Effects of the chemical structure of polyfluorene on selective extraction of semiconducting single-walled carbon nanotubes

Semiconducting Carbon Nanotube Extraction Enabled by Alkylated Cellulose Wrapping

With the growing demand for postsilicon electronics, the purification of single-walled carbon nanotubes (SWCNTs) in terms of their chirality, which defines their atomic and electronic structure, is becoming increasingly important. Herein, we demonstrate the selective extraction of high-quality semiconducting SWCNTs using alkyl cellulose as a dispersant in organic solvents. We investigated the separation factors of dispersant structures, such as the degree of substitution (DS) and molecular weight, and clarified the appropriate dispersant structures, such as moderately substituted hexyl cellulose, for selective semiconducting SWCNT extraction. Due to the improved purity and quality of the semiconducting SWCNTs obtained by this method, their films exhibit excellent thermoelectric power factors, outperforming not only unsorted SWCNTs but also conducting polymer-sorted SWCNTs. This sorting technology paves the way for supplying high-quality semiconducting SWCNTs in a viable manner.

Polymethyl(1-Butyric acidyl)silane-Assisted Dispersion and Density Gradient Ultracentrifugation Separation of Single-Walled Carbon Nanotubes

Individual single–walled carbon nanotubes (SWNTs) with distinct electronic types are crucial for the fabrication of SWNTs–based electronic and magnetic devices. Herein, the water–soluble polymethyl(1–butyric acidyl)silane (BA–PMS) was synthesized via the hydrosilylation reaction between 3–butenoic acid and polymethylsilane catalyzed by 2,2′–azodibutyronitrile. As a new dispersant, BA–PMS displayed a quite good dispersing capacity to arc–discharged SWNTs and moderate selectivity for metallic species. The application of sucrose–DGU, the density gradient ultracentrifugation with sucrose as the gradient medium, to the co–surfactants (BA–PMS and sodium dodecyl sulfonate) individually dispersed SWNTs yielded metallic SWNTs of 85.6% purity and semiconducting SWNTs of 99% purity, respectively. This work paves a path to the DGU separation of the SWNTs dispersed by polymer–based dispersants with hydrophobic alkyl chains.

Enrichment of high-purity large-diameter semiconducting single-walled carbon nanotubes

Semiconducting single-walled carbon nanotubes SWCNTs (s-SWCNTs) are considered one of the most promising alternatives to traditional silicon-based semiconductors. In particular, large-diameter s-SWCNTs (>1.2 nm) exhibit more advantages over small-diameter ones in high-performance electronic applications because of their higher charge carrier mobility and reduced Schottky barrier height. Great efforts have been made to enriching large-diameter s-SWCNTs from mass-produced raw CNTs that contain both metallic SWCNTs and s-SWCNTs. Among separation technologies, the effective and scalable ones are conjugated polymer wrapping (CPW), gel permeation chromatography (GC), aqueous two-phase extraction (ATPE), and density gradient ultracentrifugation (DGU). In this review, we survey recent progress on enriching large-diameter s-SWCNTs using those methods and outline the strategies and challenges in the separation according to the electronic type and chirality of SWCNTs. Finally, we highlight some applications of the enriched large-diameter s-SWCNTs and outlook for the future of SWCNT-based electronic devices.

Separation of Semiconducting Carbon Nanotubes Using Conjugated Polymer Wrapping

In the past two decades, single-walled carbon nanotubes (SWNTs) have been explored for electronic applications because of their high charge carrier mobility, low-temperature solution processability and mechanical flexibility. Semiconducting SWNTs (s-SWNTs) are also considered an alternative to traditional silicon-based semiconductors. However, large-scale, as-produced SWNTs have poor solubility, and they are mixtures of metallic SWNTs (m-SWNTs) and s-SWNTs, which limits their practical applications. Conjugated polymer wrapping is a promising method to disperse and separate s-SWNTs, due to its high selectivity, high separation yield and simplicity of operation. In this review, we summarize the recent progress of the conjugated polymer wrapping method, and discuss possible separation mechanisms for s-SWNTs. We also discuss various parameters that may affect the selectivity and sorting yield. Finally, some electronic applications of polymer-sorted s-SWNTs are introduced. The aim of this review is to provide polymer chemist a basic concept of polymer based SWNT separation, as well as some polymer design strategies, influential factors and potential applications.

