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Trajectory of the Selective Dissolution of Charged Single-Walled Carbon Nanotubes

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† Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
‡ Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
§ Linde LLC, Murry Hill, New Jersey 07974, United States
Cite this: J. Phys. Chem. C 2017, 121, 39, 21703–21712
Publication Date (Web):September 4, 2017
https://doi.org/10.1021/acs.jpcc.7b06553

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY.

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Abstract

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Single-walled carbon nanotubes (SWCNTs) are typically produced as a mixture of different lengths and electronic types. Methods for sorting these as-produced mixtures typically require damaging and unscalable techniques to first overcome the strong bundling forces between the nanotubes. Previously, it has been shown that negatively charging SWCNTs can lead to their thermodynamically driven, gentle dissolution in polar solvents, and moreover that this process can selectively dissolve different SWCNT species. However, there are several conflicting claims of selectivity that must be resolved before the full potential of this method for scalably postprocessing SWCNTs can be realized. Here we carefully investigate dissolution as a function of charge added to the as-produced SWCNT sample, using a range of complementary techniques. We uncover a far richer dependence on charge of SWCNT dissolution than previously imagined. At low values of charge added, amorphous carbons preferentially dissolve, followed sequentially by metallic, larger diameter (>9 Å) semiconducting SWCNTs, and finally smaller diameter semiconducting SWCNTs as more charge is added. At an optimal stoichiometry of NaC10, the dissolution yield is maximized across all species. However, at higher charge the larger diameter and metallic SWCNTs are so highly charged that they are no longer soluble, leaving smaller diameter SWCNTs in solution. Our results clearly demonstrate two interconnected mechanisms for dissolution: the sequential charging of the SWCNTs and their solution thermodynamics. This work reconciles conflicting reports in the literature, demonstrates that upon charge added the different SWCNTs behave like discrete molecular species, and points toward selective dissolution as a scalable method for SWCNT separation.

1 Introduction

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Despite advances in selective synthesis, (1-3) as-grown single-walled carbon nanotube (SWCNT) samples typically consist of a mixture of lengths (0.1 μm–several μm), diameters (0.4–5 nm), and electronic types (approximately 1:2 metallic, m-SWCNTs to semiconducting sc-SWCNTs), (4) tightly held together in bundles which are decorated in residual metal catalyst and amorphous carbon. (5) Many “postsynthesis” methods that aim to refine this mix into homogeneous samples have been explored. However, most of these methods rely on processes that are detrimental to the pristine properties of the SWCNTs and/or are difficult to scale. For example, initial purification steps, necessary to remove carbonaceous materials or catalyst particles, are often performed with nitric acid refluxing (5) or acid/base mixtures, (6) which can damage the atomic framework of the SWCNT. (7) Furthermore, a prerequisite for most enrichment procedures is the individualization of SWCNTs from their bundles. Dispersions are typically achieved by energetic processes such as ultrasonication to separate the SWCNTs from one another and then ultracentrifugation to separate the unbundled fraction. (8, 9) The sonication damages and shortens the nanotubes, (10) while the (sometimes multiple) ultracentrifugation step is difficult to scale. (11)
An alternative to shear-based processing is reductive chemistry, a versatile route for the efficient solubilization of a range of important nanomaterials. (12-15) When applied to SWCNTs, reductive processing (i.e., controllably adding electrons to the nanomaterials) can lead to spontaneous, gentle dissolution of individualized SWCNTs in polar aprotic solvents, avoiding the damage associated with other methods. (12, 13, 16, 17) The reduced SWCNTs can be generated either chemically (with sodium naphthalide, (12) alkali metal, (18) or alkali metal–ammonia (13) to form so-called nanotubide salts) or electrochemically. (16) The dissolution is spontaneous so requires no damaging energy input or centrifugation, and the process is intrinsically scalable. Moreover, there are several reports that specific SWCNTs dissolve preferentially; i.e., reductive dissolution can be used for scalable postsynthesis separation of SWCNTs. Unfortunately, there are clear inconsistencies between the reported enrichments. SWCNTs charged in alkali metal–ammonia and subsequently dissolved in N,N-dimethylformamide (DMF) manifested only a selective dissolution of the m-SWCNTs. (13, 18) However, in one case this result is contradicted by the same apparent separation observed in the undissolved material. (18) Another study on metal–ammonia reduced SWCNTs reported the preferential chemical functionalization of both m-SWCNTs and smaller diameter sc-SWCNTs upon charging. (19) In contrast, another study showed that sodium naphthalide reduced nanotubide salts could lead to the preferential dissolution of larger diameter nanotubes of both types. (20) Electrochemically reduced SWCNTs samples showed a preferential dissolution of amorphous carbon, but not of the different nanotube species. (16) Potential sources of these discrepancies are the reliance on Raman spectroscopy to assess separation, (13, 17, 18, 20) the physical agitation often used to encourage the dispersion of the SWCNTs, (18) a range of charging stoichiometries used, and the dependence of the process on the exact composition of the as-synthesized SWCNT starting material which can vary with the production method and batch. Raman spectroscopy can be misleading, as it examines a small sample of SWCNT types for each laser excitation, it is highly sensitive to the environment, and the effect of diameter and metal/semiconductor enrichment are often indiscriminable. (21, 22) Bundling can narrow and shift the resonant energies of the SWCNTs, causing peak intensity and position shifts and making full population analysis prohibitively difficult. Furthermore, functionalization can mask the unique SWCNT electronic structure, and selectivity induced by sample heating/damage from laser irradiation can affect both the radial breathing mode (RBM) signals and the intensity ratio between the sp2-associated G-mode and the defect-induced D-mode (IG+/ID ratio). (23-27) It is also important to note that dispersions obtained via stirring or sonication will impede assessment of the truly selectively dissolved fraction.
To develop a full understanding of the selective dissolution of reduced SWCNTs, a multitechnique analysis of the spontaneously dissolved fraction as a function of charge added to SWCNT was performed. In this way, it can be unambiguously demonstrated that the selective dissolution is highly dependent on charging stoichiometry. Furthermore, these results highlight the two competing dissolution mechanisms: (i) selective charging based on the relative electron affinities of the different SWCNTs and (ii) the thermodynamic instability of the solutions at high charge densities.
Nanotubide salts with carefully controlled charge/carbon ratios were prepared via reducing SWCNT samples with solvated electrons generated by dissolving alkali metals in liquid ammonia (methods, Scheme 1). In brief, the SWCNTs and a stoichiometric amount of sodium are loaded into a vessel into which anhydrous ammonia was condensed. The ammonia was then evaporated to leave a nanotubide salt. These salts were subsequently dissolved in DMF in an argon glovebox for 3 days, without any stirring or physical agitation. Nanotubide salts were prepared with the stoichiometry NaCx (where x = 200, 150, 100, 40, 20, 10, 5). These ratios give a charge density (electrons per carbon atom) of 0.005, 0.0067. 0.01, 0.025, 0.05, 0.1, and 0.2, respectively. For clarity, dissolved SWCNT fractions from each salt will be referred to as (NaCx)DIS and undissolved fractions as (NaCx)UND, where NaCx is the stoichiometry of the nanotubide salt after ammoniation.

Scheme 1

Scheme 1. Formation of SWCNT Nanotubide, NaCx, Following Reductive Treatment with Solvated Electrons from Sodium–Ammonia Solutions, Followed by Dissolution in a Dry Polar Aprotic Solvent (DMF)
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2 Experimental Details

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2.1 Chemicals

HiPco SWCNTs Batch R2-172 and Batch R1-821 were from NanoIntegris Technologies Inc. N,N-Dimethylformamide (DMF, ≥ 99.8%), sodium ingot (≥99.95%), anhydrous ammonia (NH3 ≥ 99.95%), nitric acid (HNO3, 70%), hydrochloric acid (HCl, 37%), sodium deoxycholate (DOC, ≥ 97%), and deuterium oxide (D2O, 99.9 atom %) were all from Sigma-Aldrich Co and used as supplied unless otherwise stated.

2.2 Ammonia Condensation

2.2.1 SWCNT Preparation

As-received HiPco SWCNTs were dried in a vacuum oven at 80 °C to remove residual ethanol before being vacuum annealed at 500 °C under a turbo-vacuum to at least 1 × 10–6 mbar for a minimum of 12 h to remove oxygen and moisture. The SWCNTs were taken to the argon glovebox with no further atmospheric exposure.

2.2.2 Nanotubide Production

The SWCNTs were added to a quartz reaction tube with the stoichiometric amount of sodium ingot and sealed at the valve before being removed from the glovebox. The reaction tube was immersed in an isopropyl alcohol bath at −50 °C and attached to the ammonia rig, which was evacuated with a turbopump to 1 × 10–6 mbar before the tube was opened to the vacuum. The ammonia was then expanded into a known buffer volume (300 cc) on the rig and the pressure recorded, followed by the condensation onto the sample. This expansion step was repeated until 15 bar liters was condensed. A blue color was observed showing the solvation of the electron (and the sodium cation), before the ammonia lost color due to the reduction of the SWCNTs. This condensation was left for 12 h before the ammonia lecture bottle was cooled using liquid nitrogen and the ammonia was cryopumped back from the reaction tube. When all the ammonia had evaporated from the sample, the reaction tube was sealed and taken back into the glovebox. In a typical experiment, ∼100 mg of nanotubide (mass corrected to subtract the mass due to sodium) was spontaneously dissolved in 100 mL of anhydrous DMF with no stirring or sonication. After 3 days, the concentrations remained constant, and these supernatants were investigated for chiral distributions.

2.3 Yield Measurements

2.3.1 TGA-Mass Spectrometer

An aliquot, 80 μL, of the supernatants was decanted into alumina crucibles and inserted into a Mettler Toledo TGA/DSC 1LF/UMX with an attached HIDEN HPR20-QIC EGA mass spectrometer. The samples were held at 200 °C in air until no DMF signature (M = 73 g mol–1) was detected from the evolved gas. The sample was then heated to 600 °C in air at 10 °C min–1 to degrade the carbon and leave residual metal oxides and hydroxides from the iron catalyst and sodium.

2.3.2 Mass Filtration

A portion, 5 mL, of the supernatant was mixed with 250 mL of DMF in a Büchner funnel and vacuum filtrated on top of a 0.2 μm pore size poly(tetrafluoroethylene) (PTFE) filter paper. The SWCNT film produced on the paper was washed with 250 mL each of ethanol (99.8%), chloroform (≥99%), and deionized water (18 MΩ) before drying in the oven at 80 °C to remove residual water. A balance with an accuracy of ±5 μg was used to measure the mass of the PTFE filter paper before and after filtration, under ambient conditions.

2.3.3 UV–Vis–NIR Absorption

To measure concentration, the supernatants were decanted into a 10 mm path length, sealed quartz cuvette and the lid was further wrapped in Parafilm. Scans were performed on a PerkinElmer Lambda 950. If the absorbance exceeded 3 AU, samples were diluted 10 times using DMF and repeated.

2.4 Chiral Analysis

2.4.1 Raman Spectroscopy

Drops of 1 mL of nanotubide solutions were deposited onto glass microscope slides and allowed to dry in an argon glovebox atmosphere. Raman spectroscopy was performed on each dissolved fraction. Spectra were collected on a Renishaw inVia micro-Raman spectrometer with 532 nm (2.33 eV) DPSS diode (numerical aperture 0.8/100×, sample power 3.2 mW, WiRE 4.1 HF7241 software 2014 interface, Renishaw PLC, U.K.) in backscattered geometry, with laser spot diameters 0.8 μm. Raman spectra were processed and Raman maps produced using WiRE software (using spectra that were background corrected and normalized to the G+-mode unless otherwise stated). For each sample, a 400 μm2 map was recorded on a point by point basis, with each spectrum on a 1 μm2 region.

