer NWs precisely controlled in both location and number between source and drain electrode were achieved by direct electrohydrodynamic NW printing. The polymer NW Fe-FETs with a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) exhibited non-volatile ON/OFF current margin at zero gate voltage of approximately 10(2) with time-dependent data retention and read/write endurance of more than 10(4) seconds and 10(2) cycles, respectively. Furthermore, our device showed characteristic bistable current hysteresis curves when being deformed with various bending radii and multiple bending cycles over 1000 times.

Transparent and Flexible Graphene Pressure Sensor with Self-Assembled Topological Crystalline Ionic Gel.

This study demonstrates transparent and flexible capacitive pressure sensors using a high-k ionic gel composed of an insulating polymer (poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene), P(VDF-TrFE-CFE)) blended with an ionic liquid (IL; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide, [EMI][TFSA]). The thermal melt recrystallization of the P(VDF-TrFE-CFE):[EMI][TFSA] blend films develops the characteristic topological semicrystalline surface of the films, making them highly sensitive to pressure. Using optically transparent and mechanically flexible graphene electrodes, a novel pressure sensor is realized with the topological ionic gel. The sensor exhibits a sufficiently large air dielectric gap between graphene and the topological ionic gel, resulting in a large variation in capacitance before and after the application of various pressures owing to the pressure-sensitive reduction of the air gap. The developed graphene pressure sensor exhibits a high sensitivity of 10.14 kPa-1 at 20 kPa, rapid response times of <30 ms, and durable device operation with 4000 repeated ON/OFF cycles. Furthermore, broad-range detections from lightweight objects to human motion are successfully achieved, demonstrating that the developed pressure sensor with a self-assembled crystalline topology is potentially suitable for a variety of cost-effective wearable applications.

Controlled polymer crystal/two-dimensional material heterostructures for high-performance photoelectronic applications.

The control of atomically thin two-dimensional (2D) crystal-based heterostructures wherein the interfaces of 2D nanomaterials are vertically stacked with other thin functional materials via van der Waals interactions is highly important for not only optimizing the excellent properties of 2D nanomaterials, but also for utilizing the functionality of the contact materials. In particular, when 2D nanomaterials are combined with soft polymeric components, the resulting photoelectronic devices are potentially scalable and mechanically flexible, allowing the development of a variety of prototype soft-electronic devices, such as solar cells, displays, photodetectors, and non-volatile memory devices. Diverse polymer/2D heterostructures are frequently employed, but the performance of the devices with heterostructures is limited, mainly because of the difficulty in controlling the molecular structures of the polymers on the 2D surface. Thus, understanding the crystal interactions of polymers on atomically flat and dangling-bond-free surfaces of 2D materials is essential for ensuring high performance. In this study, the recent progress made in the development of thin polymer films fabricated on the surfaces of various 2D nanomaterials for high-performance photoelectronic devices is comprehensively reviewed, with an emphasis on the control of the molecular and crystalline structures of the polymers on the 2D surface.

Artificially Intelligent Tactile Ferroelectric Skin.

Lightweight and flexible tactile learning machines can simultaneously detect, synaptically memorize, and subsequently learn from external stimuli acquired from the skin. This type of technology holds great interest due to its potential applications in emerging wearable and human-interactive artificially intelligent neuromorphic electronics. In this study, an integrated artificially intelligent tactile learning electronic skin (e-skin) based on arrays of ferroelectric-gate field-effect transistors with dome-shape tactile top-gates, which can simultaneously sense and learn from a variety of tactile information, is introduced. To test the e-skin, tactile pressure is applied to a dome-shaped top-gate that measures ferroelectric remnant polarization in a gate insulator. This results in analog conductance modulation that is dependent upon both the number and magnitude of input pressure-spikes, thus mimicking diverse tactile and essential synaptic functions. Specifically, the device exhibits excellent cycling stability between long-term potentiation and depression over the course of 10 000 continuous input pulses. Additionally, it has a low variability of only 3.18%, resulting in high-performance and robust tactile perception learning. The 4 × 4  device array is also able to recognize different handwritten patterns using 2-dimensional spatial learning and recognition, and this is successfully demonstrated with a high degree accuracy of 99.66%, even after considering 10% noise.

