Single-walled Carbon Nanotubes Material Research Articles

SWCNT fibers decorated with Au nanoparticles as voltammetric sensors for arsenic (III) determination

We developed a new method to produce fiber electrodes from single-walled carbon nanotube (SWCNT) films, which can be employed as a novel sensing platform for electrochemical investigations and analysis. By assembling SWCNT bundles in the form of a strong free-standing fiber with almost entire surface accessible to an analyte, we are able to avoid the interference of substrate and improve detection efficiency. We examined the influence of physical, chemical and electrochemical pretreatments of the new electrode SWCNT material on its properties. Transmission and scanning electron microscopy, Raman spectroscopy and cyclic voltammetry were used to characterize the SWCNT fibers. The modification of the SWCNT fiber electrode with gold nanoparticles provides the necessary analytical characteristics including high repeatability and sensitivity and low detection limit (1.3 – 2.1 μg L-1) for arsenic determination by anodic stripping voltammetry. Arsenic detection parameters such as deposition potential, deposition time and scan rate were optimized. Linear responses were found in concentration ranges from 3 to 210 μg L-1 and 7 to 125 μg L-1 for gold modified SWCNT fiber sensors (the SWCNTs were synthesized using CO or ethanol as precursors) at a relatively low deposition time of 60 s. The developed Au-SWCNT sensors allow rapid As determination in real samples and exhibit high durability and long service life.

Conjugated Polymers with Self-Immolative Sidechain Linkers for Carbon Nanotube Dispersion.

Single-walled carbon nanotubes (SWNTs) are promising materials for generating high-performance electronic devices. However, these applications are greatly restricted by their lack of purity and solubility. Commercially available SWNTs are a mixture of semi-conducting (sc-) and metallic (m-) SWNTs and are insoluble in common solvents. Conjugated polymers can selectively disperse either sc- or m-SWNTs and increase their solubility; however, the conductivity of conjugated polymer-wrapped SWNTs is largely affected by the polymer side chains. Here, a poly(fluorene-co-phenylene) polymer that contains a self-immolative linker as part of its sidechains is reported. The self-immolative linker is stabilized with a tert-butyldimethylsilyl ether group that, upon treatment with tetra-n-butylammonium fluoride (TBAF), undergoes a 1,6-elimination reaction to release the sidechain. Sonication of this polymer with SWNTs in tetrahydrofuran (THF) results in concentrated dispersions that are used to prepare polymer-SWNT thin films. Treatment with TBAF caused side-chain cleavage into carbon dioxide and the corresponding diol, which can be easily removed by washing with solvent. This process is characterized by a combination of absorption and Raman spectroscopy, as well as four-point probe measurements. The conductance of the SWNT thin films increased ≈60-fold upon simple TBAF treatment, opening new possibilities for producing high-conductivity SWNT materials for numerous applications.

On the surface area per volumetric loading: Its pronounced improvement in densely-packed SWCNT by double-function purification

Volumetric loading is often a critical parameter in process design rather than weight. In this work, we have assessed the volumetric textural parameters of purified single-walled carbon nanotube materials (SWCNT). Purification is a necessary step in the SWCNT manufacturing process as they contain a metal residue inherent to their synthesis. Nitric acid treatment was applied for both metal removal and carbon structural/textural modification. Results show that the volumetric BET area is enhanced in ca. 500% with respect to the non-purified SWCNT (ca. 160% per mass), where both volumetric microporosity and external surfaces are enhanced. For such optimal material, the SWCNT structure remains well-defined though changes are observed (densification, more interstitial space, cutting of the tubes and amorphous carbon being formed). Three intrinsic factors contribute to the volumetric BET's enhancement: the bulk density and the mass-based surface parameters; microporous and external surfaces. The bulk density is enhanced due to a structural densification, thus more carbon is available per volume despite heavier metals (Ni, Y) being removed. One indirect factor, the MOx-removal effect, affects both intrinsic surface parameters. After studying this effect in depth, it was found that the microporosity is truly and largely enhanced due to newly-formed interstitial space. The external surface area is slightly improved but to a much lesser extent than microporosity. Overall, the factors dominating the volumetric BET for our system and applied experimental conditions are the bulk density, microporosity and MOx-removal effect. Concerning the conventional mass-based BET, microporosity and MOx-removal effect are the dominating factors. The study also reveals that mesoporosity control in these materials is possible, in comparison to previous studies.