2.4.2 Aqueous Dispersions of SWCNTS for PL Spectroscopy and UV/Vis/NIR Absorption

The supernatants were dried in an evaporation dish in the argon glovebox on a hot plate at 160 °C, and the SWCNTs were recovered. The SWCNT solute was then placed into a reaction tube and outgassed at 500 °C for 24 h under a static vacuum of 1 × 10–6 mbar to sublime the sodium metal. The sublimed sodium condensed as a mirror at the cold end of the reaction tube. The SWCNTs were then exposed to atmosphere and 15 mg mixed with 30 mL of a solution of 1 wt/vol % DOC/D2O. The mixture was placed in an isopropyl alcohol bath at 0 °C and ultrasonicated for 2 × 15 min using a Sonics & Materials VCX750-220 V tip probe (225 W). The resultant dispersions were ultracentrifuged with a Beckmann Coulter Optima L-90K with SW41Ti swinging bucket rotor at 120 000 g for 40 min, and only the top 50% of the supernatants were recovered.

2.4.3 PL Spectroscopy

The nanotube dispersions were investigated using an Applied Nanofluorescence NS1 Nanospectralyzer. A cuvette with a 4 mm path length was filled with 2 mL of SWCNT dispersion was scanned with a 500 ms integration time on 3 lasers (638, 691, and 782 nm). Scans were averaged over 30 measurements, and emission was detected with an InGaAs detector cooled to −18 °C. Fitting was performed by the software supplied with the NS1, and abundances were calculated from predetermined optical absorption cross sections.

2.4.4 UV–Vis–NIR

Background scans used the 1 wt/vol % DOC/D2O solution. The dispersions used were the same as for the PL spectra and performed on a PerkinElmer Lambda 950.

2.5 Additional Characterization Techniques

2.5.1 ICP-AES

Supernatants of nanotubide solutions were dried in evaporation dishes at ambient temperature in the glovebox before exposure to the atmosphere. The powders were then thermally degraded via TGA using the same methodology as the yield measurement, to obtain the carbon mass. The residue was dissolved in aqua regia (3 parts 37% hydrochloric acid to 1 part 72% nitric acid), and ICP-AES was performed on a PerkinElmer ICP 2000 DV OES.

2.5.2 Electron Microscopy

Imaged SWCNT samples were recovered after vacuum annealing (500 °C, 1 × 10–6 mbar). The SWCNT powders were adhered using silver paint (Acheson silver DAG 1415 M) to Al stubs with all preparatory supplies purchased from Agar Scientific, U.K. Field emission gun SEM (Leo Gemini 1525 with SmartSEM software interface V05.05.03.00, 2010, Carl Zeiss NTS Ltd., U.K.) was operated in secondary electron detection mode at 5 keV at a working distance of approximately 10 mm.

3 Results and Discussions

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3.1 Yield of Dissolution

The percentages of the nanotubide salts that spontaneously dissolved in the DMF (Figure 1) were obtained via three methods: ultraviolet–visible–near-infrared (UV–vis–NIR) absorption, membrane filtration of the solute, and thermogravimetric analysis (TGA) of the nanotubide solutions. The most suitable method to evaluate the yield of dissolution was determined to be membrane filtration, since it measures the entirety of the SWCNT sample (nanotubes, catalyst particles, and amorphous carbon, but no sodium). TGA leaves the iron oxide residue from the catalyst and sodium (hydr)oxide used in the charging process and therefore underestimates the yield, (28) and absorbance measurements introduce issues of extra light scattering due to doping, causing an overestimation (29) (a revised value for the extinction coefficients can be found in Figure S1). The maximum yield of dissolution obtained was ∼80 wt % from a salt of stoichiometry of NaC10. Notably, this yield is much higher than previous reports for a purely spontaneous dissolution (13, 30) (i.e., with no sonication or stirring). With low charge added (between 0.005 and 0.01 e/C, NaC200–100) the yield of dissolution is low (<18 wt %). As the amount of charge added increases, so too does the concentration of solution, with a maximum of 79.5 wt % occurring at NaC10 (0.1 e/C). Interestingly, higher charging results in a decreased yield of 76 wt %. Although relatively small, this decrease is observed in all three methods, indicating that it is a real effect. To investigate the extent to which the dependence of the yield of dissolution on charge added is due to the preferential dissolution of different species, Raman, UV–vis–NIR absorption, and photoluminescence (PL) spectroscopies were utilized.

Figure 1

Figure 1. Nanotubide salt dissolution yields in the solvent DMF, as a function of charge added to the as-produced powder for different characterization techniques. Black squares represent measurements based on the absorbance at 660 nm. Red circles represent the mass measurements via membrane filtration. Blue triangles represent mass measurements from TGA after oxidative decomposition. The solid lines shown are a guide to the eye.

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3.2 Raman Spectroscopy: sp2 Purity

Raman spectroscopy can be used to probe the proportion of defective material in a SWCNT sample, by evaluating the intensity ratio of the structural order/disorder modes IG+/ID, (31) and also to determine the relative population of the different SWCNT types via analysis of the RBMs. (32) Here, this technique was used to assess the defectiveness of the dissolved species and as a qualitative guide for selectivity, the latter requiring corroboration with the other spectroscopic techniques used in this work, as Raman spectroscopy by itself can be misleading. Measurements were taken from the spontaneously dissolved SWCNTs from each salt, dried onto substrates and exposed to dry air to oxidize the reduced SWCNTs. (33-35)
The dissolved SWCNTs from the lowcharge stoichiometry salt (NaC100)DIS give the lowest IG+/ID ratio, indicative of the highest amount of amorphous carbon and/or defective material (Figure 2b, c). As the concentration of sodium in the salt increases, the IG+/ID ratio of the dissolved fraction monotonically increases, with the highest ratio from (NaC5)DIS. To better understand the origin of the decreased IG+/ID ratio, scanning electron microscopy (SEM) was performed on the undissolved and dissolved fractions of the NaC100 salt (Figure 2d–f). The dissolved fraction (Figure 2e) consists mostly of nontubular material, with very few visible nanotube bundles present. Contrastingly, the undissolved fraction (Figure 2f) is comprised of nanotube bundles albeit with a denser morphology than the starting material (Figure 2d) and an improved sp2 structural quality from the corresponding Raman IG+/ID ratio (Figure S2). This indicates that amorphous carbon, rather than defective SWCNTs, preferentially dissolves in the lowest charging regime. (36)

Figure 2

Figure 2. (a) Dissolved fraction Raman spectra with varying charge stoichiometry using a 532 nm laser, showing the RBM region (100–350 cm–1), normalized to the largest feature. (b) The D-mode disorder peak (∼1350 cm–1) and G-mode graphitic peaks (∼1510–1580 cm–1) for each dissolved fraction, normalized to the intensity of the G+-mode. (c) The IG+/ID ratio as a function of charge stoichiometry showing increasing purity with increasing charging. (d) SEM of the starting HiPco SWCNT material. (e) SEM of the dissolved (NaC100)DIS nanotubide, with few visible SWCNT bundles. (f) SEM of the undissolved (NaC100)UND nanotubide showing the change in morphology from the starting material.

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Concerning the selective dissolution of SWCNTs, a clear reduction is observed in the relative intensity of the peak at ∼268 cm–1, (attributed to the RBM of the (7,6) sc-SWCNT) (37) relative to the peak at 233 cm–1 (attributed to the RBM of the (9,6) m-SWCNT) upon increasing charge (Figure 2a). The change indicates an enrichment of m-SWCNTs, in agreement with previous reports. (13, 18) However, this decrease is also consistent with the conclusion that the larger diameter SWCNTs dissolve first, in agreement with other studies. (20) The multicomponent G–-mode at ∼1510–1550 cm–1 in Figure 2b is also dependent on SWCNT type in the excited (resonant) population. The decreasing intensity of the mode at ∼1522 cm–1 relative to the ∼1545 cm–1 mode at lower charging again indicates that the dissolved SWCNTs are larger in diameter and the overall diameter distribution is narrower. (38) Furthermore, the broadening and asymmetry of the G–-mode can also be attributed to a relative increase in the population of metallic species. (39, 40)

3.3 UV–vis–NIR Spectroscopy: m-SWCNT Enrichment

PL and UV–vis–NIR absorption can be used to quantitatively analyze the (n, m) enrichment of a SWCNT sample, being less sensitive to environmental effects than resonant Raman spectroscopy (although PL only probes sc-SWCNTs (41)). For both techniques it was necessary to first separate, decharge, dry, and redisperse the spontaneously dissolved fraction in water using standard protocols (see methods). (42) The decharging is required because the optical transitions of individual SWCNTs depend on their electronic structure, which will change upon charge doping. (29, 43) Different species of isolated SWCNTs absorb light in the UV–vis–NIR regime at different characteristic energies based on their band structure, enabling an analysis of populations of different SWCNTs in a mixed as-produced sample. However, the absorption peaks obtained can be broad and overlapping, complicating the analysis. This challenge is compounded by the difficulty in deconvoluting the wavelength-dependent background, usually performed with a fitted λ–a (where λ is the wavelength and a is the fitted variable) or an exponential line shape. (44, 45) However, toward higher wavenumbers (>20 000 cm–1) the background has an extra contribution from a π-band surface plasmonic excitation present in all carbon sp2 materials. In addition there is extra scattering intrinsic to the presence of m-SWCNTs, scattering from amorphous carbon, catalytic impurities, damaged and/or functionalized SWCNTs from the ultrasonication process, and bundled SWCNTs. As a result, the fitting of a single function across the entire sample becomes prohibitive. In this work, these factors are addressed by use of an ISO standard (46) of linear background subtraction, as is further discussed in the Supporting Information.
At the lowest salt charge stoichiometries (NaC200–150)DIS, no features were observed in the UV–vis–NIR spectra, consistent with the dissolution of only amorphous and defective material. For all other stoichiometries, the resultant spectra showed characteristic SWCNT absorption transitions (Figure 3). The percentage ratios of m-/sc-SWCNTs in the dissolved fractions were calculated by the ratio of the areas of the M11 and S11 transitions (Table 1). (46-48) The as-produced material was found to have a metallic fraction of 26.2%, in good agreement with previous literature values. (49) For (NaC100)DIS, a dramatic increase in the m-SWCNT fraction is found, more than doubling to 55.9%. However, at higher charging (NaC40–5)DIS, the metallic enrichment is indiscernible, with a third of the sample now being made up of m-SWCNTs.

Figure 3

Figure 3. UV–vis–NIR spectra with varying NaCx ratios showing preferential dissolution of metallic SWNTs at low charge stoichiometries. Top: semiconducting SWCNT S11 transitions. Middle: sc-SWCNT S22 transitions. Bottom: m-SWCNT M11 transitions. All spectra shown are baseline corrected and normalized to the largest peak intensity. The NaC100 shows the biggest change, with a decrease for peaks in the S11 region and increase in the M11 region, showing metallic enrichment for this charge ratio.

