Separation of Small-Diameter Single-Walled Carbon Nanotubes in

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Separation of Small Diameter Single Walled Carbon Nanotubes in 1-3 Steps with Aqueous Two-Phase Extraction Han Li, Georgy Gordeev, Oisin Garrity, Stephanie Reich, and Benjamin S. Flavel ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09579 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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Separation of Small Diameter Single Walled Carbon Nanotubes in 1 – 3 Steps with Aqueous Two-Phase Extraction Han Li,† Georgy Gordeev,§ Oisin Garrity,§ Stephanie Reich§ and Benjamin S. Flavel†,‡ † Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany. § Department of Physics, Freie Universität Berlin, Berlin, Germany. ‡ Institute of Materials Science, Technische Universität Darmstadt, Darmstadt, Germany. KEYWORDS: chiral sorting, ATPE, SWCNT, CNT, dextran, PEG, pH. CORRESPONDING AUTHORS: [email protected] & [email protected]

ABSTRACT: An aqueous two-phase extraction (ATPE) technique capable of separating small diameter single walled carbon nanotubes in 1, 2 or at the most 3 steps is presented. Separation is performed in the well-studied two-phase system containing polyethylene glycol (PEG) and dextran but it is achieved without changing the global concentration or ratio of co-surfactants. Instead, the technique is reliant upon the different surfactant shell around each nanotube diameter at a fixed surfactant concentration. The methodology to obtain a single set of surfactant conditions is provided and strategies to optimize these for other diameter regimes are discussed. In total 11

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different chiralities in the diameter range 0.69 – 0.91 are separated. These include semiconducting and both armchair and non-armchair metallic nanotube species.

Titration of co-surfactant

suspensions reveal separation to be driven by the pH of the suspension with each (n,m) species partitioning at a fixed pH. This allows for an (n,m) separation approach to be presented that is as simple as pipetting known volumes of acid into the ATPE system.

The efficacy at which carbon nanotubes (CNTs) can be dispersed, enriched and separated according to their diameter,1 length,2 wall-number,3 electronic property,4 chirality,5 and even enantiomeric type6 has come a long way in the last two decades. Highly selective separation techniques have been developed in both aqueous and organic solvents and it is now possible to fabricate single chirality devices from single walled carbon nanotubes (SWCNTs).7-9 Despite this progress in separation, most devices consist of the same few (n,m) species and this is a result of the widespread adoption of organic polymer extraction by the community. Using this two-step technique laboratories with limited sorting background can easily obtain nanotube species such as (6,5) or (7,5) by simply combining commercially available raw materials with highly (n,m) specific polymers.10 The simplicity of polymer extraction has lead researchers to search for other polymer/nanotube combinations,11, 12 but the requirement of in-house synthesis or the development of new catalysts systems pose a barrier to most groups. Consequently, the full potential of carbon nanotubes, being a material with richly tailorable optical and electronic properties, has been prevented from being realized in devices. Alternatively, the finesse of aqueous based techniques far surpasses that of polymer extraction and affords not only a larger library of single chirality semiconducting but also metallic SWCNTs.13 Traditionally these techniques have been avoided by the device community due to their lower semiconducting content,14, 15 but the real disadvantage


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was the increased level of experimental complexity, which left the separation to the realm of experts. In light of recent work to further enrich an aqueous sorted fraction by subsequent polymer wrapping16, 17 the challenge to widespread use now appears to be in reducing the experimental complexity. As outlined by Doorn and co-workers, the goal is to develop a ‘dial in’18 method comparable to polymer extraction which remain accessible to the non-expert.

The complexity of aqueous methods stems from the use of surfactants such as sodium dodecyl sulfate (SDS), sodium cholate (SC), sodium deoxycholate (DOC), or co-surfactant mixtures thereof1,

17, 19

to disperse the raw nanotube material. Unlike selective polymers, surfactants

disperse all (n,m) species in the raw material but within these there exist small structural differences in their coating around different diameter CNTs,20 around the metallic and semiconducting subpopulations21 and to a lesser extent around (n,m) species.22 It is these differences in surfactant shell which modulate the interaction of the CNTs with a third medium for separation.23 For example, in 2013 Zheng et al.24 introduced the aqueous two-phase extraction (ATPE) process to separate CNTs and this has since proven to be a rapid, highly sensitive and scalable technique. Although other two-phase combinations have been investigated,25 ATPE is reliant upon aqueous solutions of polyethylene glycol (PEG) and dextran (DX). These polymers are miscible for concentration ratios below a so-called ‘tie-line’ and phase separate into a hydrophobic (PEG rich) top and hydrophilic (DX rich) bottom phase for all ratios above.26 Separation sensitivity is then related to the different solvation energy of these two phases for CNTs19 which can be related to their surfactant coating.18 ATPE was initially developed with SDS24 and partitioning was explained by SDS ordering around small and large diameter CNTs,24


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but joint efforts by the research teams around Fagan and Zheng have extended the approach to include co-surfactant mixtures of SDS, DOC, and SC for small27 and large diameter nanotubes.20

Experimentally ATPE relies upon the sequential removal and re-addition of CNT containing top or bottom phases to clean opposing phases. A modulation of the SDS/DOC/SC ratio is used to control the surfactant shell around the CNT and thereby the phase in which they are found. Likewise, the addition of strong oxidants, reductants,28 salts,18 or changes in temperature can influence CNT partitioning.18 By partitioning CNTs between the two phases followed by the removal of undesired species, it is possible to eventually arrive at conditions where only one (n,m) species is isolated in either the PEG or DX. As shown by Subbaiyan et al.18 the necessary differences in surfactant shell are extremely small and even a difference of only 0.003% is enough to change the experimental result. Faced with a vast number of experimental combinations it is challenging to optimize the current ATPE approach given that in some cases up to 8 sequential extraction steps20 are required obtain an (n,m) pure fraction. Complicating this issue is the fact that despite a visible phase separation, an uneven distribution is actually obtained and PEG and DX remain miscible with each other in unknown concentration.26 Likewise, within the two phases an unknown distribution of surfactant occurs.20 Therefore, with each additional step it becomes increasingly difficult to make any statement about the PEG/DX/surfactant ratio. In an effort to overcome these problems a mimic separation is prepared in parallel and used to obtain clean mimic phases. In this way, even though the surfactant ratio is unknown the two phases remain selfconsistent. Unfortunately, despite the use of mimic phases the uneven distribution of PEG and DX can result in the PEG:DX ratio falling below the tie line and the formation of a single phase. The experimentalist must therefore control the concentration of many different chemicals and


