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.

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.

Self-assembled block copolymer micelles with silver-carbon nanotube hybrid fillers for high performance thermal conduction.

The development of polymer-filled composites with an extremely high thermal conductivity (TC) that is competitive with conventional metals is in great demand due to their cost-effective process, light weight, and easy shape-forming capability. A novel polymer composite with a large thermal conductivity of 153 W m(-1) K(-1) was prepared based on self-assembled block copolymer micelles containing two different fillers of micron-sized silver particles and multi-walled carbon nanotubes. Simple mechanical mixing of the components followed by conventional thermal compression at a low processing temperature of 160 °C produced a novel composite with both structural and thermal stability that is durable for high temperature operation up to 150 °C as well as multiple heating and cooling cycles of ΔT = 100 °C. The high performance in thermal conduction of our composite was mainly attributed to the facile deformation of Ag particles during the mixing in a viscous thermoplastic medium, combined with networked carbon nanotubes uniformly dispersed in the nanoscale structural matrix of block copolymer micelles responsible for its high temperature mechanical stability. Furthermore, micro-imprinting on the composite allowed for topographically periodic surface micropatterns, which offers broader suitability for numerous micro-opto-electronic systems.

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.

Non-volatile organic memory with sub-millimetre bending radius.

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 µm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

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.

Extremely bright full color alternating current electroluminescence of solution-blended fluorescent polymers with self-assembled block copolymer micelles.

Electroluminescent (EL) devices operating at alternating current (AC) electricity have been of great interest due to not only their unique light emitting mechanism of carrier generation and recombination but also their great potential for applications in displays, sensors, and lighting. Despite great success of AC-EL devices, most device properties are far from real implementation. In particular, the current state-of-the art brightness of the solution-processed AC-EL devices is a few hundred candela per square meter (cd m(-2)) and most of the works have been devoted to red and white emission. In this manuscript, we report extremely bright full color polymer AC-EL devices with brightness of approximately 2300, 6000, and 5000 cd m(-2) for blue (B), green (G), and red (R) emission, respectively. The high brightness of blue emission was attributed to individually networked multiwalled carbon nanotubes (MWNTs) for the facile carrier injection as well as self-assembled block copolymer micelles for suppression of interchain nonradiative energy quenching. In addition, effective FRET from a solution-blended thin film of B-G and B-G-R fluorescent polymers led to very bright green and red EL under AC voltage, respectively. The solution-processed AC-EL device also worked properly with vacuum-free Ag paste on a mechanically flexible polymer substrate. Finally, we successfully demonstrated the long-term operation reliability of our AC-EL device for over 15 h.

Wafer-scale arrays of nonvolatile polymer memories with microprinted semiconducting small molecule/polymer blends.

Nonvolatile ferroelectric-gate field-effect transistors (Fe-FETs) memories with solution-processed ferroelectric polymers are of great interest because of their potential for use in low-cost flexible devices. In particular, the development of a process for patterning high-performance semiconducting channel layers with mechanical flexibility is essential not only for proper cell-to-cell isolation but also for arrays of flexible nonvolatile memories. We demonstrate a robust route for printing large-scale micropatterns of solution-processed semiconducting small molecules/insulating polymer blends for high performance arrays of nonvolatile ferroelectric polymer memory. The nonvolatile memory devices are based on top-gate/bottom-contact Fe-FET with ferroelectric polymer insulator and micropatterned semiconducting blend channels. Printed micropatterns of a thin blended semiconducting film were achieved by our selective contact evaporation printing, with which semiconducting small molecules in contact with a micropatterned elastomeric poly(dimethylsiloxane) (PDMS) mold were preferentially evaporated and absorbed into the PDMS mold while insulating polymer remained intact. Well-defined micrometer-scale patterns with various shapes and dimensions were readily developed over a very large area on a 4 in. wafer, allowing for fabrication of large-scale printed arrays of Fe-FETs with highly uniform device performance. We statistically analyzed the memory properties of Fe-FETs, including ON/OFF ratio, operation voltage, retention, and endurance, as a function of the micropattern dimensions of the semiconducting films. Furthermore, roll-up memory arrays were produced by successfully detaching large-area Fe-FETs printed on a flexible substrate with a transient adhesive layer from a hard substrate and subsequently transferring them to a nonplanar surface.

