Conjugated Polymer Nanostructures Research Articles

Kinetically Controlled Formation of Semi-crystalline Conjugated Polymer Nanostructures

Conjugated polymer (CP) materials that are considered as an attractive choice for various electronic and optoelectronic applications possess highly heterogeneous complex structures that can span several orders of magnitude in length scale. This is due to the intricate coupling effects of weak secondary interactions and entropic forces that determine molecular organization in bulk polymer materials. Understanding the influence of molecular interactions on the emergence and evolution of nano- and microscale structures is of paramount importance to the guided design and development of condensed CP materials. Such understanding is even more critical for the rational design of hierarchical systems away from equilibrium, where weak molecular interactions can guide the system along competing kinetic pathways toward local energy minima and metastable architectures. In this work, we studied a promising concept for accessing various kinetically stabilized semi-crystalline CP nanostructures that are formed in the process of controlled chain-growth Kumada catalyst-transfer polymerization initiated by a small-molecule catalytic species which can exist in equilibrium between free and aggregated states. Specifically, we chose a small-molecule perylenedicarboximide (PDCI) polymerization catalytic initiator which has a strong tendency toward reversible supramolecular assembly–disassembly that can be controlled by temperature. Addition of a thiophene monomer to a solution of the PDCI initiator starts the chain-growth polymerization process, which produces distinct nanoscale semi-crystalline polythiophene structures formed under predominant kinetic control. We demonstrated that, depending on the reaction temperature (which affects the fine balance between the position of PDCI assembly–disassembly equilibrium, rate of polymerization, and solvent–solute interactions for the growing polythiophene chains), a range of kinetically trapped hierarchically organized semi-crystalline CP systems with substantially varying optoelectronic properties could be obtained. In addition to spectroscopic and electron microscopic studies, in order to better reveal the structural aspects of the generated polymer nanoscale systems, we carried out a series of X-ray diffraction and neutron scattering experiments which indicated complex hierarchical organization in these kinetically stabilized CP assembled materials. We expect that this in situ polymerization-based approach can lead to a general way to expand access to various semi-crystalline CP nanostructured materials with broadly tunable electronic and optical properties.

Carbon/graphene quantum dot and conjugated polymer nanostructures impart unprecedented high efficiencies in routine P3HT:PCBM photovoltaics

Graphene quantum dot (GQD) and carbon quantum dot (CQD) nanoparticles were employed to fabricate the crab-like GQD/poly(3-hexylthiophene) (P3HT) and semicrab-like CQD/P3HT short chain, pellet-like GQD/P3HT and CQD/P3HT long chain, GQD/P3HT nanofiber and CQD/P3HT nanofiber supramolecules. The P3HT crystals were composed of face-on ((100) and (002) prisms) and edge-on ((020) and (002) growth planes) oriented chains. The lowest d-spacing values were acquired for the QD decorated P3HT nanofibers (d(100) = 13.41–13.46 Ǻ and d(020) = 3.39–3.43 Ǻ). To investigate the function of designed supramolecules in roles of morphology modifier, donor, and acceptor, the ternary and binary photovoltaics of P3HT: (Meissner and Wohrle, 1991; Meissner and Wohrle, 1991)-Phenyl C71 butyric acid methyl ester (PC71BM):supramolecule, P3HT:supramolecule, and PC71BM:supramolecule were prepared. The GQD-based nanostructures acted better than the CQD-based ones, originated from their thinner and disk-like nature, and the highest photovoltaic characteristics were detected in P3HT:PC71BM:GQD/P3HT nanofiber systems (14.06 mA cm−2, 0.70 V, 66.11%), leading to a performance of 6.50%. Thin films modified by crab-like GQD/P3HT short chain (5.47%), CQD/P3HT nanofiber (4.56%), and semicrab-like CQD/P3HT short chain (3.46%) nanostructures also reflected the high-quality devices. The systems manipulated by the crab/semicrab-like (668 and 462 Ω.cm2) and decorated fibrillar (553 and 395 Ω.cm2) nanostructures demonstrated the faster electron transport and, thereby the lower carrier recombination. Even the well-designed supramolecules of GQD/P3HT nanofiber and crab-like GQD/P3HT short chain led to the low-quality binary photovoltaics (3.68% and 2.29%) with respect to corresponding ternary systems. When the QDs and their associated nano-hybrids were employed as the electron donors, the weakest cells were fabricated and the highest efficacy was 2.01% for PC71BM:GQD/P3HT.