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Table 1. Change in Ratio of m-SWCNT/sc-SWCNT with Changing Charge Stoichiometry of the Dissolved Fractions from the UV–Vis–NIR Analysis
sample% m-SWCNT ± 0.5% sc-SWCNT ± 0.5
HiPco R2–17226.273.8
NaC5 dissolved32.567.5
NaC10 dissolved31.268.8
NaC20 dissolved34.865.2
NaC40 dissolved30.369.7
NaC100 dissolved55.944.1
NaC150 dissolvedn/an/a
NaC200 dissolvedn/an/a
To investigate the enrichment for the (NaC100)DIS sample in more detail, the relative percentage changes of individual SWCNT (n, m) populations (indexed from empirical data from UV–vis–NIR (50)), with respect to the raw sample, were calculated (Figure 4). The m-SWCNTs all show enrichment in the dissolved fraction as expected, though a slight preferential dissolution of the larger diameter m-SWCNTs can be observed. For sc-SWCNTs, there is a dramatic diameter effect: the fraction of larger diameter sc-SWCNTs increases, whereas the fraction of smaller sc-SWCNTs decreases. The same analysis was performed on the undissolved fraction, (NaC100)UND, since previous studies have had issues with observation of the same enrichment on both fractions. (18) Crucially, here we found the complementary trend, that is, a decrease in larger diameter population in the (NaC100)UND material (Figure S4) confirming a genuine enrichment. For sc-SWCNTs, it is possible to quantify the (n, m) population changes with greater accuracy using PL spectroscopy.

Figure 4

Figure 4. UV–vis–NIR relative percentage change of the (NaC100)DIS dissolved fractions relative to the starting material of assigned m-SWCNTs (black) and sc-SWCNTs, (red) showing the preferential dissolution of larger diameter SWCNTs. The solid lines are linear fits of the data as a guide to the eye.

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3.4 Photoluminescence Spectroscopy: sc-SWCNT Diameter Enrichment

PL spectroscopy was performed for all samples, and the spectra were analyzed using fits for relative abundances from Weisman et al. (51)Figure 5 shows the relative percentage enrichment of the dissolved SWCNTs, grouped over four different diameter regimes, as a function of salt charge stoichiometries. The PL spectra were fitted for all species present in the samples and quantified as a percentage change in PL intensity compared to the as-received SWCNT (n, m) population. SWCNT species with similar diameters showed similar trends as the charge stoichiometry changed. The (NaC100)DIS dissolved fraction showed an enrichment of the largest diameter sc-SWCNTs (>10 Å) with a similar enhancement of the second largest set (9.0–10 Å) albeit to a lesser magnitude. Conversely, the smallest SWCNTs in the distribution (<8.3 Å) showed the largest decrease with the second smallest set (8.3–9.0 Å) showing a smaller decrease in population. The most abundant SWCNT species changed from the (7, 6) in the raw material to the larger (11, 1) nanotube, demonstrating preferential dissolution of the larger diameter sc-SWCNTs at low charge added. As the charge on the salt is increased, for all other stoichiometries the most abundant species reverts to the (7, 6), and relative abundances tend toward that of the starting (n, m) distribution, due to the successive dissolution of the smaller diameter sc-SWCNTs. This analysis is consistent with the measured increase in yield of dissolution. Furthermore, the change in the PL abundance of the (7, 6) SWCNT corroborates the Raman data, which showed the increase of the (7, 6) sc-SWCNT RBM with increasing charge.

Figure 5

Figure 5. PL relative percentage change of the dissolved fractions for the eight most abundant SWCNTs in the HiPco raw batch R2-172, separated into four diameter regimes. The NaC100 dissolved shows an enhancement of the larger diameter and a decrease of the smaller diameter sc-SWCNTs. Further charge moves the diameter distribution back toward that of the starting material.

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Importantly, diameter selectivity as determined by PL shows excellent agreement with that determined from UV–vis–NIR as shown in Figure S5. This corroboration gives confidence in the analysis technique and validates the method for background subtraction for the UV–vis–NIR spectra.
The average diameter of the sc-SWCNTs from the as-produced powder was found to be 9.32 Å. The average diameters for each fraction were calculated from the PL data and are shown in Figure 6. The average sc-SWCNT diameter is largest for the (NaC100)DIS and decreases monotonically as the charge increases. For the two salts with the highest yields of dissolution, NaC10 and NaC5, the diameter distribution is the same within error as the starting material.

Figure 6

Figure 6. Mean SWCNT diameter and standard error of the various dissolved fractions as a function of charge, showing that the average diameter increases from that of the raw material as the charge level is decreased. The black dotted line represents the mean of the raw SWCNT sample population, and the red line is a guide for the eye.

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3.5 e-DOS Analysis: Preferential Charging of Different SWCNT Species

To understand the measured selective dissolution of different SWCNTs, a combined electronic density of states (e-DOS) was produced from extended tight binding models (52, 53) summed over the SWCNT species present in the starting material. The e-DOS for the individual sc-SWCNTs were scaled according to their relative abundances, as experimentally determined by PL, and again by 73.8% for the overall proportion of sc-SWCNTs determined by the UV–vis–NIR absorption. The m-SWCNTs were scaled to 26.2% of the overall distribution, with each (n, m) index within the diameter range determined by the PL given an equal weighting. The combined density of states is shown in Figure 7a. By integrating this function, the number of charges per carbon on a SWCNT as a function of chemical potential can be extracted separately for m-SWCNTs, small diameter (<9.0 Å) and larger diameter (>9.0 Å) sc-SWCNTs, shown in Figure 7b and compared to the enrichment measured in the spontaneously dissolve fractions as a function of charge.

Figure 7

Figure 7. (a) Combined electronic density of states (1D e-DOS) for the HiPco R2–172 batch, derived empirically from PL and UV–vis–NIR data, separated into three components (small-diameter sc-SWCNT, large-diameter sc-SWCNT, and m-SWCNT). (b) Integrated DOS to give the proportion of electronic states per carbon for the individual components (small sc-SWCNT, large sc-SWCNT, and m-SWCNT) for a given stoichiometry of the total HiPco system.

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The nominal charge stoichiometries for NaC100 and NaC5 are marked in Figure 7b, neglecting effects due to quantum capacitance (54) and charge lost to the amorphous carbon (which would cause a shift of the stoichiometries to lower charge per carbon values). For the NaC100 salt, nearly half of the occupied electronic states are from m-SWCNTs. This indicates that at this charge ratio, m-SWCNTs will be reduced in preference to sc-SWCNTs as they have the most accessible states directly above the Fermi level. As uncharged SWCNTs are insoluble, the selective dissolution of the m-SWCNTs from the NaC100 salt can be attributed to preferential charging of these species. However, above a Na:C ratio of about NaC40, the proportion of electronic states that are attributed to m-SWCNTs remains relatively constant. This effect explains the lack of metallic enrichment at higher charge ratios observed from UV–vis–NIR (in Table 1). A similar argument can be made for the larger diameter sc-SWCNTs. As Figure 7b shows, aside from a small contribution from the smaller sc-SWCNTs for the NaC100 stoichiometry, the rest of the occupied states arise from larger diameter sc-SWCNTs. The larger diameter sc-SWCNTs have states closer to the Fermi level and will charge preferentially compared to smaller sc-SWCNTs, but after most m-SWCNTs. However, when the charge added is further increased, the smaller diameter SWCNTs will also be charged, and the diameter distribution returns to that of the starting material as measured in Figure 6.

3.6 Cation Distribution

To further investigate the mechanism behind the selective dissolution, the distribution of charge between the dissolved and undissolved SWCNTs was determined. To do this, the metal to carbon ratio for the dissolved and undissolved fractions of the same salt was measured using elemental analysis via inductively coupled plasma atomic emission spectroscopy (ICP-AES). Since a larger sample mass was needed, three selected nanotubides salts NaC5, NaC10, and NaC30 were prepared from a new batch of HiPco SWCNTs, R1-821, and dissolved in DMF as before. In this batch, mass filtration measurements (see Figure S6) determined the yields of dissolution to be 54.8%, 89.1%, and 60.7% respectively, showing an even more pronounced reduction in yield of dissolution at higher charging than before (Figure 1). The difference in yield of dissolution between this batch and the previous one is due to the different amorphous carbon content, with the IG+/ID ratio larger in the newer batch (see Figure S7). The solutions were separated into dissolved and undissolved fractions before elemental analysis, and PL was performed (see Experimental Details).
Table 2 shows the measured sodium content of the fractions as a function of SWCNT diameter. For the NaC30 salt, the system is in the regime below the maximum dissolution yield and the average diameter is, as expected, higher in the dissolved fraction. The dissolved fraction has a measured Na:C ratio of 1:20.0, compared to the undissolved 1:42.2. The differing values confirm the hypothesis that the larger SWCNTs, in the dissolved fraction, are preferentially charged. For NaC5, we find a smaller average sc-SWCNT diameter and smaller dissolution yield in the dissolved fraction, as before. Interestingly, the dissolved fraction has a similar Na:C ratio of 1:23.2 as the Na:C ratio for the dissolved fraction of the NaC30 salt. However, in the corresponding undissolved fraction, we now find signficiantly more sodium with a ratio of 1:1.5. This important finding clearly demonstrates that at very high charging the SWCNTs are not soluble, explaining the decrease in yield of dissolution. Moreover, this time it is the largest SWCNTs that do not dissolve. This result indicates that the metallic and large-diameter SWCNTs continue to be doped preferentially, but although first to be solubilized, eventually these tubes become too highly charged to dissolve.
Table 2. Stoichiometry of the Dissolved and Undissolved Fractions for an NaC5 and an NaC30 Salt, with the Corresponding Average Diameter for the Dissolved Fraction Compared to the As-Received Batch
 saltdissolvedundissolved
measured stoichiometryNaC5NaC23.2NaC1.5
average sc-SWCNT diameter (Å)9.409.259.74
measured stoichiometryNaC30NaC20.0NaC42.2
average sc-SWCNT diameter (Å)9.409.619.36
This observation can be understood by considering the thermodynamics of the dissolution process. An ionic salt (such as sodium nanotubide) will dissolve if the free energy associated with the solvent-coordinated species is less than that of the combined free energy of the isolated ionic salt and solvent. The free energy of the salt will be largely dominated by its lattice enthalpy, as the magnitude of the entropic term will be small for an ordered crystal. In turn, the lattice enthalpy will be dependent on the ionic species involved; in general, the greater the charge density of the ions, the greater the magnitude of the lattice enthalpy. (55) This trend has been shown, for charged C60 (fulleride) solutions, via Monte Carlo computer simulations studying the dissolution of fulleride salts as a function of charge. (14, 56, 57) In the case where the C60 molecules are highly charged, the lattice enthalpy term is so great that there is no favorable gain of free energy upon solvation and thus they are not soluble. Similarly, here the SWCNTs that charge preferentially dissolve first but are the first to become too charged to dissolve at high levels of reduction, resulting in solutions of smaller diameter SWCNTs.

4 Conclusions

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In summary, the selective dissolution of SWCNTs has been carefully investigated as a function of charge added to the as-produced SWCNT mix. From low to high charge, the selective dissolution trajectory is seen to favor first the amorphous carbon and defective material, followed by the m-SWCNTs, large-diameter sc-SWCNTs, and finally smaller diameter sc-SWCNTs, with an optimum charging value to give a maximized dissolution yield. The complex dissolution dependence on charging arises from the range of electronic structures of the SWCNT species in the sample. At low doping, the SWCNTs with the states available closest to the Fermi level are preferentially charged and therefore preferentially dissolve. At higher metal loadings, the excess charge on the large-diameter SWCNTs prevents their dissolution, as confirmed by the observation of charge partitioning between dissolved and undissolved fractions.
The analysis of the enriched fractions clearly shows that reliance on Raman spectroscopy to determine separation can be misleading, and a valid picture necessitates a range of complementary methods including UV–vis–NIR and PL. The trajectory of nanotube dissolution also demonstrates that SWCNTs within a mixed sample can themselves be viewed as discrete molecular species, with the equilibrium charge distribution that results upon fixed doping being governed by the differing electron affinities of the SWCNTs. The results also highlight the importance of the balance of the thermodynamics of the dissolution process, in particular demonstrating that for very highly charged SWCNTs dissolution is unfavorable. This conclusion will be an important consideration for controlling concentrations and dissolution via alkali-metal reduction not just for SWCNTs (58) but also graphene (33, 59, 60) and other 2d-materials. (15)

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06553.

