Reproducible Dry Stamping Transfer of PEDOT:PSS Transparent Top Electrode for Flexible Semitransparent Metal Halide Perovskite Solar Cells.

A semitransparent flexible metal halide perovskite (MHP) solar cells were demonstrated by reproducible dry stamping transfer of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS, PH1000) transparent flexible top electrode onto poly(ethylene terephthalate) (PET)/indium tin oxide (ITO)/PEDOT:PSS (AI4083)/MHP/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The reproducible transfer of the PEDOT:PSS top electrode was enabled by the modification of PEDOT:PSS with poly(ethylene imine) (PEI)/2-methoxyethanol (2-MEA) solution. In addition, the PEI/2-MEA modification to PEDOT:PSS resulted in improved conductivity and reduced work function of the top electrode. Therefore, we could fabricate highly efficient flexible semitransparent MHP solar cells with >13% (active area = 1 cm2) power conversion efficiency.

Tissue engineering with electrospun electro-responsive chitosan-aniline oligomer/polyvinyl alcohol.

Mimicking the native tissue is an ultimate goal in tissue engineering. In this study, conductive chitosan was synthesized by coupling with aniline oligomers, and then conductive nanofibers were fabricated using electrospinning technique to mimic the tissue structure and properties. The conductivity of the resulting biomaterial was adjusted to ca. 10-5 S/cm, which can recapitulate electrical properties of the tissue. The structure of nanofiber was evaluated using scanning electron microscopy noticing that the aniline oligomer addition to the system decreased the diameter of the nanofiber because of its hydrophobic nature. Conductive nanofiber exhibited on-demand drug release feature of the conductive webs, signaled by 40% rise in the drug release at 40 min after electrical stimulation in comparison with non-stimulated webs, characteristic of a promising drug release platform. Moreover, biocompatibility evaluation using MTT assay revealed that the conductive substrate provides a higher cellular activity to the platform with respect to non-conductive substrates. Such platforms are the harbingers of the emerging new generation, which can revolutionize the tissue engineering satisfying an enhanced tissue regeneration.

Self-gelling electroactive hydrogels based on chitosan-aniline oligomers/agarose for neural tissue engineering with on-demand drug release.

Designing biomimetic scaffolds is an intellectual challenge of the realm of regenerative medicine and tissue engineering. An electroactive substrate should meet multidisciplinary mimicking the mechanical, electrical, and electrochemical properties of neural tissues. Hydrogels have been known platforms to regulate neural interface modulus, but the lack of conductivity always hampered their applications; hence, developing conductive hydrogels with on-demand drug release has become a concern of tissue engineering. In this work, electroactive hydrogels based on chitosan-aniline oligomer and agarose with self-gelling properties were synthesized, and their electrical, thermal, and electrochemical properties were characterized by Fourier transform infrared (FTIR), cyclic voltammetry (CV), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), and four probe method . The conductivity of the as-prepared aniline oligomer-based hydrogel was ∼10-4 S/cm; which fell within the range of conductivities appropriate for applications in tissue engineering. The aniline oligomer played a key role in controlling the hydrogel properties by regulating the glass transition temperature and thermal properties. In addition, the swelling and degradation rates were decreased because of the hydrophobic properties of the aniline oligomer. The swelling capacity of the pristine hydrogel was ∼800%, while that of the conductive hydrogel decreased to ∼300%. The conductivity of the hydrogel was regulated by modifying the macromolecular architecture through aniline oligomer incorporation thanks to its conductivity on-demand drug release was observed by electrical stimulation, in which a large amount of the drug was released by voltage application. Biocompatibility analysis of the designed hydrogel was indicative of the conductivity enhancement, as reflected in the growth and proliferation of cellular activity.

Synthesis of Single-Crystalline Hexagonal Graphene Quantum Dots from Solution Chemistry.

