Light-driven self-assembly of spiropyran-functionalized covalent organic framework

Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response. Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework (COF). The COF contains a spiropyran in each pore which exhibits excellent reversible photoswitching behavior to its merocyanine form in the solid state in response to UV/Vis light. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence. This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications.

Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers

Funneling excitation energy toward lower energy excited states is a key concept in photosynthesis, which is often realized with at most two chemically different types of pigment molecules. However, current synthetic approaches to establish energy funnels, or gradients, typically rely on Förster-type energy-transfer cascades along many chemically different molecules. Here, we demonstrate an elegant concept for a gradient in the excited-state energy landscape along micrometer-long supramolecular nanofibers based on the conjugated polymer poly(3-hexylthiophene), P3HT, as the single component. Precisely aligned P3HT nanofibers within a supramolecular superstructure are prepared by solution processing involving an efficient supramolecular nucleating agent. Employing hyperspectral imaging, we find that the lowest-energy exciton band edge continuously shifts to lower energies along the nanofibers' growth direction. We attribute this directed excited-state energy gradient to defect fractionation during nanofiber growth. Our concept provides guidelines for the design of supramolecular structures with an intrinsic energy gradient for nanophotonic applications.

Design of a Smart Conducting Nanocomposite with an Extended Strain Sensing Range by Conjugating Hybrid Structures

In recent years, flexible and wearable strain sensors, consisting of a polymer matrix and a conducting filler, have received extensive attention owing to their physical advantages, such as being lightweight, stretchable, and having the potential for application to complex forms. However, achieving a low hysteresis of the relative change in resistance, wide sensing range, and reduced plastic deformation is still challenging. To address these issues, in this study, we developed hybrid conducting composites with a wide range of sensing abilities and low hysteresis. The bi-layer composites, comprising a carbon nanotube (CNT) composite layer with reinforced/conducting properties, and a natural rubber-based layer with extreme strain properties, could effectively circumvent their limitations. Compared to single-layer CNT composites, the bi-layer structure could increase the tensile strain with reduced plastic deformation, resulting in the prevention of surface cracks on the CNT composite. In addition, it has the benefit of measuring a wider sensing range, which cannot be measured in a single-CNT composite system. A cyclic stretching/releasing test was performed to demonstrate that the strain sensor exhibited excellent reproducibility. Our results can function as a useful design guide for stretchable sensor applications.

Controlling the organization and stretchability of poly(3-butylthiophene) spherulites

In this work, we report a simple strategy to readily prepare poly(3-butylthiophene) (P3BT) films with patterned spherulites by brushing the P3BT film surface and annealing the film with carbon disulfide (CS₂) vapor. The spherulites nucleated preferentially at the mechanically scratched areas over the unscratched region of the film. The ridge (formed at the side of the scratch) hinders the diffusion of the P3BT molecules, promoting the aggregation and nucleation of P3BT along the ridge to form spherulites upon the CS₂ vapor-annealing. The sizes of the ridge and the scratch have no effect on the nucleation and crystallization of the patterned spherulites. We evaluated the crack formation of the P3BT films with patterned spherulites in response to mechanical stretching along different directions. When the stretching direction was parallel to the scratching direction, cracks appeared preferentially at the boundary between the ordered spherulites. In contrast, cracks occurred first at the boundary of stochastic nucleated spherulites located away from the patterned spherulites, when the stretching direction was perpendicular to the scratching direction. The patterned spherulites with regulated mechanical properties may find applications in the design and fabrication of stretchable organic optoelectronic devices with enhanced stability and durability.

Hybrid conjugated polymer/magnetic nanoparticle composite nanofibers through cooperative non-covalent interactions

Hybrid organic-inorganic composites possessing both electronic and magnetic properties are promising materials for a wide range of applications. Controlled and ordered arrangement of the organic and inorganic components is key for synergistic cooperation toward desired functions. In this work, we report the self-assemblies of core-shell composite nanofibers from conjugated block copolymers and magnetic nanoparticles through the cooperation of orthogonal non-covalent interactions. We show that well-defined core-shell conjugated polymer nanofibers can be obtained through solvent induced self-assembly and polymer crystallization, while hydroxy and pyridine functional groups located at the shell of nanofibers can immobilize magnetic nanoparticles via hydrogen bonding and coordination interactions. These precisely arranged nanostructures possess electronic properties intrinsic to the polymers and are simultaneously responsive to external magnetic fields. We applied these composite nanofibers in organic solar cells and found that these non-covalent interactions led to controlled thin film morphologies containing uniformly dispersed nanoparticles, although high loadings of these inorganic components negatively impact device performance. Our methodology is general and can be utilized to control the spatial distribution of functionalized organic/inorganic building blocks, and the magnetic responsiveness and optoelectronic activities of these nanostructures may lead to new opportunities in energy and electronic applications.

