Structurally Analogous Degradable Version of FluoreneâBipyridine

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Structurally Analogous Degradable Version of Fluorenebipyridine Copolymer with Exceptional Selectivity for Large Diameter Semiconducting Carbon Nanotubes Catherine Kanimozhi, Gerald J Brady, Matthew J Shea, Peishen Huang, Yongho Joo, Michael S. Arnold, and Padma Gopalan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14115 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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ACS Applied Materials & Interfaces

Structurally Analogous Degradable Version of Fluorene-bipyridine Copolymer with Exceptional Selectivity for Large Diameter Semiconducting Carbon Nanotubes Catherine Kanimozhi, † Gerald J. Brady, † Matthew J. Shea, † Peishen Huang, † Yongho Joo, † Michael S. Arnold, † and Padma Gopalan*, † †

Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison,

Wisconsin 53706, United States. ABSTRACT: Separation of electronically pure, narrowly dispersed, pristine semiconducting single walled carbon nanotubes (S-CNT) from a heterogeneous as-synthesized mixture is essential for various semiconducting technologies and biomedical applications. While conjugated polymer wrappers are often utilized to facilitate electronic type sorting, it is highly desirable to remove organic residues from the resulting devices. We report here the design and synthesis of a mild acid degradable π-conjugated polyimine polymer (PFO-N-BPy) that is structurally analogous to the commonly used and commercially available poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6’-(2,2’-bipyridine))] (PFO-BPy). An acid cleavable imine link (-HC=N-) was introduced in the PFO-N-BPy backbone to impart degradability, which is absent in PFO-BPy. PFO-N-BPy was synthesized via a metal catalyst free Aza-Wittig reaction in high yields. PFON-BPy with a degree of polymerization of just ~10 showed excellent (> 99% electronic purity) selectivity for both large diameter (1.3-1.7 nm) arc-discharge S-CNTs and smaller diameter (0.8-1.2 nm) HiPCO SCNTs. Overall, selectivity for semiconducting species is similar to that of PFO-BPy but with an advantage of complete depolymerization under mild acidic conditions into recyclable monomers. We further show by UV-Vis, X-ray photoelectron spectroscopy (XPS), and SEM that the PFO-N-BPy wrapped S-CNTs can be aligned into a monolayer array on gate dielectrics using a floating evaporative self-assembly process from which the polymer can be completely removed. Short channel FETs were

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fabricated from the polymer-stripped aligned S-CNT arrays which further confirmed the semiconducting purity on the order of 99.9% or higher. Keywords: Nanotube electronics, selective sorting, degradable conjugated polymers, aligned arrays, evaporative self-assembly


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INTRODUCTION A broad range of semiconducting technologies based on single-walled carbon nanotubes (CNTs) require access to low-cost electronically pure bare semiconducting (S-) CNTs with a narrow polydispersity in band-gap and chirality.1-3 CNTs synthesized via chemical vapor deposition (CVD)4 and arc-discharge,5 among others, are electronically heterogeneous, with metallic (M-) CNTs comprising up to a third of CNTs. As a result, many dispersion and sorting techniques have been developed to isolate S-CNTs, especially the use of small molecules and polymers that assemble around individual CNTs to disperse them in solution. However, only a subset of these molecules are selective for specific chirality and electronic type. Bile salts and single-stranded DNA6 are among the most widely used in aqueous solutions.7 Their selectivity during dispersion is relatively weak; however, after dispersion, the packing density and interactions of these species with CNTs vary subtly with electronic-type and chiral indices (n, m), providing a handle for sorting via chromatography,8 density gradient ultracentrifugation (DGU),9 or two-phase separation.10 In contrast, some families of conjugated polymer strongly differentiate among S- and M-CNTs upon dispersion in organic solvents. This direct and selective dispersion of CNTs is simpler, faster, scalable, and avoids the need for a subsequent separation step. Recently, we11-17 and others18-25 have explored conjugated polymers as CNT-differentiating agents. Specifically, conjugated polymers like polyfluorenes and their derivative copolymers are capable of differentiating by electronic-type, and under the right conditions they will not disperse M-CNTs,17 leading to electronic type purities in excess of 99.98%.17,26 Unfortunately, the conjugated polymers used for sorting are difficult to remove completely from the nanotube surface,12 hindering device performance. For example, thin-film transistors27-30 require electronically pure S-CNTs in the diameter range 1.3-1.7 nm with no M-CNTs to achieve a high on/off ratio and high on-state conductance,31-33 where polymer residues are known to increase contact resistance.34,35 Solar cells 36 require monochiral,16 smaller diameter (0.7-1.2 nm) S-CNTs that absorb nearinfrared photons, where polymer removal can aid in improved transport of electrons and excitons. Other