Graphene-based carbon nanostructures with nanometer dimensions have been of great interest due to the existence of a bandgap. So far, well-ordered edge structure and uniformly synthesized graphene quantum dots (GQDs) with a hexagonal single-crystalline structure have not been directly observed owing to the limited precision of current synthesis approaches. Herein, we report on a novel approach not just for the synthesis of the size-controlled single-crystalline GQDs with hexagonal shape but also for a new discovery on constructing 2D and 3D graphene single crystal structures from d-glucose via catalytic solution chemistry. With size-controlled single-crystalline GQDs, we elucidated the crucial role of edge states on luminescence from the correlation between their crystalline size and exciton lifetime. Furthermore, blue-emissive single-crystalline GQDs were used as an emitter on light-emitting diodes and exhibit stable deep-blue emission regardless of the voltage and doping level.

Zigzag-Shaped Silver Nanoplates: Synthesis via Ostwald Ripening and Their Application in Highly Sensitive Strain Sensors.

Zigzag-shaped Ag nanoplates display unique anisotropic planar structures with unusual jagged edges and relatively large lateral dimensions. These characteristics make such nanoplates promising candidates for metal inks in printed electronics, which can be used for realizing stretchable electrodes. In the current work, we used a one-pot coordination-based synthetic strategy to synthesize zigzag-shaped Ag nanoplates. In the synthetic procedure, cyanuric acid was used both as a ligand of the Ag+ ion, hence producing complex structures and controlling the kinetics of the reduction of the cation, and as a capping agent that promoted the lateral growth of the Ag nanoplates. Hence, cyanuric acid played a crucial role in the formation of zigzag-shaped nanoplates. In contrast to previous studies that reported oriented attachment to be the predominant mechanism responsible for the growth of zigzag-shaped nanoplates, Ostwald ripening was the dominant growth mechanism in the current work. Our findings on the particle morphology and crystalline structure of the Ag nanoplates motivated us to use them as conductive materials for stretchable strain sensors. Strain sensors based on nanocomposites of our zigzag-shaped Ag nanoplate and polydimethylsiloxane in the form of a sandwich structure were successfully produced by following a simple, low-cost, and solution-processable method. The strain sensors exhibited extremely high sensitivity (gauge factor ≈ 2000), high stretchability with a linear response (≈27%), and high reliability, all of which allowed the sensor to monitor diverse human motions, including joint movement and phonation.

A new method for mapping the three-dimensional atomic distribution within nanoparticles by atom probe tomography (APT).

We present a new method of preparing needle-shaped specimens for atom probe tomography from freestanding Pd and C-supported Pt nanoparticles. The method consists of two steps, namely electrophoresis of nanoparticles on a flat Cu substrate followed by electrodeposition of a Ni film acting as an embedding matrix for the nanoparticles. Atom probe specimen preparation can be subsequently carried out by means of focused-ion-beam milling. Using this approach, we have been able to perform correlative atom probe tomography and transmission electron microscopy analyses on both nanoparticle systems. Reliable mass spectra and three-dimensional atom maps could be obtained for Pd nanoparticle specimens. In contrast, atom probe samples prepared from C-supported Pt nanoparticles showed uneven field evaporation and hence artifacts in the reconstructed atom maps. Our developed method is a viable means of mapping the three-dimensional atomic distribution within nanoparticles and is expected to contribute to an improved understanding of the structure-composition-property relationships of various nanoparticle systems.

A High Aspect Ratio Serpentine Structure for Use As a Strain-Insensitive, Stretchable Transparent Conductor.

The development of strain-insensitive stretchable transparent conductors (TCs) is essential for manufacturing stretchable electronics. Despite recent progress, achieving a high optoelectronic performance under applied strain of 50% continues to present a significant challenge in this research field. Herein, an ultratall and ultrathin high aspect ratio serpentine metal structure is described that exhibits a remarkable stretching ability (the resistance remains constant under applied strain of 100%) and simultaneously provides an excellent transparent conducting performance (with a sheet resistance of 7.6 Ω î¨-1 and a transmittance of 90.5%). It is demonstrated that the highly stretchable transparent conducting properties can be attributed to the high aspect ratio feature. A high aspect ratio (aspect ratio of 17-367) structure permits facile deformation of the serpentine structure with in-plane motion, leading to a high stretching ability. In addition, this structural feature avoids the classic tradeoff between optical transmittance and electrical conductance, providing a high electrical conductance without decreasing the optical transmittance. The practical utility of these devices is tested by using these TCs as stretchable interconnectors among LEDs or in wearable VOC gas sensors.

