Removable and Recyclable Conjugated Polymers for Highly Selective and High-Yield Dispersion and Release of Low-Cost Carbon Nanotubes

Degradable π-Conjugated Polymers

The integration of next-generation electronics into society is rapidly reshaping our daily interactions and lifestyles, revolutionizing communication and engagement with the world. Future electronics promise stimuli-responsive features and enhanced biocompatibility, such as skin-like health monitors and sensors embedded in food packaging, transforming healthcare and reducing food waste. Imparting degradability may reduce the adverse environmental impact of next-generation electronics and lead to opportunities for environmental and health monitoring. While advancements have been made in producing degradable materials for encapsulants, substrates, and dielectrics, the availability of degradable conducting and semiconducting materials remains restricted. π-Conjugated polymers are promising candidates for the development of degradable conductors or semiconductors due to the ability to tune their stimuli-responsiveness, biocompatibility, and mechanical durability. This perspective highlights three design considerations: the selection of π-conjugated monomers, synthetic coupling strategies, and degradation of π-conjugated polymers, for generating π-conjugated materials for degradable electronics. We describe the current challenges with monomeric design and present options to circumvent these issues by highlighting biobased π-conjugated compounds with known degradation pathways and stable monomers that allow for chemically recyclable polymers. Next, we present coupling strategies that are compatible for the synthesis of degradable π-conjugated polymers, including direct arylation polymerization and enzymatic polymerization. Lastly, we discuss various modes of depolymerization and characterization techniques to enhance our comprehension of potential degradation byproducts formed during polymer cleavage. Our perspective considers these three design parameters in parallel rather than independently while having a targeted application in mind to accelerate the discovery of next-generation high-performance π-conjugated polymers for degradable organic electronics.

Removing Conjugated Polymers from Aligned Carbon Nanotube Arrays

Aligned carbon nanotube (A-CNT) with high semiconducting purity and high-density have been considered as one of the most promising active channels for field-effect transistors (FETs), but conjugated polymer dispersant residues on the surface of A-CNT have become the main obstacle for its further development in electronics applications. In this work, a series of removable conjugated polymers (CPs) are designed and synthesized to achieve favorable purification and alignment for CNT arrays with a high density of ≈360 CNTs/µm. Furthermore, a removal process of CPs on the CNT array film is developed. Raman spectra show that the CNTs in array film are almost not damaged after the removal process, and the G/D ratio is as high as 35. The field-effect transistors (FETs) are fabricated with a saturation current density up to 600 µA µm-1 and a current on-off ratio of ≈105, even with a relatively long channel length of ≈3 µm.

Polymer removal and dispersion exchange of (10,5) chiral carbon nanotubes with enhanced 1.5 μm photoluminescence

Singe-chirality single-walled carbon nanotubes (SWCNTs) produced by selective polymer extraction have been actively investigated for their semiconductor applications. However, to fulfil the needs of biocompatible applications, the organic solvents in polymer-sorted SWCNTs impose a limitation. In this study, we developed a novel strategy for organic-to-aqueous phase exchange, which involves thoroughly removing polymers from the sorted SWCNTs, followed by surfactant covering and redispersing of the cleaned SWCNTs in water. Importantly, the obtained aqueous system allows us to perform sp3 functionalization of the SWCNTs, leading to a strong photoluminescence emission at 1550 nm from the defect sites of (10,5) SWCNTs. These functionalized SWCNTs as infrared light emitters show considerable potential for bioimaging applications. This exchange-and-functionalization strategy opens the door for future biocompatible applications of polymer-sorted SWCNTs.

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance

High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

Azomethine-Containing Pyrrolo[3,2-b]pyrrole Copolymers for Simple and Degradable Conjugated Polymers

Conjugated polymers have received significant attention as potentially lightweight and highly tailorable alternatives to inorganic semiconductors, but their synthesis is often complex, produces toxic byproducts, and they are not typically designed to be degradable or recyclable. These drawbacks necessitate dedicated efforts to discover materials with design motifs that enable targeted and efficient degradation of conjugated polymers. In this vein, the synthetic simplicity of 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) is exploited to access azomethine-containing copolymers via a benign acid-catalyzed polycondensation protocol. Polymerizations involve reacting a dialdehyde-functionalized dihydropyrrolopyrrole with p-phenylenediamine as the comonomer using p-toluenesulfonic acid as a catalyst. The inherent dynamic equilibrium of the azomethine bonds subsequently enabled the degradation of the polymers in solution in the presence of acid. Degradation of the polymers is monitored via NMR, UV-vis absorbance, and fluorescence spectroscopies, and the polymers are shown to be fully degradable. Notably, while absorbance measurements reveal a continued shift to higher energies with extended exposure to acid, fluorescence measurements show a substantial increase in the fluorescence response upon degradation. Results from this study encourage the continued development of environmentally-conscious polymerizations to attain polymeric materials with useful properties while simultaneously creating polymers with structural handles for end-of-life management or/and recyclability.