Conjugated polymer nanostructures displaying highly photoactivated antimicrobial and antibiofilm functionalities.

This work reports the use of conjugated polymer nanostructures (CPNs) as photoactivated antimicrobial compounds against Gram-positive and Gram-negative microorganisms. Two representative CPNs of polythiophene (PEDOT) and polyaniline (PANI) were prepared as nanofibres with an average diameter of 40 nm and length in the micrometer range. Both CPNs exhibited strong antimicrobial activity under UVA irradiation with the same fluence rate as the UVA component of the solar spectrum. The effect was tested using the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli. The reduction of colony forming units (CFUs) reached >6 log for PEDOT concentrations as low as 33 ng mL-1. For PEDOT nanofibers, a complete inhibition of S. aureus and E. coli growth was reached at 883 ng mL-1 and 333 ng mL-1 respectively. The photoactivation effect of PANI nanofibres on S. aureus and E. coli was also high, with a CFU reduction of about 7 log and 4 log respectively for an exposure concentration of 33 ng mL-1. The antimicrobial activity was only high under light irradiation and was almost negligible for bulk PEDOT and PANI. The effect of polymeric nanofibers could be attributed to the photoinduced generation of reactive oxygen species, which may induce cell membrane damage, eventually leading to bacterial impairment and inhibition of their biofilm forming capacity. CPN PEDOT and PANI coatings were able to keep surfaces free of bacterial attachment and growth even after 20 h of previous contact with exponentially growing cultures in the dark. PEDOT and PANI CPNs demonstrated good cytocompatibility with human fibroblasts and the absence of hemolytic activity. The materials demonstrated advantages in terms of broad antibacterial spectrum, biofilm inhibition, and the absence of acute toxicity for biomedical applications.

Synthesis and Self-Assembly of Conjugated Block Copolymers.

In the past two decades, conjugated polymers (CPs) have drawn great attention due to their excellent conductivity and charge mobility, rendering them broad applications in organic electronics. Controlling over the morphologies and nanostructures of CPs is very important to improve the performance of CP-based devices, which is still a tremendously difficult task. Conjugated block copolymers (cBCPs), composed of different CP blocks or CP coupled with coiled polymeric blocks, not only maintain the advantages of high conductivity and mobility but also demonstrate features of morphological versatility and tunability. Due to the strong π–π interaction and crystallinity of the conjugated backbones, the self-assembly behaviors of cBCPs are very complicated and largely remain to be explored. In this tutorial review, we first summarize the general synthetic methods for different types of cBCPs. Then, recent studies on the self-assembly behaviors of cBCPs are discussed, with an emphasis on the structural factors that affect the morphologies of cBCPs both in bulk and thin film states. Finally, we briefly provide our outlook on the future research of the self-assembly of cBCPs.

Bottom-Up Approaches for Precisely Nanostructuring Hybrid Organic/Inorganic Multi-Component Composites for Organic Photovoltaics

Achieving control over the morphology of conjugated polymer (CP) blends at nanoscale is crucial for enhancing their performances in diverse organic optoelectronic devices, including thin film transistors, photovoltaics, and light emitting diodes. However, the complex CP chemical structures and intramolecular interactions often make such control difficult to implement. We demonstrate here that cooperative combination of non-covalent interactions, including hydrogen bonding, coordination interactions, and π-π interactions, etc., can be used to effectively define the morphology of CP blend films, in particular being able to achieve accurate spatial arrangement of nanoparticles within CP nanostructures. Through UV-vis absorption spectroscopy and transmission electron microscopy, we show strong attachment of fullerene molecules, CdSe quantum dots, and iron oxide nanoparticles, onto well-defined CP nanofibers. The resulting core/shell hybrid nanofibers exhibit well-defined donor/acceptor interface when employed in photovoltaic devices, which also contributes to enhanced charge separation and transport. These findings provide a facile new methodology of improving CP/nanoparticle interfacial properties and controlling blend morphology. The generality of this methodology demonstrated in current studies points to a new way of designing hybrid materials based on organic polymers and inorganic nanoparticles towards applications in modern electronic devices.