  • Calculation of the extinction coefficient as a function of charge on the nanotubes (Figure S1); Raman Spectroscopy of the SEM sample shown in Figure 2e, f (Figure S2); UV–vis–NIR spectra of the dissolved SWCNT solutions after aqueous redispersion and corresponding discussion (Figure S3); photoluminescence (PL) analysis showing the opposite trends in the undissolved and dissolved SWCNTs for two different stoichiometries (Figure S4); comparison of the diameter enrichment for UV–vis–NIR and PL (Figure S5); yield of dissolution measurements for the newer batch as a function of stoichiometry (Figure S6); Raman spectroscopy comparison of the two batches used in this study (Figure S7); explanation of the calculation for Figure 7b (Figure S8) (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
    • Christopher A. Howard - Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
  • Authors
    • David J. Buckley - Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
    • Stephen A. Hodge - Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
    • Martina De Marco - Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
    • Sheng Hu - Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
    • David B. Anthony - Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
    • Patrick L. Cullen - Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
    • Kevin McKeigue - Linde LLC, Murry Hill, New Jersey 07974, United States
    • Neal T. Skipper - Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
    • Milo S. P. Shaffer - Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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We acknowledge the EPSRC for funding the project as well as Linde Nanomaterials for assistance and discussion.

Abbreviations

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DMF

N,N-dimethylformamide

DOC

sodium deoxycholate

e-DOS

electronic density of states

ICP-AES

inductively coupled plasma atomic emission spectroscopy

m-

metallic

PL

photoluminescence

RBM

radial breathing mode

sc-

semiconducting

SEM

scanning electron microscopy

TGA

thermogravimetric analysis

UV–vis–NIR

ultraviolet–visible–near-infrared

References

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    Howard, C. A.; Wasse, J. C.; Skipper, N. T.; Thompson, H.; Soper, A. K. The solvation structure of fulleride C605- anions in potassium ammonia solution J. Phys. Chem. C 2007, 111, 5640– 5647 DOI: 10.1021/jp066743+
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    Cullen, P. L.; Cox, K. M.; Bin Subhan, M. K.; Picco, L.; Payton, O. D.; Buckley, D. J.; Miller, T. S.; Hodge, S. A.; Skipper, N. T.; Tileli, V.; Howard, C. A. Ionic solutions of two-dimensional materials Nat. Chem. 2017, 9, 244– 249 DOI: 10.1038/nchem.2650
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    15
    Ionic solutions of two-dimensional materials
    Cullen, Patrick L.; Cox, Kathleen M.; Bin Subhan, Mohammed K.; Picco, Loren; Payton, Oliver D.; Buckley, David J.; Miller, Thomas S.; Hodge, Stephen A.; Skipper, Neal T.; Tileli, Vasiliki; Howard, Christopher A.
    Nature Chemistry (2017), 9 (3), 244-249CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Strategies for forming liq. dispersions of nanomaterials typically focus on retarding reaggregation, for example via surface modification, as opposed to promoting the thermodynamically driven dissoln. common for mol.-sized species. Here we demonstrate the true dissoln. of a wide range of important 2D nanomaterials by forming layered material salts that spontaneously dissolve in polar solvents yielding ionic solns. The benign dissoln. advantageously maintains the morphol. of the starting material, is stable against reaggregation and can achieve solns. contg. exclusively individualized monolayers. Importantly, the charge on the anionic nanosheet solutes is reversible, enables targeted deposition over large areas via electroplating and can initiate novel self-assembly upon drying. Our findings thus reveal a unique soln.-like behavior for 2D materials that enables their scalable prodn. and controlled manipulation.
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    Hodge, S. A.; Fogden, S.; Howard, C. A.; Skipper, N. T.; Shaffer, M. S. P. Electrochemical processing of discrete single-walled carbon nanotube anions ACS Nano 2013, 7, 1769– 1778 DOI: 10.1021/nn305919p
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    Jiang, C.; Saha, A.; Marti, A. A. Carbon nanotubides: an alternative for dispersion, functionalization and composites fabrication Nanoscale 2015, 7, 15037– 15045 DOI: 10.1039/C5NR03504J
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    Gebhardt, J.; Bosch, S.; Hof, F.; Hauke, F.; Hirsch, A.; Görling, A. Selective reduction of SWCNTs—Concepts and insights J. Mater. Chem. C 2017, 5, 3937– 3947 DOI: 10.1039/C5TC01407G
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    Selective reduction of SWCNTs - concepts and insights
    Gebhardt, Julian; Bosch, Sebastian; Hof, Ferdinand; Hauke, Frank; Hirsch, Andreas; Goerling, Andreas
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (16), 3937-3947CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    The charging of single-walled carbon nanotube (SWCNT) mixts. by redn. via alkali metal atoms is an established first step towards covalent SWCNT functionalization. In this combined d.-functional theory and exptl. study, we investigate this redn. with respect to differences occurring between tubes of different electronic type (metallic (m) and semiconducting (s.c.) tubes, resp.). We find that metals, specifically potassium, adsorb stronger to m- than s.c.-SWCNTs, which can be explained by the different band structures of both tube types. We investigate this trend in detail for a variety of different chiral SWCNTs, finding a potassium coverage dependent preference of m- over s.c.-SWCNTs, which is predicted to allow for a selective charging of metallic tubes for K/C ratios ≤ 1/200. This selective charging can be translated into the enrichment of m-SWCNTs during dispersion of SWCNT mixts., since only reduced tubes are dissolved from the bulk material. The results for isolated tubes can be generalized to SWCNT bundle arrangements, which means that the theor. predicted selective charging is transferable also to this more realistic description of the exptl. systems. The theor. findings regarding an electronic type selective charging of SWCNTs have been verified by an exptl. study. By a combination of Raman and absorption/emission spectroscopic anal., a preferential dispersion of charged metallic carbon nanotubes in THF as solvent was found for the predicted low potassium concns. Our results lead to the conclusion that previous m/s.c. selective reductive functionalization reactions cannot be explained on the basis of an electronic type selective charging step, as these reactions used much higher alkali metal concns.
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    Wunderlich, D.; Hauke, F.; Hirsch, A. Preferred functionalization of metallic and small-diameter single walled carbon nanotubes via reductive alkylation J. Mater. Chem. 2008, 18, 1493– 1497 DOI: 10.1039/b716732f
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    19
    Preferred functionalization of metallic and small-diameter single walled carbon nanotubes via reductive alkylation
    Wunderlich, D.; Hauke, F.; Hirsch, A.
    Journal of Materials Chemistry (2008), 18 (13), 1493-1497CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    A selectivity investigation of the covalent sidewall functionalization of SWNTs by reductive alkylation (Billups reaction) is reported. The functionalized tubes Rn-SWNT were characterized by UV-Vis-NIR absorption, NIR emission and Raman spectroscopy. The redn. of SWNTs with sodium and the subsequent alkylation of the reduced tubes SWNTn- with Bu iodide reveal a pronounced SWCNT diam. dependence, i.e., SWNTs with smaller diam. are considerably more reactive than tubes with larger diam. Moreover, this reaction sequence also favors the preferred functionalization of metallic over semiconducting tubes. Treatment of the reduced intermediates SWNTn- with protons, via the use of ethanol, instead of alkyl iodide leads to hydrogenated tubes. However, in this case the degree of functionalization is considerably lower than that obsd. for the corresponding alkylation. Also no pronounced preference of the reaction of metallic and small diam. tubes was obsd. during the hydrogenation process.
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    Voiry, D.; Drummond, C.; Pénicaud, A. Portrait of carbon nanotube salts as soluble polyelectrolytes Soft Matter 2011, 7, 7998– 8001 DOI: 10.1039/c1sm05959a
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    O’Connell, M. J.; Sivaram, S.; Doorn, S. K. Near-infrared resonance Raman excitation profile studies of single-walled carbon nanotube intertube interactions: A direct comparison of bundled and individually dispersed HiPco nanotubes Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 235415 DOI: 10.1103/PhysRevB.69.235415
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    21
    Near-infrared resonance Raman excitation profile studies of single-walled carbon nanotube intertube interactions: A direct comparison of bundled and individually dispersed HiPco nanotubes
    O'Connell, Michael J.; Sivaram, Saujan; Doorn, Stephen K.
    Physical Review B: Condensed Matter and Materials Physics (2004), 69 (23), 235415/1-235415/15CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Complete Raman excitation profiles for single-walled C nanotube radial breathing modes were obtained for bundled HiPco C nanotube samples at 700-985 nm excitation. Results are compared to similar profiles generated from individual C nanotubes dispersed in aq. soln., allowing a direct detn. of intertube interaction effects on electronic properties for 12 specific semiconducting nanotube chiralities. Red shifts in the excitation profiles (ranging from 54 to 157 meV) are obsd. on going from isolated individual to bundled nanotubes. Addnl., bundling is found to broaden the electronic transitions by an av. factor of 2.4 compared to individualized nanotube bandwidths. These results compare well with recent theor. predictions for bundling effects. A study of bundling effects on radial breathing mode frequencies for 17 different nanotube chiralities finds no evidence for significant perturbation of these frequencies resulting from intertube interactions. Results demonstrate that previously reported radial breathing mode frequency shifts are apparent shifts only, resulting from red shifting of the resonant electronic transitions for bundled nanotubes. Bundle inhomogeneity, packing efficiency, orientational disorder, and symmetry redn. are indicated as important factors in detg. the degree of intertube interaction.
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    Araujo, P. T.; Fantini, C.; Lucchese, M. M.; Dresselhaus, M. S.; Jorio, A. The effect of environment on the radial breathing mode of supergrowth single wall carbon nanotubes Appl. Phys. Lett. 2009, 95, 261902 DOI: 10.1063/1.3276909
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    Filho, A. G. S.; Jorio, A.; Samsonidze, G. G.; Dresselhaus, G.; Saito, R.; Dresselhaus, M. S. Raman spectroscopy for probing chemically/physically induced phenomena in carbon nanotubes Nanotechnology 2003, 14, 1130 DOI: 10.1088/0957-4484/14/10/311
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    Müller, M.; Maultzsch, J.; Wunderlich, D.; Hirsch, A.; Thomsen, C. Raman spectroscopy on chemically functionalized carbon nanotubes Phys. Status Solidi B 2007, 244, 4056– 4059 DOI: 10.1002/pssb.200776119
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    Hennrich, F.; Krupke, R.; Lebedkin, S.; Arnold, K.; Fischer, R.; Resasco, D. E.; Kappes, M. M. Raman spectroscopy of individual single-walled carbon nanotubes from various sources J. Phys. Chem. B 2005, 109, 10567– 10573 DOI: 10.1021/jp0441745
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    Raman Spectroscopy of Individual Single-Walled Carbon Nanotubes from Various Sources
    Hennrich, Frank; Krupke, Ralph; Lebedkin, Sergei; Arnold, Katharina; Fischer, Regina; Resasco, Daniel E.; Kappes, Manfred M.
    Journal of Physical Chemistry B (2005), 109 (21), 10567-10573CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Resonance Raman spectroscopy/microscopy was used to study individualized single-walled carbon nanotubes (SWNTs) both in aq. suspensions as well as after spin-coating onto Si/SiO2 surfaces. Four different SWNT materials contg. nanotubes with diams. ranging from 0.7 to 1.6 nm were used. Comparison with Raman data obtained for suspensions shows that the surface does not dramatically affect the electronic properties of the deposited tubes. Raman features obsd. for deposited SWNTs are similar to what was measured for nanotubes directly fabricated on surfaces using chem. vapor deposition (CVD) methods. In particular, individual semiconducting tubes could be distinguished from metallic tubes by their different G-mode line shapes. It could also be shown that the high-power, short-time sonication used to generate individualized SWNT suspensions does not induce defects in great quantities. However, (addnl.) defects can be generated by laser irradn. of deposited SWNTs in air, thus giving rise to an increase of the D-mode intensity for even quite low power densities (∼104 W/cm2).
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    Gebhardt, B.; Syrgiannis, Z.; Backes, C.; Graupner, R.; Hauke, F.; Hirsch, A. Carbon nanotube sidewall functionalization with carbonyl compounds—modified Birch conditions vs the organometallic reduction approach J. Am. Chem. Soc. 2011, 133, 7985– 7995 DOI: 10.1021/ja2016872
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    26
    Carbon Nanotube Sidewall Functionalization with Carbonyl Compounds-Modified Birch Conditions vs the Organometallic Reduction Approach
    Gebhardt, Benjamin; Syrgiannis, Zois; Backes, Claudia; Graupner, Ralf; Hauke, Frank; Hirsch, Andreas