Highly Conductive Robust Carbon Nanotube Networks for Strong Buckypapers and Transparent Electrodes

AbstractWhile individual single‐wall carbon nanotubes (SWCNTs) have remarkable strength and electrical conductivity, SWCNT networks fabricated from dispersions have inferior properties due to nanotube bundling, limiting the potential applications of SWCNT materials. Herein, a common dye molecule (purpurin) is used to exfoliate SWCNTs via noncovalent functionalization and to fabricate SWCNT materials by a simple solution‐based process. The advantageous noncovalent interactions result in efficient exfoliation and metallic SWCNT enrichment, affording SWCNT materials with high mechanical robustness and electrical conductivity. This method is used to prepare mechanically robust SWCNT films and flexible transparent conductive electrodes.

Influence of Excess Conjugated Wrapping Polymer in Semiconducting Single-Walled Carbon Nanotube Dispersions

Single-walled carbon nanotubes (SWNTs) are promising nanomaterials for incorporation into organic electronic devices (OEDs), with the potential to fabricate flexible devices while exploiting inexpensive solution-processing techniques. As-synthesized SWNTs are insoluble and comprised of a mixture of metallic and semiconducting SWNTs (sc-SWNTs), necessitating dispersal and purification before integration into OEDs. Refinement of conjugated polymer extraction techniques has allowed for the isolation of sc-SWNTs from metallic in a reproducible and scalable manner. The availability of highly-pure sc-SWNT materials has facilitated the production of thin-film transistors (TFTs) with very high charge carrier mobilities, outperforming organic small molecule and polymer semiconductors. However, the realization of commercial OED applications of polymer-sorted sc-SWNTs have not yet been achieved, partially due to the prohibitive time and materials costs associated with purifying sc-SWNTs.Current protocols for dispersing sc-SWNTs with conjugated polymers involve three broad steps: (1) dispersion of bulk SWNT material, (2) removal of non-dispersed carbonaceous materials, and (3) removal of excess polymer through filtration or centrifugation. The final step of removal of excess polymer is time-consuming and wasteful, but viewed as necessary for preparing high-performing TFTs, as the conjugated polymer has much lower performance compared to SWNTs.In this study we performed the first systematic investigation of the effect of excess polymer on SWNT TFT performance. Three SWNT concentrations were investigated, with varying ratios of excess polymer added to each. TFT device performance was monitored using several metrics, including: mobility, threshold voltage, on/off ratios and hysteresis. Characterization of large numbers of replicate TFT devices determined that below a threshold amount of excess polymer the presence of excess polymer did not have a negative impact on device performance. Detailed analysis of the sc-SWNT films through Raman spectroscopy and atomic force microscopy (AFM) confirmed that a simple rinsing step was sufficient to remove all the unbound conjugated polymer from the substrate surface without affecting the sc-SWNT network. The volume of solvent required for the rinsing step was substantially lower than that required for filtration or centrifugation steps. Furthermore, at higher SWNT concentrations the excess polymer prevented nanotube bundling, resulting in moderate improvements in both mobility and on/off ratios. Our results were reproducible for two different conjugated polymer sc-SWNT systems, demonstrating the versatility of this procedure.

(Invited) Discussion on Preparation of Single-Walled Carbon Nanotubes for High-Performance Electronics

The unique structure and outstanding properties of single-walled carbon nanotubes (SWCNTs) make them top candidates for future high-performance electronics. Highly pure (>99.9999%) and dense (>125tubes/micron) SWCNTs are desired to present performance surpassing silicon-based devices and chips. Material preparation is the prerequisite of electronic applications. Selective synthesis, purification/sorting, and controlled assembling are procedures that associated with pathways to obtain SWCNT materials for high-performance electronics. However, it’s still not clear that what is the feasible strategy. More discussions and critical thinkings are in urgent need. I will try to review some of the recent developments in the field and discuss the possible directions in future study.