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emerging applications such as drug delivery vehicles37-39 and chemical sensors40,41 require electronic grade bare S-CNTs for efficient chemical/biological functionalization.16 One method to ensure complete polymer removal is to incorporate degradable sub-units that may be activated after sorting to cleave the polymer, which releases from the CNT surface.42 Fluorene copolymers that change conformation upon protonation,43 HF cleavable alternating copolymers,44 and metal-complexation45,46 or hydrogen-bond47-50 driven polymers have been reported to efficiently sort, then decouple from S-CNTs. Recently, S-CNTs have been shown to be dispersed by several acid degradable poly(azomethinyl) polymers,18 where an alkylated fluorene is copolymerized with phenyl,51 biphenyl, stilbene, or azo-benzene containing comonomers. The wrapping behavior of these polymers in some cases showed partial (n, m) selectivity (e.g. to (8,7) and (7,6) S-CNTs). Overall, it is challenging to rationally incorporate both degradability and selectivity in a single conjugated polymer, while keeping the synthesis simple, as polymer-nanotube interactions are very sensitive to even small changes in the chemical structure of the polymer backbone. In this study we report a structurally analogous degradable version of the commonly used PFO-BPy, with nearly identical SCNT selectivity which can be degraded under mild acidic conditions and subsequently completely removed from the nanotube surface. The new polymer, poly[(9,9-di-n-octyl-2,7-fluoren-dinitrilomethine)alt-co-(6,6'-{2,2'-bipyridyl-dimethine})], which we term PFO-N-BPy, has fluorene and BPy alternating repeat units connected via an imine bond (-CH=N-). PFO-N-BPy is synthesized using a metal catalyst free synthesis approach, in contrast to most π-conjugated polymers used for sorting S-CNTs, including PFO-BPy, which are synthesized using metal complex catalyzed C-C cross coupling reactions such as Suzuki or Stile.52 As the imine functional group is susceptible to hydrolysis by mild organic acids such as trifluoro acidic acid (TFA), the polymer can be easily degraded and removed from the S-CNT surface post-sorting. We synthesize PFO-N-BPy by a modified Wittig reaction known as Aza-Wittig, using fluorene diazide and bipyridine dialdehyde as monomers. The metal catalyst free synthesis of this new polymer is appealing for applications in which even trace metals severely impact device performance.51


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We further show the sorting of CNTs synthesized via both arc discharge (Arc-D) (diameter range 1.31.7 nm) and high pressure carbon monoxide disproportionation reaction (HiPCO) (diameter range 0.8-1.2 nm) using PFO-N-BPy. Among both diameter ranges, PFO-N-BPy shows high selectivity for S-CNTs, as evidenced by imperceptible M-CNT optical absorption features. While the conventional wisdom in this field is to have at least 25-30 repeat53,54 units in the conjugated polymer to effectively wrap and solubilize the CNTs, we find that PFO-N-BPy (Figure 1) with a molecular weight of ~ 7K (Mn, Ð = 2.01) which is only ~10 units is sufficient to solubilize and separate S-CNTs. The degradable polymer PFO-N-BPy showed a higher degree of crystallinity as measured by X-ray diffraction studies (XRD) and a 25-30 °C higher glass transition temperature (Tg) than the non-degradable polymer PFO-BPy (100 repeat units). The sorted PFO-N-BPy wrapped S-CNTs were aligned into arrays by a floating evaporative self-assembly (FESA) process, with a nanotube packing density of 35-40 tubes/µm.55 Subsequently, PFO-N-BPy was completely removed from the CNT surface by treatment with TFA, characterized by UV-Vis spectroscopy. Field effect transistor (FET) devices fabricated from these aligned arrays were used to confirm the high semiconducting electronic-type purity of the PFO-N-BPy wrapped S-CNTs.



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Figure 1: Schematic showing the sorting cycle for S-CNTs using PFO-N-BPy and its recycling into monomers. RESULTS AND DISCUSSION Synthesis of PFO-N-BPy. Our polymer design uses fluorene and bipyridine aromatic units in the polymer backbone with a degradable imine (-HC=N-) connecting the two. PFO-N-BPy was synthesized (Scheme 1 c) by a condensation reaction between reactive 2,7-diazido-9,9-dioctylfluorene (M1) and 2,2’-bipyridine -4,4’dicarboxaldehyde (M2), resulting in an imine link. Following previous literature reports,56 both Monomers M1 and M2 (Scheme 1a, b) synthesis results in moderate to good yields (65-85%). For the synthesis of PFO-N-BPy, 2,7-diazido-9,9-dioctylfluorene (M1) or 2,7-diamino-9,9-dioctylfluorene (defined as M1*) can be used as one of the monomers with M2. In this work we use a one-step high yielding conversion of aryl diboronic acid to aryl diazide under mild reaction conditions to synthesized M1.56 Unlike M1, synthesis of M1* requires harsh reaction conditions; for the synthesis of the dinitro derivative followed by reduction to the diamine.57 In addition, Aza-Wittig condensation reaction is a relatively simple, metal catalyst-free route for achieving high molecular weight polyimines. It is well known that the presence of transition metal catalyst impurities (Palladium (II), Nickel (I); etc) even in ppm levels can act as dopants and trapping sites in optoelectronic devices.58 Hence, a metal catalyst free synthesis in the long run is beneficial in eliminating the need for extensive purification of semiconducting polymers.59 This is also true for semiconducting devices based on CNTs, composite materials, and other biomedical applications which require heavy metal free materials.60 Though the condensation reaction for PFO-N-BPy synthesis does not involve heavy metals it does require a phosphine derivative as dehydrating agent. These derivatives are shown to be toxic61 due to the formation of the by-product ‘phosphine oxide’ which is quite insoluble in many organic solvents. Previous studies18 have predominantly used methyl diphenyl phosphine (Me(Ph)2P) as the dehydrating agent where the by6 ACS Paragon Plus Environment