Structurally Analogous Degradable Version of Fluorene-Bipyridine Copolymer with Exceptional Selectivity for Large-Diameter Semiconducting Carbon Nanotubes

Separation of electronically pure, narrowly dispersed, pristine, semiconducting single-walled carbon nanotubes (CNTs) from a heterogeneous as-synthesized mixture is essential for various semiconducting technologies and biomedical applications. Although conjugated polymer wrappers are often utilized to facilitate electronic-type sorting, it is highly desirable to remove organic residues from the resulting devices. We report here the design and synthesis of a mild acid-degradable π-conjugated polyimine polymer, poly[(9,9-di-n-octyl-2,7-fluoren-dinitrilomethine)-alt-co-(6,6'-{2,2'-bipyridyl-dimethine})] (PFO-N-BPy), that is structurally analogous to the commonly used and commercially available poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-(2,2'-bipyridine))] (PFO-BPy). An acid cleavable imine link (-HC═N-) was introduced in the PFO-N-BPy backbone to impart degradability, which is absent in PFO-BPy. PFO-N-BPy was synthesized via a metal catalyst-free aza-Wittig reaction in high yields. PFO-N-BPy with a degree of polymerization of just ∼10 showed excellent (>99% electronic purity) selectivity for both large-diameter (1.3-1.7 nm) arc-discharge semiconducting CNTs (S-CNTs) and smaller diameter (0.8-1.2 nm) high-pressure carbon monoxide disproportionation reaction S-CNTs. Overall, the selectivity for the semiconducting species is similar to that of PFO-BPy but with an advantage of complete depolymerization under mild acidic conditions into recyclable monomers. We further show by ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy that the PFO-N-BPy-wrapped S-CNTs can be aligned into a monolayer array on gate dielectrics using a floating evaporative self-assembly process from which the polymer can be completely removed. Short channel field effect transistors were fabricated from the polymer-stripped aligned S-CNT arrays, which further confirmed the semiconducting purity on the order of 99.9% or higher.

Self-Organized Micro-Spiral of Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are reported to spontaneously align in a rotational pattern by drying a liquid droplet of toluene containing polyfluorene as a dispersant. By situating a droplet of an SWCNT solution around a glass bead, spiral patterns are generated. The parallel alignment of SWCNTs along one stripe of such a pattern is confirmed using scanning electron microscopy and polarized optical microscopy. The orientation order increases toward the outer edge of a stripe. The stripe width in the pattern is proportional to the solute concentration, and the width and position of the stripes follow geometric sequences. The growth of the rotational pattern is also observed in real time. The process of spiral pattern formation is visualized, indicating the role of the annihilation of counter-traveling accompanied by continuous depinning. The geometric sequences for the stripe width and position are explained by the near-constant traveling speed and solute enrichment at the droplet periphery.

Specific Molecular Interaction and Recognition at Single-Walled Carbon Nanotube Surfaces

Carbon nanotubes (CNTs) are carbon clusters (polymers) with huge molecular weight and have been the central material in the field of nanomaterials science and nanotechnology because of their remarkable electronic, thermal, mechanical, optical, and electrical properties. In this review article, we first focus on the formation of self-assembled CNT superstructures and spontaneous conductive CNT-honeycomb structure formation from CNT/long-chain ammonium lipids by simple solvent casting. We also summarized our recent studies on specific molecular interactions and recognition at single-walled carbon nanotube surfaces and CNT chirality recognition using specific polymers. For such studies, the key issue is to develop a methodology to solubilize/disperse them in solvent because as-synthesized CNTs form tightly bundled structures as a result of their strong van der Waals interactions and are insoluble in many solvents. For the analysis of molecules and CNT surfaces, the introduction of thermodynamic treatment and an HPLC method using CNT-coated silica as a stationary phase was powerful.

Suppression of Polyfluorene Photo-Oxidative Degradation via Encapsulation of Single-Walled Carbon Nanotubes

Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are well-known dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3^'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN-SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.

Diameter-Selective Dispersion of Carbon Nanotubes via Polymers: A Competition between Adsorption and Bundling

The mechanism of the selective dispersion of single-walled carbon nanotubes (CNTs) by polyfluorene polymers is studied in this paper. Using extensive molecular dynamics simulations, it is demonstrated that diameter selectivity is the result of a competition between bundling of CNTs and adsorption of polymers on CNT surfaces. The preference for certain diameters corresponds to local minima of the binding energy difference between these two processes. Such minima in the diameter dependence occur due to abrupt changes in the CNT's coverage with polymers, and their calculated positions are in quantitative agreement with preferred diameters reported experimentally. The presented approach defines a theoretical framework for the further understanding and improvement of dispersion/extraction processes.