Sorption of metal elements by single-walled carbon nanotubes and x-ray absorption spectroscopy analysis

Single-walled carbon nanotubes (SWCNTs) were used for the sorption of monovalent K(I), divalent Pb(II), Zn(II), and Cd(II) in addition to hexavalent Cr(VI) metal elements from aqueous solutions. X-ray absorption spectroscopy (XAS) was employed for the preliminary characterization of the metals sorbed SWCNTs materials. The adsorption capacities of the investigated metals are associated with the functional groups involved in the sorption mechanism. The sorption and XAS results revealed that hexavalent Cr(VI) was highly adsorbed by SWCNTs. The sorption mechanism of Cr(VI) is associated with the reduction of Cr(VI) to trivalent Cr(III). XAS also suggested that in the environment nearest to the sorbed metals Cr, Pb, Zn, and K, the existing binding interactions are Cr–O, Pb–O, Zn–O, and K–O respectively.

Electronic and optical properties of SWCNTs and spin-orbit coupling effect on their electronic structures: First-principle computing

In this research, electronic and optical properties of armchair (7,7), zigzag (7,0), and chiral (4,2) configurations of Single-Walled Carbon Nanotubes (SWCNT) and the effect of spin-orbit coupling (SOC) on their electronic structures were studied with density functional theory (DFT) computing. The armchair, zigzag, and chiral SWCNT structures were built using the CASTEP software. Then, this code was used to calculate the band structures, density of states and optical properties of these systems. Electronic and optical properties of SWCNT material could be influenced by its configuration, such as the bandgap energy, total density of states (TDOS) and partial density of states (PDOS), absorption coefficient, dielectric function, optical conductivity, complex refractive index, and the loss function. In the absence or presence of SOC effect, armchair, zigzag and chiral nanotubes are semiconductors. SOC causes the bandgap energy to increase and the TDOS of "armchair, zigzag, and chiral SWCNTs" to alter. These results provide crucial physical information regarding the control of the electronic properties of SWCNTs using SOC.

One-step separation of high-purity single-chirality single-wall carbon nanotubes using sodium hyodeoxycholate

Chirality separation of single-wall carbon nanotubes (SWCNTs) is key to enabling applications in single photon sources, optoelectronic devices and bioimaging, and it generally requires multiple separation steps to obtain single chirality species with the desired purity. In this work, we report the use of a novel sodium hyodeoxycholate (SHC) surfactant for separation of single-chirality SWCNTs. The results demonstrated that chirality-selective adsorption and desorption of SWCNTs on a gel column can be achieved readily by modulating the concentrations of the SHC-based mixed surfactants. Through an optimized one-step procedure, single-chirality (6,5) SWCNTs were successfully separated, and the chirality purity was estimated to be 97% by using a spectroscopic approach, exceeding most reported purities for traditional surfactant-based one- or multiple-step separations. Due to the ultrahigh chirality resolution of the present method, the (6,5) SWCNTs can also be effectively separated from different SWCNT materials with various diameter or chirality distributions, even from materials containing very few (6,5) SWCNTs. The high-purity chirality separation demonstrated in this work lays an important foundation for the industrial production and applications of single-chirality SWCNTs.

Highly-Selective Harvesting of (6,4) SWCNTs Using the Aqueous Two-Phase Extraction Method and Nonionic Surfactants.

Monochiral single-walled carbon nanotubes (SWCNTs) are indispensable for advancing the technology readiness level of nanocarbon-based concepts. In recent times, many separation techniques have been developed to obtain specific SWCNTs from raw unsorted materials to catalyze the development in this area. This work presents how the aqueous two-phase extraction (ATPE) method can be enhanced for the straightforward isolation of (6,4) SWCNTs in one step. Introducing nonionic surfactant into the typically employed mixture of anionic surfactants, which drive the partitioning, is essential to increasing the ATPE system's resolution. A thorough analysis of the parameter space by experiments and modeling reveals the underlying interactions between SWCNTs, surfactants, and phase-forming agents, which drive the partitioning. Based on new insight gained on this front, a separation mechanism is proposed. Notably, the developed method is highly robust, which is proven by isolating (6,4) SWCNTs from several raw SWCNT materials, including SWCNT waste generated over the years in the laboratory.