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product methyl diphenyl phosphine oxide (Me(Ph)2PO) is difficult to remove from the final polymer. To circumvent this purification problem, we choose a tri n-butyl phosphine (n-butyl)3P)62 as the dehydrating agent as tri butyl phosphine oxide (n-butyl)3PO) is highly soluble in methanol and can be completely removed from the final product. As a result, the polymer purification involves simple precipitation of the reaction mixture in methanol and soxhlet extraction of the polymer in hot methanol. The purified polymers show number averaged molecular weight (Mn) ranging from ~7 k to 15 k and a dispersity of ~2 (Table 1). Both polymers exhibit good solubility of 20 to 40 mg/mL in toluene (40-50 mg/mL in chloroform and tetrahydrofuran) at room temperature which is essential for solubilizing CNTs. The detailed chemical characterization of the monomers and the polymers are included in the supporting information Figures S1-S7.

Scheme 1: Synthetic scheme for (a) monomers M1 and M2 (b) and (c) their polymerization into PFO-NBPy.

Optical, thermal, and structural properties of PFO-N-BPy. Absorption spectra of PFO-N-BPy shows an absorption maximum (λmax) at 391 and 410 nm in solution and in thin film, respectively (Figure S8 a). The red shifted λmax in thin-film as compared to solution is attributable to improved inter-molecular interactions due to increased coplanarity of the backbone. This 7 ACS Paragon Plus Environment


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coplanarity is also evident from the vibrational fine structure in the absorption spectrum in the solid state. In toluene solution PFO-N-BPy shows a red shifted λmax compared to the parent PFO-BPy again due to the extended conjugation via the -HC=N- link (Figure S8 b). The glass transition temperature (Tg) (Table 1) of this new polymer PFO-N-BPy with a degree of polymerization (DP) of 10 is ~145 °C which is 25-35 °C higher compared to PFO-BPy (Tg ~ 115 °C) with a DP of ~100 (Figure S9 a). The favorable inter-molecular interaction (π-stacking) between the PFO-NBPy chains and the extended π-conjugation observed in the absorption spectrum correlates well with the increase in Tg. The thermal decomposition characteristics measured by TGA under nitrogen atmosphere (Figure S9 b) shows a single step decomposition for both the copolymers. However, PFO-N-BPy exhibits a lower decomposition temperature (Td ~ 360 °C) and ~ 25% higher weight loss when compared to its non-degradable polymer analog PFO-BPy (Td = 385 °C) with only 40% total weight loss. The weight loss of ~ 40% observed in PFO-BPy corresponds to the loss of alkyl side chains, but in PFO-NBPy the greater weight loss of ~ 65% is due to simultaneous decomposition of the side chains and main chain. Hence, the introduction of the imine linker makes the polymer thermally labile, providing an additional handle for polymer removal. X-ray diffraction studies of drop casted thin-films of PFO-N-BPy show distinct diffraction peaks at 2θ = 7.95°, 14.3°, 19.40° and 21.62° which correspond to the d-spacing of 11.12 Å, 6.19 Å, 4.58 Å and 4.11 Å respectively. In contrast a single broad peak at 2θ = 21.3° is observed for PFO-BPy with a dspacing of 4.19 Å. Hence, PFO-BPy is largely amorphous in nature (Figure S10), whereas the presence of imine in the backbone of PFO-N-BPy effectively separates (Figure S11) the fluorene and BPy units by ~ 4.04 Å (bond lengths of C-C=N-C) which is three times longer than ~ 1.4 Å (avg of C-C and C=C) in PFO-BPy. The increased separation between the fluorene and BPy groups in PFO-N-BPy effectively reduces the steric hindrance from the peri-hydrogen atoms,63 leading to a more planar structure which facilitates the inter-chain interactions. As a result, PFO-N-BPy is expected to show more crystallinity than PFO-BPy.


