Probing the Selectivity of Azomethine Imine Cycloaddition to Single‐Walled Carbon Nanotubes by Resonance Raman Spectroscopy

Selective covalent surface modification of single‐walled carbon nanotubes (SWNTs) is of great importance to various carbon nanotube‐based applications as it might offer an alternative method for enriching metallic and semiconducting nanotubes. Herein, we report on the surface modification of SWNTs through 1,3‐dipolar cycloaddition of 3‐phenyl‐phthalazinium‐1‐olate, which is a stable and reactive azomethine imine. For this reaction, microwave heating was found to be more efficient than conventional and solvent‐free heating. The sensitivity of cycloaddition to the molecular structure of SWNTs was probed using resonance Raman spectroscopy with three different laser excitations. Based on the obtained results, azomethine imine addition to the surface of nanotubes is selective for metallic and large‐diameter semiconducting SWNTs. Thermogravimetric analysis coupled with mass spectrometry showed that fragments released at high temperatures corresponded to the phenylphthalazine group, thus confirming the covalent surface functionalization. Modified SWNTs were further characterized by X‐ray photoelectron and UV/Vis‐NIR spectroscopies.     

On the applicability of cluster models to study the chemical reactivity of carbon nanotubes

We have performed a comparative study on the reactivity of metallic and semiconducting nanotubes using infinite and finite models. Infinite models were created using periodic boundary conditions while finite ones were constructed by means of hydrogen terminated nanotubes sections. Cluster models systematically underestimate the reactivity of metallic single wall carbon nanotube (SWCNT)s. We have confirmed that metallic nanotubes are more reactive than semiconducting species, in disagreement with previous works. The differences can be attributed to the presence of an instability in the singlet ground state of the wavefunction corresponding to semiconducting nanotubes clusters. When lower electronic states of the pristine cluster are considered, semiconducting nanotubes become less reactive as compared with metallic SWCNTs. Particularly, if an antiferromagnetic solution is considered for the semiconducting (10,0) SWCNT cluster, it becomes less reactive than the (5,5) SWCNT, as observed for infinite models. Because semiconducting nanotubes are less reactive than metallic counterparts, their reaction energies converge faster to the values observed for graphene. For a 1.6-nm diameter semiconducting nanotube, the addition energy is comparable with graphene. Thus, semiconducting nanotubes with diameters larger than 1.6 nm are going to be as reactive as graphene and the effects of curvature will be unimportant.     

Electronically selective chemical functionalization of carbon nanotubes: correlation between Raman spectral and electrical responses

Single-walled carbon nanotubes (SWNTs) demonstrate remarkable electronic and mechanical properties useful in developing areas such as nanoelectromechanical systems and flexible electronics. However, the highly inhomogeneous electronic distribution arising from different diameters and chirality in any given as-synthesized SWNT samples imposes severe limitations. Recently demonstrated selective chemical functionalization methods may provide a simple scalable means of eliminating metallic tubes from SWNT transistors and electronic devices. Here, we report on combined electron transport and Raman studies on the reaction of 4-bromobenzene diazonium tetrafluoroborate directly with single and networks of SWNT transistors. First, Raman studies are carried out on isolated individual SWNTs grown on SiO2/Si substrates by chemical vapor deposition with and without metal contacts. Metallic tubes are found to have, on average, higher reactivity toward diazonium reagents. However, a considerable degradation of electrical properties of semiconducting tubes occurs if the reaction is carried out to the point where the conductivity of metallic tubes is significantly suppressed. Insights from single-tube studies are then applied to elucidate the electrical and the Raman responses of SWNT random network transistors of different channel lengths to chemical functionalization.     

Influence of Polymer Electronics on Selective Dispersion of Single‐Walled Carbon Nanotubes

The separation and isolation of semiconducting and metallic single‐walled carbon nanotubes (SWNTs) on a large scale remains a barrier to many commercial applications. Selective extraction of semiconducting SWNTs by wrapping and dispersion with conjugated polymers has been demonstrated to be effective, but the structural parameters of conjugated polymers that dictate selectivity are poorly understood. Here, we report nanotube dispersions with a poly(fluorene‐co‐pyridine) copolymer and its cationic methylated derivative, and show that electron‐deficient conjugated π‐systems bias the dispersion selectivity toward metallic SWNTs. Differentiation of semiconducting and metallic SWNT populations was carried out by a combination of UV/Vis‐NIR absorption spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and electrical conductivity measurements. These results provide new insight into the rational design of conjugated polymers for the selective dispersion of metallic SWNTs.     