Mild acid-based surfactant-free solutions of single-walled carbon nanotubes: Highly viscous, less toxic, and humidity-insensitive solutions

Owing to their exceptional physical properties, single-walled carbon nanotubes (SWNTs) are widely applicable if translated into macroscopically assembled SWNT materials. The assembly method involves SWNT solutions prepared using surfactants or superacids. As both variations of the method are difficult to industrialize, a novel SWNT solution is prepared herein using mild polyphosphoric acid (PPA) without surfactant. Although the dissolution of SWNTs in PPA is thermodynamically unfavorable because of lower acidity compared to superacids, high viscosity kinetically contributes to phase stabilization when SWNTs are de-bundled upon high shearing. Interestingly, the SWNT/PPA solution is humidity-insensitive, which is beneficial for handling. In the 0.01–2.0 wt% range, the critical concentration and saturation point, at which phase transitions of isotropic-liquid crystalline (LC) and LC-gel states occur, are observed at 0.1 and 1.0 wt%, respectively. According to the steady shear rheology, the optimal concentrations for fiber spinning are 0.3–0.7 wt%. Polarized Raman and WAXD analyses show that the wet-spun SWNT fibers from the 0.5 wt% solution exhibit the highest fiber orientation, correlating with the lowest extrusion shear stress of the solution. The KevlarTMp-aramid dissolved in PPA and blended with SWNTs shows a negligible effect on the solution rheology but higher fiber modulus at high fractions.

Conductivity vs functionalization in single-walled carbon nanotube films

Diazo functionalization is a chemical method that changes the conductance of metallic single-walled carbon nanotubes (SWCNTs) by disrupting the C–C double bonds. Its application to native mixtures of metallic and semiconducting SWCNTs is a promising way of large-scale production of semiconducting SWCNTs for use in electronics. This has been well studied on isolated SWCNTs, but the implications on the conductivity of SWCNT materials are still unclear. Here, we study the conductivity of such functionalized SWCNT films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films. At low functionalization degree (below 0.2 mol%), the conductivity is dominated by a subnetwork of metallic SWCNTs through two parallel mechanisms: a Luttinger liquid mechanism and a Variable Range Hopping process. Higher functionalization (over 0.4 mol%) destroys the Luttinger liquid mechanism, and a second parallel Variable Range Hopping process arises, attributed to the conduction through the semiconducting SWCNTs. At these high functionalization degrees, the SWCNT film behaves as a material with the desired semiconducting properties.Graphical abstractWe studied the conductivity of chemically functionalized Single Walled Carbon Nanotube films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films.

Large thermoelectric power factor in wafer-scale free-standing single-walled carbon nanotube films

Single-walled carbon nanotubes (SWCNTs) have the potential for application in thermoelectric energy generators owing to their advantages, such as good charge-carrier transport properties, mechanical flexibility and robustness, and tunability of polarity. However, the fabrication of SWCNTs still remains a problem due to its complexity and high cost. In this paper, we propose an approach for the direct formation of free-standing SWCNT films from as-grown SWCNT mats without any dispersion or separation processes. We used this approach to develop high-performance SWCNT-based thermoelectric leg materials. The as-grown SWCNT mats were synthesized by an enhanced direct injection pyrolytic synthesis (eDIPS) method. The selectivity of the tube diameter for the eDIPS method clarified the dependence of the thermoelectric performance of the free-standing SWCNT films on the tube diameter. The Seebeck coefficients and thermal conductivities were found to correlate with the tube diameter and agreed with the theoretical predictions. Owing to the dispersion-free film formation, our SWCNT films afforded large thermoelectric power factors. In particular, a power factor of 350 μW/(m K2) was obtained for the mean tube diameter of 1.7 nm without any semiconductor extraction or doping treatments. Our approach allowed the fabrication of thermoelectric legs with an arbitrary size; thus, it offers a useful strategy for the simpler, cheaper, and low-waste manufacturing of high-performance organic thermoelectric devices.

