Beomjin Jeong

High Through-Plane Thermal Conduction of Graphene Nanoflake Filled Polymer Composites Melt-Processed in an L-Shape Kinked Tube

Design of materials to be heat-conductive in a preferred direction is a crucial issue for efficient heat dissipation in systems using stacked devices. Here, we demonstrate a facile route to fabricate polymer composites with directional thermal conduction. Our method is based on control of the orientation of fillers with anisotropic heat conduction. Melt-compression of solution-cast poly(vinylidene fluoride) (PVDF) and graphene nanoflake (GNF) films in an L-shape kinked tube yielded a lightweight polymer composite with the surface normal of GNF preferentially aligned perpendicular to the melt-flow direction, giving rise to a directional thermal conductivity of approximately 10 W/mK at 25 vol % with an anisotropic thermal conduction ratio greater than six. The high directional thermal conduction was attributed to the two-dimensional planar shape of GNFs readily adaptable to the molten polymer flow, compared with highly entangled carbon nanotubes and three-dimensional graphite fillers. Furthermore, our composite with its density of approximately 1.5 g/cm(3) was mechanically stable, and its thermal performance was successfully preserved above 100 °C even after multiple heating and cooling cycles. The results indicate that the methodology using an L-shape kinked tube is a new way to achieve polymer composites with highly anisotropic thermal conduction.

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.

Solvent-Assisted Gel Printing for Micropatterning Thin Organic–Inorganic Hybrid Perovskite Films

While tremendous efforts have been made for developing thin perovskite films suitable for a variety of potential photoelectric applications such as solar cells, field-effect transistors, and photodetectors, only a few works focus on the micropatterning of a perovskite film which is one of the most critical issues for large area and uniform microarrays of perovskite-based devices. Here we demonstrate a simple but robust method of micropatterning a thin perovskite film with controlled crystalline structure which guarantees to preserve its intrinsic photoelectric properties. A variety of micropatterns of a perovskite film are fabricated by either microimprinting or transfer-printing a thin spin-coated precursor film in soft-gel state with a topographically prepatterned elastomeric poly(dimethylsiloxane) (PDMS) mold, followed by thermal treatment for complete conversion of the precursor film to a perovskite one. The key materials development of our solvent-assisted gel printing is to prepare a thin precursor film with a high-boiling temperature solvent, dimethyl sulfoxide. The residual solvent in the precursor gel film makes the film moldable upon microprinting with a patterned PDMS mold, leading to various perovskite micropatterns in resolution of a few micrometers over a large area. Our nondestructive micropatterning process does not harm the intrinsic photoelectric properties of a perovskite film, which allows for realizing arrays of parallel-type photodetectors containing micropatterns of a perovskite film with reliable photoconduction performance. The facile transfer of a micropatterned soft-gel precursor film on other substrates including mechanically flexible plastics can further broaden its applications to flexible photoelectric systems.

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.

Electrically Tunable Soft-Solid Block Copolymer Structural Color.

One-dimensional photonic crystals based on the periodic stacking of two different dielectric layers have been widely studied, but the fabrication of mechanically flexible polymer structural color (SC) films, with electro-active color switching, remains challenging. Here, we demonstrate free-standing electric field tunable ionic liquid (IL) swollen block copolymer (BCP) films. Placement of a polymer/ionic liquid film-reservoir adjacent to a self-assembled poly(styrene-block-quaternized 2-vinylpyridine) (PS-b-QP2VP) copolymer SC film allowed the development of red (R), green (G), and blue (B) full-color SC block copolymer films by swelling of the QP2VP domains by the ionic liquid associated with water molecules. The IL-polymer/BCP SC film is mechanically flexible with excellent color stability over several days at ambient conditions. The selective swelling of the QP2VP domains could be controlled by both the ratio of the IL to a polymer in the gel-like IL reservoir layer and by an applied voltage in the range of -3 to +6 V using a metal/IL reservoir/SC film/IL reservoir/metal capacitor type device.

A field-induced hole generation layer for high performance alternating current polymer electroluminescence and its application to extremely flexible devices

The performance of alternating current driven electroluminescent devices significantly depends on the total amount of injected carriers as well as the balance of the number of injected carriers in an emission layer, which requires a careful design of the relative energy level structures of constituent layers. Here, we demonstrate a new field-induced hole generation layer between an emission layer and an insulator for high performance alternating current polymer electroluminescence (AC-PEL). Our hole generation layer of doped poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) in the presence of multi-walled carbon nanotubes was also able to supply sufficient holes, giving rise to a good balance with the number of electrons readily injected from a top electrode. The resulting AC-PEL device exhibits high electroluminescence performance with a low turn-on root-mean-square voltage of 8.8 Vrms, a maximum luminance of 40 919 cd m−2, a maximum current efficiency of 3.74 cd A−1 and a power efficiency of 3.25 lm W−1. Other buffer layers such as WO3 and MoO3 are also suitable as field-induced hole generation layers. Moreover, our hole generation layer enables us to develop an extremely flexible and even foldable AC-PEL device when combined with a polymer insulator as well as a flexible transparent electrode based on the Ag nanowire network.

