39 research outputs found

    Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates

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    The resolution of inkjet printing technology is determined by wetting and evaporation processes after the jet drop contacts the substrate. Here, the wetting of different picoliter solubilized polymer droplets jetting onto one-end-closed porous alumina was investigated. The selected polymers are commonly used in inkjet ink. The synergistic effects of the hierarchical structure and substrate surface modification were used to control the behavior of polymer-based ink drops. A model that invokes the effect of surface tension was applied to calculate the amount of polymer solution penetrating into the pores. The calculation corroborates experimental observations and shows that the volume of polymer solution in the pores increases with an increase in pore radius and depth, resulting in less solution remaining on the substrate surface. The structure of the porous substrate coupled with intrinsic polymer properties and surface modifications all contribute to the resolution that can be achieved via inkjet printing

    Creating and Optimizing Interfaces for Electric-Field and Photon-Induced Charge Transfer

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    We create and optimize a structurally well-defined electron donor–acceptor planar heterojunction interface in which electric-field and/or photon-induced charge transfer occurs. Electric-field-induced charge transfer in the dark and exciton dissociation at a pentacene/PCBM interface were probed by <i>in situ</i> thickness-dependent threshold voltage shift measurements in field-effect transistor devices during the formation of the interface. Electric-field-induced charge transfer at the interface in the dark is correlated with development of the pentacene accumulation layer close to PCBM, that is, including interface area, and dielectric relaxation time in PCBM. Further, we demonstrate an <i>in situ</i> test structure that allows probing of both exciton diffusion length and charge transport properties, crucial for optimizing optoelectronic devices. Competition between the optical absorption length and the exciton diffusion length in pentacene governs exciton dissociation at the interface. Charge transfer mechanisms in the dark and under illumination are detailed

    Regioregularity and Intrachain Ordering: Impact on the Nanostructure and Charge Transport in Two-Dimensional Assemblies of Poly(3-hexylthiophene)

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    The properties of supramolecular assemblies in conjugated macromolecular systems are strongly dependent on single chain effects. We report that differences in regioregularity (RR) of side chain attachment in poly­(3-hexylthiophene) (P3HT) as small as ca. 4% are sufficient to induce dramatic changes in the electronic and morphological properties of the material. Casting the electronic absorption spectra in the framework of Spano’s model reveals that the conjugation length is surprisingly sensitive to RR, with differences in free exciton bandwidth between the two P3HT samples approaching 73 meV. The enhanced main chain planarization motivates a concomitant increase in nanofibril width as well as crystallinity observed in thin films of the higher RR variant. This observation correlates well with the field effect mobilities that are attenuated by 1 to 2 orders of magnitude in the lower RR polymer film. We suggest that the increased intrachain order coupled with a reduced fraction of grain boundaries in the higher RR film is responsible for the reported differences

    Memory and Photovoltaic Elements in Organic Field Effect Transistors with Donor/Acceptor Planar-Hetero Junction Interfaces

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    Interfacial charge transfer at organic/organic planar-hetero junctions allows access to device structures that create new opportunities for flexible electronic devices. Fundamental characteristics of a pentacene/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) interface are explored via a comprehensive study of charge transfer between these two materials using the field-effect transistor (FET) geometry both in the dark and under illumination. Organic memory elements in a field effect transistor are demonstrated for a device fabricated with a pentacene/PCBM interface. Electric field induced charge transfer at the interface, in the dark, induced a nonvolatile memory effect with a large hysteresis characterized by a memory window of 43 V in the transfer characteristics. A photoinduced threshold voltage shift induced by exciton dissociation at the interface, in the absence of a gate electric field, is consistent with the formation of the photoinduced conducting channel in pentacene

    Automated Analysis of Orientational Order in Images of Fibrillar Materials

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    Nanofibers are a ubiquitous structural motif in a variety of functional materials. In the field of organic electronics, π–π-stacking of conjugated polymers leads to fibrillar morphologies with a wide array of fiber packing behavior. Fiber orientation and alignment are known to influence the charge transport properties of devices such as organic field effect transistors. The solution processing methods used to create these devices give rise to large variations in these structural parametershowever, they are only observable with imaging techniques such as atomic force microscopy (AFM). To bring more rigorous quantification of orientation and alignment to these materials, a comprehensive image analysis tool is introduced to quantify the two-dimensional orientation and alignment of nanofibers from AFM images. It has been developed in MATLAB and packaged as a stand-alone application, so that researchers with no computational expertise can produce publication-ready figures directly from their images. AFM frequently yields images with low contrast and moderate noise, making quantitative feature extraction a significant challenge. In this protocol, each image is analyzed in the context of an Orientation Map, in which nanofibers are thinned to single-pixel width and an orientation is extracted for each of these pixels. The Orientation Map is obtained through a five-step process: fiber smoothing by anisotropic diffusion filtering, contrast enhancement by top hat filtering, binarization by adaptive thresholding, skeletonization, and recovery of orientations from the result of diffusion filtering. Each step involves parameters that can be set using physical heuristics, which are examined in detail. This Orientation Map yields an orientation distribution and a plot of <i>S</i><sub>2D</sub>, an orientational order parameter, as a function of frame size. The image analysis procedure is used to quantify differences in P3HT nanofiber morphology induced by various solution processing recipes, as well as the effect of spin-coating when used to deposit solutions of nanofibers. All examples presented in this protocol can be reproduced from beginning to end using the included software, with visualizations produced at each stage of processing

    Anisotropic Assembly of Conjugated Polymer Nanocrystallites for Enhanced Charge Transport