Nucleophilic-alkylation-reoxidation: a functionalization sequence for single-wall carbon nanotubes

A new reaction sequence for the chemical functionalization of single-wall carbon nanotubes (SWNTs) consisting of the nucleophilic addition of t-BuLi to the sidewalls of the tubes and the subsequent reoxidation of the intermediates t-Bu(n)SWNT(n-) leading to t-Bu(n)SWNT was developed. During the formation of the t-Bu(n)SWNT(n-), a homogeneous dispersion in benzene was formed due to the electrostatic repulsion of the negatively charged intermediates causing debundling. The entire reaction sequence can be repeated, and the degree of functionalization of the products (t-Bu(n))(m)SWNT (m = 1-3) increases with increasing m. Degrees of functionalization expressed as the carbon-to-addend ratio of up to 31 were reached. The reaction was studied in detail by photoelectron spectroscopy, Raman spectroscopy, and scanning tunneling microscopy (STM). The C 1s core level spectra reveal that the nucleophilic attack of the t-BuLi leads to negatively charged SWNTs. Upon oxidation, this negative charge is removed. The valence band spectra of the functionalized samples exhibit a significant reduction in the pi-derived density of states. In STM, the covalently bonded t-butyl groups attached to the sidewalls have been visualized. Raman spectroscopy reveals that addition of the nucleophile to metallic tubes is preferred over the addition to semiconducting tubes.     

Band‐Gap Opening in Metallic Single‐Walled Carbon Nanotubes by Encapsulation of an Organic Salt

The encapsulation of viologen derivatives into metallic single‐walled carbon nanotubes (SWNTs) results in the opening of a band gap, making the SWNTs semiconducting. Raman spectroscopy, thermogravimetric analysis, and aberration‐corrected high‐resolution transmission electron microscopy confirm the encapsulation process. Through the fabrication of field‐effect transistor devices, the change of the electronic structure of the tubes from metallic to semiconducting upon the encapsulation is confirmed. The opening of a gap in the band structure of the tubes was not detected in supramolecular controls.     

Spectroscopic analysis of single-walled carbon nanotubes and semiconjugated polymer composites

Interactions between arc discharge single-walled carbon nanotubes within polymer composites have been well documented. Here hybrid systems of the conjugated organic polymer poly(p-phenylene vinylene-co-2,5-dioctyloxy-m-phenylene vinylene) (PmPV) and HiPco SWNTs are explored using UV/vis/NIR and Raman spectroscopy at 514.5 and 632.8 nm to determine specific interactions. An examination of the radial breathing modes at 514.5 nm shows similar tube diameters of 1.28 and 1.35 nm selected for both the arc discharge and HiPco composites. The corresponding G lines of both composites show no specific type of tubes being selected. At 514.5 nm, the G line of the HiPco composite (1% mass fraction) shows contributions from semiconducing and metallic tubes, and the arc discharge composite (1% mass fraction) is dominated by semiconducting nanotubes. At 632.8 nm, the G line of the HiPco composite (1% mass fraction) is dominated by semiconducting tubes, and the arc discharge composite (1% mass fraction) shows strong contributions from metallic tubes. This finding is a strong indication that the selection process is dependent on tube diameter rather than backbone structure. The solubility limits of both composites are determined by investigating the G lines of both composites and have been found to be greater than 1% mass fraction by weight for the arc discharge composite and greater than 0.1% mass fraction by weight for the HiPco composite.     

Separation of semiconducting from metallic carbon nanotubes by selective functionalization with azomethine ylides

A mild and efficient method for the functionalization of SWNTs by cycloaddition of azomethine ylides derived from trialkylamine-N-oxides is described. Selective reaction of semiconducting carbon nanotubes was achieved by preorganizing the starting N-oxides on the nanotube surface prior to generating the reactive ylides. Separation of met-SWNTs from functionalized sem-SWNTs was successfully accomplished by inducing solubilization of sem-SWNTs in the presence of lignoceric acid.     