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Table 1: Characterization of molecular weights, thermal and electronic properties for PFO-BPy and PFON-BPy Mn (Kg/mol)a

Ða

PFO-BPy

50

~2

PFO-N-BPy

6.6

2.01

Polymer

a

λmax (nm)

b

(eV)

Td (° C)

100

355

3.2

382

40

117

11

394

2.73

360

66

145

DPna

b

Eg

c

(%)Weight loss

d

Tg (°C)

Molecular weights were obtained by GPC in THF at 30 °C using polystyrene standards. bObtained from UV-vis

absorption spectrum in toluene. cDetermined by TGA at a heating rate of 10 °C/min in N2 atmosphere. dDetermined by DSC from the second heating cycle under N2 atmosphere.

Dispersion and selective sorting of S-CNTs. We use PFO-N-BPy to disperse both Arc-D (Arc-D@PFO-N-BPy) (Table 2) and HiPCO (HiPCO@PFON-BPy) nanotubes in toluene solution. The absorption spectra of the resulting dispersions are shown in Figure 2, and contain three distinct regions. In the solution containing Arc-D@PFO-N-BPy, absorption regions corresponding to the S33 (400-650 nm), M11 (650-800 nm), and S22 (900-1200 nm) transitions are displayed in Figure 2a. In the solution containing HiPCO@PFO-N-BPy, regions corresponding to the M11 (450-550 nm), S22 (620-900 nm), and S11 (950-1600 nm) are displayed in Figure 2b. In both spectra, absorption signatures of M-CNTs (M11) are undetectable, suggesting that the S-CNT purity in all spectra exceed 99%. Additionally, we use PFO-BPy to disperse both sets of nanotubes to discern the differences, if any, between the two polymers. In both nanotube ensembles, we observe few differences in either electronic type selectivity, evidenced by the low M11 regions in all solutions, or chirality. In both the Arc-D and HiPCO ensembles, PFO-N-BPy exhibits a slight preference for S-CNTs with larger diameters compared to PFO-BPy, perhaps as a consequence of it’s slightly longer repeat unit length as well as differences in the rigidity of the backbone. However, further studies are needed to understand the influence of molecular weights, repeat unit length and flexibility on the selectivity. The slight preference of PFO-N-BPy for larger diameter nanotubes from the HiPCO ensemble is corroborated by comparing the excitation9 ACS Paragon Plus Environment


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emission photoluminescence maps of HiPCO@PFO-N-BPy with HiPCO@PFO-BPy (Figure 2c-d). Notably, the (10,2), (13,3) and (14,1) are present in only HiPCO@PFO-N-BPy but not in HiPCO@ PFOBPy. The (10,3) seems to be present in HiPCO@PFO-BPy but not (or much less) in HiPCO@PFO-NBPy. PFO-N-BPy seems to have lower affinity for (6,5) compared to PFO-BPy. The highest peak intensity in HiPCO@PFO-BPy is (8,6), while in HiPCO@PFO-N-BPy it is (8,7). In addition, HIPCO@PFO-N-BPy has much more (9,7) than in HiPCO@PFO-BPy, while (10,6) is present in HiPCO@PFO-BPy more than HiPCO@PFO-N-BPy. The three highest emission intensities are in the order of (8,7) > (9,7) > (8,6) for HiPCO@PFO-N-BPy, and (8,6) > (10,6) > (9,7) for HiPCO@PFO-BPy. Their diameter order follows64: HiPCO@PFO-N-BPy {1.02 nm > 1.09 nm > 0.95 nm}; and HiPCO@PFO-N-BPy {0.95 nm > 1.1 nm > 1.09 nm}.65 We quantify the yield, in µg/mL, for each dispersion, using the optical cross sections of Sanchez et al. for the HiPCO ensemble,66 and Mistry et al. for the Arc-D ensemble.1 It is notable that even though PFO-N-BPy has similar selectivity and yield as PFO-BPy, the DP of the two polymers vary by an order of magnitude (DP of 10 in PFO-N-BPy Vs 100 for PFO-BPy). Given that the aromatic subunits namely fluorene and BPy are common in the two polymers, the linker length, type, and hence the flexibility of the linker might be playing a critical role in maximizing the π-π interactions with the CNT.


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Figure 2: Absorption spectra of (a) ArcD S-CNTs and (b) HiPCO S-CNTs dispersed by PFO-BPy (black) and PFO-N-BPy (red) in toluene along with the chemical structure of the polymers. PLE maps of HiPCO SWCNTs dispersed in toluene (c) HiPCO @ PFO-BPy and (d) HiPCO @ PFO-N-BPy Table 2: Summary of CNT sorting Sorting polymer

a

Mn (kg/mol)

~DP

PFO-BPy

50

100

PFO-N-BPy a

6.6

CNT

Semiconducting purity

Yield (µg/mL)

Arc-D

> 99.98%

7.31

HiPCO

> 99.9%

3.66

Arc-D

> 99.9%

8.13

HiPCO

> 99.9%

5.24

10

Molecular weights were obtained by GPC in THF at 30 °C using polystyrene standards.