Two-component polymer sorting to obtain high-purity s-SWCNTs for all-carbon photodetectors

The advancement of carbon-based electronics is reliant on the development of semiconducting carbon nanotubes with high purity and yield. We developed a new extraction strategy to efficiently sort SWCNTs with superior yields and purity. The approach uses two polymers, poly[N-(1-octylnonyl)-9H-carbazol-2,7-diyl](PCz) and poly(9,9-n-dihexyl-2,7-fluorene-alt-9-phenyl-3,6-carbazole)(PDFP), and two sonication processes to eliminate surface polymer contamination. PCz selectively wraps large-diameter s-SWCNTs, with PDFP added as an enhancing molecule to increase sorting efficiency at 4-fold compared to the efficiency of only PCz alone sorting. The purity of the sorted s-SWCNTs was confirmed to be above 99 % using absorption and Raman spectra. Field-effect transistors and photodetectors made from the sorted s-SWCNTs exhibited excellent semiconductor properties and broad-spectrum detection, with good long-term stability. Furthermore, a photodetector using large-tube diameter s-SWCNTs achieved broad-spectrum detection, which the photoresponsivity is 0.35 mA/W and the detectivity is 4.7×106 Jones. The s-SWCNTs/graphene heterojunction photodetector achieved a photoresponsivity of 3 mA/W and a detectivity of 6.3×106 Jones. This new strategy provides a promising approach to obtain high-purity and high-yield s-SWCNTs for carbon-based photodetectors.

Biobased, Degradable, and Conjugated Poly(Azomethine)s

Carotenoids are a class of biobased conjugated molecules that bear a resemblance to the substructure of polyacetylene, a well-known conductive but insoluble polymer. Solubility is an important physical attribute for processing materials using different techniques. To impart solubility in polymers, alkyl side chains are often included in the molecular design. While these design strategies are well explored in conjugated systems, they have not been implemented with carotenoids as a building block in polymers. Here, we show a series of carotenoid-based polymers with varying side chain lengths to tune solubility. Using carotenoid and p-phenylenediamine-based monomers, degradable and biobased poly(azomethine)s were synthesized via imine polycondensation. Maximum solubilities corresponding to the varying alkyl chain lengths were quantitatively determined by ultraviolet-visible (UV-vis) absorption spectroscopy. Since carotenoids are biobased with known degradation products, the effect of acidic and artificial sunlight-promoted degradation was systematically investigated using UV-vis spectroscopy, ¹H nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, gel permeation chromatography (GPC), and high-resolution mass spectroscopy (HRMS). Our polymer system was found to have two modes of on-demand degradation, with acid hydrolysis accelerating the rate of polymer degradation and artificial sunlight generating additional degradation products. This work highlights carotenoid monomers as viable candidates in the design of biobased, degradable, and conjugated polymers.

Decoration of Polyfluorene-Wrapped Carbon Nanotubes with Photocleavable Side-Chains

Functionalizing polyfluorene-wrapped carbon nanotubes without damaging their properties is effective via Copper-Catalyzed Azide-Alkyne Cycloaddition (CuAAC). However, the length and nature of polymer side-chains can impact the conductivity of polyfluorene-SWNT films by preventing close contact between the nanotubes. Here, we investigate the functionalization of a polyfluorene-SWNT complex using photocleavable side-chains that can be removed post-processing. The cleavage of the side-chains containing an ortho-nitrobenzyl ether derivative is efficient when exposed to a UV lamp at 365 nm. The photoisomerization of the o-nitrobenzyl ether linker into the corresponding o-nitrosobenzaldehyde was first monitored via UV-Vis absorption spectroscopy and ¹H-NMR spectroscopy on the polymer, which showed efficient cleavage after 2 h. We next investigated the cleavage on the polyfluorene-SWNT complex via UV-Vis-NIR absorption spectroscopy. The precipitation of the nanotube dispersion and the broad absorption peaks after overnight irradiation also indicated effective cleavage. In addition, Raman spectroscopy post-irradiation showed that the nanotubes were not damaged upon irradiation. This paper reports a proof of concept that may find applications for SWNT-based materials in which side-chain removal could lead to higher device performance.

Single-chirality of single-walled carbon nanotubes (SWCNTs) through chromatography and its potential biological applications

Gel chromatography is used to separate single-chirality and selective-diameter SWCNTs. We also explore the use of photothermal therapy and biosensor applications based on single-chirality, selected-diameter, and unique geometric shape.

Controlled Preparation of Single-Walled Carbon Nanotubes as Materials for Electronics

Single-walled carbon nanotubes (SWCNTs) are of particular interest as channel materials for field-effect transistors due to their unique structure and excellent properties. The controlled preparation of SWCNTs that meet the requirement of semiconducting and chiral purity, high density, and good alignment for high-performance electronics has become a key challenge in this field. In this Outlook, we outline the efforts in the preparation of SWCNTs for electronics from three main aspects, structure-controlled growth, selective sorting, and solution assembly, and discuss the remaining challenges and opportunities. We expect that this Outlook can provide some ideas for addressing the existing challenges and inspire the development of SWCNT-based high-performance electronics.