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surfactants at the same time in order to control partitioning, whilst simultaneously attempting to maintain an appropriate DX:PEG ratio, and with every new addition or extraction the experimental conditions become increasingly uncertain.20

In an effort to eliminate the experimental uncertainty of ATPE, Subbaiyan et al.18 provided conditions to achieve mono chiral (6,5) or (7,5) fractions from different as-received CNTs in 1 or 2 steps. Similar to the work of Zheng and Fagan this was achieved by making a first cut in diameter in step 1 followed by the use of optimized surfactant ratios (SDS:DOC of 20.4:1 for (6,5) and 10.4:1 for (7,5)) to achieve (n,m) pure fractions in a subsequent step. Certainly it may be possible to find optimized conditions for other (n,m) species but given the infinite number of possibilities a rational design of parameters is challenging. In the current work a different direction is taken and both the PEG:DX and co-surfactant ratios are fixed. In this approach the initial differences in surfactant shell provide the resolution for the extraction. The first step provides a rough cut in diameter, the second step selects the diameter of the (n,m) species of interest and the third is used to ensure metallic/semiconducting separation. The separation is modulated by the pH of the two phases, which through the addition of either acid or base provides a level of flexibility, control and reversibility currently missing to the ATPE method.

Results and Discussion The use of pH to modulate CNT partitioning is shown in a simplified experiment in Figure 1 (a). As-received CoMoCAT powder suspended in 1% DOC was mixed to achieve a final SDS:DOC ratio of 10:1 ratio with an SDS/DOC concentration of 0.5 % / 0.05 % in a centrifuge tube containing 8% (m/m) PEG and 4% DX. At these SDS:DOC conditions all of the SWCNTs begin


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in the DX phase. Sequential additions of 0.5M HCl were then introduced and both the PEG and DX phase were measured by absorption spectroscopy. Here it is instructive to follow spectral changes in the PEG phase. As more acid is added more (n,m) species partition into the PEG until eventually the DX is absent of SWCNTs. In this experiment the (n,m) species in the PEG is additive due to the re-combination of PEG and DX between steps and a chiral identification is difficult. Nevertheless, the order in which (n,m) species partition into the PEG phase is clearly diameter controlled. Within the DX the overwhelming concentration of (6,5) (Dt=0.76nm) in the CoMoCAT material make the comparison of spectral changes difficult until (6,5) partitions into the PEG (5μL of HCl) and only the smaller species such as (7,3), (6,4) and (5,5) remain. The ability to partition CNTs into the PEG without altering the global surfactant concentration, in particular through volumetric dilution, dramatically simplifies the ATPE process but it is clear that the acid must be causing changes to the surfactant shell. A detailed discussion of the separation mechanism is provided later along with a description of how the optimum surfactant conditions of 0.5% SDS / 0.05 % DOC were determined.

To achieve chiral separation the experimental procedure was altered and is depicted in Figure 2. The same surfactant conditions of 0.5% SDS / 0.05 % DOC were used but instead of recombining the PEG and DX between steps, a known volume of HCl was added to partition all SWCNTs with a diameter larger than the target species into the PEG (T1, T=top phase, step 1) and these were removed from the DX phase (B1, B=bottom phase, step 1). With the aid of a mimic top phase once again a known volume of HCl was added and both semiconducting and metallic CNTs with the same diameter were partitioned into the PEG (T2) whilst leaving behind all smaller diameter species in the bottom phase (B2).

Once isolated in the PEG phase a simple


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metallic/semiconducting separation was achieved by combining T2 with the remaining mimic bottom phase and adding sodium hypochlorite (NaClO). NaClO has been shown to initiate the reorganization of the surfactant shell and lead to a strong electronic separation.28 In the example provided in Figure 2, pure (6,5) is obtained in the DX (B3) and (7,4) in the PEG (T3). A similar process can be repeated for other chiralities as shown in Figure 3, where knowledge of the appropriate amount of acid for steps 1 and 2 is all that is required. Supplementary Table S1 summarizes the necessary steps and the required volume of HCl to obtain other chiralities. A clear trend of decreased pH for smaller diameters can be seen. In order to improve the clarity of Figure 3 only phases T1, T2, T3 and B3 are shown and additional spectra can be found in Figure S1.

Several peculiarities are apparent in Figure 3. For the two metallic species (6,6) and (5,5); less spectra are shown in (d) and (h), respectively. This is due to less steps being required for their isolation. (5,5) is the smallest diameter species of appreciable concentration in the CoMoCAT material (Dt=0.688nm) such that after the diameter cut in step 1 (T1), (5,5) is the only species remaining in the dextran (B1). Likewise, (6,6) is the largest metallic species (Dt=0.825nm) of appreciable concentration. After step 1 (T1), (6,6) is the only metallic species in T1 and top phase can be combined with mimic bottom phase with NaClO to obtain a pure fraction. In order to avoid confusion anytime NaClO is added the notation T3/B3 is used but effectively the isolation of (6,6) is two-step and (5,5) one step. In Figure 2 and 3 0.5% SDS / 0.05 % DOC has been used and this has resulted in pure fractions of (6,5), (7,4), (7,5), (8,4), (6,6), (7,6), (8,3) and (5,5). However, for (9,4) or (6,4), (7,3) as shown in Figures 3 (a) and (g), respectively, the quality of separation is noticeably lower at 0.5% SDS / 0.05 % DOC. The reason for this is related to the diameter of these species. (9,4) has one of the largest diameters (Dt=0.916nm) in the CoMoCAT material and


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(6,4) and (7,3) are two of the smallest semiconducting species (Dt=0.692 and 0.706nm, respectively) and have a diameter difference of only 0.014nm between them. This suggests that whilst the conditions of 0.5% SDS / 0.05 % DOC have been optimized to afford a high resolution around the central average diameter of the CoMoCAT material for diameters on the edge of this distribution new initial conditions need to be found. Indeed, it is unexpected that the same surfactant conditions can be applied arbitrarily to all diameters and it will be necessary to adjust the initial SDS/DOC concentration in order to separate other raw materials. In the next section strategies to improve the separation when the diameter changes are discussed and we hope these provide experimental guidelines for other research groups wishing to replicate our work or separate SWCNTs with other diameters. It is important to stress that in all subsequent experiments that all of the nanotubes begin in the dextran phase and that after conditions for the initial global surfactant concentration are found that these are not changed during separation. Only the pH is changed and this is unlike the traditional ATPE approach, which requires a change in the global surfactant ratio or concentration to drive partitioning.

Central to the pH driven ATPE technique is the necessity to be able to optimize the addition of HCl to obtain a clear cut of diameter in step 1 and the subsequent ability to coerce only the diameter of interest into the PEG (T2) in the second step. Although a metallic/semiconducting is performed in the third step if more than one metallic and one semiconducting species are present after step 2, a single chiral separation will not be possible. Figure 4 (a) demonstrates this principle for the (9,4) species, where the experimental conditions and spectra for T1 and T2-1 are identical to T1 and T2 in Figure 3 (a), respectively. Following removal of larger diameters and replacement of T1 with mimic top phase one begins to add HCl to partition (9,4) into the PEG in step 2. For 6μL of HCl


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there was no clear presence of (9,4) (T2-1) and the PEG and DX phases were recombined before introducing an additional 1μL HCl to obtain T2-2. With the additional 1μL of acid not only has (9,4) now moved into the PEG but also a considerable amount of (7,5). Clearly the SDS/DOC surfactant shell around these two species at was very similar at 0.5% SDS / 0.05 % and less acid should have been added. However, at some point increasingly smaller volumes of HCl become experimentally difficult to realize. At this point it becomes necessary to adjust the initial surfactant shell around the nanotubes and find a new optimum that will allow better access to this diameter range. To achieve this cues can be taken from the work of Zheng and Fagan20 who suggested that for larger SWCNT diameters more DOC and less SDS should be used. Essentially the logic behind this statement can be traced back to the competition between the two surfactants around a nanotube and their preference for different diameters as discussed later. In Figure 4 (b) and (c) 0.07% and 0.1% DOC in 0.5% SDS were tested as initial conditions. In Figure 4 (b) once again T1 was removed and more acid was added. After 7μL no clear separation was observed in T2-1, but now with an additional 1μL the ratio of (9,4):(7,5) was reversed and (9,4) isolated with increased purity. The sensitivity of this process to concentration is made apparent in Figure 4 (c) with 0.1% DOC and 0.5% SDS, where once again (9,4) can no longer be resolved. Intuitively, slightly less acid in (a) or slightly more in (b) should afford similar results, practically however, problems are encountered with the concentration of the separated CNTs in the top phase. This can also be seen in (a) for the addition of 6 μL of acid which results in small peaks that could be associated to (9,4) in T2-1 but their concentration is too low to be certain. A trade-off between selectivity and concentration needs to be made and this is in agreement with the work of Doorn and co-workers18 who discuss the effect of surfactant ratio on the separation of different (n,m) species. The isolation of any (n,m) species is dependent upon finding conditions such that its surfactant shell does not


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have overlap with other species. In the event that there is an overlap the concentration of the CNT will decrease to only that fraction without an overlap. Here a different surfactant ratio is useful to reduce the overlap and thereby increase the concentration. Through this process of trial and error initial surfactant conditions can be developed for (9,4). This is the process we suggest others to follow upon trying to optimize their own initial conditions for other raw materials with a different range. All phases of the 3-step process are shown in Figure 4 (d). Noticeably there are no metallic SWCNTs with a diameter similar to (9,4) and T3 remained empty making (9,4) a two-step process.

Following the logic applied to larger diameters, the resolution of (6,4) and (7,3) in the small diameter regime should be improved by reducing the DOC concentration. Figure 5 (a) and (b) show a comparison between 0.05% or 0.025% DOC in 0.5% SDS, respectively. Although the intensity ratio between (6,4) and (7,3) in T2 or B3 of (b) decreased relative to (a), indicating that the experimental conditions were shifted in the correct direction, a noticeable improvement was not obtained for further reductions in DOC. This is attributed to the marginal difference in diameter between these two species and the exceptionally high affinity of DOC to small diameters.29 Consequently, both species are highly covered in DOC and changes to the DOC/SDS concentration do not allow for resolvable differences in their surfactant shell. Fortunately, despite having similar diameters, (6,4) and (7,3) have different chiral angles (23° and 17°, respectively) and this provides an additional lever for separation. The bile surfactants SC and DOC have a similar pKa30 but the sensitivity of SC to chiral angle29 enable (7,3) and (6,4) to be resolved as shown in Figure 5 (c) and (d). In agreement with Subbaiyan et al.18 the concentration of SC was found to have little effect on the separation and 0.5% SC was added to the DOC and SDS. It should also be noted that SC was trialed for the separation of all other (n,m) species, however, in


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the case that existing surfactant shell differences for SDS/DOC of 0.5 % / 0.05 % were enough to allow separation, the addition of SC was superfluous and only increased the experimental complexity from 2 to 3 surfactants. This agrees with Fagan et al.20 who mention that SDS and DOC are the most important surfactants for ATPE separation. However, the important point here is that if a raw material contains many species with a very small difference in their diameter the addition of SC to the initial surfactant conditions can help by affording a chiral sensitivity. An additional peculiarity of SC is that the PEG phase (T3) contains the semiconducting species after the addition of NaClO rather than the DX. This is due to DOC and SC behaving very differently in metallic/semiconducting separations20 with SC shown to have a selectivity towards metallic species.21