Selective polycarboxylation of semiconducting single-walled carbon nanotubes by reductive sidewall functionalization

The efficient and controllable synthesis, the detailed characterization, and the chemical postfunctionalization of polycarboxylated single-walled carbon nanotubes SWCNT(COOH)(n) are reported. This innovative covalent sidewall functionalization method is characterized by (a) the preservation of the integrity of the entire σ-framework of SWCNTs; (b) the possibility of achieving very high degrees of addition; (c) control of the functionalization degrees by the variation of the reaction conditions (reaction time, ultrasonic treatment, pressure); (d) the identification of conditions for the selective functionalization of semiconducting carbon nanotubes, leaving unfunctionalized metallic tubes behind; (e) the proof that the introduced carboxylic acid functionalities can serve as versatile anchor points for the coupling to functional molecules; and (f) the application of a subsequent thermal degradation step of the functionalized semiconducting tubes leaving behind intact metallic SWCNTs. Functional derivatives have been characterized in detail by means of Raman, UV-vis/nIR, IR, and fluorescence spectroscopy as well as by thermogravimetric analysis combined with mass spectrometry, atomic force microscopy, and zeta-potential measurements.     

Relative optical absorption of metallic and semiconducting single-walled carbon nanotubes

While it is well-known that tube-tube interaction causes changes (peak red-shift and suppression) in the optical absorption of single-walled carbon nanotubes (SWNTs), we found in this work that, upon bundling, the optical absorption of metallic SWNTs (M11) is less affected compared to their semiconducting counterparts (S11 or S22), resulting in enhanced absorbance ratio of metallic and semiconducting SWNTs (A(M)/A(S)). Annealing of the SWNTs increases this ratio due to the intensified tube-tube interaction. We have also found that the interaction between SWNTs and the surfactant Triton X-405 has a similar effect. The evaluation of SWNT separation by types (metallic or semiconducting) based on the optical absorption should take these effects into account.     

Separation of Semiconducting Single‐Walled Carbon Nanotubes by Using a Long‐Alkyl‐Chain Benzenediazonium Compound

We designed and synthesized 4‐dodecyloxybenzenediazonium tetrafluoroborate ( 1 ), which preferentially reacts with metallic single‐walled carbon nanotubes (SWNTs) by kinetic control. We first determined the suitable experimental conditions for the preferential reaction of 1 with individually dissolved SWNTs by monitoring the decrease in absorbance for the metallic SWNT in the range of 400–650 nm in the absorption spectrum of the SWNTs. The reacted SWNTs were thoroughly rinsed with THF to obtain THF‐insoluble SWNTs. The Raman spectrum of the THF‐insoluble SWNTs showed a strong peak near 180 cm^?1, which corresponds to a semiconducting breathing band. The metallic breathing bands (≈220 cm^?1) and Breit–Wingner–Fano (BWF) modes (1520 cm^?1) corresponding to the metallic SWNTs were much weaker than those of the pristine SWNTs. We also confirmed that metallic peaks in the range of 400–650 nm in the absorption spectrum of THF‐insoluble SWNTs that were individually dissolved in an aqueous micelle of sodium cholate were almost nondetectable. All the results indicate that the THF‐insoluble SWNTs are semiconducting.     

Selective interactions of porphyrins with semiconducting single-walled carbon nanotubes

A derivatized porphyrin with long alkyl chains, 5,10,15,20-tetrakis(hexadecyloxyphenyl)-21H,23H-porphine, is selective toward semiconducting single-walled carbon nanotubes (SWNTs) in presumably noncovalent interactions, resulting in significantly enriched semiconducting SWNTs in the solubilized sample and predominantly metallic SWNTs in the residual solid sample according to Raman, near-IR absorption, and bulk conductivity characterizations.     

Electronic properties of n-type carbon nanotubes prepared by CF4 plasma fluorination and amino functionalization

Single-walled carbon nanotubes (SWNTs) have been fluorinated by CF4 plasma exposure and further functionalized with 1,2-diaminoethane. The degree of amino functionalization is dependent on the degree of initial fluorination rather than oxygen or carbon defects. Reaction at both ends of 1,2-diaminoethane was observed to increase with fluorine content. Back-gated SWNT devices have shown p-type semiconducting behavior for CF4-functionalized SWNTs and n-type semiconducting behavior for amino-functionalized SWNTs. The degree of n-type behavior increases with the amount of nitrogen attached to the SWNTs.     