Organic light emitting board for dynamic interactive display

Interactive displays involve the interfacing of a stimuli-responsive sensor with a visual human-readable response. Here, we describe a polymeric electroluminescence-based stimuli-responsive display method that simultaneously detects external stimuli and visualizes the stimulant object. This organic light-emitting board is capable of both sensing and direct visualization of a variety of conductive information. Simultaneous sensing and visualization of the conductive substance is achieved when the conductive object is coupled with the light emissive material layer on application of alternating current. A variety of conductive materials can be detected regardless of their work functions, and thus information written by a conductive pen is clearly visualized, as is a human fingerprint with natural conductivity. Furthermore, we demonstrate that integration of the organic light-emitting board with a fluidic channel readily allows for dynamic monitoring of metallic liquid flow through the channel, which may be suitable for biological detection and imaging applications.

Solution-processed electron-only tandem polymer light-emitting diodes for broad wavelength light emission

Polymer light-emitting diodes (PLEDs) have been of great interest for flexible mobile displays and large area solid-state lighting due to the possibility of achieving low production costs using solution processes combined with various printing technologies. Although monochromatic operation of PLEDs with high luminous efficiency and device stability has been achieved using numerous strategies, the development of color-tunable PLEDs capable of emitting a broad range of light upon the application of external stimuli still remains challenging. Here, we present a solution-processed broad range color-tunable tandem PLED of inverted and regular light emitting units (LEUs) stacked in series sharing a floating polymer electrode as a charge injection layer between two LEUs. Fine and broad wavelength color control from pure blue to pure orange is achieved when an AC field is applied with different positive-to-negative polarity heights in our tandem PLED. Our AC-driven tandem PLEDs offer a facile route to color-tunable polymer EL devices with a maximum current efficiency (CE) and luminance of 2.5 cd A−1 and 1300 cd m−2 for blue emission under DC reverse and 6.7 cd A−1 and 6000 cd m−2 for orange emission under DC forward as well as 3.1 cd A−1 and 6000 cd m−2 for white emission under AC, respectively.

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.

Block copolymer structural color strain sensor

AbstractThe development of electrically responsive sensors based on the capacitance, voltage, and resistance that can detect and simultaneou sly visualize the large strain involved with human motion is in great demand. Here, we demonstrate a highly stretchable, large strain capacitive sensor that can visualize strain based on the strain-responsive structural color (SC). Our device contains an elastomeric sensing film that produces a capacitance change under strain, in which a self-assembled block copolymer (BCP) photonic crystal (PC) film with 1D periodic in-plane lamellae aligned parallel to the film surface is embedded for the efficient visualization of strain. The capacitance change arises from changes in the dimensions of the elastomer film under strain. The mechanochromic BCP PC film responds to strain, giving rise to an SC change with strain. The initial red SC of the sensor blueshifts and turns blue when the sensor is stretched to 100%, resulting in a full-color SC alteration as a function of the strain. Our BCP SC strain sensor exhibits a fast strain response with multi-cycle reliability of both the capacitance and SC changes over 1000 cycles. This property allows for efficient visible recognition not only of the strained positions during finger bending and poking with a sharp object but also of the shapes of the strained objects.Sensors: Getting the blues under strainA strain sensor that changes color when stretched has been developed by a team in Korea. Flexible sensors that measure pressure or strain could find application as wearable health monitors. One route to achieve such sensors is to use dielectric films whose capacitance changes in response to mechanical stimulus. However, an easy way is needed for wearers to read the output. Cheolmin Park from Yonsei University and colleagues constructed a polymer-based material with a periodic structure that reflects light of different colors depending on the degree to which it is extended. Specifically, they created a block copolymer soft-solid film with one-dimensional periodic layers. The capacitance changes under strain as a result of the dimensional change of the elastomer film, causing the initially red sensor to turn blue when fully stretched.A novel block copolymer structural color strain sensor was developed, capable of electrically sensing a strain in capacitance change with simultaneous visualization of the strain. A thin mechanochromic bilayer of a block copolymer structural film with 1-D periodic lamellae placed on an ionic gel layer allowed for direct visualization of strain while sensing in capacitance when embedded in an elastomeric dielectric medium. Our block copolymer structural strain sensor was suitable for a reflective mode electronic skin which readily recognized human motion of the finger, elbow, and knee.