4-quinolin-8-yloxy Linked Triphenylamine Based Polyimides: Blue Light Emissive and Potential Hole-Transport Materials.

A series of fluorescent donor- acceptor (D-A) alternating copolyimides (P1, P2, P3 and P4) with 4-quinolin-8-yloxy linked triphenylamine main polymer chain have been synthesized by conventional polycondensation. All the synthesized co-polyimides were characterized by elemental, gel permeation chromatography and FTIR spectral analysis. These newly prepared PIs possess HOMO energy levels in range of - 4.74 to - 4.78 eV and have medium optical band gaps. The photoluminescence spectral analysis revealed blue to violet emission with appreciable efficiency with lower onset oxidation potentials suitable for the facile hole injection materials. All the photophysical and electrochemical properties were also explored in context of effect of the pendant 4- quinolin-8-yloxy, indicating suitable combination of donor (TPA) on one hand and imide and pendant as acceptor on both ends.Graphical Abstract.

Facile Synthesis of Composition-Controlled Graphene-Supported PtPd Alloy Nanocatalysts and Their Applications in Methanol Electro-Oxidation and Lithium-Oxygen Batteries.

A new and simple approach is reported for the synthesis of uniformly dispersed PtPd alloy nanocatalysts supported on graphene nanoplatelets (GNPs) (PtPd-GNPs) through the introduction of bifunctional materials, which can modify the GNP surface and simultaneously reduce metal ions. With the use of poly(4-styrenesulfonic acid), poly(vinyl pyrrolidone), poly(diallyldimethylammonium chloride), and poly(vinyl alcohol) as bifunctional materials, PtPd-GNPs can be produced through a procedure that is far simpler than previously reported methods. The as-prepared nanocrystals on GNPs clearly exhibit uniform PtPd alloy structures of around 2 nm in size, which are strongly anchored and well distributed on the GNP sheets. The Pt/Pd atomic ratio and loading density of the nanocrystals on the GNPs are controlled easily by changing the metal precursor feed ratio and the mass ratio of GNP to the metal precursor, respectively. As a result of the synergism between Pt and Pd, the as-prepared PtPd-GNPs exhibit markedly enhanced electrocatalytic performance during methanol electro-oxidation compared with monometallic Pt-GNP or commercially available Pt/C. Furthermore, the PtPd-GNP nanocatalysts also show greatly enhanced catalytic activity toward the oxygen reduction/evolution reaction in a lithium-oxygen (Li-O2 ) process, resulting in greatly improved cycling stability of a Li-O2 battery.

Synthesis and Photophysical Study of New Green Fluorescent TPA Based Poly(azomethine)s.

A series of poly(azomethine)s (PAMs) were synthesized from N1-(4-aminophenyl)-N1-(4-phenoxyphenyl)benzene-1,4-diamine (DA) and various dialdehydes to investigate the influence of structure of polymer chain and triphenylamine-based phenoxy pendant group on the optoelectronic properties. The structural characterization of the resulting poly(azomethine)s was carried out by solubility test, gel permeation chromatography, viscosity measurement, fourier transform infrared (FTIR) spectral and CHN elemental analysis. The photophysical and electrochemical properties of the materials were scrutinized by UV-vis, photoluminescence, time correlation photon counting spectral analysis (TCSP) and cyclic voltammetry. The thermal stability of the poly(azomethine)s was assessed by differential scanning calorimetry and thermogravimetric analysis found to be stable upto 300 °C. These polymers exhibit moderate inherent viscosity range from 0.99 to 1.15 g dL- 1 and appreciable organosolubility. The presence of triphenylamine and azomethine (CH = N) linkage in our synthesized materials rendered them fluorescent, emitting green light upon excitation at 375 nm with quantum efficiencies of 3.9-8.5%. The pendant phenoxy group at para-position in new poly(azomethine)s has also lowered the onset oxidation potentials and elevated the HOMO levels. Additionally, the presence of conjugation increases the fluorescence time of the excited state in conjugated polymers which was found in the range 9.22-11.17 ns, sufficient to be use in future optoelectronic applications.