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    The anisotropic assembly of P3HT nanocrystallites into longer nanofibrillar structures was demonstrated via sequential UV irradiation after ultrasonication to the pristine polymer solutions. The morphology of resultant films was studied by atomic force microscopy (AFM), and quantitative analysis of intra- and intermolecular ordering of polymer chains was performed by means of static absorption spectroscopy and quantitative modeling. Consequently, the approach to treat the precursor solution enhanced intra- and intermolecular ordering and reduced the incidence of grain boundaries within P3HT films, which contributed to the excellent charge carrier transport characteristics of the corresponding films (μ ≈ 12.0 × 10<sup>–2</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for 96% RR P3HT)

    Domed Silica Microcylinders Coated with Oleophilic Polypeptides and Their Behavior in Lyotropic Cholesteric Liquid Crystals of the Same Polypeptide

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    Liquid crystals can organize dispersed particles into useful and exotic structures. In the case of lyotropic cholesteric polypeptide liquid crystals, polypeptide-coated particles are appealing because the surface chemistry matches that of the polymeric mesogen, which permits a tighter focus on factors such as extended particle shape. The colloidal particles developed here consist of a magnetic and fluorescent cylindrically symmetric silica core with one rounded, almost hemispherical end. Functionalized with helical poly­(γ-stearyl-l-glutamate) (PSLG), the particles were dispersed at different concentrations in cholesteric liquid crystals (ChLC) of the same polymer in tetrahydrofuran (THF). Defects introduced by the particles to the director field of the bulk PSLG/THF host led to a variety of phases. In fresh mixtures, the cholesteric mesophase of the PSLG matrix was distorted, as reflected in the absence of the characteristic fingerprint pattern. Over time, the fingerprint pattern returned, more quickly when the concentration of the PSLG-coated particles was low. At low particle concentration the particles were “guided” by the PSLG liquid crystal to organize into patterns similar to that of the re-formed bulk chiral nematic phase. When their concentration increased, the well-dispersed PSLG-coated particles seemed to map onto the distortions in the bulk host’s local director field. The particles located near the glass vial–ChLC interfaces were stacked lengthwise into architectures with apparent two-dimensional hexagonal symmetry. The size of these “crystalline” structures increased with particle concentration. They displayed remarkable stability toward an external magnetic field; hydrophobic interactions between the PSLG polymers in the shell and those in the bulk LC matrix may be responsible. The results show that bio-inspired LCs can assemble suitable colloidal particles into soft crystalline structures

    Ultrasound-Induced Ordering in Poly(3-hexylthiophene): Role of Molecular and Process Parameters on Morphology and Charge Transport

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    Facile methods for controlling the microstructure of polymeric semiconductors are critical to the success of large area flexible electronics. Here we explore ultrasonic irradiation of solutions of poly­(3-hexylthiophene) (P3HT) as a simple route to creating ordered molecular aggregates that result in a one to two order of magnitude improvement in field effect mobility. A detailed investigation of the ultrasound induced phenomenon, including the role of solvent, polymer regioregularity (RR) and film deposition method, is conducted. Absorption spectroscopy reveals that the development of low energy vibronic features is dependent on both the regioregularity as well as the solvent, with the latter especially influential on the intensity and shape of the band. Use of either higher regioregular polymer or ultrasonic irradiation of lower regioregular polymer solutions results in high field effect mobilities that are nearly independent of the dynamics of the film formation process. Surprisingly, no distinct correlation between thin-film morphology and macroscopic charge transport could be ascertained. The relationships between molecular and process parameters are very subtle: modulation of one effects changes in the others, which in turn impact charge transport on the macroscale. Our results provide insight into the degree of control that is required for the development of reproducible, robust materials and processes for advanced flexible electronics based on polymeric materials

    Imparting Chemical Stability in Nanoparticulate Silver via a Conjugated Polymer Casing Approach

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    Only limited information is available on the design and synthesis of functional materials for preventing corrosion of metal nanostructures. In the nanometer regime, even noble metals are subject to chemical attack. Here, the corrosion behavior of noble metal nanoparticles coated with a conjugated polymer nanolayer was explored for the first time. Specifically, electrochemical corrosion and sulfur tarnishing behaviors were examined for Ag-polypyrrole (PPy) core–shell nanoparticles using potentiodynamic polarization and spectrophotometric analysis, respectively. First, the Ag-PPy nanoparticles exhibited enhanced resistance to electrochemically induced corrosion compared to their exposed silver counterparts. Briefly, a neutral PPy shell provided the highest protection efficiency (75.5%), followed by sulfate ion- (61.3%) and dodecylbenzenesulfonate ion- (53.6%) doped PPy shells. However, the doping of the PPy shell with chloride ion induced an adverse effect (protection efficiency, −120%). Second, upon exposure to sulfide ions, the Ag-PPy nanoparticles preserved their morphology and colloidal stability while the bare silver analog underwent significant structural deformation. To further understand the function of the PPy shell as a protection layer for the silver core, the catalytic activity of the nanostructures was also evaluated. Using the reduction of 4-nitrophenol as a representative example of a catalytic reaction, the rate constant for that reduction using the PPy encased Ag nanoparticles was found to be 1.1 × 10<sup>–3</sup> s<sup>–1</sup>, which is approximately 33% less than that determined for the parent silver. These results demonstrate that PPy can serve as both an electrical and chemical barrier for mitigating undesirable chemical degradation in corrosive environments, as well as provide a simple physical barrier to corrosive substances under appropriate conditions
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