Recent Advances in Structure Separation of Single-Wall Carbon Nanotubes and Their Application in Optics, Electronics, and Optoelectronics

Structural control of single-wall carbon nanotubes (SWCNTs) with uniform properties is critical not only for their property modulation and functional design but also for applications in electronics, optics, and optoelectronics. To achieve this goal, various separation techniques have been developed in the past 20 years through which separation of high-purity semiconducting/metallic SWCNTs, single-chirality species, and even their enantiomers have been achieved. This progress has promoted the property modulation of SWCNTs and the development of SWCNT-based optoelectronic devices. Here, the recent advances in the structure separation of SWCNTs are reviewed, from metallic/semiconducting SWCNTs, to single-chirality species, and to enantiomers by several typical separation techniques and the application of the corresponding sorted SWCNTs. Based on the separation procedure, efficiency, and scalability, as well as, the separable SWCNT species, purity, and quantity, the advantages and disadvantages of various separation techniques are compared. Combined with the requirements of SWCNT application, the challenges, prospects, and development direction of structure separation are further discussed.

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.

Host-Guest Molecular Interaction Enabled Separation of Large-Diameter Semiconducting Single-Walled Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (s-SWCNTs) with a diameter of around 1.0-1.5 nm, which present bandgaps comparable to silicon, are highly desired for electronic applications. Therefore, the preparation of s-SWCNTs of such diameters has been attracting great attention. The inner surface of SWCNTs has a suitable curvature and large contacting area, which is attractive in host-guest chemistry triggered by electron transfer. Here we reported a strategy of host-guest molecular interaction between SWCNTs and inner clusters with designed size, thus selectively separating s-SWCNTs of expected diameters. When polyoxometalate clusters of ∼1 nm in size were filled in the inner cavities of SWCNTs, s-SWCNTs with diameters concentrated at ∼1.3-1.4 nm were selectively extracted with the purity of ∼98% by a commercially available polyfluorene derivative. The field-effect transistors built from the sorted s-SWCNTs showed a typical behavior of semiconductors. The sorting mechanisms associated with size-dependent electron transfer from nanotubes to inner polyoxometalate were revealed by the spectroscopic and in situ electron microscopic evidence as well as the theoretical calculation. The polyoxometalates with designable size and redox property enable the flexible regulation of interaction between the nanotubes and the clusters, thus tuning the diameter of sorted s-SWCNTs. The present sorting strategy is simple and should be generally feasible in other SWCNT sorting techniques, bringing both great easiness in dispersant design and improved selectivity.

Conformally Gated Surface Conducting Behaviors of Single-Walled Carbon Nanotube Thin-Film-Transistors

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have gathered significant interest in various emerging electronics due to their outstanding electrical and mechanical properties. Although large-area and low-cost fabrication of s-SWCNT field effect transistors (FETs) can be easily achieved via solution processing, the electrical performance of the solution-based s-SWCNT FETs is often limited by the charge transport in the s-SWCNT networks and interface between the s-SWCNT and the dielectrics depending on both s-SWCNT solution synthesis and device architecture. Here, we investigate the surface and interfacial electro-chemical behaviors of s-SWCNTs. In addition, we propose a cost-effective and straightforward process capable of minimizing polymers bound to s-SWCNT surfaces acting as an interfering element for the charge carrier transport via a heat-assisted purification (HAP). With the HAP treated s-SWCNTs, we introduced conformal dielectric configuration for s-SWCNT FETs, which are explored by a carefully designed wide array of electrical and chemical characterizations with finite-element analysis (FEA) computer simulation. For more favorable gate-field-induced surface and interfacial behaviors of s-SWCNT, we implemented conformally gated highly capacitive s-SWCNT FETs with ion-gel dielectrics, demonstrating field-effect mobility of ~8.19 cm2/V⋅s and on/off current ratio of ~105 along with negligible hysteresis.

A NIR laser induced self-healing PDMS/Gold nanoparticles conductive elastomer for wearable sensor

Self-healing conductive elastomers have been widely used in smart electronic devices, such as wearable sensors. However, nano fillers hinder the flow of polymer segments, which make the development of conductive elastomer with rapid repair and high ductility a challenge. In this work, thioctic acid (TA) was grafted onto amino-modified polysiloxane (PDMS-NH₂) by dehydration condensation of amino group and carboxyl group. By introducing gold nanoparticles, a dynamic network based on S-Au interaction was constructed. The dynamic gold cross-linking could effectively dissipate the energy exerted by external force and improve the extensibility of conductive elastomer. In addition, S-Au interaction had a good optothermal effect, so that the elastomer rapidly healed under NIR irradiation, and the repair efficiency reached 92%. We further evaluated the performance of the conductive elastomer as a strain sensor. The sample could accurately monitor the bending of human joints and small muscle state changes. This kind of self-healable conductive elastomer based on dynamic S-Au interaction has great potential in the fields of interpersonal interaction and health monitoring.