Dispersion of single‐walled carbon nanotubes using nucleobase‐containing poly(acrylamide) polymers

Single‐walled carbon nanotubes (SWNTs) possess extraordinary properties, but suffer from poor solubility and a lack of purity. Of the possible routes available to solubilize and purify nanotube samples, the use of noncovalent functionalization is ideal as carbon nanotube properties are not deleteriously affected. A multitude of different dispersants have been investigated thus far, but of particular interest is deoxyribonucleic acid (DNA), which has previously been demonstrated to effectively separate metallic and semiconducting carbon nanotubes. Here, we investigate the ability of synthetic nucleobase‐containing poly(acrylamide) polymers to produce stable nanotube dispersions in organic solvents. Polymers bearing different nucleobase and backbone structures, as well as block copolymers with different block sequences were investigated. Polymer:SWNT mass ratios and solvent compositions were optimized for the nucleobase‐functionalized polymers, and semiconducting and metallic SWNT populations were identified by a combination of UV‐Vis‐NIR absorption, Raman, and fluorescence spectroscopy. These results demonstrate the capacity for synthetic DNA analogues to disperse SWNTs in organic media. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2611–2617     

Separation of single-walled carbon nanotubes on silica gel. Materials morphology and Raman excitation wavelength affect data interpretation

In this report, procedures are discussed for the enrichment of single-walled carbon nanotube (SWNT) types by simple filtration of the functionalized SWNTs through silica gel. This separation uses nanotube sidewall functionalization employing two different strategies. In the first approach, a crude mixture of metallic and semiconducting SWNTs was heavily functionalized with 4-tert-butylphenyl addends to impart solubility to the entire sample of SWNTs. Two major polarity fractions were rapidly filtered through silica gel, with the solvent being removed in vacuo, heated to 700 degrees C to remove the addends, and analyzed spectroscopically. The second approach uses two different aryldiazonium salts (one with a polar grafting group and one nonpolar), appended selectively onto the different SWNTs by means of titration and monitoring by UV analysis throughout the functionalization process. The different addends accentuate the polarity differences between the band-gap-based types permitting their partial separation on silica gel. Thermal treatment regenerated pristine SWNTs in enriched fractions. The processed samples were analyzed and characterized by Raman spectroscopy. A controlled functionalization method using 4-fluorophenyl and 4-iodophenyl addends was performed, and XPS analyses yielded data on the degree of functionalization needed to affect the van Hove singularities in the UV/vis/NIR spectra. Finally, we demonstrate that relative peak intensity changes in Raman spectra can be caused by morphological changes in SWNT bundling based on differing flocculation or deposition methods. Therefore a misleading impression of separations can result, underscoring the care needed in assessing efficacies in SWNT enrichment and the prerequisite use of multiple excitation wavelengths and similar flocculation or deposition methods in comparative analyses.     

Separated metallic and semiconducting single-walled carbon nanotubes: opportunities in transparent electrodes and beyond

Ever since the discovery of single-walled carbon nanotubes (SWNTs), there have been many reports and predictions on their superior properties for use in a wide variety of potential applications. However, an SWNT is either metallic or semiconducting; these properties are distinctively different in electrical conductivity and many other aspects. The available bulk-production methods generally yield mixtures of metallic and semiconducting SWNTs, despite continuing efforts in metallicity-selective nanotube growth. Presented here are significant advances and major achievements in the development of postproduction separation methods, which are now capable of harvesting separated metallic and semiconducting SWNTs from different production sources with sufficiently high enrichment and quantities for satisfying at least the needs in research and technological explorations. Opportunities and some available examples for the use of metallic SWNTs in transparent electrodes and semiconducting SWNTs in various device nanotechnologies are highlighted and discussed.     

A route for bulk separation of semiconducting from metallic single-wall carbon nanotubes

Substantial separation of single-wall carbon nanotubes (SWNTs) according to type (metallic versus semiconducting) has been achieved for HiPco and laser-ablated SWNTs. We presently argue that stable dispersions of SWNTs with octadecylamine (ODA) in tetrahydrofuran (THF) originate from the physisorption and organization of ODA along the SWNT sidewalls in addition to the originally proposed zwitterion model. Furthermore, the reported affinity of amine groups for semiconducting SWNTs, as opposed to their metallic counterparts, contributes additional stability to the physisorbed ODA. This provides a venue for the selective precipitation of metallic SWNTs upon increasing dispersion concentration, as indicated by Raman investigations.     