Polymer wrapping versus well-oriented crystal growth of polythiophenes onto multi-wall carbon nanotubes via surface chemical modification and regioregularity deliberation

Distinct nano-hybrids including double-fibrillar, shish–kebab, shish–kebab–shish, stem–leaf and stem–leaf–stem were developed using P3HT and CNTs.

Control of Molecular Ordering, Alignment, and Charge Transport in Solution-Processed Conjugated Polymer Thin Films

Morphology of conjugated polymers is a critical factor that significantly affects intrinsic charge transport characteristics and in turn performance of polymer-based devices. Morphological defects including misaligned crystalline grains and grain boundaries significantly impede efficient charge hopping between transport sites, resulting in degradation of device performance. Therefore, one important challenge is to control morphology of active polymer thin-films for achieving high performance flexible electronic devices. In the past decade, significant progress has been achieved in morphology control of conjugated polymer thin-films using solution-based processing techniques. This review focuses on recent advances in processing strategies that can tune the morphologies and thus impact charge transport properties of conjugated polymer thin films. Of the available processing strategies, polymer solution treatments and film deposition techniques will be mainly highlighted. The correlation between processing conditions, active layer morphologies, and device performance will be also be discussed.

Air-Liquid Interfacial Self-Assembly of Non-Amphiphilic Poly(3-hexylthiophene) Homopolymers

Here, we demonstrate that the self-assembly of poly(3-hexylthiophene) (P3HT) at the air-water interface can lead to free-standing films of densely packed P3HT nanowires. Interfacial self-assembly on various liquid subphases, such as water, diethylene glycol, and glycerol, indicates that the viscosity of the subphase is an important factor for the formation of well-ordered nanostructures. The thin-film morphology is also sensitive to the concentration of P3HT, its molecular weight (MW), and the presence of oxidative defects. The densely packed nanowire films can be easily transferred to solid substrates for device applications. The ultrathin films of P3HT prepared by the interfacial assembly showed significantly higher hole mobility (∼3.6 × 10^-2 cm²/V s) in a field-effect transistor than comparably thin spin-cast films. This work demonstrates that the air-liquid interfacial assembly is not limited to amphiphilic polymers and can, under optimized conditions, be applied to fabricate ultrathin films of widely used conjugated polymers with controlled morphologies.

Assembly of polythiophenes on responsive polymer microgels for the highly selective detection of ammonia gas

A novel material that allows highly selective ammonia-to-conductance signal transduction is prepared by the assembly of polythiophenes on responsive polymer microgels.

Hierarchical Polymer-Carbon Nanotube Hybrid Mesostructures by Crystallization-Driven Self-Assembly

Multistep crystallization-driven self-assembly has great potential to enable the construction of sophisticated hybrid mesostructures. During the assembly procedure, each step modifies the properties of the overall structure. Here, we demonstrate the flexibility and efficiency of this approach by preparing polymer-carbon nanotube (CNT) hybrid mesostructures. We started by growing polyferrocenyldimethylsilane (PFS) homopolymer crystals onto multiwalled CNTs. This first step facilitated the redispersion of the coated CNTs in both polar (2-propanol) and nonpolar (decane) solvents. In the second step of hybrid construction, a unimer solution of a PFS block copolymer was added into the PFS-CNT solution. The PFS coating on the CNT initiated the growth of elongated micelles, resulting in structures that resembled hairy caterpillars. PFS-b-P2VP (P2VP = poly-2-vinylpyridine) micelles were grown from the surface of PFS-CNT hybrids in 2-propanol, and PFS-b-PI (PI = polyisoprene) micelles were grown from these hybrids in decane. These micelles, by transmission electron microscopy were seen to have an unusual wavy kinked structure, very different from the uniform smooth structures normally formed by both block copolymers. For hybrids with PFS-b-PI micelles, cross-linking of the micelle coronas locked the whole structure in place and allowed us to use the partial oxidation of PFS components to grow metal nanoparticles in the core of these micelles. We finally investigated the influence of the corona-forming block used to grow the micelles on the wettability of films made from these mesostructures. Films formed with CNT hybrids grafted with PFS-b-PI micelles were superhydrophobic (contact angle, 152°). In contrast, the surface of the films was much more hydrophilic (contact angle, 54°) when they were prepared from CNT hybrids grafted with PFS-b-P2VP micelles.