First principles study on modification of Ni composite SWCNT material system for gas adsorption

Single wall Carbon nanotubes (SWCNT) have been extensively researched and discussed due to its unique nanostructure and electrical properties. Meanwhile, the composite of transitional magnetic elements has a unique effect on the properties of materials. In this paper, density functional theory (DFT) is used to study compound of magnetic element Ni with SWCNT (Ni-SWCNT). The calculated results of Ni-SWCNT possess selective adsorption interactions with reducing gases such as H2 and CH4 compared without composited systems, and has a great change in electric conductivity after adsorption reducing gases. Through analysis, the reason of such excellent performance is spin-polarized nonlocal electrons of the magnetic element Ni atom transferred to SWCNT and paired with electrons of the reducing gas to form a stable system. Therefore, the Ni-SWCNT material system has great research potential for sensitive detection of dangerous reducing gases and high-efficiency catalytic materials, and is worthy of further confirmation by experimental research. This article reveals the mechanism of magnetic transition elements modification SWCNT and provides a theoretical basis for compound and modification of materials with magnetic elements.

High Adsorption of Benzoic Acid on Single Walled Carbon Nanotube Bundles

Removal of harmful chemicals from water is paramount to environmental cleanliness and safety. As such, need for materials that will serve this purpose is in the forefront of environmental research that pertains to water purification. Here we show that bundles of single walled carbon nanotubes (SWNTs), synthesized by direct thermal decomposition of ferrocene (Fe(C5H5)2), can remove emerging contaminants like benzoic acid from water with high efficiencies. Experimental adsorption isotherm studies indicate that the sorption capacity of benzoic acid on these carbon nanotubes (CNTs) can be as high as 375 mg/g, which is significantly higher (in some cases an order of magnitude) than those reported previously for other adsorbents of benzoic acid such as activated carbon cloth, modified bentonite and commercially available graphitized multiwall carbon nanotubes (MWNTs). Our Molecular Dynamics (MD) simulation studies of experimental scenarios provided major insights related to this process of adsorption. The MD simulations indicate that, high binding energy sites present in SWNT bundles are majorly responsible for their enhanced adsorptive behavior compared to isolated MWNTs. These findings indicate that SWNT materials can be developed as scalable materials for efficient removal of environmental contaminants as well as for other sorption-based applications.

(Invited) Controlling Carbon Nanotubes By DNA: From Separation to Integration

A unique feature of single-wall carbon nanotube (SWCNT) material is its structure diversity: within the same rolled-up graphene sheet structure framework one finds SWCNTs of different electronic bandgaps: metals with zero bandgap, quasi-metals with meV bandgap and semiconductors with ~ 1 eV bandgap. However, making use of SWCNT structure diversity requires a method to obtain SWCNTs of defined structures. In this contribution, we present our current state of DNA-based sorting of SWCNTs. In particular, we show our ability to put any DNA sequence on any SWCNT species, enabling DNA-based assembly of high-density SWCNT devices. In the end, we discuss the design of a device we call Photon Perceptron Array (PPA). PPA is an array of micron-sized spectrometer units that can measure the spectrum of incoming photon in the wavelength range of 200 nm to 2000 nm (and possibly going all the way to THz), with a spatial resolution comparable to the size of a single spectrometer unit. Each spectrometer unit is composed of a set of distinct photodetectors made of SWCNTs of defined atomic structures, placed in parallel with tube-to-tube separations controlled at nanometer precision by DNA origami technology. A high-density array of identical spectrometers can be constructed by placing DNA origami/carbon nanotube complexes in photolithographically-defined locations. PPA may be used for spectral imaging in many fields of science and technology.

(Invited) Challenges in Quantifying the Purity of Semiconducting Single-Walled Carbon Nanotubes

Semiconducting carbon nanotubes (sc-SWCNT) sit amongst the most promising nanomaterials for a new generation of electronic devices. Progress towards commercial applications is combined with challenges related to materials development and characterization, fabrication methods as well as device modeling. This talk will focus primarily on the enrichment process and the related ink formulation for printed electronics and integrated circuits. [1] The quality of carbon nanotube ink solutions is closely tied with the rapid feedback provided by optical spectroscopy methods: UV-vis, Raman and fluorescence. [2] I will describe our efforts on improving the material’s assessment when purity exceeds well beyond 99%. [3] [1] Ding, J.; Li, Z.; Lefebvre, J.; Cheng, F.; Dubey, G.; Zou, S.; Finnie, P.; Hrdina, A.; Scoles, L.; Lopinski, G. P.; Kingston, C. T.; Simard, B.; Malenfant, P. R. L. Enrichment of Large-Diameter Semiconducting SWCNTs by Polyfluorene Extraction for High Network Density Thin Film Transistors. Nanoscale 2014, 6, 2328-2339.[2] J. Lefebvre, J.; Finnie, P.; Fagan, J.; Zheng, M.; Hight Walker, A. R. Metrological Assessment of Single-Wall Carbon Nanotube Materials by Optical Methods. Handbook of Carbon Nanomaterials 2019, 9, R. B. Weisman and J. Kono editors (World Scientific) ISBN 978-981-3235-45-8.[3] Lefebvre, J.; Ding, J.; Li, Z.; Finnie, P.; Lopinski, G.; Malenfant, P. R. L. High-Purity Semiconducting Single-Walled Carbon Nanotubes: A Key Enabling Material in Emerging Electronics. Acc. Chem. Res. 2017, 50 (10), 2479-2486. Figure 1