High-temperature operating non-volatile memory of printable single-wall carbon nanotubes self-assembled with a conjugate block copolymer.

Printable non-volatile polymer memories are fabricated with solution-processed nanocomposite films of poly(styrene-block-paraphenylene) (PS-b-PPP) and single-wall carbon nanotubes (SWNTs). The devices show stable data retention at high temperatures of up to 100 °C without significant performance degradation due to the strong, non-destructive, and isomorphic π-π interactions between the SWNTs and PPP block.

Functionalized soft nanoporous materials through supramolecular assembly of end-functionalized polymer blends.

Supramolecular assembly through complementary interaction between molecular subgroups belonging to phase-separating polymer species offers a great opportunity, not only for constructing nanoscale soft templates reminiscent of conventional block copolymer morphologies, but also for tailoring surface properties by facile removal of one of the structure components by cleaving complementary interactions. Herein we report the fabrication of a novel, organic, nanoporous film through supramolecular assembly of two complementarily, end-interacting, mono-end-functionalized polymers under solvent annealing. The film of end-functionalized polymer blends under solvent annealing yielded phase-separated nanodomains that resemble nanoscopically ordered structures of block copolymers, but that are more advantageous due to easily cleavable and exchangeable links between the phase-separated domains. The removal of one of the components of the precursor structure formed from the end-functionalized polymers through cleavage of complementary interactions allowed us to fabricate mono- or multilayered nanoporous structures in which the chemically useful end-functionalities of the remnant polymers are rich on the surface of the pores. The resultant, organic, nanoporous films with tailored surface functionality offer a useful platform for various chemical and biological applications.

Coassembly of metal and titanium dioxide nanocrystals directed by monolayered block copolymer inverse micelles for enhanced photocatalytic performance.

Functional nanostructures of self-assembled block copolymers (BCPs) incorporated with various inorganic nanomaterials have received considerable attention on account of their many potential applications. Here we demonstrate the two-dimensional self-assembly of anisotropic titanium dioxide (TiO(2)) nanocrystals (NCs) and metal nanoparticles (NPs) directed by monolayered poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymer inverse micelles. The independent position-selective assembly of TiO(2) NCs and silver nanoparticles (AgNPs) preferentially in the intermicelle corona regions and the core of micelles, respectively, for instance, was accomplished by spin-coating a mixture solution of PS-b-P4VP and ex situ synthesized TiO(2) NCs, followed by the reduction of Ag salts coordinated in the cores of micelles into AgNPs. Hydrophobic TiO(2) NCs with a diameter and length of approximately 3 nm and 20-30 nm, respectively, were preferentially sequestered in the intermicelle nonpolar PS corona regions energetically favorable with the minimum entropic packing penalty. Subsequent high-temperature annealing at 550 °C not only effectively removed the block copolymer but also transformed the TiO(2) NCs into connected nanoparticles, thus leading to a two-dimensionally ordered TiO(2) network in which AgNPs were also self-organized. The enhanced photocatalytic activity of the AgNP-decorated TiO(2) networks by approximately 27 and 44 % over that of Ag-free TiO(2) networks and randomly deposited TiO(2) nanoparticles, respectively, was confirmed by the UV degradation property of methylene blue.

Flexible non-volatile ferroelectric polymer memory with gate-controlled multilevel operation.

A flexible field-effect transistor with a poly(3-hexylthiophene) (P3HT) active channel and a ferroelectric poly(vinlyidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) insulator exhibits gate-voltage-controllable multilevel non-volatile memory characteristics with highly reliable data retention and endurance.

Compression of cross-linked poly(vinylidene fluoride-co-trifluoro ethylene) films for facile ferroelectric polarization.