Theoretical investigation of the stability,electronic and magnetic properties of thiolated single‐wall carbon nanotubes

The addition of SH and OH groups to single‐wall carbon nanotubes (SWCNTs) was investigated employing first principles calculations. In the case of the semiconducting (10, 0) SWCNT the SWCNT‐SH binding energy is weak, 2–4 kcal/mol. However, for the metallic (5, 5) SWCNT it is larger, 7–9 kcal/mol. Thus metallic SWCNTs seem to be more reactive to SH than the semiconducting ones. Indeed, the (6, 6) SWCNT is more reactive to SH than the (10, 0) SWCNT, by 2–3 kcal/mol, something that can be explained only considering the electronic structure of the tube, because the (6, 6) has a larger diameter. The binding energies are larger for the addition of the OH group, 25 and 30 kcal/mol for the (10, 0) and (5, 5) SWCNTs, respectively. When a single OH or SH group is attached to the metallic SWCNTs, we observe important changes in the DOS at the Fermi level. However, when multiple SH groups are attached, the changes in the electronic and magnetic properties depend on the position of the SH groups. The small binding energy found for the SH addition indicates that the successful functionalization of SWCNTs with SH, SCH₃, and S(CH₂)_nSH groups is mostly due to the presence of defects created after acid treatment and to a minor extent by the metallic tubes present in the samples. Perfect semiconducting SWCNTs showed very low reactivity against the SH group. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Spectroscopic Characteristics of Differently Produced Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes (SWNTs) synthesized with different methods are investigated by using multiple characterization techniques, including Raman scattering, optical absorption, and X‐ray absorption near edge structure, along with X‐ray photoemission by following the total valence bands and C 1s core‐level spectra. Four different SWNT materials (produced by arc discharge, HiPco, laser ablation, and CoMoCat methods) contain nanotubes with diameters ranging from 0.7 to 2.8 nm. The diameter distribution and the composition of metallic and semiconducting tubes of the SWNT materials are strongly affected by the synthesis method. Similar sp² hybridization of carbon in the oxygenated SWNT structure can be found, but different surface functionalities are introduced while the tubes are processed. All the SWNTs demonstrate stronger plasmon resonance excitations and lower electron binding energy than graphite and multiwalled carbon nanotubes. These SWNT materials also exhibit different valence‐band X‐ray photoemission features, which are considerably affected by the nanotube diameter distribution and metallic/semiconducting composition.     

A theoretical study on nBuLi/lithium aminoalkoxide aggregation in hexane and THF

Theoretical calculations on aggregation of nBuLi/lithium aminoalkoxide superbases, such as nBuLi/LiDMAE (LiDMAE = Me(2)N(CH(2))(2)OLi) and nBuLi/LiPM (LiPM = Li-N-methyl-2-pyrrolidine methoxide) in gas phase and solution are reported. The combination of equimolar amounts of each component in hexane induced unusual reactivity of the resulting superbase, which remains misunderstood. In order to elucidate the corresponding reaction mechanisms, it is imperative to get a deeper insight into the energetics of aggregation and the effect of the medium on equilibrium constants. In the present study, we compute and compare the stability of (nBuLi)(n), (LiPM)(n), and equimolecular mixed aggregates (nBuLi:LiPM)(n) in gas phase, hexane, and THF. Calculations have been carried out at the density functional theory level (B3LYP/6-31G(d)) using continuum and discrete continuum models of solvation. Higher-level calculations (MP2/aug-ccpVQZ) have been done in some cases for test purposes. Enthalpic and entropic contributions have been discussed and were shown to play an opposite role in hexane (or gas phase) and THF. The characteristics of LiPM and mixed nBuLi/LiPM solutions are found to be significantly different from those of nBuLi solutions. These calculations are in accordance with experimental data in both hexane and THF. Further comparison of theoretical and experimental results for gas-phase Li(+)-THF and Li(+)-DME complexes has enabled a discussion on computational errors for entropic contributions in THF. The value for the release of a THF solvent molecule is proposed to be DeltaS approximately 23 eu. These results provide new insights to the aggregation of organolithium compounds in solution and will be useful for the investigation of other systems.