Enantiomeric Separation of Semiconducting Single-Walled Carbon Nanotubes by Acid Cleavable Chiral Polyfluorene

Helical wrapping by conjugated polymer has been demonstrated as a powerful tool for the sorting of single-walled carbon nanotubes (SWCNTs) according to their electronic type, chiral index, and even handedness. However, a method of one-step extraction of left-handed (M) and right-handed (P) semiconducting SWCNTs (s-SWCNTs) with subsequent cleavage of the polymer has not yet been published. In this work, we designed and synthesized one pair of acid cleavable polyfluorenes with defined chirality for handedness separation of s-SWCNTs from as-produced nanotubes. Each monomer contains a chiral center on the fluorene backbone in the 9-position, and the amino and carbonyl groups in the 2- and 7-positions maintain the head-to-tail regioselective polymerization resulting in polyimines with strictly all-(R) or all-(S) configuration. The obtained chiral polymers exhibit a strong recognition ability toward left- or right-handed s-SWCNTs from commercially available CoMoCAT SWCNTs with a sorting process requiring only bath sonication and centrifugation. Interestingly, the remaining polymer on each single nanotube, which helps to prevent aggregation, does not interfere with the circular dichroism signals from the nanotube at all. Therefore, we observed all four interband transition peaks (E₁₁, E₂₂, E₃₃, E₄₄) in the circular dichroism (CD) spectra of the still wrapped optically enriched left-handed and right-handed (6,5) SWCNTs in toluene. Binding energies obtained from molecular dynamics simulations were consistent with our experimental results and showed a significant preference for one specific handedness from each chiral polymer. Moreover, the imine bonds along the polymer chains enable the release of the nanotubes upon acid treatment. After s-SWNT separation, the polymer can be decomposed into monomers and be cleanly removed under mild acidic conditions, yielding dispersant-free handedness sorted s-SWNTs. The monomers can be almost quantitatively recovered to resynthesize the chiral polymer. This approach enables high selective isolation of polymer-free s-SWNT enantiomers for their further applications in carbon nanotube (CNT) devices.

Selective Chemistry-Based Separation of Semiconducting Single-Walled Carbon Nanotubes and Alignment of the Nanotube Array Network under Electric Field for Field-Effect Transistor Applications

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are considered as a replacement for silicon in field-effect transistors (FETs), solar cells, logic circuits, and so forth, because of their outstanding electronic, optical, and mechanical properties. Herein, we have studied the reaction of pristine SWCNTs dispersed in a pluronic F-68 (PF-68) polymer solution with para-amino diphenylamine diazonium sulfate (PADDS) to separate nanotubes based on their metallicity. The preferential selectivity of the reactions was monitored by changes in the semiconducting (S22 and S33) and metallic (M11) bands by ultraviolet-visible-near infrared spectroscopy. Metallic selectivity depended on the concentrations of PADDS, reaction time, and the solution pH. Furthermore, separation of pure s-SWCNTs was confirmed by Raman spectroscopy and Fourier-transform infrared spectroscopy. After the removal of metallic SWCNTs, direct current electric field was applied to the pure s-SWCNT solution, which effectively directed the nanotubes to align in one direction as nanotube arrays with a longer length and high density. After that, electrically aligned s-SWCNT solution was cast on a silicon substrate, and the length of the nanotube arrays was measured as ∼2 to ∼14 μm with an areal density of ∼2 to ∼20 tubes/μm of s-SWCNTs. Next, electrically aligned s-SWCNT arrays were deposited on the channel of the FET device by drop-casting. Field-emission scanning electron microscopy and electrical measurements have been carried out to test the performance of the aligned s-SWCNTs/FETs. The fabricated FETs with a channel length of 10 μm showed stable electrical properties with a field-effect mobility of 30.4 cm2/Vs and a log10 (I on/I off) current ratio of 3.96. We envisage that this new chemical-based separation method and electric field-assisted alignment could be useful to obtain a high-purity and aligned s-SWCNT array network for the fabrication of high-performance FETs to use in digital and analog electronics.

Sorting and decoration of semiconducting single-walled carbon nanotubes via the quaternization reaction

A study for the selective separation and functionalization of alcohol-soluble semiconducting single-walled carbon nanotubes (sc-SWCNTs) is carried out by polymer main-chain engineering. Introducing tertiary amine groups endows the functionalized sc-SWCNTs with alcohol-soluble properties and introducing the pyrimidine rings allows to increase the selective purity of sc-SWCNTs. In this study, a series of poly[(9,9-dioctylfluorene)-2,7-(9,9-bis(3'-(N,N-dimethylamino)propyl)-fluorene)] m -alt-[2-methylpyrimidine-2,7-(9,9-dioctylfluorene)] n (PFPy) are used for the selective dispersion of semiconducting single-walled carbon nanotubes, where n and m are the composition ratio of the copolymer. When m = n, the effective isolation of sc-SWCNTs with purity greater than 99% is achieved. The alcohol-soluble sc-SWCNTs with a diameter in the range of 1.1-1.4 nm are obtained through designing reasonable molecular structure. Moreover, the particular preference of PFPy (m = n) for sc-SWCNTs was studied via density functional theory (DFT) calculations and it was proved to be a promising method for the separation and functionalization of sc-SWCNTs.