Photocatalytic degradation of organic pollutant with polypyrrole nanostructures under UV and visible light

Conjugated polymer nanostructures (CPNs) emerge as a new class of photocatalysts for organic pollutant degradation under UV and visible light. Polyprrole (PPy), as a conjugated polymer, exhibits a wide range of applications. We present here the first demonstration of employing pure PPy nanostructures as a very efficient photocatalyst for water depollution. PPy nanostructures were synthesized in hexagonal mesophases (used as soft templates) by chemical polymerization (PPy-NS-c), obtained by radiolysis (PPy-NS-γ), and synthesized without any template via chemical method (PPy-bulk) as bulk. The different PPy samples were characterized by SEM, TEM, FTIR and UV–vis absorption spectroscopy. The photocatalytic activity of both PPy nanostructures (PPy-NS-c and PPy-NS- γ), which remain very stable after several cycles, was evaluated for the degradation of organic pollutant in aqueous solution (phenol and methyl orange were taken as a model pollutant). PPy nanostructures show high photocatalytic activity under both UV and visible light while bulk PPy (PPy-bulk) has no appreciable activity. PPy-NS-c present the highest activity for photodegradation of phenol under UV light, while PPy-NS-γ exhibit the best photocatalytic activity under visible light. We demonstrate here that the nanostructuration of these polymers is an important factor for their application in photocatalysis.

Facile crosslinking of polythiophenes by polyethylenimine via ester aminolysis for selective Cu(II) detection in water

Functionalization of π-conjugated polymers is dispensable for solubilization of the rigid and hydrophobic backbones in water. However, polymer aggregation is always present and leads to issues with complication and reproducibility in spectral properties. Herein, we reported a simple and robust method to make a series of conjugated polymer nanostructures by a crosslinking strategy. In favor of multivalency effect, polythionenes with various chain lengths were functionalized with branched polyethylenimine (PEI) via ester aminolysis reaction under mild conditions. Photophysical studies revealed the conjugated backbones could be well stabilized and dispersed in water. By taking advantage of intermolecular recognition interaction between copper ions and cationic PEI, we applied crosslinked polythiophenes as a nano probe at very low concentration (0.01 mg/mL) to fluorescently detect copper ions with high sensitivity up to 10 nM and selectivity over other metal ions in aqueous solutions, without occurrence of detectable aggregates. The overall performance of our nano probes outperforms reported water-soluble polymers-based probes, particularly in probe availability and manipulation as well as selective copper detection capability.

Highly active poly(3-hexylthiophene) nanostructures for photocatalysis under solar light

Conjugated polymer nanostructures have recently emerged as a new class of very active photocatalysts under solar light. Poly(3-hexylthiophene) (P3HT) is one of the commonly used conjugated polymers for photovoltaics applications. P3HT nanostructures synthesized in soft templates provided by hexagonal mesophases show high photocatalytic activity for degradation of pollutants under both UV and visible light. These photocatalysts are very stable even after repeated cycling. Addition of scavengers and mechanistic studies show that O2− is the main radical responsible for degradation of phenol taken as a model pollutant. P3HT nanostructures can be easily deposited on flat supports such as quartz for photocatalytic applications avoiding a separation step by centrifugation. Most interestingly, the photocatalytic activity of these P3HT nanostructures is highly enhanced when they are supported on a solid surface opening new perspectives in photocatalytic reactors and self-cleaning surfaces.

Confinement Effects on the Optical Properties and Chain Conformations of Poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) Nanotubes

Although many fabrication methods of conjugated polymer nanostructures have been developed, the effects of confinement on the morphologies, chain conformations, and optical properties of conjugated polymers are still not well understood. In this work, poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) (F8BT), a light‐emitting conjugated polymer, is used to fabricate nanotubes by a solution wetting method using anodic aluminum oxide templates. The property changes under the nanoscale confinement have been investigated. The morphologies of F8BT nanostructures are characterized by scanning electron microscope, transmission electron microscope, and laser scanning confocal microscope. The data show the hollow morphologies and no or low crystallinity of the nanostructures. The optical properties are also measured using UV–vis absorption, photoluminescence, and confocal Raman spectroscopy. The spectroscopy data indicate that F8BT polymer chains have less intermolecular interactions and coplanarity under the confinement effect. image

(Invited) Conjugated Polymer Nanostructures for Li-Battery

In this presentation, I will talk about the prepareation, characterization and self-assembly of organic conjugated polymers and find their applications in Li-batteries.

