Metal-oxide assisted surface treatment of polyimide gate insulators for high-performance organic thin-film transistors.

We developed a facile method for treating polyimide-based organic gate insulator (OGI) surfaces with self-assembled monolayers (SAMs) by introducing metal-oxide interlayers, called the metal-oxide assisted SAM treatment (MAST). To create sites for surface modification with SAM materials on polyimide-based OGI (KPI) surfaces, the metal-oxide interlayer, here amorphous alumina (α-Al2O3), was deposited on the KPI gate insulator using spin-coating via a rapid sol-gel reaction, providing an excellent template for the formation of a high-quality SAM with phosphonic acid anchor groups. The SAM of octadecylphosphonic acid (ODPA) was successfully treated by spin-coating onto the α-Al2O3-deposited KPI film. After the surface treatment by ODPA/α-Al2O3, the surface energy of the KPI thin film was remarkably decreased and the molecular compatibility of the film with an organic semiconductor (OSC), 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), was increased. Ph-BTBT-C10 molecules were uniformly deposited on the treated gate insulator surface and grown with high crystallinity, as confirmed by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The mobility of Ph-BTBT-C10 thin-film transistors (TFTs) was approximately doubled, from 0.56 ± 0.05 cm2 V-1 s-1 to 1.26 ± 0.06 cm2 V-1 s-1, after the surface treatment. The surface treatment of α-Al2O3 and ODPA significantly decreased the threshold voltage from -21.2 V to -8.3 V by reducing the trap sites in the OGI and improving the interfacial properties with the OSC. We suggest that the MAST method for OGIs can be applied to various OGI materials lacking reactive sites using SAMs. It may provide a new platform for the surface treatment of OGIs, similar to that of conventional SiO2 gate insulators.

Surface grafting of octylamine onto poly(ethylene-alt-maleic anhydride) gate insulators for low-voltage DNTT thin-film transistors.

This study investigates a spin-coating method for modifying the surface properties of a poly(ethylene-alt-maleic anhydride) (PEMA) gate insulator. The 60 nm-thick PEMA thin film exhibits excellent electrical insulating properties, and its surface properties could be easily modified by surface grafting of octylamine. Due to surface treatment via spin-coating, the surface energy of the PEMA gate insulator decreased, the crystal quality of the organic semiconductor improved, and consequently the performance of low-voltage organic thin-film transistors (TFTs) was enhanced. Our results suggest that the surface treatment of the PEMA gate insulator could be a simple and effective method for enhancing the performance of organic TFTs.

Stretchable and Skin-Conformal Thermoelectric Generator with Highly Flexible and Plastically Bendable Silver Selenide Films.

Among inorganic thermoelectric materials, flexible thermoelectric materials have attracted considerable attention. In this study, highly flexible and plastically bendable silver selenide films with excellent thermoelectric performance at room temperature are presented. The flexibility of the freestanding silver selenide films was significantly improved through a simple annealing treatment. The highly flexible silver selenide films with a thickness of 26.0 µm displayed outstanding n-type thermoelectric performance, achieving an in-plane zT value of 0.38 at room temperature. Because silver selenide films are plastically bendable with a bending radius of less than 1 mm, they can be shaped into various forms. To achieve stretchability and skin-conformality in the thermoelectric generator, S-shaped silver selenide strips were used as an n-type thermoelectric element. Effective harvesting of electricity from heat of the human body was successfully demonstrated.

Surface modification of a polyimide gate insulator with an yttrium oxide interlayer for aqueous-solution-processed ZnO thin-film transistors.

We report a simple approach to modify the surface of a polyimide gate insulator with an yttrium oxide interlayer for aqueous-solution-processed ZnO thin-film transistors. It is expected that the yttrium oxide interlayer will provide a surface that is more chemically compatible with the ZnO semiconductor than is bare polyimde. The field-effect mobility and the on/off current ratio of the ZnO TFT with the YOx/polyimide gate insulator were 0.456 cm(2)/V·s and 2.12 × 10(6), respectively, whereas the ZnO TFT with the polyimide gate insulator was inactive.

Surface modification of polyimide gate insulators for solution-processed 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C10-BTBT) thin-film transistors.