Non-covalent polymer wrapping of carbon nanotubes and the role of wrapped polymers as functional dispersants

Carbon nanotubes (CNTs) have been recognized as a promising material in a wide range of applications from biotechnology to energy-related devices. However, the poor solubility in aqueous and organic solvents hindered the applications of CNTs. As studies have progressed, the methodology for CNT dispersion was established. In this methodology, the key issue is to covalently or non-covalently functionalize the surfaces of the CNTs with a dispersant. Among the various types of dispersions, polymer wrapping through non-covalent interactions is attractive in terms of the stability and homogeneity of the functionalization. Recently, by taking advantage of their stability, the wrapped-polymers have been utilized to support and/or reinforce the unique functionality of the CNTs, leading to the development of high-performance devices. In this review, various polymer wrapping approaches, together with the applications of the polymer-wrapped CNTs, are summarized.

Hybrids of copolymers of fluorene and C60 -carrying-carbazole with semiconducting single-walled carbon nanotubes

Three different copolymers of C60 -carrying-carbazole and fluorene units with different copolymer composition ratios were designed and synthesized. On the basis of photoluminescence, atomic force microscopy, and Vis-NIR and Raman spectroscopic analysis, we found that these copolymers solubilize only semiconducting single-walled carbon nanotubes (sem-SWNTs) to form copolymer/sem-SWNT hybrids, in which energy transfer from the copolymer/C60 moieties to the SWNTs was revealed. By comparing two possible hybrid structures with molecular-mechanics simulations, the greatest stabilization was found when the C60 moieties lay on the sem-SWNT surfaces.

Polymer-sorted semiconducting carbon nanotube networks for high-performance ambipolar field-effect transistors

Efficient selection of semiconducting single-walled carbon nanotubes (SWNTs) from as-grown nanotube samples is crucial for their application as printable and flexible semiconductors in field-effect transistors (FETs). In this study, we use atactic poly(9-dodecyl-9-methyl-fluorene) (a-PF-1-12), a polyfluorene derivative with asymmetric side-chains, for the selective dispersion of semiconducting SWNTs with large diameters (>1 nm) from plasma torch-grown SWNTs. Lowering the molecular weight of the dispersing polymer leads to a significant improvement of selectivity. Combining dense semiconducting SWNT networks deposited from an enriched SWNT dispersion with a polymer/metal-oxide hybrid dielectric enables transistors with balanced ambipolar, contact resistance-corrected mobilities of up to 50 cm(2)·V(-1)·s(-1), low ohmic contact resistance, steep subthreshold swings (0.12-0.14 V/dec) and high on/off ratios (10(6)) even for short channel lengths (<10 μm). These FETs operate at low voltages (<3 V) and show almost no current hysteresis. The resulting ambipolar complementary-like inverters exhibit gains up to 61.

Selective extraction of metallic arc-discharged single-walled carbon nanotubes by a water soluble polymethylsilane derivative

Arc-discharged metallic SWNTs are selectively extracted with an aqueous solution of polymethyl(1-undecylic acidyl)silane by the formation of a charge donor–acceptor complex.

Effect of side-chain halogenation on the interactions of conjugated polymers with SWNTs

Halogenation of polyfluorene side-chain ends with bromine or iodine causes significant differences in the nanotube species that are dispersed in solvent, indicating that subtle changes in polymer structure can affect polymer-nanotube interactions.

Semiconducting single-walled carbon nanotubes sorting with a removable solubilizer based on dynamic supramolecular coordination chemistry

Highly pure semiconducting single-walled carbon nanotubes (SWNTs) are essential for the next generation of electronic devices, such as field-effect transistors and photovoltaic applications; however, contamination by metallic SWNTs reduces the efficiency of their associated devices. Here we report a simple and efficient method for the separation of semiconducting- and metallic SWNTs based on supramolecular complex chemistry. We here describe the synthesis of metal-coordination polymers (CP-Ms) composed of a fluorene-bridged bis-phenanthroline ligand and metal ions. On the basis of a difference in the 'solubility product' of CP-M-solubilized semiconducting SWNTs and metallic SWNTs, we readily separated semiconducting SWNTs. Furthermore, the CP-M polymers on the SWNTs were simply removed by adding a protic acid and inducing depolymerization to the monomer components. We also describe molecular mechanics calculations to reveal the difference of binding and wrapping mode between CP-M/semiconducting SWNTs and CP-M/metallic SWNTs. This study opens a new stage for the use of such highly pure semiconducting SWNTs in many possible applications.