Printed carbon nanotube thin-film transistors: progress on printable materials and the path to applications

Printing technologies have attracted significant attention owing to their potential use in the low-cost manufacturing of custom or large-area flexible electronics. Among the many printable electronic materials that have been explored, semiconducting carbon nanotubes (CNTs) have shown increasing promise based on their exceptional electrical and mechanical properties, relative stability in air, and compatibility with several printing techniques to form semiconducting thin films. These attractive attributes make printed CNT thin films promising for applications including, but not limited to, sensors and display backplanes - at the heart of which is electronics' most versatile device: the transistor. In this review, we present a summary of recent advancements in the field of printed carbon nanotube thin-film transistors (CNT-TFTs). In addition to an introduction of different printing techniques, together with their strengths and limitations, we discuss key aspects of ink/material selection and processing of various device components, including the CNT channels, contacts, and gate insulators. It is clear that printed CNT-TFTs are rapidly advancing, but there remain challenges, which are discussed along with current techniques to resolve them and future developments towards practical applications from these devices. There has been interest in low-cost, printable transistors for many years and the CNT-TFTs show great promise for delivering, but will not become a reality without further research advancement.

Chirality-Enriched Carbon Nanotubes for Next-Generation Computing

For the past half century, silicon has served as the primary material platform for integrated circuit technology. However, the recent proliferation of nontraditional electronics, such as wearables, embedded systems, and low-power portable devices, has led to increasingly complex mechanical and electrical performance requirements. Among emerging electronic materials, single-walled carbon nanotubes (SWCNTs) are promising candidates for next-generation computing as a result of their superlative electrical, optical, and mechanical properties. Moreover, their chirality-dependent properties enable a wide range of emerging electronic applications including sub-10 nm complementary field-effect transistors, optoelectronic integrated circuits, and enantiomer-recognition sensors. Here, recent progress in SWCNT-based computing devices is reviewed, with an emphasis on the relationship between chirality enrichment and electronic functionality. In particular, after highlighting chirality-dependent SWCNT properties and chirality enrichment methods, the range of computing applications that have been demonstrated using chirality-enriched SWCNTs are summarized. By identifying remaining challenges and opportunities, this work provides a roadmap for next-generation SWCNT-based computing.

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.

Engineering Supramolecular Polymer Conformation for Efficient Carbon Nanotube Sorting

Supramolecular polymer sorting is a promising approach to separating single-walled carbon nanotubes (CNTs) by electronic type. Unlike conjugated polymers, they can be easily removed from the CNTs after sorting by breaking the supramolecular bonds, allowing for isolation of electronically pristine CNTs as well as facile recycling of the sorting polymer. However, little is understood about how supramolecular polymer properties affect CNT sorting. Herein, chain stoppers are used to engineer the conformation of a supramolecular sorting polymer, thereby elucidating the relationship between sorting efficacy and polymer conformation. Through NMR and UV-vis spectroscopy, small-angle X-ray scattering (SAXS), and thermodynamic modeling, it is shown that this supramolecular polymer exhibits ring-chain equilibrium, and that this equilibrium can be skewed toward chains by the addition of chain stoppers. Furthermore, by controlling the stopper-monomer ratio, the sorting yield can be doubled from 7% to 14% without compromising the semiconducting purity (>99%) or properties of sorted CNTs.

Supramolecular Chemistry-Based One-Pot High-Efficiency Separation of Solubilizer-Free Pure Semiconducting Single-Walled Carbon Nanotubes: Molecular Strategy and Mechanism

Single-walled carbon nanotubes (SWCNTs) have the potential to revolutionize nanoscale electronics and power sources; however, their low purity and high separation cost limit their use in practical applications. Here we present a supramolecular chemistry-based one-pot, less expensive, scalable, and highly efficient separation of a solubilizer/adsorbent-free pure semiconducting SWCNT (sc-SWCNT) using flavin/isoalloxazine analogues with different substituents. On the basis of both experimental and computational simulations (DFT study), we have revealed the molecular requirements of the solubilizers as well as provided a possible mechanism for such a highly efficient selective sc-SWCNT separation. The present sorting method is very simple (one-pot) and gives a promising sc-SWCNT separation methodology. Thus, the study provides insight for the molecular design of an sc-SWCNT solubilizer with a high (n,m)-chiral selectivity, which benefits many areas including semiconducting nanoelectronics, thermoelectric, bio and energy materials, and devices using solubilizer-free very pure sc-SWCNTs.

Silicon-Waveguide-Integrated Carbon Nanotube Optoelectronic System on a Single Chip

Monolithic optoelectronic integration based on a single material is a major pursuit in the fields of nanophotonics and nanoelectronics in order to meet the requirements of future fiber-optic telecommunication systems and on-chip optical interconnection systems. However, the incompatibility between silicon-based electronics and germanium or compound semiconductor-based photonics makes it very challenging to realize optoelectronic integration based on a single material. Here, the integration between silicon waveguides and a carbon nanotube (CNT) optoelectronic system is demonstrated. Waveguide-integrated photodetectors based on the CNT exhibit 12.5 mA/W photoresponsivity at 1530 nm, which presents an improvement of 97.6 times enhanced absorption efficiency compared to that without the waveguide. Multiplied output signals of cascading photodetectors are used to control the output of CNT-based logic gates, thereby demonstrating that the CNT-based optoelectronic integration system is compatible with silicon photonics. Our work indicates that carbon nanotubes have the potential for future integration between nanophotonics and nanoelectronics on a single chip.