Enhanced Sensitivity of Patterned Graphene Strain Sensors Used for Monitoring Subtle Human Body Motions.

With the growth of the wearable electronics industry, structural modifications of sensing materials have been widely attempted to improve the sensitivity of sensors. Herein, we demonstrate patterned graphene strain sensors, which can monitor small-scale motions by using the simple, scalable, and solution-processable method. The electrical properties of the sensors are easily tuned via repetition of the layer-by-layer assembly, leading to increment of thickness of the conducting layers. In contrast to nonpatterned sensors, the patterned sensors show enhanced sensitivity and the ability to distinguish subtle motions, such as similar phonations and 81 beats per minute of pulse rate.

Capacity Decay Mitigation by Asymmetric Positive/Negative Electrolyte Volumes in Vanadium Redox Flow Batteries.

Capacity decay in vanadium redox flow batteries during charge-discharge cycling has become an important issue because it lowers the practical energy density of the battery. The battery capacity tends to drop rapidly within the first tens of cycles and then drops more gradually over subsequent cycles during long-term operation. This paper analyzes and discusses the reasons for this early capacity decay. The imbalanced crossover rate of vanadium species was found to remain high until the total difference in vanadium concentration between the positive and negative electrolytes reached almost 1 mol dm-3 . To minimize the initial crossover imbalance, we introduced an asymmetric volume ratio between the positive and negative electrolytes during cell operation. Changing this ratio significantly reduced the capacity fading rate of the battery during the early cycles and improved its capacity retention at steady state. As an example, the practical energy density of the battery increased from 15.5 to 25.2 Wh L-1 simply after reduction of the positive volume by 25 %.

Highly stretchable and wearable graphene strain sensors with controllable sensitivity for human motion monitoring.

Because of their outstanding electrical and mechanical properties, graphene strain sensors have attracted extensive attention for electronic applications in virtual reality, robotics, medical diagnostics, and healthcare. Although several strain sensors based on graphene have been reported, the stretchability and sensitivity of these sensors remain limited, and also there is a pressing need to develop a practical fabrication process. This paper reports the fabrication and characterization of new types of graphene strain sensors based on stretchable yarns. Highly stretchable, sensitive, and wearable sensors are realized by a layer-by-layer assembly method that is simple, low-cost, scalable, and solution-processable. Because of the yarn structures, these sensors exhibit high stretchability (up to 150%) and versatility, and can detect both large- and small-scale human motions. For this study, wearable electronics are fabricated with implanted sensors that can monitor diverse human motions, including joint movement, phonation, swallowing, and breathing.

Synthesis of chestnut-bur-like palladium nanostructures and their enhanced electrocatalytic activities for ethanol oxidation.

We report a facile method for the synthesis of Pd nanostructures with highly open structure and huge surface area by reducing Na2PdCl4 with ascorbic acid and using cetylpyridinium chloride (CPC) as a surfactant in an aqueous solution. The prepared Pd nanostructures had an average overall size of 70 nm and were composed of dozens of needle-like thin arms, originating from the same core, with an average thickness of 2.3 nm; the arms looked like chestnut-burs. Time evolution of Pd nanostructures implied that small Pd particles generated at the early stage of the reaction by fast reduction grew via the particle attachment growth mechanism. The morphology and size of the Pd nanostructures could be readily controlled by varying the concentration of CPC; depending on the amount of CPC, the reduction rates varied the morphology of the Pd nanostructures. Because of the huge surface area and possible catalytically active sites, the prepared chestnut-bur-like Pd nanostructures exhibited greater electrocatalytic activity toward ethanol electrooxidation compared to other Pd nanocatalysts, including cubic and octahedral Pd nanocrystals, and even commercial Pd/C.