    Journal of the American Chemical Society (2011), 133 (20), 7985-7995CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Covalent addn. reactions turned out to be one of the most important functionalization techniques for a structural alteration of single walled carbon nanotube (SWCNT) scaffolds. During the last years, several reaction sequences based on an electrophilic interception of intermediately generated SWCNTn- carbanions, obtained via Birch redn. or by a nucleophilic addn. of organometallic species, were developed. Nevertheless, the scope and the variety of potential electrophiles is limited due to the harsh reaction conditions requested for a covalent attachment of the functional entities onto the SWCNT framework. Herein, the authors present a significant modification of the reductive alkylation/arylation sequence, the so-called Billups reaction, which extends the portfolio of electrophiles for covalent sidewall functionalization to carbonyl compds.-ketones, esters, and even carboxylic acid chlorides. Also, these carbonyl-based electrophiles can also be used as secondary functionalization reagents for anionic SWCNT intermediates, derived from a primary nucleophilic addn. step. This directly gives mixed functional SWCNT architectures, equipped with hydroxyl or carbonyl anchor groups, suitable for ongoing derivatization reactions. A correlated absorption and emission spectroscopic study elucidates the influence of the covalent sidewall functionalization degree onto the excitonic transition features of carbon nanotubes. The characterization of the different SWCNT adducts was carried out by Raman, UV-visible/nIR, and fluorescence spectroscopy as well as by TGA combined with mass spectrometry and XPS anal.
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    Arepalli, S.; Freiman, S. W.; Hooker, S. A.; Migler, K. D. Measurement issues in single-wall carbon nanotubes; National Institute of Standards and Technology Special Publication (NIST-SP), 2008, 960. 19.
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    Chen, F.; Xue, Y.; Hadjiev, V. G.; Chu, C.; Nikolaev, P.; Arepalli, S. Fast characterization of magnetic impurities in single-walled carbon nanotubes Appl. Phys. Lett. 2003, 83, 4601– 4603 DOI: 10.1063/1.1630854
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    O’Connell, M. J.; Eibergen, E. E.; Doorn, S. K. Chiral selectivity in the charge-transfer bleaching of single-walled carbon-nanotube spectra Nat. Mater. 2005, 4, 412– 418 DOI: 10.1038/nmat1367
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    Chiral selectivity in the charge-transfer bleaching of single-walled carbon-nanotube spectra
    O'Connell, Michael J.; Eibergen, Ezra E.; Doorn, Stephen K.
    Nature Materials (2005), 4 (5), 412-418CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Chiral selective reactivity and redox chem. of carbon nanotubes are two emerging fields of nanoscience. These areas hold strong promise for producing methods for isolating nanotubes into pure samples of a single electronic type, and for reversible doping of nanotubes for electronics applications. Here, the authors study the selective reactivity of single-walled carbon nanotubes with org. acceptor mols. The authors observe spectral bleaching of the nanotube electronic transitions consistent with an electron-transfer reaction occurring from the nanotubes to the org. acceptors. The reaction kinetics have a strong chiral dependence, with rates being slowest for large-bandgap species and increasing for smaller-bandgap nanotubes. The chiral-dependent kinetics can be tuned to effectively freeze the reacted spectra at a fixed chiral distribution. Such tunable redox chem. may be important for future applications in reversible noncovalent modification of nanotube electronic properties and in chiral selective sepns.
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    Clancy, A. J.; Melbourne, J.; Shaffer, M. S. P. A one-step route to solubilised, purified or functionalised single-walled carbon nanotubes J. Mater. Chem. A 2015, 3, 16708– 16715 DOI: 10.1039/C5TA03561A
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    30
    A one-step route to solubilized, purified or functionalized single-walled carbon nanotubes
    Clancy, A. J.; Melbourne, J.; Shaffer, M. S. P.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (32), 16708-16715CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Reductive dissoln. is a promising processing route for single walled carbon nanotubes (SWCNTs) that avoids the damage caused by ultrasonication and aggressive oxidn. while simultaneously allowing access to a wealth of SWCNT functionalization reactions. Here, reductive dissoln. has been simplified to a single one-pot reaction through the use of sodium naphthalide in dimethylacetamide allowing direct synthesis of SWCNT Na+ solns. Gram quantities of SWCNTs can be dissolved at concns. over 2 mg mL-1. These reduced SWCNT solns. can easily be functionalized through the addn. of alkyl halides; reducing steric bulk of the grafting moiety and increasing polarizability of the leaving group increases the extent of functionalization. An optimized abs. sodium concn. of 25 mM is shown to be more important than carbon to metal ratio in detg. the max. degree of functionalization. This novel dissoln. system can be modified for use as a non-destructive purifn. route for raw SWCNT powder by adjusting the degree of charging to dissolve carbonaceous impurities, catalyst particles and defective material, before processing the remaining SWCNTs.
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    Dresselhaus, M. S.; Jorio, A.; Souza Filho, A. G.; Saito, R. Defect characterization in graphene and carbon nanotubes using Raman spectroscopy Philos. Trans. R. Soc., A 2010, 368, 5355– 5377 DOI: 10.1098/rsta.2010.0213
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    Defect characterization in graphene and carbon nanotubes using Raman spectroscopy
    Dresselhaus, M. S.; Jorio, A.; Souza Filho, A. G.; Saito, R.
    Philosophical Transactions of the Royal Society, A: Mathematical, Physical & Engineering Sciences (2010), 368 (1932), 5355-5377CODEN: PTRMAD; ISSN:1364-503X. (Royal Society)
    This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic expts. with microscopy studies and from the development of new theor. models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amts. of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, resp. Raman spectroscopy is used to study the 'at. structure' of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be obsd. by near-field optical measurements on the G' feature for doped single-walled carbon nanotubes.
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    Maultzsch, J.; Telg, H.; Reich, S.; Thomsen, C. Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 205438 DOI: 10.1103/PhysRevB.72.205438
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    Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment
    Maultzsch, J.; Telg, H.; Reich, S.; Thomsen, C.
    Physical Review B: Condensed Matter and Materials Physics (2005), 72 (20), 205438/1-205438/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The authors present a comprehensive study of the chiral-index assignment of C nanotubes in aq. suspensions by resonant Raman scattering of the radial breathing mode. The authors det. the energies of the 1st optical transition in metallic tubes and of the 2nd optical transition in semiconducting tubes for >50 chiral indexes. The assignment is unique and does not depend on empirical parameters. The systematics of the so-called branches in the Kataura plot are discussed; many properties of the tubes are similar for members of the same branch. The radial breathing modes obsd. in a single Raman spectrum can be easily assigned based on these systematics. Empirical fits provide the energies and radial breathing modes for all metallic and semiconducting nanotubes with diams. between 0.6 and 1.5 nm. The authors discuss the relation between the frequency of the radial breathing mode and tube diam. Finally, from the Raman intensities the authors obtain information on the electron-phonon coupling.
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    Catheline, A.; Vallés, C.; Drummond, C.; Ortolani, L.; Morandi, V.; Marcaccio, M.; Iurlo, M.; Paolucci, F.; Pénicaud, A. Graphene solutions Chem. Commun. 2011, 47, 5470– 5472 DOI: 10.1039/c1cc11100k
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    Graphene solutions
    Catheline, Amelie; Valles, Cristina; Drummond, Carlos; Ortolani, Luca; Morandi, Vittorio; Marcaccio, Massimo; Iurlo, Matteo; Paolucci, Francesco; Penicaud, Alain
    Chemical Communications (Cambridge, United Kingdom) (2011), 47 (19), 5470-5472CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Thermodn. drive the spontaneous dissoln. of a graphite intercalation compd. (GIC) KC8 in NMP to form stable solns. Redn. potential of graphene is measured at +22 mV vs. SCE. Single layer graphene flakes (∼1 μm2) were unambiguously identified by electron diffraction.
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    Pénicaud, A.; Valat, L.; Derré, A.; Poulin, P.; Zakri, C.; Roubeau, O.; Maugey, M.; Miaudet, P.; Anglaret, E.; Petit, P. Mild dissolution of carbon nanotubes: composite carbon nanotube fibres from polyelectrolyte solutions Compos. Sci. Technol. 2007, 67, 795– 797 DOI: 10.1016/j.compscitech.2005.12.032
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    Mild dissolution of carbon nanotubes: composite carbon nanotube fibres from polyelectrolyte solutions
    Penicaud, A.; Valat, L.; Derre, A.; Poulin, P.; Zakri, C.; Roubeau, O.; Maugey, M.; Miaudet, P.; Anglaret, E.; Petit, P.; Loiseau, A.; Enouz, S.
    Composites Science and Technology (2007), 67 (5), 795-797CODEN: CSTCEH; ISSN:0266-3538. (Elsevier B.V.)
    Polyelectrolyte carbon nanotube solns. are spontaneously obtained upon the dissoln. of alkali metal nanotube salts in polar org. solvents. Air exposure followed by hydrochloric acid treatment restores the neutral pristine state of the nanotubes. Spinning of the polyelectrolyte nanotube solns. into polyvinyl alc. solns. produces composite carbon nanotube/polyvinyl alc. fibers, the properties of which are compared to those of fibers obtained from sodium dodecyl sulfate aq. dispersions.
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    Hodge, S. A.; Buckley, D. J.; Yau, H. C.; Skipper, N. T.; Howard, C. A.; Shaffer, M. S. P. Chemical routes to discharging graphenides Nanoscale 2017, 9, 3150– 3158 DOI: 10.1039/C6NR10004J
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    Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. Purification and characterization of single-wall carbon nanotubes (SWNTs) obtained from the gas-phase decomposition of CO (HiPco Process) J. Phys. Chem. B 2001, 105, 8297– 8301 DOI: 10.1021/jp0114891
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    36
    Purification and Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process)
    Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H.
    Journal of Physical Chemistry B (2001), 105 (35), 8297-8301CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    A purifn. method is given for extg. the Fe metal catalyst and non-SWNT carbon from nanotubes produced by the HiPco process. A multistage purifn. method has been investigated. Sample purity is documented by ESEM, TEM, TGA, Raman and UV-vis-near-IR spectroscopy. Metal catalyzed oxidn. at low temp. has been shown to selectively remove non-SWNT carbon and permit extn. of iron with concd. HCl. Prolonged catalyzed oxidn. has been found to preferentially remove smaller diam. tubes. The onset of oxidn. of purified smaller diam. HiPco SWNTs is also found to be approx. 100 °C lower than for purified larger diam. tubes produced in the laser-oven process.
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    Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Structure-assigned optical spectra of single-walled carbon nanotubes Science 2002, 298, 2361– 2366 DOI: 10.1126/science.1078727
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    Structure-Assigned Optical Spectra of Single-Walled Carbon Nanotubes
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    Science (Washington, DC, United States) (2002), 298 (5602), 2361-2366CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
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    The authors report G-band resonance Raman spectra of single-wall C nanotubes (SWNTs) at the single-nanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diams. dt ranging between 0.95 nm and 2.62 nm, the authors have conclusively detd. the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diam. dependent (ωG+=1591 cm-1), while the lower-frequency peak is given by ωG-=ωG+-C/dt2, with C being different for metallic and semiconducting SWNTs (CM>CS). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1<IωG-/IωG+<0.3 for most of the isolated SWNTs (∼90%). Unusually high or low IωG-/IωG+ ratios are obsd. for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E44S interband transition and scattered photons are in resonance with E33S. Since the Eii values depend sensitively on both nanotube diam. and chirality, the (n,m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theor. predictions and exptl. observations about these special G-band phenomena gives addnl. support for the (n,m) assignment from resonance Raman spectroscopy.
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    The sources of broad backgrounds in visible-near-IR absorption spectra of single-walled C nanotube (SWCNT) dispersions are studied through controlled expts. Chem. functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chem. changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with av. lengths between 120 and 650 nm. Broad background absorption from amorphous C is obsd. and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples contg. many SWCNT structural species. A spectral modeling method is described for sepg. the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coeff. are quant. assessed. The results obtained here suggest procedures for prepg. SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.
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  • Abstract