Hydrolytic degradation of PFO-N-BPy with mild acid. The depolymerization to the monomers due to hydrolytic degradation of the imine link occurs instantaneously upon the addition of a mild acid such as TFA to a toluene solution of PFO-N-BPy. 1H 11 ACS Paragon Plus Environment


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NMR analysis in deuterated toluene after TFA addition shows peaks only for fluorene units from M1 as M2 is insoluble in toluene and crashes out. However upon addition of DMF to the same solution M2 goes into solution and peaks from both monomers are visible by NMR (Figure S12). We note that M1 is now regenerated as 2,7-diamino-9,9-dioctylfluorene (M1*). The identity of these recovered monomers following isolation and purification was confirmed by 1H NMR. Both M1* and M2 were isolated and recycled with a yield of 70 and 90% respectively. Our preferred route for recycling M1* and M2 following degradation is to convert M1* to M1 in the presence of tret-butylnitrite (t-BuONO) and azidotrimethylsilane (TMSN3) in acetonitrile67 followed by coupling with M2 to resynthesize PFO-NBPy. While, M1* can be directly coupled with M2 using PTSA as catalyst, this reaction is slow as it takes over 48 hrs to get 4-5 K molecular weights with a recycling yield of 70%. Hence the Aza-wittig reaction using aromatic diazides offers advantages over the condensation reaction using aromatic diamines. We also monitor the degradation of PFO-N-BPy by the disappearance of the absorption peak at 394 nm in solution (Figure 3a) and at 402 nm in thin-films (Figure S13). Post TFA treatment, the solution absorption spectra shows only the monomer absorption peaks at 320 and 280 nm, confirming the quantitative degradation of PFO-N-BPy in solution. When the polymer is wrapped on the CNTs, e.g. in Arc@PFO-N-BPy solution, the TFA treatment is followed by washing with toluene, chloroform, and DMF to remove any residual monomers (Figure 3b, Scheme S1), to yield pristine polymer free S-CNTs.

Alignment of polymer wrapped S-CNT arrays by FESA and polymer removal from S-CNT surface. Next, we fabricate aligned S-CNT arrays using floating evaporative self-assembly (FESA)55 from Arc@PFO-N-BPy dispersed in chloroform. The concentration of Arc@PFO-N-BPy in chloroform (~10 µg/ml) obtained through the sorting process is indeed high enough to generate aligned S-CNT arrays (Figure 3d-f). FESA of Arc@PFO-N-BPy solution results in mostly isolated and well aligned tubes, with a density of 35-40 tubes µm-1 (obtained from SEM images), which is comparable to Arc@PFO-BPy based S-CNT arrays with 46 tubes µm-1.37 Implementation of the polymer removal from these aligned CNT 12 ACS Paragon Plus Environment

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arrays using TFA can be challenging when compared to the solution due to the substrate confinement effects. To study the effectiveness of this method we treated the FESA aligned Arc@PFO-N-BPy S-CNT arrays to a catalytic amount (0.3% v/v) of TFA in toluene to hydrolyze the imine bond and depolymerize the PFO-N-BPy. This step is followed by rinsing with non-halogenated solvents such as dimethylformamide and isopropanol to prevent potential doping from the halogenated solvents68. Additional thermal annealing is implemented at 400 °C under vacuum to remove any adsorbed or trapped solvent molecules. Comparison of the SEM images of the “as-deposited” FESA aligned Arc@PFO-NBPy films (Figure 3d), with those at different stages of post-processing namely: TFA treatment + rinsed, and TFA treatment + rinsed + annealed (Figure 3e-f) show that all the residues are effectively removed and the morphology of the aligned arrays is preserved. Polymer removal is further confirmed by X-ray photoelectron spectroscopy (XPS) where the N(1s) signal that arises primarily from PFO-N-BPy disappeared from the surface treated FESA aligned Arc@PFO-N-BPy (Figure 3c). The presence of any unintentional doping of S-CNTs during these processing steps is monitored by Raman spectra. Though both G and 2D bands upshift by ~ 2 cm-1 upon treatment with TFA, subsequent rinsing with solvent reverses this effect, leaving an undoped pristine S-CNT (Figure S14 and Table S1).