Covalent Surface Modification Effects on Single‐Walled Carbon Nanotubes for Targeted Sensing and Optical Imaging

AbstractOptical nanoscale technologies often implement covalent or noncovalent strategies for the modification of nanoparticles, whereby both functionalizations are leveraged for multimodal applications but can affect the intrinsic fluorescence of nanoparticles. Specifically, single‐walled carbon nanotubes (SWCNTs) can enable real‐time imaging and cellular delivery; however, the introduction of covalent SWCNT sidewall functionalizations often attenuates SWCNT fluorescence. Recent advances in SWCNT covalent functionalization chemistries preserve the SWCNT's pristine graphitic lattice and intrinsic fluorescence, and here, such covalently functionalized SWCNTs maintain intrinsic fluorescence‐based molecular recognition of neurotransmitter and protein analytes. The covalently modified SWCNT nanosensor preserves its fluorescence response towards its analyte for certain nanosensors, presumably dependent on the intermolecular interactions between SWCNTs or the steric hindrance introduced by the covalent functionalization that hinders noncovalent interactions with the SWCNT surface. These SWCNT nanosensors are further functionalized via their covalent handles with a targeting ligand, biotin, to self‐assemble on passivated microscopy slides, and these dual‐functionalized SWCNT materials are explored for future use in multiplexed sensing and imaging applications.

Single-walled carbon nanotube membranes for optical applications in the extreme ultraviolet range

In this paper, we explore the possibility of using free-standing thin films from single-walled carbon nanotube (SWCNT) material in optics of the extreme ultraviolet (EUV) range. Test samples were fabricated using an aerosol chemical vapor deposition method. Synchrotron radiation was used to record the transmittance spectra of samples in the EUV range. The measured transmittance for a film 40 nm thick almost monotonously increases from 76% at a wavelength of 20 nm–99% at a wavelength of 1 nm. The measured stress-strain curve for the test samples shows that the SWCNT-based thin films have rather high ductility as opposite to fragile films made of conventional solid state materials. We use numerical simulations to demonstrate that the film strain occurs mainly by straightening and sliding of the nanotubes past each other without forming of strain localization responsible for fragile behavior. The combination of high radiation transmittance and unique mechanical properties makes the SWCNT-based thin films very promising for use in the EUV optics. In particular, such films can be used to protect delicate optical elements for EUV lithography from their contamination with debris particles.

Capacitive and Charge Transfer Effects of Single‐Walled Carbon Nanotubes in TiO2 Electrodes

The transfer of nanoscale properties from single-walled carbon nanotubes (SWCNTs) to macroscopic systems is a topic of intense research. In particular, inorganic composites of SWCNTs and metal oxide semiconductors are being investigated for applications in electronics, energy devices, photocatalysis, and electroanalysis. In this work, a commercial SWCNT material is separated into fractions containing different conformations. The liquid fractions show clear variations in their optical absorbance spectra, indicating differences in the metallic/semiconducting character and the diameter of the SWCNTs. Also, changes in the surface chemistry and the electrical resistance are evidenced in SWCNT solid films. The starting SWCNT sample and the fractions as well are used to prepare hybrid electrodes with titanium dioxide (SWCNT/TiO2 ). Raman spectroscopy reflects the optoelectronic properties of SWCNTs in the SWCNT/TiO2 electrodes, while the electrochemical behavior is studied by cyclic voltammetry. A selective development of charge transfer characteristics and double-layer behavior is achieved through the suitable choice of SWCNT fractions.