Two-dimensional TiO2 honeycomb structure for enhanced light extraction from polymer light-emitting diodes.

We have demonstrated the preparation of an ordered hole pattern via a simple colloidal assembly technique and a sol-gel process without the use of any special equipment to enhance light extraction from polymer light-emitting diodes (LEDs). From two-dimensional (2D) polystyrene (PS) colloidal crystals, 2D TiO2 honeycomb structures were easily obtained after depositing TiO(x) sol solution onto the colloidal crystals by a doctor blade technique and removing the PS colloidal particles. In order to optimize the thickness of the deposited TiO(x) materials for fabricating 2D TiO2 honeycomb structures, the concentration of the TiO(x) sol solution was controlled. By applying the 2D TiO2 honeycomb structure on the backside of the glass substrate, the efficiency of a polymer LED, compared to the device without the structure, was increased without a significant change in the electroluminescence spectrum. This enhancement is attributed to the improved light extraction, which is due to the suppression of total internal reflection at the interface between the glass substrate and the air.

Hemispherical arrays of colloidal crystals fabricated by transfer printing.

The self-assembly of colloidal spheres is an effective strategy for producing nanopatterns. To use colloidal crystals in lithographic applications, the key challenge is to fabricate a monolayer of colloidal crystals uniformly over a large area. A simple and effective method for fabricating a colloidal crystals monolayer from a 3D colloidal crystal is described. The top layer of a surface-etched 3D colloidal crystal is picked up on a PDMS stamp, and by simply heating the receiving substrate, the polymeric colloidal crystal can be easily transferred to many types of substrates, including curved or flexible materials, without utilizing a glue layer. Moreover, the colloidal spheres are deformed to hemispheres during the transfer process, which is a suitable form of a lithographic mask for both dry and wet etching processes. An array of silicon nanocones and gold dots is demonstrated by pattern transfer from an array of hemispherical polymeric particles. In addition, it is also shown that the transferred hemispherical array has good antireflective properties.

Color-stable white-light-emitting diodes doped with phosphorescent dopants via enhanced energy transfer through homogeneous morphology.

We demonstrate that white light-emitting diodes using polyfluorene-based blue-emitting conjugated copolymer and small molecule as co-host doped with green and red-emitting phosphorescent dyes. The energy transfer from co-host to phosphorescent dyes is enhanced by blending with small molecule due to the suppressed phase separation between conjugated polymer and Ir complexes as well as the high triplet state of co-host. We obtained voltage-independent pure white light emission with homogeneous morphology in the blended single active layer system by reducing charge trapping in Ir complexes and enhancing the efficient Förster energy transfer.

Foldable graphene electronic circuits based on paper substrates.

Graphene electronic circuits are prepared on paper substrates by using graphene nanoplates and applied to foldable paper-based electronics. The graphene circuits show a small change in conductance under various folding angles and maintain an electronic path on paper substrates after repetition of folding and unfolding. Foldable paper-based applications with graphene circuits exhibit excellent folding stability.

Stamping transfer of a quantum dot interlayer for organic photovoltaic cells.

An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode.

Improved light extraction efficiency in organic light emitting diodes with a perforated WO3 hole injection layer fabricated by use of colloidal lithography.

We present an organic light emitting diode with a perforated WO3 hole injection layer to improve the light extraction efficiency. The two-dimensionally perforated WO3 layer was fabricated by use of colloidal lithography. The light extraction efficiency was improved due to Bragg scattering of waveguide modes and surface plasmon polaritons, and the operating voltage was also decreased. As a result, the external quantum efficiency and the power efficiency were increased as compared with those of conventional organic light emitting diodes without WO3 layer. The angular dependence of emission characteristics was investigated by measuring radiant intensity profiles for emission angles and azimuthal angles.