Conjugated polymer chain and crystallite orientation induced by vertically aligned carbon nanotube arrays

We report a method for controlling the orientations of conjugated polymers in the active layer of organic thin-film transistors (OTFTs) by annealing the film at the melting temperature in a vertically aligned multiwalled carbon nanotube (VA-CNT) template under various load pressures. Poly(3-hexylthiophene) [P3HT] molecules are effectively aligned in the melting state annealing at 240 °C for 30 min, as a result of intermolecular π-π and CH3-π interactions between the polymer and the VA-CNTs, which are separated from the conjugated polymer film after cooling to room temperature. In-plane and out-of-plane X-ray diffraction results show that the melt-annealed P3HT film with VA-CNTs has better crystallite ordering than a pristine 80 °C baked film and a melt-annealed film without VA-CNTs, and a larger number of crystallites in the treated P3HT film are oriented in the [100] direction, which is normal to the substrate. When we used the melt-aligned P3HT film with VA-CNTs as the active layer in OTFTs, the P3HT OTFT exhibits a better field-effect mobility value of 0.12 cm(2)/(V s) than a simply melt-annealed device without VA-CNTs [0.06 cm(2)/(V s)].

Self-assembly of double helical nanostructures inside carbon nanotubes

We use molecular dynamics (MD) simulations to show that a DNA-like double helix of two poly(acetylene) (PA) chains can form inside single-walled carbon nanotubes (SWNTs). The computational results indicate that SWNTs can activate and guide the self-assembly of polymer chains, allowing them to adopt a helical configuration in a SWNT through the combined action of the van der Waals potential well and the π-π stacking interaction between the polymer and the inner surface of SWNTs. Meanwhile both the SWNT size and polymer chain stiffness determine the outcome of the nanostructure. Furthermore, we also found that water clusters encourage the self-assembly of PA helical structures in the tube. This molecular model may lead to a better understanding of the formation of a double helix biological molecule inside SWNTs. Alternatively, it could form the basis of a novel nanoscale material by utilizing the 'empty' spaces of SWNTs.

Growth of polythiophene/perylene tetracarboxydiimide donor/acceptor shish-kebab nanostructures by coupled crystal modification

Self-assembled crystalline organic nanostructures containing electron donor and acceptor materials hold promise as building blocks for photovoltaic devices. We show that coupled crystallization of poly(3-hexyl thiophene) (P3HT) and perylene tetracarboxydiimide (PDI) induced by solvent evaporation, wherein both components modify crystallization of the other, gives rise to donor/acceptor "shish-kebabs" with tunable nanostructures. P3HT kinetically stabilizes supersaturated solutions of PDI and modifies the growth of PDI crystals, leading to formation of extended PDI shish nanowires that in turn serve as heterogeneous nucleation sites for fibrillar P3HT kebabs during solvent casting. The dimensions of these nanostructures can be tailored through variations in donor/acceptor ratio or solvent quality, and the method is shown to be general to several other poly(3-alkyl thiophenes) and perylene derivatives, thus providing a simple and robust route to form highly crystalline nanophase separated organic donor/acceptor assemblies.

Nanoscale investigation of the electrical properties in semiconductor polymer-carbon nanotube hybrid materials

The morphology and electrical properties of hybrids of a semiconducting polymer (namely poly(3-hexylthiophene) P3HT) and carbon nanotubes are investigated at the nanoscale with a combination of Scanning Probe Microscopy techniques, i.e., Conductive Atomic Force Microscopy (C-AFM) and time-resolved Current Sensing Force Spectroscopy Atomic Force Microscopy (CSFS-AFM, or PeakForce TUNA™). This allows us to probe the electrical properties of the 15 nm wide P3HT nanofibers as well as the interface between the polymer and single carbon nanotubes. This is achieved by applying controlled, low forces on the tip during imaging, which allows a direct comparison between the morphology and the electrical properties at the nanometre scale.

Hierarchical self-assembly of amphiphilic semiconducting polymers into isolated, bundled, and branched nanofibers

Herein, we report a high-yield click synthesis and self-assembly of conjugated amphiphilic block copolymers of polythiophene (PHT) and polyethylene glycol (PEG) and their superstructures. A series of different length PHT(m)-b-PEG(n) with well-defined relative block lengths was synthesized by a click-coupling reaction and self-assembled into uniform and stably suspended nanofibers in selective solvents. The length of nanofibers was controllable by varying the relative block lengths while keeping other dimensions and optical properties unaffected for a broad range of f(PHT) (0.41 to 0.82), which indicates that the packing of PHT dominates the self-assembly of PHT(m)-b-PEG(n). Furthermore, superstructures of bundled and branched nanofibers were fabricated through the self-assembly of PHT(m)-b-PEG(n) and preformed PHT nanofibers. The shape, length, and density of the hierarchical assembly structures can be controlled by varying the solvent quality, polymer lengths, and block copolymer/homopolymer ratio. This work demonstrates that complex superstructures of organic semiconductors can be fabricated through the bottom-up approach using preformed nanofibers as building blocks.

Fabrication of organic field effect transistor by directly grown poly(3 hexylthiophene) crystalline nanowires on carbon nanotube aligned array electrode

We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm(2)/(V s) and 3.1 × 10(5), respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure.