Giyoung Song

Flexible transition metal dichalcogenide nanosheets for band-selective photodetection.

The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 μm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra.

Gas-Driven Ultrafast Reversible Switching of Super-hydrophobic Adhesion on Palladium-Coated Silicon Nanowires

A gas-driven ultrafast adhesion switching of water droplets on palladium-coated Si nanowire arrays is demonstrated. By regulating the gas-ambient between the atmosphere and H2 , the super-hydrophobic adhesion is repeatedly switched between water-repellent and water-adhesive. The capability of modulating the super-hydrophobic adhesion on a super-hydrophobic surface with a non-contact mode could be applicable to novel functional lab-on-a-chip platforms.

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.

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.

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.

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.

Controlled Nanopores in Thin Films of Nonstoichiometrically Supramolecularly Assembled Graft Copolymers

Herein, nanometer-scale morphologies of graft-copolymer-like supramolecular thin films, composed of sulfonic acid terminated polystyrene (SPS) and poly(2-vinylpyridine) (P2VP), and their application to antireflection coatings were investigated. The intermolecular complexes of SPS and P2VP, formed through nonstoichiometric multiple hydrogen bonding between the sulfonic acid group of SPS and the nitrogen atom in pyridine unit of P2VP, occurring in film deposition allowed for the formation of spherical micelles (with SPS and P2VP as the corona and core, respectively) in the thin film. Interestingly, the domain size of the micelles was tunable from approximately 20 to 90 nm on average by controlling either the blend ratio of components or the concentration of polymer solution. Furthermore, nanoporous thin films could be easily prepared by removing the core of micelle-based nanostructures by using a simple solvent etching process, leaving sulfonic acid groups on the surface of nanopores, which can be utilized as potential functional sites. Those resultant nanoporous thin films were conveniently employed as an antireflection layer on a glass substrate, giving a maximum 97.8 % transmittance in the visible wavelength range.

High performance thermal conduction of silver microparticles thermos-compressed in three-dimensionally interconnected polystyrene beads

We demonstrate a facile strategy to prepare polymer composite with anomalously increased thermal conductivity based on Ag network and structured polymer matrix. Conventional thermal compression process combined with colloidal type thermoplastic beads as structural matrix enabled efficient interconnected thermal conduction pathways of Ag microparticles, giving rise to extremely high thermal conductivity of 120 W/mK at 70 vol% of Ag particles comparable with one third of the value of pure Ag. The systematic investigation with different types of polymer matrix, such as thermoset epoxy and cross-linked colloidal bead, suggests that appropriate frictional force exerted on Ag particles with polymer matrix during compression is of prime importance for developing Ag conduction pathways. Furthermore, the thermoplastic PS beads matrix also provides sufficient mechanical strength to preserve the networked Ag pathways with firm interfaces.

Surface functionalized nanostructures: Via position registered supramolecular polymer assembly

Versatile control of cylindrical nanostructures formed by supramolecular assembly of end-functionalized polymer blends is demonstrated not only in their orientation over large areas but also in their surface chemical functionalities. Two binary blends consisting of supramolecular analogues of diblock copolymers with complementary end-sulfonated and aminated groups are investigated, viz., mono-end-functionalized polymers of (i) one-end-sulfonated polystyrene (SPS) and one-end-aminated poly(butadiene) (APBD) and (ii) one end-aminated polystyrene (APS) and one end-sulfonated poly(butadiene) (SPBD). The orientation of the cylinders with respect to the substrate surface depends on the solvent annealing time; either hexagonally ordered vertical cylinders or in-plane ones are readily obtained by controlling the solvent annealing time. Selective chemical etching of one of the polymers provides four different chemically modified nanostructures, viz., hexagonally ordered cylindrical holes and cylindrical posts with either sulfonate or amine surface functional groups. Additional supramolecular assembly is successfully achieved by solution coating either polymers or organic dyes that complementarily interact with the functional groups on the nanostructures. Furthermore, the supramolecularly assembled nanostructures are controlled by confining them to topographically pre-patterned Si substrates with pattern geometries of various shapes and sizes to produce globally ordered vertical or in-plane cylinders with chemical functionalities on their surfaces.

Multifunctional Woven Structure Operating as Triboelectric Energy Harvester, Capacitive Tactile Sensor Array, and Piezoresistive Strain Sensor Array

This paper presents a power-generating sensor array in a flexible and stretchable form. The proposed device is composed of resistive strain sensors, capacitive tactile sensors, and a triboelectric energy harvester in a single platform. The device is implemented in a woven textile structure by using proposed functional threads. A single functional thread is composed of a flexible hollow tube coated with silver nanowires on the outer surface and a conductive silver thread inside the tube. The total size of the device is 60 × 60 mm2 having a 5 × 5 array of sensor cell. The touch force in the vertical direction can be sensed by measuring the capacitance between the warp and weft functional threads. In addition, because silver nanowire layers provide piezoresistivity, the strain applied in the lateral direction can be detected by measuring the resistance of each thread. Last, with regard to the energy harvester, the maximum power and power density were measured as 201 μW and 0.48 W/m2, respectively, when the device was pushed in the vertical direction.