The surface property of a polyimide gate insulator was successfully modified with an n-octadecyl side-chain. Alkyl chain-grafted poly(amic acid), the polyimide precursor, was synthesized using the diamine comonomer with an alkyl side-chain. By adding a base catalyst to the poly(amic acid) coating solution, the imidization temperature of the spin-coated film could be reduced to 200 °C. The 350 nm-thick polyimide film had a dielectric constant of 3.3 at 10 kHz and a leakage current density of less than 8.7 × 10(-10) A cm(-2), while biased from 0 to 100 V. To investigate the potential of the alkyl chain-grafted polyimide film as a gate insulator for solution-processed organic thin-film transistors (TFTs), we fabricated C(10)-BTBT TFTs. C(10)-BTBT was deposited on the alkyl chain-grafted polyimide gate insulator by spin-coating, forming a well-ordered crystal structure. The field-effect mobility and the on/off current ratio of the TFT device were measured to be 0.20-0.56 cm(2) V(-1) s(-1) and >10(5), respectively.

Substrate-Free Thermoelectric 25 µm-Thick Ag2Se Films with High Flexibility and In-Plane zT of 0.5 at Room Temperature.

Thermoelectric inorganic films are flexible when sufficiently thin. By removing the substrate, that is, making them free-standing, the flexibility of thermoelectric films can be enhanced to the utmost extent. However, studies on the flexibility of free-standing thermoelectric inorganic films have not yet been reported. Herein, the high thermoelectric performance and flexibility of free-standing thermoelectric Ag2Se films are reported. Free-standing Ag2Se films with a thickness of 25.0 ± 3.9 µm exhibited an in-plane zT of 0.514 ± 0.060 at room temperature. These films exhibited superior flexibility compared to Ag2Se films constrained on a substrate. The flexibility of the Ag2Se films was systematically investigated in terms of bending strain, bending radius, thickness, and elastic modulus. Using free-standing Ag2Se films, a substrate-free, flexible thermoelectric generator was fabricated. The energy-harvesting capacity of the thermoelectric generator was also demonstrated.

Zero Energy Heating of Solvent with Network-Structured Solar-Thermal Material: Eco-Friendly Palladium Catalysis of the Suzuki Reaction.

Solar-thermal materials absorb sunlight and convert it into heat, which is released into the surrounding medium. Utilization of solar energy for solvent heating can be a potential method of eco-friendly organic reactions. However, to date, significant heating of the entire volume of a solvent by 1 sun illumination has not been reported. In the present work, a network structure of solar-thermal materials has been proposed for zero energy heating of a solvent under 1 sun illumination. A network-structured solar-thermal material with an additional catalytic function was fabricated by sputtering palladium into a melamine sponge. The nanocrystalline palladium-decorated melamine sponge (Pd-sponge) has excellent sunlight absorption properties in the entire wavelength range that enable efficient solar-thermal conversion. The Pd-sponge can reduce heat loss to the surroundings by effectively blocking thermal radiation from the heated solvent. The temperature of the reaction solution with the ethanol-water mixture filled in the Pd-sponge increased from 23 to 59 °C under 1 sun illumination. The elevated temperature of the reaction solutions by solar-thermal conversion successfully accelerated the heterogeneous Pd-catalyzed Suzuki coupling reactions with high conversions. Easy and low-energy-consuming multicycle use of the solar-thermal and catalytic properties of the Pd-sponge has also been demonstrated.

One-pot bifunctionalization of silica nanoparticles conjugated with bioorthogonal linkers: application in dual-modal imaging.

Covalent surface modification of silica nanoparticles (SNPs) offers great potential for the development of multimodal nanomaterials for biomedical applications. Herein, we report the synthesis of covalently conjugated bifunctional SNPs and their application to in vivo multimodal imaging. Bis(methallyl)silane 15 with cyclopropene and maleimide, designed as a stable bifunctional linker, was efficiently synthesized by traceless Staudiger ligation, and subsequently introduced onto the surface of monodispersed SNPs via Sc(OTf)3-catalyzed siloxane formation. The bifunctional linker-grafted SNP 20 underwent both thiol-conjugated addition and tetrazine cycloaddition in one pot. Finally, positron emission tomography/computed tomography and fluorescence imaging study of dual functional SNP [125I]28 labeled with NIR dye and 125I isotope showed a prolonged circulation in mice, which is conducive to the systemic delivery of therapeutics.

In situ catalytic activity of CuO nanosheets synthesized from a surfactant-lamellar template.