A benefit of the pH driven ATPE separation is the reversibility of extraction. When using surfactants to partition SWCNTs between the two phases if too much was added, leading to an inappropriate co-surfactant ratio or an over-exaction, one was left with the difficult task of trying to re-calculate all parameters (PEG/DX/co-surfactant ratio) in order to return to the original state or place unwanted species back to the opposing phase. The removal of surfactants is also impossible. With a pH driven ATPE process it is as simple as adding NaOH to neutralize the acid. The small volume of acid or base allows for changes in the relative concentration of all other parameters to be neglected. Although it is important to mention that it is currently unclear if the DOC aggregates partition in a constant manner for a given surfactant ratio. Nevertheless, this makes it exceptionally easy to find the appropriate amount of acid to add at each step. Figure 4 (e) demonstrates the ability to do this with examples to find the correct volume of acid to extract all diameters larger than (9,4) in step 1 (e) or optimum conditions for extraction of (7,5) at step 2


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(f). Figure 6 summarizes the different (n,m) species isolated by pH modulated ATPE and Figures S2-4 provide additional photoluminescence excitation maps and Raman characterization.

There are several possible explanations for the ability of HCl to cause a partitioning of CNTs. In this section these possibilities are tested and the role of pH is discussed in line with the literature established mechanism of ATPE separation. The first possibility is related to counterion effects and an increase the ionic conductivity due to the addition of Cl- ions. Subbaiyan et al.18 added 60mM NaCl to re-organize the disordered micellular structure of SDS into a more densely packed and highly ordered structure18 and this caused partitioning of the CNTs. Furthermore the addition of NaCl has been shown to result in diameter dependent bundling of CNTs and may also drive partitioning.31 Figure S5 tests this hypothesis by adding NaCl in equal concentration to the HCl used in Figure 1. At this concentration no SWCNTs were found to partition into the PEG phase and the increase of Cl- can be excluded as a possible mechanism. However, it is important to remember that the NaCl concentration used in this work was significantly lower than that used by Subbaiyan et al.18 and that Na+ counterion was predicted to be more important than Cl-.

A second possibility is the well-known ability to p-dope SWCNTs with acid. This is an effect that has been extensively studied for different surfactants32 and the inverse relationship between diameter and bandgap ensures diameter dependent changes in the surfactant shell. Indeed, this is an effect that both our group33, 34 and others35 have utilized to purify SDS suspended SWCNTs in the past. Typically, p-doping is observed in the pH range of 6.0-2.5 and this is measured optically as a reduction in the first (S11) or second (S22) order oscillator strength of the nanotube. Figure S6 tests this hypothesis by adding the same volume of 0.5M HCl used in Figure 1 (a) and (b) to


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CoMoCAT material suspended in 0.5% SDS, 0.05% DOC and the co-surfactant mixture of 0.5% SDS/0.05% DOC. All measurements were performed in water without PEG or DX. SDS has a pKa of approximately 1.9 and is thus insensitive to changes in pH. Consequently, the SDS suspension of CNTs remained stable with decreases in pH and a clear reduction of S11 (~ 990 nm) is seen (Figure S6a). For DOC with a pKa of 6.58 the addition of acid protonates the carboxyl group on the surfactant and leads to a destabilization of the suspension.36 In fact, protonation of DOC has been shown to lead to a dramatic increase in the aggregation number from 3-5 to up to 200 at pH 6.79 as primary micelles begin to aggregate into larger secondary micelles.36, 37 The free acid is also soluble at 0.24 g/L in water at 15 °C while the sodium salt is soluble at > 333 g/L. This leads to the formation of large helical DOC aggregates in solution, these are visible to the eye as a change from a clear solution to one with a ‘silky’ or ‘milky’ appearance for pure DOC37 or the formation of CNT bundles as shown in Figure S6 (d). It is therefore difficult to determine the level of doping on the SWCNTs due to the increased background absorption (below 800 nm) from aggregation of both SWCNTs and DOC. The co-surfactant mixture of SDS/DOC is noticeably different from the two surfactants alone. First and foremost, upon adding HCl the presence of SDS has prevented the protonated DOC from forming large aggregates in solution, presumably through the formation of co-surfactant micelles,37 and the suspension remained stable without bundling of the CNTs. For the co-surfactant mixture there is also a lack of reduction in S11 (Figure S6 (c)) indicating that the SWCNTs are not doped. This is in line with the work of Duque et al.,38 who discussed the potential of molecules like SC and DOC to form uniform, highly coated surfactant shells around SWCNTs and protect them from acid doping. However, a clear blue shift of up to 8 nm (Figure S7) can be seen upon adding HCl and this is an indication of changes to the surfactant structure. The optical properties of CNTs are strongly


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influenced by their surrounding dielectric environment39, 40 and for aqueous dispersions these are dictated by the surfactant coating. By combining recent experimental and theoretical findings it is possible to speculate about how changes in pH may lead to the observed blue shift and how this is related to a diameter dependent partitioning. Both SDS and DOC are known to have a preference towards different SWCNT diameters.29 On small diameter SWCNTs and in low surfactant concentrations (surface per SDS headgroup, 0.98/nm2) SDS has been shown to lay with its hydrophobic tail parallel to the axis of the nanotube whilst leaving large uncoated regions of CNT side-wall exposed to solution. Upon increasing the surfactant concentration (surface per SDS headgroup, 0.25/nm2), the exposed regions are to a large extent preserved and SDS begins to form disordered rings of adsorbed surfactant around the already coated regions.41 This is a direct result of the energetic penalty required to bend the hydrocarbon chain around a CNT. For large diameter SWCNTs the energetic penalty for off-axis wrapping is lower and the SDS coverage is consequently higher than for smaller diameter CNTs and this is irrespective of surfactant concentration.41 Increasing the concentration only serves to further improve the surface coverage until once again disordered ad-layers of surfactant are formed.41 In all cases a disordered surfactant coating and orientation of hydrophobic chain along the length of the nanotube results in a hydrophobic object that prefers to reside in the PEG phase.24 On the other hand, limited theoretical studies on DOC make it difficult to predict its structure around a nanotube in different concentration regimes. Certainly, many studies exist for SC and the two bile surfactants have a similar amphiphilic structure. Both have been predicted to lay with their hydrophobic face on the side-wall of a CNT with their hydrophilic face into solution19 but it is not clear if theoretical insights gained for SC42 can be directly applied to DOC. Even at a very basic level, consideration of their CMC (20–25°C), 9–15mM for SC and 2–6mM for DOC, reveal that these two surfactants


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are not the same and they have been shown to be experimentally quite different. Experimentally, DOC has been shown to have a higher surface coverage around small diameter SWCNTs compared to large diameters29 whereas the opposite is true for SC.42 Additionally, SC has been shown to be selective to larger chiral angles29 and is a property not observed for DOC. Either way, the two (DOC) or three (SC) hydroxyl groups and the carboxylic acid on the hydrophilic face of these molecules ensure a hydrophilic object that prefers to reside in the DX phase.

For co-surfactant mixtures it has been shown that DOC is capable of displacing SC which in turn is capable of displacing SDS and Shastry et al. discussed exposed patches in the DOC or SC layer as being ‘plugged’ by SDS.21 In this work it is therefore predicted that the opposing preference of SDS and DOC for large and small diameters, respectively, results in a gradient of co-surfactant coatings between small (more DOC, less SDS) and large (less DOC, more SDS) diameter SWCNTs. For a fixed SDS:DOC ratio or concentration that each SWCNT diameter will have a different surfactant shell, and this will be more or less hydrophilic depending on the amount of DOC on its surface. Consequently, the SWCNT will be found in either the top or bottom phase and it is possible to choose SDS:DOC conditions such that all of the CNTs are initially in the bottom phase. In our work we intentionally choose conditions such that all nanotubes begin in the bottom phase. Essentially this is the same effect used by Zheng and Fagan27 and Subbaiyan et al.,18 where increasing SDS:DOC ratios move the large diameters to the top and the addition of DOC return them to the bottom phase. This implies that for each diameter there is a minimum amount of DOC molecules required to retain it in the DX phase and this is proposed to be key to the separation mechanism presented in this work. Schematically this is represented in Figure 1 (c). Upon adding HCl the carboxylic acids on the DOC start to become protonated, initially this occurs


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only on a fraction of the overall DOC molecules, but then by the time the pKa point is reached 50 % are protonated and then finally at the equivalence point all DOC molecules both in solution and on the SWCNTs are protonated. In this way DOC has essentially acted as buffer to resist the change in pH upon addition of HCl and prevented the doping of the nanotubes. Given that different diameter CNTs have a different DOC coating at the beginning of the experiment it is possible to imagine that the number of de-protonated DOC molecules along a CNT drop below the required number to hold it in the DX phase. At this point the CNTs partition into the PEG. An open question remains as to what happens to the protonated DOC in the mixed micelle around a CNT and what is responsible for the observed blue shift. Here it is clear that whatever occurs it must result in a more hydrophobic coating around the SWCNT and thus an increased likelihood to partition into the PEG phase. We believe that the tendency for protonated DOC to self-aggregate may be so high that it leaves the CNT surface to form aggregates in solution and the nanotubes become entirely individualized and stabilized by SDS as shown in Figure 1 (c). Certainly, an SDS coating has been shown in the literature to be more hydrophobic and this would move the CNTs into the PEG phase. Additionally, Weisman et al.43 have shown that despite the less ordered surfactant shell around the nanotube a blue shift is the hallmark that DOC has been removed and replaced by SDS. Likewise, Subbaiyan et al. have observed a red-shift upon the replacement of SDS by DOC around the nanotube.18 This effect can be seen as a diameter dependent blue-shifting of the carbon nanotubes optical properties with increasing acid addition in Figure S7. As such the mechanism responsible for partitioning is the same as that already discussed in the literature when changes in the global surfactant concentration are made except that in the case of pH the effective DOC concentration is reduced not by adding more SDS but rather by forcing the DOC to aggregate. This is an important factor differentiating this work from previous work because it


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allows for the effective DOC concentration to be reduced without having to change the SDS concentration and DOC can easily be recovered when it is required by adding NaOH.

To further clarify the role of DOC protonation in the separation, a titration curve for the 10:1 SDS:DOC ratio used in this work at an SDS/DOC concentration of 0.5 % / 0.05 % is provided in Figure S8. Furthermore, to provide the reader with an indication of the way in which the separation changes for other surfactant ratios and/or concentrations similar data for five different ratios ranging from 2.5:1 to 40:1 in different concentration regimes are also included. Absorption measurements of the PEG and DX phase for different surfactant conditions are provided in Figures S9-11. This data is provided to give the reader an impression of the surfactant optimization process used to arrive at the SDS/DOC conditions of 0.5 % / 0.05 % that we have used throughout our work and it is an important point to consider. Due to the resolution of the pH-based separation being entirely defined by the initial coating of SDS and DOC it is important to investigate what happens either side of these optimum conditions and we hope that the following section will assist other groups to implement our approach to CNTs with a different average diameter. We would also like to once again stress that the separation shown is a result of pH changes and not a result of modulation of the surfactant conditions. Once optimum surfactant conditions are found for a certain diameter range it is no longer necessary to make changes in order to obtain (n,m) pure fractions. In Figure S8 it can be seen that up to the equivalence point DOC acts to buffer the pH upon addition of HCl and higher concentrations of DOC only increase this capacity. For comparison a titration curve of a 0.5% SDS solution is shown in Figure S12 where a rapid drop in pH can be seen. In agreement with the work of Zheng and Fagan20, 27 several trends can be seen in Figures S9-11. When the concentration of SDS is high (i.e. Figure S9 (c) or Figure S10 (c)),


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SDS out-competes DOC and the raw CoMoCAT suspension is either partially or completely in the PEG phase prior to adding HCl. When the concentration of DOC ratio is high (i.e. Figure S10 (a) or S11 (a)) all of the CNTs begin in the bottom phase. In order to determine the appropriate surfactant conditions for separation of (n,m) species within our diameter range the partitioning of three groups of CNTs was monitored with increasing acid. These were the comparatively larger species such as (9,4), (8,4), (7,5) and (6,6) with diameter of 0.8–0.9nm followed by (8,3), (6,5) and (7,4) with a diameter of 0.7–0.8nm and lastly (7,3), (6,4) and (5,5) with a diameter of 0.6– 0.7nm. Absorption spectra of these three groups are shown in Figure S8 (b). In our work, the appropriate surfactant conditions were deemed as those where the three groups could be well resolved from each other without overlap but where this spectral cut is made in the future will depend on the raw material under investigation. This is shown in Figure S8 (c) and (d) where the pH range over which these groups partition into the PEG is shown. In agreement with Subbaiyan et al.18 in can be seen that an SDS:DOC ratio of 10:1 was found to best. However, within these set of experiments not only is the SDS:DOC ratio important but also the concentration of the surfactants. Once an sensitive ratio is found it is necessary to find the appropriate concentration to further resolve these groups by (n,m) type. This is shown in more detail in Figure S13 by comparing the partitioning of (6,5) and (8,3) at an SDS:DOC ratio of 10:1. It can be seen for an SDS/DOC concentration of 0.5%/0.05% that (6,5) and (8,3) do not overlap. It is for this reason that we consider these SDS/DOC conditions to provide the best resolution between nanotubes in the CoMoCAT raw material.

For those readers wishing to repeat our method or scale-up the process to achieve large quantities of (n,m) pure samples several important questions arise. Firstly, as with other surfactant sorted


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samples17 we find that this method is compatible with further processing steps, such as the removal of the surfactants and subsequent polymer wrapping, and it is expected that these can be employed to prepare (n,m) pure samples with device relevant semiconducting content.17 In the past the yield of the ATPE method has also been shown to be limited by interfacial trapping of CNTs between the PEG and dextran. The use of pH to modulate CNT partitioning does not resolve this issue and an example is shown in Figure S14 for the experiment presented in Figure 1. Typically, for the surfactant conditions of 0.5 % SDS / 0.05 % DOC it was found that HCl additions greater than 5μL as well as multiple extraction steps lead to increased interfacial trapping. In this work the ability to achieve a final separation in a maximum of 3 steps helps to reduce the interfacial trapping compared to a multistep or sequential sorting process. Nevertheless, the sequential extraction of different (n,m) species (large to small diameter), where each (n,m) species is first completely extracted and then fresh top phase is used to extract the next chirality is possible and is shown in Figure S15. The addition of NaOH and HCl allow for the experimentalist to ensure that the extraction is complete before moving to the next species but this process does require more extractions for increasingly smaller diameters.

The traditional ATPE method involving

modulation of the surfactant concentration has also been shown to be sensitive to different enantiomers and it is certainly expected that these will be achievable with pH modulation. Each enantiomer is expected to have a slightly different surfactant shell and this should in principle be sufficient for their extraction with the addition of HCl. This will be the focus of our future work but in Figure S16 we provide first indications of the possibility for enantiomeric separation of (6,5) and (7,5). Towards the scale up of the (n,m) separation many of the fractions presented in this work are also of noticeably low concentration and this is related to the low concentration of those species in the raw material. For example, the CoMoCAT material is highly enriched in (6,5)


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whereas (9,4) is only present in low concentration. Therefore, in the event that (9,4) were to be targeted for a large-scale separation a different raw material, possibly HiPco, with a slightly larger average diameter would be better suited and would lead to a more economical use of the chemicals required for separation. The HCl additions are also very small to ensure that the change in concentration of all other ATPE components is essentially unchanged. For example, if one considers a 4 mL suspension containing 8 % PEG, 4 % DX, 0.05 % DOC, 0.5 % SDS and 0.5 % SC the addition of 5 uL of HCl will change the concentration of the system to 7.99 % PEG, 3.995 % DX, 0.0499 % DOC, 0.4994 % SDS and 0.4994 % SC. These changes are too small to partition the CNTs into the PEG but the small volume of HCl raises issues regarding reproducibility and accuracy when applied to larger volumes. To demonstrate this a scaled-up experiment involving 20 mL is shown in Figure S17

Conclusion Using a simple 1-3 step pH driven ATPE process 11 different (n,m) species have been isolated. Not only were semiconducting species isolated but also metallic and rare non-armchair metallic species such as (7,4). It is our hope that the conditions provided in this work will allow others to easily repeat the separation in their own laboratory. Differences in water pH and/or chemical impurities between laboratories may lead to slight changes in the required volume of HCl but this can easily be obtained using the addition of NaOH. In our future work, we will focus on the separation of larger diameter CNTs. Here it is clear that the same SDS/DOC concentrations will not be appropriate but it is expected that it will be possible to find a different optimum in the surfactant conditions using the approach describe here. However, for nanotubes with Dt>0.9nm the diameter difference between (n,m) species will no longer be as large and it is likely that SC


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will be required to introduce an additional sensitivity to chiral angle. The sensitivity of SC to chirality may also allow for the separation of left and right handed (n,m) species in the future.

Methods 20mg powder of CoMoCAT SG65i SWCNTs (SouthWest Nanotechnologies, Lot no. SG65iL58) was suspended in 20mL of aqueous 1% (m/v) DOC (BioChemica) by tip sonication (Weber Ultrasonics, 35 kHz, 16 W in continuous mode) for 1 h while immersed in an ice bath. The resulting dispersion was centrifuged at 45560g (Beckman Optima L-80 XP, SW 40 Ti rotor) for 1 h and the supernatant collected for ATPE.