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Figure 3: Absorption spectra of (a) PFO-N-BPy (black), TFA treated PFO-N-BPy (PFO-N-BPy +TFA, green) and monomer M1 (blue) in toluene, (b) ArcD@PFO-N-BPy before (red) and after TFA treatment (green) in toluene (inset show that the S33 peaks from S-CNTs remain after polymer removal), (c) XPS spectra showing the disappearance of the N(1s) peak from the as-deposited (black) Arc@PFO-N-BPy aligned arrays, after TFA treatment and rinsing (green), and after an additional thermal annealing step TFA+rinsed+annealed (orange). SEM images (d-f) of FESA aligned arrays which show the effective removal of the polymer residue as a funciton of surface treatment. Field effect transistors from FESA aligned S-CNTs. To confirm the electronic purity of the semiconducting tubes we analyze FETs fabricated from aligned S-CNT arrays subjected to TFA + rinsed + annealed treatment using a backgate architecture (Figure 4a). SEM images of the aligned CNT channels showed well aligned and polymer residue free CNTs (Figure 4b). The IDS – VGS characteristics for three representative devices with channel length (LCH) of 200 nm are compared in Figure 4c. The conductance of the channels turns “on” at negative gate bias (VGS) consistent with the on-state p-type behavior expected when using Pd source and drain contacts. The on-state conductance density of the FETs (G/W) were not optimized but are compared in Figure 4d as a function of LCH. The G/W in the LCH range of 150 – 300 nm (where contact resistance is more dominant) was 220 ± 100 µS µm-1. The µ in the long channel regime (LCH > 400 nm) where contact resistance is less significant, assuming a parallel plate capacitance which underestimates mobility, was 25 ± 10 cm2V-1s-1. We analyze the off-state conductance of FETs with LCH =150 – 300 nm to quantify the semiconducting purity. A total of 1058 CNTs spanned the source-drain gap of these FETs, which have an on/off of 104.1±0.5. The off-state conductance of only one of the FETs exceeded the 1 µS expected for a M-CNT (in which the on/off ratio of the FET was only 102.5), indicating that the S-CNT purity is on the order of 99.9% or higher.


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Figure 4: (a) FET device architecture, (b) Scanning electron micrograph image of CNT films integrated into FETs with surface treatment, “TFA+rinsed+annealed”, (c) Representative IDS – VGS characteristics for FETs constructed, (d) Conductance per width (G/W) measured over LCH = 100 – 800 nm for FETs incorporating “TFA+rinsed+annealed” surface treatment.

CONCLUSION In conclusion, we have synthesized a degradable version (PFO-N-BPy) of a commonly used commercial conjugated polymer PFO-BPy by introducing an imine link that is readily degraded into recyclable monomers using mild acid. PFO-N-BPy exhibits nearly identical selectivity for Arc-D and HiPCO S-CNTs to its structural analog PFO-BPy. The introduction of the imine linker between the aromatic fluorene and BPy units results in an increase in crystallinity of the polymer and changes in the



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thermal degradation profile. We successfully fabricate aligned arrays of Arc-D@PFO-N-BPy with a density of ~ 40 S-CNTs/µm and quantitatively remove the polymer from the arrays. FET device measurements from these polymer free S-CNT arrays show that the semiconducting purity is on the order of 99.9% or higher. The high yield and high purity synthesis of the polyimine π-conjugated polymer along with its complete removal post-processing from devices makes it an attractive generalizable approach towards electronics-grade sorting of S-CNTs. The importance of this work lies in the fact that while the performance of the FETs is preserved after the polymer removal process, the result is bare CNTs that are now available for chemical or biofunctionalization or for forming high quality interfaces for example with gate dielectric layers. MATERIALS AND METHODS Characterization. All solvents and starting materials were purchased from commercial sources and used without further purification unless mentioned. 1 H NMR, 13C NMR and 31P NMR spectra were recorded using a Bruker AVance-400MHz spectrometer, in CDCl3 with TMS (0.1% v) as an internal standard. Gel permeation chromatography (GPC) was performed using a Viscotek 2210 system equipped with three Waters columns (HR 4, HR 4E, HR 3) and a 1 mL/min flow rate of THF as eluent at 30 °C. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 using a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q100 using a heating and cooling rate of 10 °C/min for three cycles. Glass transition temperatures were determined from the third heating cycle. Samples for thin-film X-ray diffraction were prepared by drop casting 10mg/mL PFO-BPy or PFO-N-BPy in chloroform on Si substrates. The films were then thermally annealed at 40 °C for overnight under vacuum. UV-Vis measurements were performed using a Shimadzu PC-2401 spectrophotometer and home-built setup, in which an input monochromator was used to produce a beam of a wavelength (10 nm resolution) and was used to scan over the range 300 nm to 1500 nm. Raman spectra of carbon nanotubes were obtained using Thermo Scientific DXRxi Raman imaging microscope at an excitation wavelength of 532 nm and 6 W laser 16 ACS Paragon Plus Environment