Chirality Pure Carbon Nanotubes: Growth, Sorting, and Characterization

Single-walled carbon nanotubes (SWCNTs) have been attracting tremendous attention owing to their structure (chirality) dependent outstanding properties, which endow them with great potential in a wide range of applications. The preparation of chirality-pure SWCNTs is not only a great scientific challenge but also a crucial requirement for many high-end applications. As such, research activities in this area over the last two decades have been very extensive. In this review, we summarize recent achievements and accumulated knowledge thus far and discuss future developments and remaining challenges from three aspects: controlled growth, postsynthesis sorting, and characterization techniques. In the growth part, we focus on the mechanism of chirality-controlled growth and catalyst design. In the sorting part, we organize and analyze existing literature based on sorting targets rather than methods. Since chirality assignment and quantification is essential in the study of selective preparation, we also include in the last part a comprehensive description and discussion of characterization techniques for SWCNTs. It is our view that even though progress made in this area is impressive, more efforts are still needed to develop both methodologies for preparing ultrapure (e.g., >99.99%) SWCNTs in large quantity and nondestructive fast characterization techniques with high spatial resolution for various nanotube samples.

Effect of nanotube coupling on exciton transport in polymer-free monochiral semiconducting carbon nanotube networks

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are attractive light-harvesting components for solar photoconversion schemes and architectures, and selective polymer extraction has emerged as a powerful route to obtain highly pure s-SWCNT samples for electronic applications. Here we demonstrate a novel method for producing electronically coupled thin films of near-monochiral s-SWCNTs without wrapping polymer. Detailed steady-state and transient optical studies on such samples provide new insights into the role of the wrapping polymer on controlling intra-bundle nanotube-nanotube interactions and exciton energy transfer within and between bundles. Complete removal of polymer from the networks results in rapid exciton trapping within nanotube bundles, limiting long-range exciton transport. The results suggest that intertube electronic coupling and associated exciton delocalization across multiple tubes can limit diffusive exciton transport. The complex relationship observed here between exciton delocalization, trapping, and long-range transport, helps to inform the design, preparation, and implementation of carbon nanotube networks as active elements for optical and electronic applications.

Imine and metal-ligand dynamic bonds in soft polymers for autonomous self-healing capacitive-based pressure sensors

In this work, a facile and simple yet effective method to generate intrinsic autonomous self-healing polymers was developed, leading to new materials that can be easily fine-tuned both mechanically and chemically. The new materials were designed to incorporate two dynamic and reversible types of chemical bonds, namely dynamic imine and metal-coordinating bonds, to enable autonomous self-healing, controlled degradability and ultra-high tunable stretchability (up to 800% strain) based on the ratio of metal to ligand incorporated. Through an easy condensation reaction, imine bonds are generated at the end-termini of a short siloxane chain. The new dynamic system was characterized by a variety of techniques, including tensile-pull strain testing, atomic force microscopy and UV-Vis spectroscopy, which showed that the highly dynamic imine bonds, combined with coordination with Fe²⁺ ions, allow for the material to regenerate 88% of its mechanical strength after physical damage. The materials were also controlled to be degraded in mild acidic conditions. Lastly, application in self-healable electronics was demonstrated through the fabrication of a capacitive-based pressure sensor, which shows good sensitivity and dynamic response (∼0.33 kPa^-1) before and after healing.

Enrichment of highly pure large-diameter semiconducting SWCNTs by polyfluorene-containing pyrimidine ring

The use of copolymers to extract and purify semiconducting SWCNTs (sc-SWCNTs) and metallic SWCNTs (m-SWCNTs) is widely employed. In this paper, the performances of two pyrimidine-alt-dioctylfluorene conjugated polymers in the enrichment of semiconducting SWCNTs are compared, and the subtle structural effects on them are discussed. It was found that both pyrimidine-polymers were more effective in wrapping the semiconducting SWCNTs than the metallic SWCNTs under the co-action of the pyrimidine and fluorene rings. Moreover, methyl groups on the pyrimidine ring of the polymer slightly contributed to the semiconducting purity, and the minor differences of sc-SWCNTs extraction between two pyrimidine-polymers are compared. Additionally, the average diameter of the selected SWCNTs is larger than that of the raw SWCNTs. The experimental results show the excellent selectivity for sc-SWCNT from both co-polymers: the index Φ i values for determining the purity of sc-SWCNTs were increased from 0.408 for P2 to 0.465 for P1, of which the selected sc-SWCNT purity is up to 99.9%. The resulting purity and the inexpensive availability of pyrimidine co-polymers make them suitable to be considered for effective sc-SWCNT enrichment.

Enrichment of large-diameter semiconducting single-walled carbon nanotubes by a mixed-extractor strategy

In this work, we report a new mixed-extractor strategy to improve the sorting yield of large-diameter semiconducting single-walled carbon nanotubes (s-SWCNTs) with high purity. In the new mixed-extractor strategy, two kinds of conjugated polymers with different rigidity, poly(9,9-n-dihexyl-2,7-fluorene-alt-9-phenyl-3,6-carbazole) (PDFP) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (P8BT), are used to sort large-diameter s-SWCNTs through two simple sonication processes. To our surprise, although PDFP itself shows no selectivity toward s-SWCNTs, it can greatly enhance the sorting yield of P8BT. Using the PDFP/P8BT mixed-extractor method, the yield of sorted s-SWCNTs has been enhanced by 5 times with a purity above 99 % in comparison to that using P8BT single-extractor method. In addition, the photoluminescence (PL) excitation maps shows that the PDFP/P8BT mixed-extractor system not only enhances the sorting yield substantially, but also tends to be enrichment of (15,4) SWCNTs with the diameter of 1.36 nm.