ATPE was performed at a concentration of 4 % (m/m) dextran (Mw 70000 Da, TCI), 8% (m/m) PEG (Mw 6000 Da, Alfa Aesar) with different SDS (Sigma-Aldrich) (0.25 % , 0.5 % , 1 % m/v) and DOC (0.025 % , 0.05 % , 0.1 % m/v) concentrations. For example, for a 4 mL scale separation with 0.05 % DOC and 0.5 % SDS: 0.8 mL PEG (40 % m/m), 0.8 mL dextran (20 % m/m), 0.2 mL DOC (1 % m/v) dispersed SWCNTs, 0.2 mL SDS (10 % m/v) and 2 mL water were mixed. The mimic suspension was prepared in the same way but without SWCNTs. The prepared CNT and mimic suspensions are used for further pH modulation and 3-step separation. The pH value of was measured in 20 mL ATPE suspensions with SWCNTs by a portable pH meter (Thermo Scientific, Orion Star A320). During the measurement, the suspension was stirred to prevent phase separation. The micropipettes used throughout this work have an accuracy of ±1% (+/- 0.12 µL for 2-20 µL and +/- 6 µL for 100-1000 µL). Absorption spectra were recorded on a Varian Cary 500 spectrophotometer and appropriate mimic phases were used the subtract the background. PEG and Dextran backgrounds were subtracted respectively when top and bottom phases were measured.


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Raman measurements were carried out in the back-scattering macro-configuration. Enriched samples suspended were excited at 576 and 650 nm. The backscattered light was dispersed by a t64000 Horiba spectrometer in triple grating configuration equipped with 900 grooves per millimeter grating and a Silicon CCD. The integration time varied from 2 to 5 minutes. The intensity and position of Raman lines were calibrated on a standard benzonitrile reference. The PLE measurements were performed with a Horiba Nanolog set up. Multiple excitation lines from a HgXe short arc lamp were selected using a pre-monochromator. An iHR Horiba spectrometer equipped with 150 grooves per millimeter grating and a nitrogen-cooled InGaAs detector were used to record the spectra. The dark spectra were subtracted in all maps and the emission lines of the (6,4) nanotube were additionally calibrated on the Tungsten light source to account for a wavelength dependent spectral sensitivity.

Step 1: A certain volume of HCl (0.5M) was added to the ATPE system to remove the nanotubes with a diameter larger than the target species. The suspension was then shaken for 1 min at 2000 rpm using a vortex mixer (Scientific Industries, Digital Vortex-Genie 2) to allow all the components to mix homogeneously. This was repeated for a mimic ATPE system without CNTs. Both were centrifuged for 2 min at 5800 rpm in a fixed angle bench top centrifuge (Hettich, EBA 12) to accelerate phase separation.

Step 2: After centrifugation, the top (PEG) phase from the CNT ATPE experiment (T1) was removed and replaced by the top phase of the mimic. Once again, a fixed volume of 0.5M HCl was added


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to extract metallic and semiconducting CNTs with the target diameter. After the identical mixing and centrifugation process used in step 1, the target species partitioned into the top phase (T2) together.

Step 3: T2 was then mixed with the remaining bottom (DX) phase of the mimic suspension. A fixed volume of sodium hypochlorite (NaClO, Honeywell) with 10-15 % available chloride was then added with a known volume of compensated HCl to the suspension. After mixing and centrifugation, the purified semiconducting species remained in the bottom phase whilst the metallic species partitioned to the top phase.

The details of exact HCl volume added to a 4 mL suspension throughout the different steps are shown in Supplementary Table S1-S3. All separations were performed at room temperature.

For the separation of (6,4) and (7,3), 0.5 % (m/v) of SC (Sigma-Aldrich) was also added to the suspension and the steps were same except that in the end the semiconducting species were found in the top phase.


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Figure 1. Absorption spectra of the (a) PEG and (b) DX phases after centrifugation with sequential addition of 0.5M HCl. 0.2mL of CoMoCAT material suspended in 1% (m/v) DOC was added to 0.8mL of 40% (m/m) PEG, 0.8mL of 20% (m/m) DX and 0.2mL 10% (m/v) SDS to achieve an SDS:DOC ratio of 10:1 at a concentration of 0.5 % SDS / 0.05 % DOC. The pH of the mixed PEG/DX system as well as the total volume of 0.5M HCl are indicated. After each HCl addition the PEG and DX phases were recombined. In the PEG phase, comparatively larger species such as (9,4), (8,4), (7,5) and (6,6) with diameter (Dt) of 0.8–0.9nm partition first (2–3μL HCl), followed by (8,3), (6,5) and (7,4) with Dt=0.7–0.8nm (4–5μL HCl) and lastly (7,3), (6,4) and (5,5) with Dt=0.6–0.7nm (5–6μL HCl). The spectra are normalised and vertically offset. (c) Proposed mechanism for pH driven ATPE separation showing small and large diameter CNTs wrapped in SDS/DOC before and after the addition of HCl. Protonation of the carboxylic acid on DOC leads to a dramatic increase in its aggregation number and the formation of DOC aggregates in solution. Consequently, DOC is replaced by SDS around the nanotube. This process is diameter dependent and drives the nanotubes into the PEG phase.


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18 cm a

Raw SWCNTs

b

Mimic

T1

HCl T0 B0

Shake

8 % PEG

Step 1

4 % Dextran

Step 2

Centrifuge

B1 Centrifuge

T1

HCl Transfer

B1 Larger D t

T2 (7,4) (6,5) B2 Smaller D t

Transfer

Centrifuge

Transfer

Step 3

Normalized absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

T2

B2

Transfer

T3

T3 (7,4) Centrifuge

B3

(6,5) HCl NaClO

B3

400 400

600 600

Metallic / Semiconducting

800 800

1000 1000

1200 1200

Wavelength / nm

Figure 2. (a) Experimental procedure for the simultaneous three step separation of (6,5) and (7,4) from the CoMoCAT raw material. (b) Absorption spectra at each stage of an example separation with 0.5% SDS and 0.05% DOC. Pure fractions of (6,5) and (7,4) are found in B3 and T3, respectively. The spectra are normalized and vertically offset for better comparison.