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power. The silicon peak at 520 cm−1 was used to calibrate the wavenumber, and the spot size was ∼1 µm2. Thermo Scientific K-alpha XPS with micro-focused monochromated Al Kα X-ray source was used for compositional analysis of the films. The 125 mm mean radius full 180° hemispherical analyzer was operated in constant analyzer energy with 400 µm selected area aperture. Survey spectra were collected with pass energy 50 eV. The resulting data were analyzed by Avantage software where fully integrated control, acquisition, and peak positioning were characterized by fitting multiplex spectra with Voigt functions. Synthesis of monomers (M1, M2) and PFO-N-BPy Synthesis of 2,7-Diazido-9,9-dioctylfluorene (M1). A 100 mL round bottom flask was charged with sodium azide (1.35 g, 20 mmol) and copper (II) sulfate pentahydrate (0.208 g, 0.83 mmol) at room temperature. To this 8 mL of methanol was added to disperse the salt which was followed by the addition of 9,9-dioctyl-9H-fluorene-2,7-diboronic acid (2.0 g, 4 mmol). The resulting reaction mixture was stirred at room temperature for 48 hs in open air. After completion of the reaction the solvent was removed under vacuum. The residue was dissolved by washing with large excess of petroleum ether (200 mL) and the undissolved insoluble inorganic salts removed by filtration. The solvent was rotary evaporated and resulted in 2,7-diazido-9,9-dioctylfluorene as a pale yellow solid. The solid was further purified by silica gel column chromatography using hexane as a solvent (yield, 1.7 g 86%). 1H NMR (400 MHz, Chloroform-d) δ 7.61-7.59 (d, 2H), 7.01-6.98 (dd, 2H), 6.95-6.94 (d, 2H), 1.93-1.89 (m, 4H), 1.22-1.05 (m, 20H), 0.84-0.81 (t, 6H), 0.62-0.51(m, 4H). 13C NMR (400 MHz, Chloroform-d δ): 152.54, 138.69, 137.57, 120.51, 117.84, 113.54, 55.42, 40.35, 31.74, 29.87, 29.17, 23.62, 22.57, 14.04. Synthesis of 2,2’-bipyridine-6,6’-dicarbaldehyde (M2). To a solution of 6,6’-dimethyl-2,2’-bipyridine (1g, 5 mmol) in DMSO (5 ml) at 50 °C was slowly added a solution of iodine (2.82 g, 11 mmol) in 25 mL of DMSO. The mixture was then added to a 250 mL round bottom flask charged with 100 mL of DMSO preheated at 145 °C. The reaction mixture was refluxed at the same temperature for 24 h, where a vigorous exothermic reaction occurred with the evolution of dimethyl sulfide. After 24 h, the excess 17 ACS Paragon Plus Environment


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iodine was quenched with saturated aqueous sodium thiosulphate solution, until a pale-yellow solution was obtained. Water was added to give a total volume of 250 ml and the resulting solution was then extracted with dichloromethane (3 x 200 ml), washed with water (2 x 100 ml), and dried over magnesium sulfate. It was filtered and concentrated under vacuum to yield a pale yellow solid. The crude product was then crystallized in dichloromethane / petroleum ether mixture to give 2,2’-bipyridine-6,6’dicarbaldehyde as pale yellow crystals (yield, 0.54 g, 47%). 1H NMR (400 MHz, Chloroform-d) δ 10.19 (s, 1H), 8.84 - 8.82 (dd, 1H), 8.09 - 8.03 (m, 2H). 13C NMR (400 MHz, Chloroform-d δ): 193.41, 155.50, 152.40, 138.20, 125.32, 122.01. Synthesis of PFO-N-BPy. An oven dried 100 mL round bottom flask was charged with M1 (200 mg, 1mmol) and M2 (88 mg, 1 mmol). The flask was evacuated and refilled with argon. To this 15 mL of freshly distilled anhydrous toluene was added under argon atmosphere. This toluene reaction mixture was purged with argon for 15 min to remove any dissolved oxygen followed by the addition of excess of tri-nbutyl phosphine ((n-Bu)3 P) (88 mg, 2 mmol). Immediately after the addition of (n-Bu)3 P the reaction color changed from colorless to dark red with an evolution of nitrogen, which indicated the formation of the reactive phosphazene intermediate. The resulting reaction mixture was then heated to 80 °C for 24 h. Toluene in the reaction mixture was reduced to half the volume and the viscous liquid was precipitated in methanol. The crude yellow solid was purified by washing with hot methanol (6h) using Soxhlet extraction to remove low molecular weight oligomers and other phosphine oxide by-products. The yellow solid was collected and dried under vacuum for 24 h (yield, 240 mg). 1H NMR (400 MHz, Chloroform-d) δ 10.21 (s, 1H), 8.88 - 8.85 (m,1H), 8.65 - 8.63 (m, 1H), 8.37 - 8.33 (m, 1H), 8.05 - 8.00 (m, 2H), 7.79 7.66 (m, 2H), 7.39 - 7.37 (m, 4H), 7.03 - 6.99 (m, 1H), 2.05 - 1.89 (m, 4H), 1.25 - 0.97 (m, 20H), 0.81 0.50 (m, 10H). Molecular weight GPC, Mn: 6,600 g/mol, Ð: 2.01 S-CNT Dispersion. Semiconducting enriched CNTs were extracted from an arc discharge carbon nanotube powder (698695, Sigma-Aldrich) or a HiPCO carbon nanotube powder (Raw, NanoIntegris R1831). A 1:1 weight ratio of 1 mg/mL of the powder and 1 mg/mL PFO-N-BPy were dispersed in 60 mL of