Chirality enriched carbon nanotubes with tunable wrapping via corona phase exchange purification (CPEP)

Single-walled carbon nanotubes (SWCNTs) have unique photophysical properties and serve as building blocks for biosensors, functional materials and devices. For many applications it is crucial to use chirality-pure SWCNTs, which requires sophisticated processes. Purification procedures such as wrapping by certain polymers, phase separation, density gradient centrifugation or gel chromatography have been developed and yield distinct SWCNT species wrapped by a specific polymer or surfactant. However, many applications require a different organic functionalization (corona) around the SWCNTs instead of the one used for the purification process. Here, we present a novel efficient and straightforward process to gain chirality pure SWCNTs with tunable functionalization. Our approach uses polyfluorene (PFO) polymers to enrich certain chiralities but the polymer is removed again and finally exchanged to any desired organic phase. We demonstrate this concept by dispersing SWCNTs in poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-{2,2'-bipyridine})] (PFO-BPy), which is known to preferentially solubilize (6,5)-SWCNTs. Then PFO-BPy is removed and recycled, while letting the SWCNTs adsorb/agglomerate on sodium chloride (NaCl) crystals, which act as a toluene-stable but water-soluble filler material. In the last step these purified SWCNTs are redispersed in different polymers, surfactants and ssDNA. This corona phase exchange purification (CPEP) approach was also extended to other PFO variants to enrich and functionalize (7,5)-SWCNTs. CPEP purified and functionalized SWCNTs display monodisperse nIR spectra, which are important for fundamental studies and applications that rely on spectral changes. We show this advantage for SWCNT-based nIR fluorescent sensors for the neurotransmitter dopamine and red-shifted sp³ defect peaks . In summary, CPEP makes use of PFO polymers for chirality enrichment but provides access to chirality enriched SWCNTs functionalized in any desired polymer, surfactant or biopolymer.

Inkjet-printed stretchable and low voltage synaptic transistor array

Wearable and skin electronics benefit from mechanically soft and stretchable materials to conform to curved and dynamic surfaces, thereby enabling seamless integration with the human body. However, such materials are challenging to process using traditional microelectronics techniques. Here, stretchable transistor arrays are patterned exclusively from solution by inkjet printing of polymers and carbon nanotubes. The additive, non-contact and maskless nature of inkjet printing provides a simple, inexpensive and scalable route for stacking and patterning these chemically-sensitive materials over large areas. The transistors, which are stable at ambient conditions, display mobilities as high as 30 cm² V⁻¹ s⁻¹ and currents per channel width of 0.2 mA cm⁻¹ at operation voltages as low as 1 V, owing to the ionic character of their printed gate dielectric. Furthermore, these transistors with double-layer capacitive dielectric can mimic the synaptic behavior of neurons, making them interesting for conformal brain-machine interfaces and other wearable bioelectronics.

The development of novel low-cost fabrication schemes for realizing stretchable transistor arrays with applicability in wearable electronics remains a challenge. Here, the authors report skin-like electronics with stretchable active materials and devices processed exclusively from ink-jet printing.

Low-voltage high-performance flexible digital and analog circuits based on ultrahigh-purity semiconducting carbon nanotubes

Carbon nanotube (CNT) thin-film transistor (TFT) is a promising candidate for flexible and wearable electronics. However, it usually suffers from low semiconducting tube purity, low device yield, and the mismatch between p- and n-type TFTs. Here, we report low-voltage and high-performance digital and analog CNT TFT circuits based on high-yield (19.9%) and ultrahigh purity (99.997%) polymer-sorted semiconducting CNTs. Using high-uniformity deposition and pseudo-CMOS design, we demonstrated CNT TFTs with good uniformity and high performance at low operation voltage of 3 V. We tested forty-four 2-µm channel 5-stage ring oscillators on the same flexible substrate (1,056 TFTs). All worked as expected with gate delays of 42.7 ± 13.1 ns. With these high-performance TFTs, we demonstrated 8-stage shift registers running at 50 kHz and the first tunable-gain amplifier with 1,000 gain at 20 kHz. These results show great potentials of using solution-processed CNT TFTs for large-scale flexible electronics.

Improving the Performance and Uniformity of Carbon-Nanotube-Network-Based Photodiodes via Yttrium Oxide Coating and Decoating

Semiconducting single-walled carbon nanotube thin films can be obtained by conjugated polymer wrapping sorting technique followed by solution deposition and can be utilized as channel materials of field-effect transistors and absorbing layers of photodiodes. However, after the deposition process, there are still polymer molecules wrapping around nanotubes, remaining between nanotubes, and remaining on the thin-film surface, which will cause large nanotube-electrode resistance and tube-tube resistance. Here, we demonstrate an yttrium oxide coating-and-decoating technique that can remove polymers only around electrodes and thus improve the performance of photodiodes without inducing new defects in the device channel. After the treatment of only the contact area, the average short-circuit current of a photodiode increases from 9.1 to 10.7 nA, whereas the average open-circuit voltage increases from 0.25 to 0.30 V. This method also improves device uniformity significantly.