In this study, we demonstrated a facile route for enhancing the ferroelectric polarization of a chemically cross-linked poly(vinylidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) film. Our method is based on thermally induced cross-linking of a PVDF-TrFE film with a 2,2,4-trimethyl-1,6-hexanediamine (THDA) agent under compression. The remanent polarization (P(r)) of a metal/ferroelectric/metal capacitor containing a cross-linked PVDF-TrFE film increased with pressure up to a certain value, whereas no change in the P(r) value was observed in the absence of THDA. A film cross-linked with 10 wt % THDA with respect to PVDF-TrFE under a pressure of 100 kPa exhibited a P(r) of approximately 5.61 µC/cm(2), which is 1.6 times higher than that in the absence of pressure. The enhanced ferroelectric polarization was attributed to highly ordered 20-nm-thick edge-on crystalline lamellae whose c-axes are aligned parallel to the substrate. The lamellae were effective for ferroelectric switching of the PVDF-TrFE when a cross-linked film was recrystallized under pressure. Furthermore, compression of a PVDF-TrFE film with a topographically prepatterned poly(dimethyl siloxane) mold gave rise to a chemically cross-linked micropattern in which edge-on crystalline lamellae were globally oriented over a very large area.

Chemically cross-linked thin poly(vinylidene fluoride-co-trifluoroethylene)films for nonvolatile ferroelectric polymer memory.

Both chemically and electrically robust ferroelectric poly(vinylidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) films were developed by spin-coating and subsequent thermal annealing with the thermal cross-linking agent 2,4,4-trimethyl-1,6-hexanediamine (THDA). Well-defined ferroelectric ß crystalline domains were developed with THDA up to approximately 50 wt %, with respect to polymer concentration, resulting in characteristic ferroelectric hysteresis polarization-voltage loops in metal/cross-linked ferroelectric layer/metal capacitors with remnant polarization of approximately 4 µC/cm(2). Our chemically networked film allowed for facile stacking of a solution-processable organic semiconductor on top of the film, leading to a bottom-gate ferroelectric field effect transistor (FeFET). A low-voltage operating FeFET was realized with a networked PVDF-TrFE film, which had significantly reduced gate leakage current between the drain and gate electrodes. A solution-processed single crystalline tri-isopropylsilylethynyl pentacene FeFET with a chemically cross-linked PVDF-TrFE film showed reliable I-V hysteresis with source-drain ON/OFF current bistablility of 1 × 10(3) at a sweeping gate voltage of ±20 V. Furthermore, both thermal micro/nanoimprinting and transfer printing techniques were conveniently combined for micro/nanopatterning of chemically resistant cross-linked PVDF-TrFE films.

Nonvolatile polymer memory with nanoconfinement of ferroelectric crystals.

We demonstrate significantly improved performance of a nonvolatile polymeric ferroelectric field effect transistor (FeFET) memory using nanoscopic confinement of poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) within self-assembled organosilicate (OS) lamellae. Periodic OS lamellae with 30 nm in width and 50 nm in periodicity were templated using block copolymer self-assembly. Confined crystallization of PVDF-TrFE not only significantly reduces gate leakage current but also facilitates ferroelectric polarization switching. These benefits are due to the elimination of structural defects and the development of an effective PVDF-TrFE crystal orientation through nanoconfinement. A bottom gate FeFET fabricated using a single-crystalline triisopropylsilylethynyl pentacene channel and PVDF-TrFE/OS hybrid gate insulator shows characteristic source-drain current hysteresis that is fully saturated at a programming voltage of ±8 V with an ON/OFF current ratio and a data retention time of approximately 10(2) and 2 h, respectively.

Block copolymer micelles with near infrared metal phthalocyanine dyes for laser induced writing.

A route has been developed to disperse metal-containing phthalocyanine dyes in a non-polar medium based on amphiphilic block copolymer micelles of poly[styrene-block-(4-vinylpyridine)] (PS-b-P4VP) and poly[styrene-block-(acrylic acid)] (PS-b-PAA) copolymers. Polar P4VP and PAA efficiently encapsulate cobalt(II), manganese(II), and nickel(II) phthalocyanine dyes by axial coordination of nitrogen and µ-oxo bridged dimerization with the transition metals, respectively. Good dispersion of the dyes is confirmed by the linear enhancement of Q-bands in UV-vis absorption spectra with dye concentration. A thin monolayered PS-b-P4VP micelle film that contained a nickel(II) phthalocyanine dye which efficiently adsorbs a laser beam on a localized area to generate a local heat higher than the glass transition temperatures of both blocks. One-dimensional laser writing on the dye-containing film allows the fabrication of a few submicrometer wide line patterns in which the self-assembled nanostructure of the block copolymer is modified by the directional heat arising from laser scanning.