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    Scheme 1

    Scheme 1. Formation of SWCNT Nanotubide, NaCx, Following Reductive Treatment with Solvated Electrons from Sodium–Ammonia Solutions, Followed by Dissolution in a Dry Polar Aprotic Solvent (DMF)
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    Figure 1

    Figure 1. Nanotubide salt dissolution yields in the solvent DMF, as a function of charge added to the as-produced powder for different characterization techniques. Black squares represent measurements based on the absorbance at 660 nm. Red circles represent the mass measurements via membrane filtration. Blue triangles represent mass measurements from TGA after oxidative decomposition. The solid lines shown are a guide to the eye.

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    Figure 2

    Figure 2. (a) Dissolved fraction Raman spectra with varying charge stoichiometry using a 532 nm laser, showing the RBM region (100–350 cm–1), normalized to the largest feature. (b) The D-mode disorder peak (∼1350 cm–1) and G-mode graphitic peaks (∼1510–1580 cm–1) for each dissolved fraction, normalized to the intensity of the G+-mode. (c) The IG+/ID ratio as a function of charge stoichiometry showing increasing purity with increasing charging. (d) SEM of the starting HiPco SWCNT material. (e) SEM of the dissolved (NaC100)DIS nanotubide, with few visible SWCNT bundles. (f) SEM of the undissolved (NaC100)UND nanotubide showing the change in morphology from the starting material.

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    Figure 3

    Figure 3. UV–vis–NIR spectra with varying NaCx ratios showing preferential dissolution of metallic SWNTs at low charge stoichiometries. Top: semiconducting SWCNT S11 transitions. Middle: sc-SWCNT S22 transitions. Bottom: m-SWCNT M11 transitions. All spectra shown are baseline corrected and normalized to the largest peak intensity. The NaC100 shows the biggest change, with a decrease for peaks in the S11 region and increase in the M11 region, showing metallic enrichment for this charge ratio.

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    Figure 4

    Figure 4. UV–vis–NIR relative percentage change of the (NaC100)DIS dissolved fractions relative to the starting material of assigned m-SWCNTs (black) and sc-SWCNTs, (red) showing the preferential dissolution of larger diameter SWCNTs. The solid lines are linear fits of the data as a guide to the eye.

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    Figure 5

    Figure 5. PL relative percentage change of the dissolved fractions for the eight most abundant SWCNTs in the HiPco raw batch R2-172, separated into four diameter regimes. The NaC100 dissolved shows an enhancement of the larger diameter and a decrease of the smaller diameter sc-SWCNTs. Further charge moves the diameter distribution back toward that of the starting material.

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    Figure 6

    Figure 6. Mean SWCNT diameter and standard error of the various dissolved fractions as a function of charge, showing that the average diameter increases from that of the raw material as the charge level is decreased. The black dotted line represents the mean of the raw SWCNT sample population, and the red line is a guide for the eye.

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    Figure 7

    Figure 7. (a) Combined electronic density of states (1D e-DOS) for the HiPco R2–172 batch, derived empirically from PL and UV–vis–NIR data, separated into three components (small-diameter sc-SWCNT, large-diameter sc-SWCNT, and m-SWCNT). (b) Integrated DOS to give the proportion of electronic states per carbon for the individual components (small sc-SWCNT, large sc-SWCNT, and m-SWCNT) for a given stoichiometry of the total HiPco system.