Solution-Processing of High-Purity Semiconducting Single-Walled Carbon Nanotubes for Electronics Devices

High-purity semiconducting single-walled carbon nanotubes (s-SWCNTs) are of paramount significance for the construction of next-generation electronics. Until now, a number of elaborate sorting and purification techniques for s-SWCNTs have been developed, among which solution-based sorting methods show unique merits in the scale production, high purity, and large-area film formation. Here, the recent progress in the solution processing of s-SWCNTs and their application in electronic devices is systematically reviewed. First, the solution-based sorting and purification of s-SWCNTs are described, and particular attention is paid to the recent advance in the conjugated polymer-based sorting strategy. Subsequently, the solution-based deposition and morphology control of a s-SWCNT thin film on a surface are introduced, which focus on the strategies for network formation and alignment of SWCNTs. Then, the recent advances in electronic devices based on s-SWCNTs are reviewed with emphasis on nanoscale s-SWCNTs' high-performance integrated circuits and s-SWCNT-based thin-film transistors (TFT) array and circuits. Lastly, the existing challenges and development trends for the s-SWCNTs and electronic devices are briefly discussed. The aim is to provide some useful information and inspiration for the sorting and purification of s-SWCNTs, as well as the construction of electronic devices with s-SWCNTs.

Controllable etching-induced contact enhancement for high-performance carbon nanotube thin-film transistors

Semiconducting single-walled carbon nanotubes (s-SWNTs) show great promises in advanced electronics. However, contact resistance between the nanotubes and metal electrode has long been a bottleneck to the development of s-SWNTs in high-performance electronic devices. Here we demonstrate a simple and controllable strategy for enhancing the electrode contact and therefore the performance of s-SWNT thin film transistors by plasma etching treatment, which effectively removes the polymer residues, including the photoresist and the conjugated molecules, adsorbed on the surface of s-SWNTs. As a result, the contact resistance is reduced by 3 times and the carrier mobility rises by up to 70%. Our method is compatible with current silicon semiconductor processing technology, making it a viable effective approach to large-scale application of s-SWNTs in the electronics industry.

Controllable plasma etching induced contact enhancement for high-performance carbon nanotube thin-film transistors and analysis of the mechanism.

Enrichment of Metallic Carbon Nanotubes Using a Two-Polymer Extraction Method

The large-scale enrichment of metallic carbon nanotubes is a challenging goal that has proven elusive. Selective dispersion of carbon nanotubes by specifically designed conjugated polymers is effective for isolating semiconducting species, but a comparable system does not exist for isolating metallic species. Here, we report a two-polymer system where semiconducting species are extracted from the raw HiPCO or plasma-torch nanotube starting material using an electron-rich poly(fluorene-co-carbazole) derivative, followed by isolation of the metallic species remaining in the residue using an electron-poor methylated poly(fluorene-co-pyridine) polymer. Characterization of the electronic nature of extracted samples was carried out via a combination of absorption, Raman, and fluorescence spectroscopy, as well as electrical conductivity measurements. Using this methodology, the metallic species in the sample were enriched 2-fold in comparison to the raw starting material. These results indicate that the use of electron-poor polymers is an effective strategy for the enrichment of metallic species.

Pillar[5]arene-Decorated Single-Walled Carbon Nanotubes

Control of single-walled carbon nanotube dispersion properties is of substantial interest to the scientific community. In this work, we sought to investigate the effect of a macrocycle, pillar[5]arene, on the dispersion properties of a polymer-nanotube complex. Pillar[5]arenes are a class of electron-rich macrocyclic hosts capable of forming inclusion complexes with electron-poor guests, such as alkyl nitriles. A hydroxyl-functionalized pillar[5]arene derivative was coupled to the alkyl bromide side chains of a polyfluorene, which was then used to coat the surface of single-walled carbon nanotubes. Noncovalent functionalization of carbon nanotubes with the macrocycle-containing conjugated polymer significantly enhanced nanotube solubility, resulting in dark and concentrated nanotube dispersions (600 μg mL-1), as evidenced by UV-vis-NIR spectroscopy and thermogravimetric analysis. Differentiation of semiconducting and metallic single-walled carbon nanotube species was analyzed by a combination of UV-vis-NIR, Raman, and fluorescence spectroscopy. Raman spectroscopy confirmed that the concentrated nanotube dispersion produced by the macrocycle-containing polymer was due to well-exfoliated nanotubes, rather than bundle formation. The polymer-nanotube dispersion was investigated using 1H NMR spectroscopy, and it was found that host-guest chemistry between pillar[5]arene and 1,6-dicyanohexane occurred in the presence of the polymer-nanotube complex. Utilizing the host-guest capability of pillar[5]arene, the polymer-nanotube complex was incorporated into a supramolecular organogel.