CuO nanosheets approximately 0.8 nm thick were synthesized under ambient conditions within a few hours using a surfactant lamellar mesophase as a soft template. In aqueous media, metal ions and anionic surfactants form a lamellar mesophase. In the lamellar layers, metal ions can crystallize without structural collapse. Highly ordered CuO nanosheet/surfactant lamellar layers formed in an aqueous solution can be easily delaminated by washing with water. The use of the delaminated CuO nanosheet catalyst instead of traditional metallic catalysts resulted in a reduction reaction of 4-nitrophenol with NaBH4 that obeyed zero-order kinetics. This indicates in situ conversion of CuO to Cu in the reaction solution. Cu in situ reduced by BH4- acted as a catalyst relaying electrons for the reduction of 4-NP. The catalytic reaction was investigated by UV-vis spectroscopy, and the reduction and crystalline structures of the nanosheets were analyzed by UV-vis spectroscopy and X-ray diffraction. These results indicate CuO nanosheets to be an attractive alternative to metal catalysts in reactions involving hydrogen.

Optimized Thermoelectric Performance of Carbon Nanoparticle-Carbon Nanotube Heterostructures by Tuning Interface Barrier Energy.

Herein, thermoelectric carbon nanoparticle (CNP)-carbon nanotube (CNT) heterostructures are introduced as a promising flexible thermoelectric material. The optimal barrier energy between the CNP and CNT increases the Seebeck coefficient (S) of the heterostructures through the energy filtering effect. For optimized thermoelectric performance, the CNP-CNT barrier energy can be effectively tuned by controlling the work function of the CNPs. The optimized p-type CNP-CNT heterostructures exhibited S and power factor (PF) of 50.6 ± 1.4 µV K-1 and 400 ± 26 µW m-1 K-2, respectively. The n-type CNP-CNT heterostructures, optimized for another work function of the CNPs, exhibited S and PF of up to -37.5 ± 3.4 µV K-1 and 214 ± 42 µW m-1 K-2, respectively. The energy harvesting capability of a thermoelectric generator prepared using p- and n-type CNP-CNT heterostructures with optimized barrier energies is demonstrated. The thermoelectric generator with 10 p-type and 9 n-type thermoelectric elements exhibited a maximum output power of 0.12 µW from a ΔT of 5 K. This work shows a facile strategy for synthesizing thermoelectric CNP-CNT heterostructures with optimized energy filtering effects. Application to the thermoelectric device on a paper substrate is also discussed.

Skin protein-derived peptide-conjugated vesicular nanocargos for selected skin cell targeting and consequent activation.

Several studies have reported that a drug nanocarrier conjugated with ligands having cell binding ability improves drug delivery performance, but multiple cell-targeting and the resultant activation in designated cells has not been investigated yet. This study reports a skin cell multi-targeting vesicular nanocargo system. We selectively conjugated several skin protein-derived cell-targeting peptides (CTPs), including KTTKS, NAP-amide, and Lam332, to amphiphilic polymer-reinforced lipid nanovesicles (PLNVs) to specifically target fibroblasts, melanocytes, and keratinocytes, respectively, through effective association with the corresponding cell membrane receptors. We then showed that CTP-conjugated PLNVs specifically bind to the designated skin cells, even in a mixture of different types of skin cells, eventually leading to skin cell multi-targeting and consequent activation. These results highlight that this CTP-conjugated PLNV system has significant potential for developing an intelligent cellular drug delivery technology for dermatological applications.

High-Performance n-Type Carbon Nanotubes Doped by Oxidation of Neighboring Sb2Te3 for a Flexible Thermoelectric Generator.

Flexible thermoelectric devices can be potentially used for flexible cooling and energy harvesting from various heat sources such as the human body. However, the development of flexible thermoelectric materials with excellent thermoelectric performance is still very challenging. In this study, a simple solution process is proposed for the preparation of flexible inorganic/carbon nanotube hybrid films with record power factors among those of the reported flexible n-type thermoelectric materials. The hybrid films fabricated by bar-coating a carbon nanotube-dispersed Sb2Te3 solution exhibit n-type power factors of up to 2440 ± 267 µV m-1 K-2 at room temperature. The dissolved Sb2Te3 recrystallizes on the carbon nanotube surfaces and form hybrid solids. The ultrahigh power factor may be originated from the effective n-doping of carbon nanotubes by the oxidation of neighboring Sb2Te3. Using the thermoelectric hybrid film, a multilayer stacked thermoelectric generator is fabricated. The flexible device with a thermal contact area of 3.8 cm2 exhibits an output power of up to 11.3 µW at a vertical ΔT of 7.5 K. This study paves the way for the realization of flexible thermoelectric devices with various device geometries.