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  • References

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      Strategies for forming liq. dispersions of nanomaterials typically focus on retarding reaggregation, for example via surface modification, as opposed to promoting the thermodynamically driven dissoln. common for mol.-sized species. Here we demonstrate the true dissoln. of a wide range of important 2D nanomaterials by forming layered material salts that spontaneously dissolve in polar solvents yielding ionic solns. The benign dissoln. advantageously maintains the morphol. of the starting material, is stable against reaggregation and can achieve solns. contg. exclusively individualized monolayers. Importantly, the charge on the anionic nanosheet solutes is reversible, enables targeted deposition over large areas via electroplating and can initiate novel self-assembly upon drying. Our findings thus reveal a unique soln.-like behavior for 2D materials that enables their scalable prodn. and controlled manipulation.
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      Hodge, S. A.; Fogden, S.; Howard, C. A.; Skipper, N. T.; Shaffer, M. S. P. Electrochemical processing of discrete single-walled carbon nanotube anions ACS Nano 2013, 7, 1769– 1778 DOI: 10.1021/nn305919p
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      Jiang, C.; Saha, A.; Marti, A. A. Carbon nanotubides: an alternative for dispersion, functionalization and composites fabrication Nanoscale 2015, 7, 15037– 15045 DOI: 10.1039/C5NR03504J
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      Gebhardt, J.; Bosch, S.; Hof, F.; Hauke, F.; Hirsch, A.; Görling, A. Selective reduction of SWCNTs—Concepts and insights J. Mater. Chem. C 2017, 5, 3937– 3947 DOI: 10.1039/C5TC01407G
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      18
      Selective reduction of SWCNTs - concepts and insights
      Gebhardt, Julian; Bosch, Sebastian; Hof, Ferdinand; Hauke, Frank; Hirsch, Andreas; Goerling, Andreas
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (16), 3937-3947CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      The charging of single-walled carbon nanotube (SWCNT) mixts. by redn. via alkali metal atoms is an established first step towards covalent SWCNT functionalization. In this combined d.-functional theory and exptl. study, we investigate this redn. with respect to differences occurring between tubes of different electronic type (metallic (m) and semiconducting (s.c.) tubes, resp.). We find that metals, specifically potassium, adsorb stronger to m- than s.c.-SWCNTs, which can be explained by the different band structures of both tube types. We investigate this trend in detail for a variety of different chiral SWCNTs, finding a potassium coverage dependent preference of m- over s.c.-SWCNTs, which is predicted to allow for a selective charging of metallic tubes for K/C ratios ≤ 1/200. This selective charging can be translated into the enrichment of m-SWCNTs during dispersion of SWCNT mixts., since only reduced tubes are dissolved from the bulk material. The results for isolated tubes can be generalized to SWCNT bundle arrangements, which means that the theor. predicted selective charging is transferable also to this more realistic description of the exptl. systems. The theor. findings regarding an electronic type selective charging of SWCNTs have been verified by an exptl. study. By a combination of Raman and absorption/emission spectroscopic anal., a preferential dispersion of charged metallic carbon nanotubes in THF as solvent was found for the predicted low potassium concns. Our results lead to the conclusion that previous m/s.c. selective reductive functionalization reactions cannot be explained on the basis of an electronic type selective charging step, as these reactions used much higher alkali metal concns.
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      Wunderlich, D.; Hauke, F.; Hirsch, A. Preferred functionalization of metallic and small-diameter single walled carbon nanotubes via reductive alkylation J. Mater. Chem. 2008, 18, 1493– 1497 DOI: 10.1039/b716732f
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      19
      Preferred functionalization of metallic and small-diameter single walled carbon nanotubes via reductive alkylation
      Wunderlich, D.; Hauke, F.; Hirsch, A.
      Journal of Materials Chemistry (2008), 18 (13), 1493-1497CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      A selectivity investigation of the covalent sidewall functionalization of SWNTs by reductive alkylation (Billups reaction) is reported. The functionalized tubes Rn-SWNT were characterized by UV-Vis-NIR absorption, NIR emission and Raman spectroscopy. The redn. of SWNTs with sodium and the subsequent alkylation of the reduced tubes SWNTn- with Bu iodide reveal a pronounced SWCNT diam. dependence, i.e., SWNTs with smaller diam. are considerably more reactive than tubes with larger diam. Moreover, this reaction sequence also favors the preferred functionalization of metallic over semiconducting tubes. Treatment of the reduced intermediates SWNTn- with protons, via the use of ethanol, instead of alkyl iodide leads to hydrogenated tubes. However, in this case the degree of functionalization is considerably lower than that obsd. for the corresponding alkylation. Also no pronounced preference of the reaction of metallic and small diam. tubes was obsd. during the hydrogenation process.
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      Voiry, D.; Drummond, C.; Pénicaud, A. Portrait of carbon nanotube salts as soluble polyelectrolytes Soft Matter 2011, 7, 7998– 8001 DOI: 10.1039/c1sm05959a
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      O’Connell, M. J.; Sivaram, S.; Doorn, S. K. Near-infrared resonance Raman excitation profile studies of single-walled carbon nanotube intertube interactions: A direct comparison of bundled and individually dispersed HiPco nanotubes Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 235415 DOI: 10.1103/PhysRevB.69.235415
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      21
      Near-infrared resonance Raman excitation profile studies of single-walled carbon nanotube intertube interactions: A direct comparison of bundled and individually dispersed HiPco nanotubes
      O'Connell, Michael J.; Sivaram, Saujan; Doorn, Stephen K.
      Physical Review B: Condensed Matter and Materials Physics (2004), 69 (23), 235415/1-235415/15CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Complete Raman excitation profiles for single-walled C nanotube radial breathing modes were obtained for bundled HiPco C nanotube samples at 700-985 nm excitation. Results are compared to similar profiles generated from individual C nanotubes dispersed in aq. soln., allowing a direct detn. of intertube interaction effects on electronic properties for 12 specific semiconducting nanotube chiralities. Red shifts in the excitation profiles (ranging from 54 to 157 meV) are obsd. on going from isolated individual to bundled nanotubes. Addnl., bundling is found to broaden the electronic transitions by an av. factor of 2.4 compared to individualized nanotube bandwidths. These results compare well with recent theor. predictions for bundling effects. A study of bundling effects on radial breathing mode frequencies for 17 different nanotube chiralities finds no evidence for significant perturbation of these frequencies resulting from intertube interactions. Results demonstrate that previously reported radial breathing mode frequency shifts are apparent shifts only, resulting from red shifting of the resonant electronic transitions for bundled nanotubes. Bundle inhomogeneity, packing efficiency, orientational disorder, and symmetry redn. are indicated as important factors in detg. the degree of intertube interaction.
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    22. 22
      Araujo, P. T.; Fantini, C.; Lucchese, M. M.; Dresselhaus, M. S.; Jorio, A. The effect of environment on the radial breathing mode of supergrowth single wall carbon nanotubes Appl. Phys. Lett. 2009, 95, 261902 DOI: 10.1063/1.3276909
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      Filho, A. G. S.; Jorio, A.; Samsonidze, G. G.; Dresselhaus, G.; Saito, R.; Dresselhaus, M. S. Raman spectroscopy for probing chemically/physically induced phenomena in carbon nanotubes Nanotechnology 2003, 14, 1130 DOI: 10.1088/0957-4484/14/10/311
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      Müller, M.; Maultzsch, J.; Wunderlich, D.; Hirsch, A.; Thomsen, C. Raman spectroscopy on chemically functionalized carbon nanotubes Phys. Status Solidi B 2007, 244, 4056– 4059 DOI: 10.1002/pssb.200776119
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      Hennrich, F.; Krupke, R.; Lebedkin, S.; Arnold, K.; Fischer, R.; Resasco, D. E.; Kappes, M. M. Raman spectroscopy of individual single-walled carbon nanotubes from various sources J. Phys. Chem. B 2005, 109, 10567– 10573 DOI: 10.1021/jp0441745
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      Raman Spectroscopy of Individual Single-Walled Carbon Nanotubes from Various Sources
      Hennrich, Frank; Krupke, Ralph; Lebedkin, Sergei; Arnold, Katharina; Fischer, Regina; Resasco, Daniel E.; Kappes, Manfred M.
      Journal of Physical Chemistry B (2005), 109 (21), 10567-10573CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Resonance Raman spectroscopy/microscopy was used to study individualized single-walled carbon nanotubes (SWNTs) both in aq. suspensions as well as after spin-coating onto Si/SiO2 surfaces. Four different SWNT materials contg. nanotubes with diams. ranging from 0.7 to 1.6 nm were used. Comparison with Raman data obtained for suspensions shows that the surface does not dramatically affect the electronic properties of the deposited tubes. Raman features obsd. for deposited SWNTs are similar to what was measured for nanotubes directly fabricated on surfaces using chem. vapor deposition (CVD) methods. In particular, individual semiconducting tubes could be distinguished from metallic tubes by their different G-mode line shapes. It could also be shown that the high-power, short-time sonication used to generate individualized SWNT suspensions does not induce defects in great quantities. However, (addnl.) defects can be generated by laser irradn. of deposited SWNTs in air, thus giving rise to an increase of the D-mode intensity for even quite low power densities (∼104 W/cm2).
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      Gebhardt, B.; Syrgiannis, Z.; Backes, C.; Graupner, R.; Hauke, F.; Hirsch, A. Carbon nanotube sidewall functionalization with carbonyl compounds—modified Birch conditions vs the organometallic reduction approach J. Am. Chem. Soc. 2011, 133, 7985– 7995 DOI: 10.1021/ja2016872
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      26
      Carbon Nanotube Sidewall Functionalization with Carbonyl Compounds-Modified Birch Conditions vs the Organometallic Reduction Approach
      Gebhardt, Benjamin; Syrgiannis, Zois; Backes, Claudia; Graupner, Ralf; Hauke, Frank; Hirsch, Andreas
      Journal of the American Chemical Society (2011), 133 (20), 7985-7995CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Covalent addn. reactions turned out to be one of the most important functionalization techniques for a structural alteration of single walled carbon nanotube (SWCNT) scaffolds. During the last years, several reaction sequences based on an electrophilic interception of intermediately generated SWCNTn- carbanions, obtained via Birch redn. or by a nucleophilic addn. of organometallic species, were developed. Nevertheless, the scope and the variety of potential electrophiles is limited due to the harsh reaction conditions requested for a covalent attachment of the functional entities onto the SWCNT framework. Herein, the authors present a significant modification of the reductive alkylation/arylation sequence, the so-called Billups reaction, which extends the portfolio of electrophiles for covalent sidewall functionalization to carbonyl compds.-ketones, esters, and even carboxylic acid chlorides. Also, these carbonyl-based electrophiles can also be used as secondary functionalization reagents for anionic SWCNT intermediates, derived from a primary nucleophilic addn. step. This directly gives mixed functional SWCNT architectures, equipped with hydroxyl or carbonyl anchor groups, suitable for ongoing derivatization reactions. A correlated absorption and emission spectroscopic study elucidates the influence of the covalent sidewall functionalization degree onto the excitonic transition features of carbon nanotubes. The characterization of the different SWCNT adducts was carried out by Raman, UV-visible/nIR, and fluorescence spectroscopy as well as by TGA combined with mass spectrometry and XPS anal.
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      Arepalli, S.; Freiman, S. W.; Hooker, S. A.; Migler, K. D. Measurement issues in single-wall carbon nanotubes; National Institute of Standards and Technology Special Publication (NIST-SP), 2008, 960. 19.
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      Chen, F.; Xue, Y.; Hadjiev, V. G.; Chu, C.; Nikolaev, P.; Arepalli, S. Fast characterization of magnetic impurities in single-walled carbon nanotubes Appl. Phys. Lett. 2003, 83, 4601– 4603 DOI: 10.1063/1.1630854
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      O’Connell, M. J.; Eibergen, E. E.; Doorn, S. K. Chiral selectivity in the charge-transfer bleaching of single-walled carbon-nanotube spectra Nat. Mater. 2005, 4, 412– 418 DOI: 10.1038/nmat1367
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      Chiral selectivity in the charge-transfer bleaching of single-walled carbon-nanotube spectra
      O'Connell, Michael J.; Eibergen, Ezra E.; Doorn, Stephen K.
      Nature Materials (2005), 4 (5), 412-418CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Chiral selective reactivity and redox chem. of carbon nanotubes are two emerging fields of nanoscience. These areas hold strong promise for producing methods for isolating nanotubes into pure samples of a single electronic type, and for reversible doping of nanotubes for electronics applications. Here, the authors study the selective reactivity of single-walled carbon nanotubes with org. acceptor mols. The authors observe spectral bleaching of the nanotube electronic transitions consistent with an electron-transfer reaction occurring from the nanotubes to the org. acceptors. The reaction kinetics have a strong chiral dependence, with rates being slowest for large-bandgap species and increasing for smaller-bandgap nanotubes. The chiral-dependent kinetics can be tuned to effectively freeze the reacted spectra at a fixed chiral distribution. Such tunable redox chem. may be important for future applications in reversible noncovalent modification of nanotube electronic properties and in chiral selective sepns.
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      Clancy, A. J.; Melbourne, J.; Shaffer, M. S. P. A one-step route to solubilised, purified or functionalised single-walled carbon nanotubes J. Mater. Chem. A 2015, 3, 16708– 16715 DOI: 10.1039/C5TA03561A
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      30
      A one-step route to solubilized, purified or functionalized single-walled carbon nanotubes
      Clancy, A. J.; Melbourne, J.; Shaffer, M. S. P.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (32), 16708-16715CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Reductive dissoln. is a promising processing route for single walled carbon nanotubes (SWCNTs) that avoids the damage caused by ultrasonication and aggressive oxidn. while simultaneously allowing access to a wealth of SWCNT functionalization reactions. Here, reductive dissoln. has been simplified to a single one-pot reaction through the use of sodium naphthalide in dimethylacetamide allowing direct synthesis of SWCNT Na+ solns. Gram quantities of SWCNTs can be dissolved at concns. over 2 mg mL-1. These reduced SWCNT solns. can easily be functionalized through the addn. of alkyl halides; reducing steric bulk of the grafting moiety and increasing polarizability of the leaving group increases the extent of functionalization. An optimized abs. sodium concn. of 25 mM is shown to be more important than carbon to metal ratio in detg. the max. degree of functionalization. This novel dissoln. system can be modified for use as a non-destructive purifn. route for raw SWCNT powder by adjusting the degree of charging to dissolve carbonaceous impurities, catalyst particles and defective material, before processing the remaining SWCNTs.
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      Dresselhaus, M. S.; Jorio, A.; Souza Filho, A. G.; Saito, R. Defect characterization in graphene and carbon nanotubes using Raman spectroscopy Philos. Trans. R. Soc., A 2010, 368, 5355– 5377 DOI: 10.1098/rsta.2010.0213
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      Defect characterization in graphene and carbon nanotubes using Raman spectroscopy
      Dresselhaus, M. S.; Jorio, A.; Souza Filho, A. G.; Saito, R.
      Philosophical Transactions of the Royal Society, A: Mathematical, Physical & Engineering Sciences (2010), 368 (1932), 5355-5377CODEN: PTRMAD; ISSN:1364-503X. (Royal Society)