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Figure 3. Absorption spectra of the PEG (T1–T3) and dextran (B3) phases at different stages of separation with 0.5% SDS and 0.05% DOC. (5,5) and (6,6) were separated with a one and two step process, respectively. The spectra are normalized and vertically offset for better comparison.


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Figure 4. Absorption spectra of the extraction of (9,4) into the PEG phase at step 2 for different volumes of 0.5M HCl (T2-1 & T2-2) at a DOC/SDS concentration of (a) 0.05%/0.5%, (b) 0.07%/0.5% and (c) 0.1%/0.5 % . (d) Optimized conditions for (9,4) showing the PEG (T1 – T3) and dextran (B3) phases at different stages of separation with 0.5% SDS/0.05% DOC. Reversible partitioning of (e) (9,4) and (f) (8,4) upon addition of 0.5M NaOH. In step 1 for (9,4), T1 must include every species up to, but not including (9,4). 0.5μL of NaOH was added to return (9,4) to


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the DX (T2-2) and the correct acid addition can be identified as 2.5μL HCl. In step 2 for (7,5), HCl is added until (8,4) partitions into the PEG (T2-2) which can be returned by addition of 0.2μL and the correct volume identified as 2.3μL. The spectra are normalized and vertically offset for better comparison.


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Figure 5. Absorption spectra of the PEG (T1 – T3) and dextran (B1 - B3) phases at different stages of separation for small diameter SWCNTs with surfactant concentrations of (a) 0.5% SDS/0.05% DOC and (b) 0.5% SDS/0.025 % DOC and optimized conditions for (c) (7,3) and (d) (6,4) with the addition of 0.5 % SC. All spectra are normalized and vertically offset for better comparison.


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Figure 6. Absorption spectra of the isolated (n,m) fractions. The spectra are normalized and vertically offset for better comparison.


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Table of Contents Graphic

Supporting Information The following files are available free of charge from the ACS Nano home page. SI Separation of Small Diameter Single Walled Carbon Nanotubes in 1 – 3 Steps with Aqueous Two-Phase Extraction (file type, PDF). Author Contributions HL and BSF conceived the idea for this work and performed the experiments. GG, OG and SR performed the Raman and photoluminescence characterization of the (n,m) pure fractions. All authors were involved in the preparation of the manuscript. Acknowledgement BSF gratefully acknowledges support from the Deutsche Forschungsgemeinschaft (DFG) under grant numbers FL 834/2-1, FL 834/2-2, FL 834/5-1 and FL 834/7-1. References


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1. Arnold, M. S.; Stupp, S. I.; Hersam, M. C., Enrichment of Single-Walled Carbon Nanotubes by Diameter in Density Gradients. Nano Lett. 2005, 5, 713-718. 2. Huang, X.; McLean, R. S.; Zheng, M., High-Resolution Length Sorting and Purification of DNA-Wrapped Carbon Nanotubes by Size-Exclusion Chromatography. Anal. Chem. 2005, 77, 6225-6228. 3. Moore, K. E.; Pfohl, M.; Hennrich, F.; Chakradhanula, V. S. K.; Kuebel, C.; Kappes, M. M.; Shapter, J. G.; Krupke, R.; Flavel, B. S., Separation of Double-Walled Carbon Nanotubes by Size Exclusion Column Chromatography. ACS Nano 2014, 8, 67566764. 4. Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C., Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1, 60. 5. Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; Mclean, R. S.; Onoa, G. B., Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNA Assembly. Science 2003, 302, 1545-1548. 6. Wei, X.; Tanaka, T.; Hirakawa, T.; Yomogida, Y.; Kataura, H., Determination of Enantiomeric Purity of Single-Wall Carbon Nanotubes Using Flavin Mononucleotide. J. Am. Chem. Soc. 2017, 139, 16068-16071. 7. Arnold, M. S.; Blackburn, J. L.; Crochet, J. J.; Doorn, S. K.; Duque, J. G.; Mohite, A.; Telg, H., Recent Developments in the Photophysics of Single-Walled Carbon Nanotubes for their use as Active and Passive Material Elements in Thin Film Photovoltaics. Phys. Chem. Chem. Phys. 2013, 15, 14896-14918. 8. Gwinner, M. C.; Jakubka, F.; Gannott, F.; Sirringhaus, H.; Zaumseil, J., Enhanced Ambipolar Charge Injection with Semiconducting Polymer/Carbon Nanotube Thin Films for LightEmitting Transistors. ACS Nano 2011, 6, 539-548. 9. Khasminskaya, S.; Pyatkov, F.; Flavel, B. S.; Pernice, W. H.; Krupke, R., Waveguide‐Integrated Light‐Emitting Carbon Nanotubes. Adv. Mater. 2014, 26, 3465-3472. 10. Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J., Highly Selective Dispersion of Single-Walled Carbon Nanotubes Using Aromatic Polymers. Nat. Nanotechnol. 2007, 2, 640. 11. Wang, H.; Bao, Z., Conjugated Polymer Sorting of Semiconducting Carbon Nanotubes and Their Electronic Applications. Nano Today 2015, 10, 737-758. 12. Samanta, S. K.; Fritsch, M.; Scherf, U.; Gomulya, W.; Bisri, S. Z.; Loi, M. A., Conjugated Polymer-Assisted Dispersion of Single-Wall Carbon Nanotubes: The Power of Polymer Wrapping. Acc. Chem. Res. 2014, 47, 2446-2456.


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Separation of Small-Diameter Single-Walled Carbon Nanotubes in

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