Insung Bae

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.

Control of thin ferroelectric polymer films for non-volatile memory applications

The article presents the recent research development in controlling molecular and microstructures of thin ferroelectric polymer films for the application of non-volatile memory. A brief overview is given of the history of ferroelectric memory and device architectures based on ferroelectric polymers particularly emphasizing on the device elements such as metal/ferroelectric/metal type capacitor, metal-ferroelectric-insulator-semiconductor (MFIS) diodes and ferroelectric field effect transistor (FeFET) with ferroelectric poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE). Key material and process issues for optimizing the memory performance in each device architecture and thus realizing non-volatile ferroelectric polymer memory are in details discussed, including the control of crystal polymorphs, film thickness, various hetero-material interfaces between ferroelectric polymer and either metal or semiconductor, crystallization and crystal orientation. The current effort of micro and nanopatterning techniques is also addressed for high density and flexible memory arrays.

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.

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.

Tailored Single Crystals of Triisopropylsilylethynyl Pentacene by Selective Contact Evaporation Printing

Organic semiconductors have great potential for application in numerous emerging low-cost and disposable electronic devices such as organic thin fi lm transistors (OTFTs), solar cells, memories, sensors, and fl exible displays. [ 1–3 ] One of the most urgent demands for the realization of such devices is the need to develop new organic semiconductors with a high carrier mobility as well as good and cost-effective processibility for highperformance devices. Chemically-modifi ed pentacene derivatives such as triisopropylsilylethynyl pentacene (TIPS-PEN) have received much attention, in particular in OTFTs. [ 4 , 5–14 ]

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.

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 β 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.

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.

Fabrication of micropatterned ferroelectric gamma poly(vinylidene fluoride) film for non-volatile polymer memory

We describe a facile and robust method for fabricating ferroelectric γ-type poly(vinylidene fluoride) (PVDF) thin films useful for non-volatile polymer memory. Our method is based on heating and cooling rate-independent melt-recrystallization of a thin PVDF film confined under a surface-energy-controlled top layer that strictly forbids paraelectric α crystals. Thin and uniform PVDF films with ferroelectric γ crystals consisting of characteristic twisted lamellae are formed with versatile top layers including metals, oxides, and even polymers. Micropatterns of ferroelectric γ PVDF domains isolated by paraelectric α domains are readily developed when pre-patterned top layers are employed. Our ferroelectric films are conveniently incorporated into arrays of either capacitor or transistor-type non-volatile memory units. Arrays of ferroelectric transistors with vacuum deposited pentacene channels are fabricated with micropatterned γ PVDF films. Furthermore, arrays of bottom-gate ferroelectric transistor memories are demonstrated, in which our ferroelectric PVDF film is directly micropatterned during crystallization under the patterned poly(3-hexyl thiophene) active channels.

High throughput modification of chemically reduced graphene oxides by a conjugated block copolymer in non-polar medium

We present a simple, but robust route to efficiently disperse very high rGO concentrations of chemically reduced graphene oxides (rGOs) in various non-polar solvents and polymers. Our method is based on the noncovalent, nondestructive modification of rGOs with a conjugated block copolymer, poly(styrene-block-paraphenylene) (PS-b-PPP). The dispersion of rGOs occurred because PPP blocks strongly adhered to basal planes of rGOs by π–π interactions, while PS blocks provided good solubility in a variety of non-polar environments. The resulting PS-b-PPP modified rGOs (PMrGOs) showed excellent solubility and dispersion stability that was dependent on the quality of the solvent with respect to the PS blocks. In particular, extremely high solubility of the rGOs, as high as 1.5 mg mL−1, was achieved in THF. Our PMrGOs and their solution blends with other non-polymer polymers such as PS, poly(methylmethacrylate) and poly(isoprene-block-styrene) were conveniently spin-coated on various substrates, giving rise to ultra-thin nanohybrid films where the amount of rGO can be systematically controlled. The scalable and simple strategy employed for fabricating rGO nanohybrid films allowed us to assemble a high performance non-volatile resistive polymer memory device in which the bias-dependent trapping and de-trapping of injected charges were efficiently manipulated on the surface of highly dispersed rGO sheets in the nanohybrid.