Sensing and memorising liquids with polarity-interactive ferroelectric sound.

The direct sensing and storing of the information of liquids with different polarities are of significant interest, in particular, through means related to human senses for emerging biomedical applications. Here, we present an interactive platform capable of sensing and storing the information of liquids. Our platform utilises sound arising from liquid-interactive ferroelectric actuation, which is dependent upon the polarity of the liquid. Liquid-interactive sound is developed when a liquid is placed on a ferroelectric polymer layer across two in-plane electrodes under an alternating current field. As the sound is correlated with non-volatile remnant polarisation of the ferroelectric layer, the information is stored and retrieved after the liquid is removed, resulting in a sensing memory of the liquid. Our pad-type allows for identifying the position of a liquid. Flexible tube-type devices offer a route for in situ analysis of flowing liquids including a human serum liquid in terms of sound.

Micropatterned Pyramidal Ionic Gels for Sensing Broad-Range Pressures with High Sensitivity.

The development of pressure sensors that are effective over a broad range of pressures is crucial for the future development of electronic skin applicable to the detection of a wide pressure range from acoustic wave to dynamic human motion. Here, we present flexible capacitive pressure sensors that incorporate micropatterned pyramidal ionic gels to enable ultrasensitive pressure detection. Our devices show superior pressure-sensing performance, with a broad sensing range from a few pascals up to 50 kPa, with fast response times of <20 ms and a low operating voltage of 0.25 V. Since high-dielectric-constant ionic gels were employed as constituent sensing materials, an unprecedented sensitivity of 41 kPa-1 in the low-pressure regime of <400 Pa could be realized in the context of a metal-insulator-metal platform. This broad-range capacitive pressure sensor allows for the efficient detection of pressure from a variety of sources, including sound waves, a lightweight object, jugular venous pulses, radial artery pulses, and human finger touch. This platform offers a simple, robust approach to low-cost, scalable device design, enabling practical applications of electronic skin.

Printable and Rewritable Full Block Copolymer Structural Color.

Structural colors (SCs) of photonic crystals (PCs) arise from selective constructive interference of incident light. Here, an ink-jet printable and rewritable block copolymer (BCP) SC display is demonstrated, which can be quickly written and erased over 50 times with resolution nearly equivalent to that obtained with a commercial office ink-jet printer. Moreover, the writing process employs an easily modified printer for position- and concentration-controlled deposition of a single, colorless, water-based ink containing a reversible crosslinking agent, ammonium persulfate. Deposition of the ink onto a self-assembled BCP PC film comprising a 1D stack of alternating layers enables differential swelling of the written BCP film and produces a full-colored SC display of characters and images. Furthermore, the information can be readily erased and the system can be reset by application of hydrogen bromide. Subsequently, new information can be rewritten, resulting in a chemically rewritable BCP SC display.

Nonvolatile Transistor Memory with Self-Assembled Semiconducting Polymer Nanodomain Floating Gates.

Organic field effect transistor based nonvolatile memory (OFET-NVM) with semiconducting nanofloating gates offers additional benefits over OFET-NVMs with conventional metallic floating gates due to the facile controllability of charge storage based on the energetic structure of the floating gate. In particular, an all-in-one tunneling and floating-gate layer in which the semiconducting polymer nanodomains are self-assembled in the dielectric tunneling layer is promising. In this study, we utilize crystals of a p-type semiconducting polymer in which the crystalline lamellae of the polymer are spontaneously developed and embedded in the tunneling matrix as the nanofloating gate. The widths and lengths of the polymer nanodomains are approximately 20 nm and a few hundred nanometers, respectively. An OFET-NVM containing the crystalline nanofloating gates exhibits memory performance with a large memory window of 10 V, programming/erasing switching endurance for over 500 cycles, and a long retention time of 5000 s. Moreover, the device performance is improved by comixing with an n-type semiconductor; thus, the solution-processed p- and n-type double floating gates capable of storing both holes and electrons allow for the multilevel operation of our OFET-NVM. Four highly reliable levels (two bits per cell) of charge trapping and detrapping are achieved using this OFET-NVM by accurately choosing the programming/erasing voltages.