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toluene using a horn tip sonicator (Thermo Fisher Scientific, Sonic Dismembrator 500) at 64 W power. The mixture was sonicated for a total of 15 min. Following the initial dispersion, the CNT solution was centrifuged (Thermo Fisher Scientific, Sorvall WX, swing bucket rotor, TH-641) at 41 krpm for 10 minutes to remove un-dispersed materials. The upper 90% of the supernatant was collected and centrifuged for an additional 30 minutes at 41 krpm. The supernatant was collected and, optionally, the toluene was distilled, rendering a gel-like polymer/S-CNT mixture. The mixture was then redispersed in 60 mL of toluene.

The solution was subsequently centrifuged and the pellet was collected and

redispersed into toluene with bath sonication. This centrifugation/redispersion was repeated four times to rinse off as much excess PFO-N-BPy as possible. For nanotube alignment studies, the final Arc-D@PFON-BPy pellet was dispersed in chloroform via horn-tip sonication for 30 seconds. Alignment of S-CNTs FET. Once the S-CNTs have been purified and collected, they can be incorporated into active or passive layers in a variety of electronic and optoelectronic devices. For many device applications, it is advantageous for the S-CNTs to be aligned along their long axes. Using the method of floating evaporative assembly, the conjugated polymer-coated S-CNTs can be aligned on a substrate, prior to the removal of the conjugated polymer. Polymer removal from FESA aligned S-CNT array. FESA aligned PFO-N-BPy wrapped S-CNTs array films (“as-deposited”) were immersed in toluene (100 mL) bath containing 0.3% v/v of trifluoroacetic acid (TFA/toluene) for 50 seconds. Subsequently FESA films were then washed with hot toluene (110 °C) and hot dimethyl formamide (80 °C) for 30 min in each solvent to remove the regenerated monomers. Finally, the films were washed in acetone and isopropanol. The above surface treatment was carried out for samples mentioned as “TFA+ rinsed”. An additional thermal annealing at 400 °C in vacuum was carried out for the samples mentioned as “TFA+ rinsed +annealed” to remove any trapped solvent or adsorbed molecules. Fabrication of S-CNT FET devices. FETs incorporating films of aligned S-CNTs as channel layers were fabricated. The S-CNTs were deposited on top of a 15 nm thick, thermally-grown SiO2 dielectric on top 19 ACS Paragon Plus Environment


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of a low-resistivity Si substrate, which acted as the gate dielectric and gate electrode, respectively. Prior to surface treatment steps the as-deposited films underwent an electron-beam lithography step which defined the channel region. The remaining PMMA covering the channel protected the underlying CNTs during a subsequent oxygen plasma etch which removed unwanted CNTs surrounding the channel area of each device. Next, surface treatments were performed. Prior to fabrication of source-drain contacts, the films underwent a mild acid rinse using TFA in toluene solvent (0.3% W:W) and 1 min. incubation followed by thermal annealing at 400 °C in vacuum. Electron-beam lithography was used to define the source-drain electrode regions and subsequently a 30 nm thick Pd film was deposited and lifted off. Figure 4a shows a schematic of the device architecture and the SEM images show the aligned S-CNT channel (right panel) spanning source-drain electrodes. Current-voltage (I-V) measurements were taken in ambient air with a gate-source voltage sweep of (-10 to 10 V) and source-drain voltage of -0.1 V. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Included detailed spectroscopic characterization data for the monomers and the polymer such as 1H NMR spectra, 13C NMR spectra, X-ray diffraction and UV-visible absorption data. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS P.G., P.H. and C.K. acknowledge support from Division of Materials Sciences and Engineering, Office of Basic Energy Science, U.S. Department of Energy under Award #ER46590 for fundamental studies


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(synthetic methodology development and characterization) on the conjugated polymer chemistry and its effect on CNT properties. The nanotube alignment and device characterization aspects of this work were supported by National Science Foundation Grant# CMMI-1462771 (G.J.B. and M.S.A.). The characterization of the dispersion of HiPCO nanotubes was supported by the U.S. Army Research Office contract W911NF-12-1-0025 (M.J.S. and M.S.A.). Partial support is also acknowledged by Y.H and J.K from University of Wisconsin-Madison Center of Excellence for Materials Research and Innovation NSF Grant No. DMR-1121288 for morphological characterization.

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