Shape-Deformable Thermoelectric Carbon Nanotube Doughs.

In this study, shape-deformable thermoelectric p- and n-type doughs are fabricated by blending single-walled carbon nanotubes with excess amounts of nonvolatile liquid surfactants for efficient energy harvesting from diverse heat sources. The shape-deformable thermoelectric doughs exhibit touch-healing properties and can be easily molded into arbitrary shapes by simple shaping methods, such as those commonly used for rubber play dough. We used cube-shaped thermoelectric doughs to fabricate a vertical thermoelectric generator. Considering the shape-deformable properties of the thermoelectric doughs, a contraction strain of ∼2% in the through-plane direction of the thermoelectric generator can be applied for an effective application of ΔT. We show that the thermoelectric generator we built with eight p-n pairs exhibits a maximum output power of 2.2 µW at a vertical ΔT of 15 K. Our results demonstrate the energy harvesting capability of thermoelectric generators with shape-deformable p- and n-type doughs. Owing to the properties of this material, thermoelectric generators with various device geometries can be fabricated for energy harvesting from a diverse range of nonflat heat sources.

Solution-Processed Carbon Nanotube Buckypapers for Foldable Thermoelectric Generators.

Freestanding single-walled carbon nanotube (SWCNT) buckypapers with thicknesses of ∼30 µm are fabricated using a simple bar-coating process. The Seebeck coefficient and electrical conductivity of the SWCNT buckypapers are affected by the composition of the dispersion solvent mixture. The maximum p-type power factor of a SWCNT buckypaper is 411 ± 13 µW m-1 K-2. The inverse relationship between the Seebeck coefficient and electrical conductivity of the SWCNT buckypapers may be explained by the number density of junctions between the SWCNT bundles. Using the SWCNT buckypapers, which can be cut, folded, and pasted, a foldable thermoelectric generator is fabricated. The thermoelectric generator folded to an area of 2.25 cm2 exhibits a maximum power of 10.3 µW at a vertical temperature difference of 30 K.

Aqueous self-assembly of amphiphilic nanocrystallo-polymers and their surface-active properties.

The self-assembly of nanocrystals using a bottom-up approach is advantageous because it is possible to control their direct structure at a nanometre length scale and their collective optical and electronic properties. Here, we present the novel fabrication and aqueous self-assembly of amphiphilic nanocrystallo-polymers. Hydrophobic nanocrystals are used as the hydrophobic component of amphiphiles, and they can drive the hydrophobic interaction-mediated direct self-assembly to create various nanostructures, such as spherical aggregates, core-shell unimolecular micelles, and cylinders. The nanocrystals can be uniformly arranged in the core of the nanostructures. We further show that the amphiphilic nanocrystallo-polymers have dynamic self-assembling and surface-active properties.

Thermoelectric fibers from well-dispersed carbon nanotube/poly(vinyliedene fluoride) pastes for fiber-based thermoelectric generators.

High-performance thermoelectric composite fibers were prepared via simple wet-spinning of single-walled carbon nanotube (SWCNT)/poly(vinylidene fluoride) (PVDF) pastes using a common solvent/coagulation system. By improving the content and dispersion state of SWCNTs in the composite fibers, the thermoelectric performance could be effectively enhanced. With n-type doping of SWCNTs using polyethylenimine, high-performance n-type SWCNT/PVDF composite fibers could be prepared. The power factors of the p- and n-type SWCNT/PVDF composite fibers with the SWCNT content of 50 wt% were 378 ± 56 and 289 ± 98 µW m-1 K-2, respectively. The electric power generation capability of an organic thermoelectric generator with the p- and n-type composite fibers was confirmed.

Solution-Processable, Thin, and High-κ Dielectric Polyurea Gate Insulator with Strong Hydrogen Bonding for Low-Voltage Organic Thin-Film Transistors.