Multilevel non-volatile data storage utilizing common current hysteresis of networked single walled carbon nanotubes.

The characteristic source-drain current hysteresis frequently observed in field-effect transistors with networked single walled carbon-nanotube (NSWNT) channels is problematic for the reliable switching and sensing performance of devices. But the two distinct current states of the hysteresis curve at a zero gate voltage can be useful for memory applications. In this work, we demonstrate a novel non-volatile transistor memory with solution-processed NSWNTs which are suitable for multilevel data programming and reading. A polymer passivation layer with a small amount of water employed on the top of the NSWNT channel serves as an efficient gate voltage dependent charge trapping and de-trapping site. A systematic investigation evidences that the water mixed in a polymer passivation solution is critical for reliable non-volatile memory operation. The optimized device is air-stable and temperature-resistive up to 80 °C and exhibits excellent non-volatile memory performance with an on/off current ratio greater than 10(4), a switching time less than 100 ms, data retention longer than 4000 s, and write/read endurance over 100 cycles. Furthermore, the gate voltage dependent charge injection mediated by water in the passivation layer allowed for multilevel operation of our memory in which 4 distinct current states were programmed repetitively and preserved over a long time period.

Highly reliable top-gated thin-film transistor memory with semiconducting, tunneling, charge-trapping, and blocking layers all of flexible polymers.

The core components of a floating-gate organic thin-film transistor nonvolatile memory (OTFT-NVM) include the semiconducting channel layer, tunneling layer, floating-gate layer, and blocking layer, besides three terminal electrodes. In this study, we demonstrated OTFT-NVMs with all four constituent layers made of polymers based on consecutive spin-coating. Ambipolar charges injected and trapped in a polymer electret charge-controlling layer upon gate program and erase field successfully allowed for reliable bistable channel current levels at zero gate voltage. We have observed that the memory performance, in particular the reliability of a device, significantly depends upon the thickness of both blocking and tunneling layers, and with an optimized layer thickness and materials selection, our device exhibits a memory window of 15.4 V, on/off current ratio of 2 × 10(4), read and write endurance cycles over 100, and time-dependent data retention of 10(8) s, even when fabricated on a mechanically flexible plastic substrate.

Organic one-transistor-type nonvolatile memory gated with thin ionic liquid-polymer film for low voltage operation.

As one of the most emerging next-generation nonvolatile memories, one-transistor (1T)-type nonvolatile memories are of great attention due to their excellent memory performance and simple device architecture suitable for high density memory arrays. In particular, organic 1T-type memories containing both organic semiconductors and insulators are further beneficial because of their mechanical flexibility with low cost fabrication. Here, we demonstrate a new flexible organic 1T-type memory operating at low voltage. The low voltage operation of a memory less than 10 V was obtained by employing a polymer gate insulator solution blended with ionic liquid as a charge storage layer. Ionic liquid homogeneously dissolved in a thin poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) film gave rise to low voltage operation of a device due to its high capacitance. Simultaneously, stable charge trapping of either anions or cations efficiently occurred in the polymer matrix, dependent upon gate bias. Optimization of ionic liquid in PVDF-TrFE thus led to an air-stable and mechanically flexible organic 1T-type nonvolatile memory operating at programming voltage of ±7 V with large ON/OFF current margin of approximately 10(3), reliable time-dependent data retention of more than 10(4) seconds, and write/read endurance cycles of 80.