      This review discusses advances that have been made in the study of defect-induced double-resonance processes in nanographite, graphene and carbon nanotubes, mostly coming from combining Raman spectroscopic expts. with microscopy studies and from the development of new theor. models. The disorder-induced peak frequencies and intensities are discussed, with particular emphasis given to how the disorder-induced features evolve with increasing amts. of disorder. We address here two systems, ion-bombarded graphene and nanographite, where disorder is represented by point defects and boundaries, resp. Raman spectroscopy is used to study the 'at. structure' of the defect, making it possible, for example, to distinguish between zigzag and armchair edges, based on selection rules of phonon scattering. Finally, a different concept is discussed, involving the effect that defects have on the lineshape of Raman-allowed peaks, owing to local electron and phonon energy renormalization. Such effects can be obsd. by near-field optical measurements on the G' feature for doped single-walled carbon nanotubes.
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      Maultzsch, J.; Telg, H.; Reich, S.; Thomsen, C. Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 205438 DOI: 10.1103/PhysRevB.72.205438
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      Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment
      Maultzsch, J.; Telg, H.; Reich, S.; Thomsen, C.
      Physical Review B: Condensed Matter and Materials Physics (2005), 72 (20), 205438/1-205438/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The authors present a comprehensive study of the chiral-index assignment of C nanotubes in aq. suspensions by resonant Raman scattering of the radial breathing mode. The authors det. the energies of the 1st optical transition in metallic tubes and of the 2nd optical transition in semiconducting tubes for >50 chiral indexes. The assignment is unique and does not depend on empirical parameters. The systematics of the so-called branches in the Kataura plot are discussed; many properties of the tubes are similar for members of the same branch. The radial breathing modes obsd. in a single Raman spectrum can be easily assigned based on these systematics. Empirical fits provide the energies and radial breathing modes for all metallic and semiconducting nanotubes with diams. between 0.6 and 1.5 nm. The authors discuss the relation between the frequency of the radial breathing mode and tube diam. Finally, from the Raman intensities the authors obtain information on the electron-phonon coupling.
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      Catheline, A.; Vallés, C.; Drummond, C.; Ortolani, L.; Morandi, V.; Marcaccio, M.; Iurlo, M.; Paolucci, F.; Pénicaud, A. Graphene solutions Chem. Commun. 2011, 47, 5470– 5472 DOI: 10.1039/c1cc11100k
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      Graphene solutions
      Catheline, Amelie; Valles, Cristina; Drummond, Carlos; Ortolani, Luca; Morandi, Vittorio; Marcaccio, Massimo; Iurlo, Matteo; Paolucci, Francesco; Penicaud, Alain
      Chemical Communications (Cambridge, United Kingdom) (2011), 47 (19), 5470-5472CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Thermodn. drive the spontaneous dissoln. of a graphite intercalation compd. (GIC) KC8 in NMP to form stable solns. Redn. potential of graphene is measured at +22 mV vs. SCE. Single layer graphene flakes (∼1 μm2) were unambiguously identified by electron diffraction.
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      Pénicaud, A.; Valat, L.; Derré, A.; Poulin, P.; Zakri, C.; Roubeau, O.; Maugey, M.; Miaudet, P.; Anglaret, E.; Petit, P. Mild dissolution of carbon nanotubes: composite carbon nanotube fibres from polyelectrolyte solutions Compos. Sci. Technol. 2007, 67, 795– 797 DOI: 10.1016/j.compscitech.2005.12.032
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      Mild dissolution of carbon nanotubes: composite carbon nanotube fibres from polyelectrolyte solutions
      Penicaud, A.; Valat, L.; Derre, A.; Poulin, P.; Zakri, C.; Roubeau, O.; Maugey, M.; Miaudet, P.; Anglaret, E.; Petit, P.; Loiseau, A.; Enouz, S.
      Composites Science and Technology (2007), 67 (5), 795-797CODEN: CSTCEH; ISSN:0266-3538. (Elsevier B.V.)
      Polyelectrolyte carbon nanotube solns. are spontaneously obtained upon the dissoln. of alkali metal nanotube salts in polar org. solvents. Air exposure followed by hydrochloric acid treatment restores the neutral pristine state of the nanotubes. Spinning of the polyelectrolyte nanotube solns. into polyvinyl alc. solns. produces composite carbon nanotube/polyvinyl alc. fibers, the properties of which are compared to those of fibers obtained from sodium dodecyl sulfate aq. dispersions.
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      Hodge, S. A.; Buckley, D. J.; Yau, H. C.; Skipper, N. T.; Howard, C. A.; Shaffer, M. S. P. Chemical routes to discharging graphenides Nanoscale 2017, 9, 3150– 3158 DOI: 10.1039/C6NR10004J
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      Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H. Purification and characterization of single-wall carbon nanotubes (SWNTs) obtained from the gas-phase decomposition of CO (HiPco Process) J. Phys. Chem. B 2001, 105, 8297– 8301 DOI: 10.1021/jp0114891
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      36
      Purification and Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process)
      Chiang, I. W.; Brinson, B. E.; Huang, A. Y.; Willis, P. A.; Bronikowski, M. J.; Margrave, J. L.; Smalley, R. E.; Hauge, R. H.
      Journal of Physical Chemistry B (2001), 105 (35), 8297-8301CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      A purifn. method is given for extg. the Fe metal catalyst and non-SWNT carbon from nanotubes produced by the HiPco process. A multistage purifn. method has been investigated. Sample purity is documented by ESEM, TEM, TGA, Raman and UV-vis-near-IR spectroscopy. Metal catalyzed oxidn. at low temp. has been shown to selectively remove non-SWNT carbon and permit extn. of iron with concd. HCl. Prolonged catalyzed oxidn. has been found to preferentially remove smaller diam. tubes. The onset of oxidn. of purified smaller diam. HiPco SWNTs is also found to be approx. 100 °C lower than for purified larger diam. tubes produced in the laser-oven process.
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      Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Structure-assigned optical spectra of single-walled carbon nanotubes Science 2002, 298, 2361– 2366 DOI: 10.1126/science.1078727
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      Structure-Assigned Optical Spectra of Single-Walled Carbon Nanotubes
      Bachilo, Sergei M.; Strano, Michael S.; Kittrell, Carter; Hauge, Robert H.; Smalley, Richard E.; Weisman, R. Bruce
      Science (Washington, DC, United States) (2002), 298 (5602), 2361-2366CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Spectrofluorometric measurements on single-walled carbon nanotubes (SWNTs) isolated in aq. surfactant suspensions have revealed distinct electronic absorption and emission transitions for more than 30 different semiconducting nanotube species. By combining these fluorimetric results with resonance Raman data, each optical transition has been mapped to a specific (n,m) nanotube structure. Optical spectroscopy can thereby be used to rapidly det. the detailed compn. of bulk SWNT samples, providing distributions in both tube diam. and chiral angle. The measured transition frequencies differ substantially from simple theor. predictions. These deviations may reflect combinations of trigonal warping and excitonic effects.
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    38. 38
      Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Ünlü, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R. G-band resonant Raman study of 62 isolated single-wall carbon nanotubes Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 155412 DOI: 10.1103/PhysRevB.65.155412
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      G-band resonant Raman study of 62 isolated single-wall carbon nanotubes
      Jorio, A.; Souza Filho, A. G.; Dresselhaus, G.; Dresselhaus, M. S.; Swan, A. K.; Unlu, M. S.; Goldberg, B. B.; Pimenta, M. A.; Hafner, J. H.; Lieber, C. M.; Saito, R.
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      The authors report G-band resonance Raman spectra of single-wall C nanotubes (SWNTs) at the single-nanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diams. dt ranging between 0.95 nm and 2.62 nm, the authors have conclusively detd. the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diam. dependent (ωG+=1591 cm-1), while the lower-frequency peak is given by ωG-=ωG+-C/dt2, with C being different for metallic and semiconducting SWNTs (CM>CS). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1<IωG-/IωG+<0.3 for most of the isolated SWNTs (∼90%). Unusually high or low IωG-/IωG+ ratios are obsd. for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E44S interband transition and scattered photons are in resonance with E33S. Since the Eii values depend sensitively on both nanotube diam. and chirality, the (n,m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theor. predictions and exptl. observations about these special G-band phenomena gives addnl. support for the (n,m) assignment from resonance Raman spectroscopy.
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      Fluorescence has been obsd. directly across the band gap of semiconducting carbon nanotubes. We obtained individual nanotubes, each encased in a cylindrical micelle, by ultrasonically agitating an aq. dispersion of raw single-walled carbon nanotubes in sodium dodecyl sulfate and then centrifuging to remove tube bundles, ropes, and residual catalyst. Aggregation of nanotubes into bundles otherwise quenches the fluorescence through interactions with metallic tubes and substantially broadens the absorption spectra. At pH less than 5, the absorption and emission spectra of individual nanotubes show evidence of band gap-selective protonation of the side walls of the tube. This protonation is readily reversed by treatment with base or UV light.
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      Bandgap fluorescence spectroscopy of aq., micelle-like suspensions of SWNTs gave access to the electronic energies of individual semiconducting SWNTs, while substantially lower is the success achieved in the detn. of the redox properties of SWNTs as individual entities. Here the authors report an extensive voltammetric and visible-NIR spectroelectrochem. study of true solns. of unfunctionalized SWNTs and det. the std. electrochem. potentials of redn. and oxidn. as a function of the tube diam. of a large no. of semiconducting SWNTs. The authors also establish the Fermi energy and the exciton binding energy for individual tubes in soln. The linear correlation found between the potentials and the optical transition energies is quantified in 2 simple equations that allow one to calc. the redox potentials of SWNTs that are insufficiently abundant or absent in the samples.
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      The sources of broad backgrounds in visible-near-IR absorption spectra of single-walled C nanotube (SWCNT) dispersions are studied through controlled expts. Chem. functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chem. changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with av. lengths between 120 and 650 nm. Broad background absorption from amorphous C is obsd. and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples contg. many SWCNT structural species. A spectral modeling method is described for sepg. the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coeff. are quant. assessed. The results obtained here suggest procedures for prepg. SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.
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      Deconvolution of the absorption spectrum of single-walled carbon nanotubes (SWNTs) into distinct (n,m) contributions is complicated because transition energies are closely spaced. The algorithm presented in this work attempts to simplify the problem by grouping nanotubes with similar transition energies and assigning wts. to their spectral contributions. Voigt line shapes were used to fit absorption spectra of sodium dodecyl sulfate suspended HiPco SWNT and CoMoCat SWNT. Line widths for the metallic (93.42 meV) and two semiconducting regions (57.96 and 29.86 meV) were obtained from the absorption spectra of DNA-wrapped SWNT fractionated by ion-exchange chromatog. The method is used to describe the reaction kinetics of certain HiPco SWNTs upon reaction with 4-chlorobenzene diazonium and 4-hydroxybenzene diazonium salts. The code for deconvolution was provided as open source in the Supporting Information for future modifications.
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      Spectrofluorimetric data for identified single-walled carbon nanotubes in aq. SDS suspension have been accurately fit to empirical expressions. These are used to obtain the first model-independent prediction of first and second van Hove optical transitions as a function of structure for a wide range of semiconducting nanotubes. To allow for convenient use in support of spectral studies, the results are presented in equation, graphical, and tabular forms. These empirical findings differ significantly from Kataura plots computed using a simple tight-binding model. It is suggested that the empirically based results should be used in preference to conventional model-based predictions in spectroscopic nanotube research.
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      Single-nanotube photometry was used to measure the product of absorption cross section and fluorescence quantum yield for 12 (n,m) structural species of semiconducting single-walled C nanotubes in aq. SDBS suspension. These products ranged from 1.7 to 4.5 × 10-19 cm2/C atom, generally increasing with optical band gap as described by the energy gap law. The findings suggest fluorescent quantum yields of ∼8% for the brightest, (10,2) species and introduce the empirical calibration factors needed to deduce quant. (n,m) distributions from bulk fluorometric intensities.
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      The higher lying bright exciton energies (EM11,ES33,ES44,EM22,ES55,ES66,EM33) of single-wall C nanotubes are calcd. by solving the Bethe-Salpeter equation within an extended tight binding method. For smaller diam. nanotubes, some higher Eii excitonic states are missing. In particular, some Eii's on the 1-dimensional Brillouin zone (cutting line) are no longer relevant to the formation of excitons and are skipped in listing the order of the Eii values. Thus the family patterns show some discontinuities in k space and this effect should be observable in Raman G' band spectroscopy. The higher exciton energies ES33 and ES44 have a large chirality dependence due to many body effects, since the self-energy becomes larger than the binding energy. Thus the chirality dependence of the higher Eii comes not only from a single particle energy but also from many-body effects.
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      The excitonic optical transition energies Eii of single wall C nanotubes, that are modified by surrounding materials around the tubes (known as the environmental effect), can be reproduced by defining a dielec. const. κ which depends on the subband index, nanotube diam., and exciton size. The environmental effects on excitons can be recognized on a plot of the functional form of κ simply by the different linear slopes obtained for different samples. This treatment should be very useful for calcg. Eii for any type of nanotube environment, hence providing an accurate assignment of many nanotube (n,m) chiralities. (c) 2010 American Institute of Physics.
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      Solns. of neg. charged graphene (graphenide) platelets were produced by intercalation of nanographite with liq. potassium-ammonia followed by dissoln. in THF. The structure and morphol. of these solns. were then investigated by small-angle neutron scattering. We found that >95 vol. % of the solute is present as single-layer graphene sheets. These charged sheets are flat over a length scale of >150 Å in soln. and are strongly solvated by a shell of solvent mols. Atomic force microscopy on drop-coated thin films corroborated the presence of monolayer graphene sheets. Our dissoln. method thus offers a significant increase in the monodispersity achievable in graphene solns.
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      Neg. charged graphene layers from a graphite intercalation compd. spontaneously dissolve in N-methylpyrrolidone, without the need for any sonication, yielding stable, air-sensitive, solns. of laterally extended atom-thick graphene sheets and ribbons with dimensions over tens of micrometers. These can be deposited on a variety of substrates. Height measurements showing single-atom thickness were performed by STM, AFM, multiple beam interferometry, and optical imaging on Sarfus wafers, demonstrating deposits of graphene flakes and ribbons. AFM height measurements on mica give the actual height of graphene (∼0.4 nm).
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    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06553.

    • Calculation of the extinction coefficient as a function of charge on the nanotubes (Figure S1); Raman Spectroscopy of the SEM sample shown in Figure 2e, f (Figure S2); UV–vis–NIR spectra of the dissolved SWCNT solutions after aqueous redispersion and corresponding discussion (Figure S3); photoluminescence (PL) analysis showing the opposite trends in the undissolved and dissolved SWCNTs for two different stoichiometries (Figure S4); comparison of the diameter enrichment for UV–vis–NIR and PL (Figure S5); yield of dissolution measurements for the newer batch as a function of stoichiometry (Figure S6); Raman spectroscopy comparison of the two batches used in this study (Figure S7); explanation of the calculation for Figure 7b (Figure S8) (PDF)


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