We developed a solution-processable, thin, and high-dielectric polyurea-based organic gate insulator for low-voltage operation and high performance of organic thin-film transistors (OTFTs). A 60 nm-thick polyurea thin film exhibited a high dielectric constant of 5.82 and excellent electrical insulating properties owing to strong hydrogen bonding. The hydrogen bonding of the synthesized polyurea was confirmed using infrared spectroscopy and was quantitatively evaluated by measuring the interactive force using atomic force microscopy. Moreover, the effect of hydrogen bonding of polyurea on the insulating properties was systematically investigated through the combination of various monomers and control of the thickness of the polyurea film. The dinaphtho[2,3- b:2',3'- f]thieno[3,2- b]thiophene-based OTFTs with the polyurea gate insulator showed excellent thin-film transistor (TFT) performance with a field-effect mobility of 1.390 cm2/V·s and an on/off ratio of ∼105 at a low operation voltage below 2 V. In addition, it is possible to fabricate flexible polymer organic semiconductor (OSC)-based TFT devices using a solution process, owing to excellent solvent stability in various organic solvents. We believe that the solution-processable polyurea gate insulator with a high dielectric constant and good insulation properties is a promising candidate for low-voltage-operated OTFTs using various OSCs.

High Thermoelectric Power Factor of a Diketopyrrolopyrrole-Based Low Bandgap Polymer via Finely Tuned Doping Engineering.

We studied the thermoelectric properties of a diketopyrrolopyrrole-based semiconductor (PDPP3T) via a precisely tuned doping process using Iron (III) chloride. In particular, the doping states of PDPP3T film were linearly controlled depending on the dopant concentration. The outstanding Seebeck coefficient of PDPP3T assisted the excellent power factors (PFs) over 200 µW m-1K-2 at the broad range of doping concentration (3-8 mM) and the maximum PF reached up to 276 µW m-1K-2, which is much higher than that of poly(3-hexylthiophene), 56 µW m-1K-2. The high-mobility of PDPP3T was beneficial to enhance the electrical conductivity and the low level of total dopant volume was important to maintain high Seebeck coefficients. In addition, the low bandgap PDPP3T polymer effiectively shifted its absorption into near infra-red area and became more colorless after doping, which is great advantage to realize transparent electronic devices. Our results give importance guidance to develop thermoelectric semiconducting polymers and we suggest that the use of low bandgap and high-mobility polymers, and the accurate control of the doping levels are key factors for obtaining the high thermoelectric PF.

3D-printable, highly conductive hybrid composites employing chemically-reinforced, complex dimensional fillers and thermoplastic triblock copolymers.

The use of 3-dimensional (3D) printable conductive materials has gained significant attention for various applications because of their ability to form unconventional geometrical architectures that cannot be realized with traditional 2-dimensional printing techniques. To resolve the major requisites in printed electrodes for practical applications (including high conductivity, 3D printability, excellent adhesion, and low-temperature processability), we have designed a chemically-reinforced multi-dimensional filler system comprising amine-functionalized carbon nanotubes, carboxyl-terminated silver nanoparticles, and Ag flakes, with the incorporation of a thermoplastic polystyrene-polyisoprene-polystyrene (SIS) triblock copolymer. It is demonstrated that both high conductivity, 22 939 S cm-1, and low-temperature processability, below 80 °C, are achievable with the introduction of chemically anchored carbon-to-metal hybrids and suggested that the highly viscous composite fluids employing the characteristic thermoplastic polymer are readily available for the fabrication of various unconventional electrode structures by a simple dispensing technique. The practical applicability of the 3D-printable highly conductive composite paste is confirmed with the successful fabrication of wireless power transmission modules on substrates with extremely uneven surface morphologies.

Enhanced diffraction efficiency in a photorefractive liquid crystal cell with poly(9-vinylcarbazole)-infiltrated mesoporous TiO2 layers.

The photorefractive effect of a layer-structured liquid crystal cell was significantly enhanced when a C60-doped poly(9-vinylcarbazole) (PVK)/TiO2 nanocomposite was used in two photoconductive layers. The C60-doped PVK/TiO2 nanocomposite film was prepared by infiltrating C60-doped PVK into a highly ordered mesoporous TiO2 layer. The addition of the TiO2 layer to the C60-doped PVK layer increased the first-order Raman-Nath diffraction efficiency from 24% to 42.9%. This enhancement of diffraction efficiency is attributed to a blocking effect of charge recombination in the composite layer. The electron transfer from the PVK layer into the TiO2 layer would decrease the recombination of photogenerated charges in the PVK layer, while charges in the PVK layer could participate in the formation of a space-charge field.