Water Processable Polythiophene Nanowires by Photo-Cross-Linking and Click-Functionalization

Replacing or minimizing the use of halogenated organic solvents in the processing and manufacturing of conjugated polymer-based organic electronics has emerged as an important issue due to concerns regarding toxicity, environmental impact, and high cost. To date, however, the processing of well-ordered conjugated polymer nanostructures has been difficult to achieve using environmentally benign solvents. In this work, we report the development of water and alcohol processable nanowires (NWs) with well-defined crystalline nanostructure based on the solution assembly of azide functionalized poly(3-hexylthiophene) (P3HT-azide) and subsequent photo-cross-linking and functionalization of these NWs. The solution-assembled P3HT-azide NWs were successfully cross-linked by exposure to UV light, yielding good thermal and chemical stability. Residual azide units on the photo-cross-linked NWs were then functionalized with alkyne terminated polyethylene glycol (PEG-alkyne) using copper catalyzed azide-alkyne cycloaddition chemistry. PEG functionalization of the cross-linked P3HT-azide NWs allowed for stable dispersion in alcohols and water, while maintaining well-ordered NW structures with electronic properties suitable for the fabrication of organic field effect transistors (OFETs).

Self-assembly of conjugated polymers for anisotropic nanostructures

As their potential applications in various electronic devices increase, the preparation of anisotropic conjugated polymer nanostructures are highly desirable. This paper presents a review of the literature and our recent results on the self-assembly of one-, two- and three-dimensional anisotropic nanostructures using conjugated polymers as building blocks, including the formation of one-dimensional (1D) nanofibers and nanotubes, two-dimensional (2D) nanoribbons and nanosheets, and three-dimensional (3D) superstructures. The mechanisms guiding the formation of various nanostructures are analyzed by a cooperative effect of π-π stacking interaction and other noncovalent interactions.

Self-assembly of conjugated polymers for anisotropic nanostructures

As their potential applications in various electronic devices increase, the preparation of anisotropic conjugated polymer nanostructures are highly desirable. This paper presents a review of the literature and our recent results on the self-assembly of one-, two- and three-dimensional anisotropic nanostructures using conjugated polymers as building blocks, including the formation of one-dimensional (1D) nanofibers and nanotubes, two-dimensional (2D) nanoribbons and nanosheets, and three-dimensional (3D) superstructures. The mechanisms guiding the formation of various nanostructures are analyzed by a cooperative effect of π-π stacking interaction and other noncovalent interactions.

A Substrate-Integrated and Scalable Templated Approach Based on Rusted Steel for the Fabrication of Polypyrrole Nanotube Arrays

We report here a facile, generalizable, and entirely scalable approach for the fabrication of vertically aligned arrays of Fe(2)O(3)/polypyrrole core-shell nanostructures and polypyrrole nanotubes. Our "all electrochemical" approach is based on the fabrication of α-Fe(2)O(3) nanowire arrays by the simple heat treatment of commodity low carbon steel substrates, followed by electropolymerization of conformal polypyrrole sheaths around the nanowires. Subsequently, electrochemical etching of the nanowires yields large-area vertically aligned polypyrrole nanotube arrays on the steel substrate. The developed methodology is generalizable to functionalized pyrrole monomers and represents a significant practical advance of relevance to the technological implementation of conjugated polymer nanostructures in electrochromics, electrochemical energy storage, and sensing.

Conjugated polymer nanostructures for organic solar cell applications

Recently, there has been tremendous progress in the development of polymer-based organic solar cells. Polymer-based solar cells have attracted a great deal of attention because they have the potential to be efficient, inexpensive, and solution processable. New materials, nanostructures, device designs, and processing methods have been developed to achieve high device efficiencies. This review focuses on the fabrication techniques of conjugated polymer nanostructures and their applications for organic solar cells. We will first introduce the fundamental knowledge of organic solar cells and emphasize the importance of nanostructures. Then we will discuss different strategies for fabricating conjugated polymer nanostructures, including topics such as polymer nanowires, nanoparticles, block copolymers, layer-by-layer deposition, nanoimprint lithography, template methods, nanoelectrodes, and porous inorganic materials. The effects of the nanostructures on the device performance will also be presented. Efficiencies higher than 10% are expected for polymer-based solar cells by using new materials and techniques.

Morphological and impedance studies on electropolymerized 3,4-(2,2-dibenzylpropylenedioxy)thiophene nanostructures on micron sized single carbon fiber

Micron sized single carbon fibers were cyclovoltammetrically coated with poly[3,4-(2,2-dibenzylpropylenedioxy)thiophene] resulting in a nanofiber network at the surface. The method provides conjugated polymer nanostructures covalently and uniformly bound to micron sized substrates. When the electropolymerization is carried out with different electrolytes in acetonitrile the dopant influences the structure of the coating layer what is proved by electrochemical impedance spectroscopy and electron microscopy. Electrodes based on poly[3,4-(2,2-dibenzylpropylenedioxy)thiophene] on single carbon fiber microelectrodes (SCFMEs) prepared in Bu4NPF6/ACN show the best capacitance performance due to their higher surface area. The improvement is attributed to the formed nanofiber network structure which results in a more efficient charge transport and collection.Impedance spectra show the typical form of ZIM vs. ZRE curves for transmission-line at a frequency of 10Hz, with transition to almost pure capacitive behavior up to 10MHz. Equivalent circuit modeling was simulated for the electrolyte/polymer film/SCFME system. A good agreement was achieved for the calculated capacitances in comparison with the experimental EIS measurement results.

Spectroscopic and electrical evaluation of poly(3-hexylthiophene) nanotubules made using template wetting nanofabrication

The unique properties of conjugated polymer nanostructures as compared to thin films of the same materials have fueled the investigation of a number of different polymer nanofabrication techniques. Here we examine the optoelectronic properties of poly(3-hexylthiophene)P3HT nanotubules made using one of the simplest of these techniques, template wetting nanofabrication. Comparison is made with results from our previous study in which in poly(2-methoxy-5-(2́-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) nanotubules prepared in this manner were shown to have dramatically different properties in comparison with thin films due to increased molecular order in the nanostructures. With semicrystalline P3HT, the differences are much more subtle, we believe due to the greater inherent order in this material. In P3HT nanotubules, a small bathochromic shift (∼0.05 eV) in the onset of Uv–vis absorption was observed indicating increased long range order and a greater effective conjugation length in the polymer. Polarized FTIR spectroscopy indicated a small degree of preferential alignment of the inter-ring C–C bond with the axis of the nanotubules as the stretching vibration for this bond showed a dichroic ratio of 1.73. The degree of increased conjugation length and chain alignment in P3HT nanotubules indicated by both the UV–vis and polarized FTIR spectra were much smaller than that previously observed for MEH-PPV. No significant improvement in mobility as measured from the current voltage characteristics of simple single carrier devices based on the P3HT nanotubules was observed, in sharp contrast to previously studied MEH-PPV where more than an order of magnitude increase was seen. Unlike in amorphous MEH-PPV, the increase in molecular order suggested by spectral data was insufficient to significantly impact mobility of semicrystalline P3HT when using template wetting nanofabrication for either of the 100 or 200 nm nanotubules examined.

Direct writing and characterization of poly(p-phenylene vinylene) nanostructures

We report the use of thermochemical nanolithography to convert a precursor polymer film to poly(p-phenylene vinylene) with sub-100 nm spatial resolution, in ambient conditions. The local thermochemical conversion is verified by Raman spectroscopy, fluorescence imaging, and atomic force microscopy. This convenient direct writing of conjugated polymer nanostructures could be desirable for the design and fabrication of future nanoelectronic, nanophotonic, and biosensing devices.

Toward a simple method for the fabrication of 1D or 3D nanostructures of polyaniline

We present a one-step chemical method for the fabrication and isolation of conjugated polymer nanostructures, in particular, polyanilines (PANIs), either in the pernigraniline or emeraldine oxidation state. The polymerization is initiated by the addition of a solid oxidant into an unstirred anilinium hydrochloride solution. This step allows a slow release of the oxidant, via its solubilization and diffusion into the undisturbed solution. The formation of visible macroscopic heterogeneous seeds promotes the homogeneous polymerization and growth of bulk nanostructures in solution. The shape of the first formed heterogeneous seeds was found to be dependent on the solubility/diffusion of the oxidant, allowing the prediction and control of the nanomorphology (1D vs. 3D or granular) of the bulk polyanilines. X-ray analyses of the as-prepared nanostructured PANIs showed that, compared to the literature data, the crystallite growth in the present study is significantly enhanced, with the 1D fibrillar morphology exhibiting a higher crystallinity per volume of nanostructure than that of a 3D granular counterpart. The electrical conductivity ( σ ) of pelletized HCl doped PANIs or emeraldine salts (ES) showed significant differences depending on the fibrillar or granular morphology, but a close relationship among number of unit cells, crystallite size and electrical conductivity is evidenced.