Surface-Selective Directed Assembly of Carbon Nanotubes Using

Sep 6, 2013 - IBM T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, United States. Chem. Mater. , 2013, 25 (18), pp...
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Surface-Selective Directed Assembly of Carbon Nanotubes Using Side-Chain Functionalized Poly(thiophene)s Jose M. Lobez* and Ali Afzali IBM T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, United States S Supporting Information *

ABSTRACT: A simple and versatile strategy for the directed assembly of carbon nanotubes (CNTs) using side-chain functionalized poly(thiophene)s is presented. Modular variations in the side-chain of the polymers can be used to fine-tune the surface properties of polymerwrapped CNTs, which yields CNTs soluble in either water or organic solvents. Directed assembly of the CNTs based on electrostatic interactions or metal−ligand interactions can be obtained, depending on the functional group on the side-chain of the polymers. Patterned thin films of CNTs with very high CNT density can be obtained with this approach, and the surface selectivity is extremely high.

KEYWORDS: carbon nanotubes, directed assembly, surface functionalization, thin films

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fabrication of homogeneous thin films of CNTs from solution, while coating only specific areas of a substrate. Here, we present a versatile strategy for the selective deposition of thin films of CNTs on specific surfaces using directed assembly mediated by side-chain functionalized poly(thiophene)s (Figure 1). These poly(thiophene)s are used to disperse the CNTs, and modular modifications on the chemical structure of the poly(thiophene) side-chain have a large impact on the surface properties of the polymer-coated CNTs. These slight chemical modifications highly influence CNT solubility and enable surface-selective CNT deposition from solution using different DA strategies. DA based on electrostatic or metal−ligand interactions is shown using these polymers: poly(thiophene)s with functional side-chains bearing negatively charged functional groups (Figure 2, 1) can be used for CNT DA based on electrostatic interactions, while poly(thiophene)s with thiols on the side-chain (Figure 2, 2) can be used for CNT DA based on metal−ligand interactions.

INTRODUCTION

Carbon nanotubes (CNTs) are promising candidates for many different applications1 such as sensors,2 supercapacitors,3 electrodes,4 drug-delivery,5 and digital logic.6 One factor limiting the widespread application of CNTs is that many of these applications would require the selective deposition of CNTs from solution on specific areas of a substrate without covering the whole substrate with a blanket film of CNTs. The selective deposition of CNTs has been difficult, due to (1) the low solubility of CNTs in most solvents, (2) the difficulty of adapting traditional solution-based processes for the fabrication of CNT thin films, and (3) the lack of proper techniques capable of yielding high-density deposition of CNTs with high selectivity for certain areas of a substrate. One possible approach for the controlled deposition of CNTs is to use directed assembly (DA), which is an interesting strategy for the fabrication of artificial supramolecular structures or the modification of surfaces.7−9 DA has been reported using building blocks as diverse as DNA10 or nanoparticles.11 Even though there are a limited number of examples for the DA of CNTs with high surface selectivity,12−14 these methods have one or several of the following drawbacks: (1) They require covalent chemical modifications that can affect the optical and electronic properties of the CNTs. (2) The CNT deposition yield is low, and the films have a very low density of CNTs. (3) They rely on the use of surfactants which have weak hydrophobic interactions with the nanotubes, and the presence of free surfactant affects the DA process, lowering the yield of CNT deposition. It is essential to develop new, industrially feasible processes that can be used for the reproducible © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Directed Assembly of CNT/Polymer 1 from Water. For the deposition of CNTs wrapped in polymer 1, 3 mg of the phosphonic acid version of polymer 1 were dispersed in 10 mL of miliQ water, to which 5 mg of tetramethylammonium hydroxide pentahydrate was added. The polymer was sonicated until completely dissolved, and 2 mg of CNTs was added. The resulting mixture was sonicated using a probe sonicator for 45 min. The CNT dispersion was centrifuged for 30 min to remove CNT bundles, and the supernatant was filtered

Received: June 10, 2013 Revised: August 10, 2013

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supernatant was used for the directed assembly of the CNTs. A few drops of the corresponding base were added right before the CNT assembly to deprotect the thiol groups of the polymer. Patterned substrates of Au on HfO2 were plasma-cleaned for 3 min and used for the deposition. Exposure of these substrates to the CNT/polymer 2 solution for 2 h led to the directed assembly of thin films of CNTs with very high density and selectivity in the Au parts of the substrate.

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RESULTS AND DISCUSSION Poly(thiophene)s can wrap around CNTs via π−π stacking,15 effectively decorating the CNT surface with the functional groups on the poly(thiophene) side-chain without disrupting the optical and electronic properties of the CNTs. Poly(thiophene)s with functional side-chains have been used in the past to modify the surface of CNTs, increasing the CNT affinity for an analyte of interest in thin film CNT sensors.16,17 Poly(thiophene)s with functional side-chains bearing negatively charged functional groups (1) or thiols (2) were synthesized from a parent poly(thiophene) polymer with alkylbromide side chains (5, Scheme 1). This parent polymer was synthesized from the corresponding 2,5-dibromo-3-(6bromohexyl)thiophene (4, Scheme 1, top) monomer using Grignard metathesis (GRIM) polymerization,18−20 which is one of the most widespread methods for the synthesis of substituted regioregular poly(thiophene)s.21 The desired functional groups were incorporated by postpolymerization modifications using a nucleophilic substitution reaction, which is a modular approach for introducing a wide variety of chemical moieties into the side-chains of conjugated polymers.22 In order to obtain watersoluble, negatively charged poly(thiophene) 1 to enable DA of CNTs using electrostatic interactions, the bromide-substituted poly(thiophene) 5 was subjected to a nucleophilic substitution reaction using triethylphosphite followed by phosphonic ester hydrolysis and deprotonation with a tetraalkylammonium hydroxide salt. To obtain a poly(thiophene) 2 decorated with thiols in the side-chain, polymer 5 was reacted with sodium thioacetate. The resulting polymer with thioester side chains (7, Scheme 1), which was soluble in organic solvents, could be hydrolyzed with an organic base such as ethanolamine in THF to yield the free thiol functional groups in the polymer sidechain. In order to study the effect of the degree of side-chain substitution for polymer 1 on the directed assembly of CNTs, random regioregular polythiophenes with different degrees of side-chain functionalization were synthesized using GRIM polymerization conditions using a mixture of 2,5-dibromo-3hexylthiophene (4) and 2,5-dibromo-3-(6-bromohexyl) thiophene monomers (Scheme 1, bottom). Treatment of the resulting copolymer (8) with triethyl phosphite followed by ester hydrolysis yielded the corresponding copolymer (9) with different degrees of phosphonic acid functionalization in the side chain. The resulting degree of side-chain functionalization was determined by the initial feed ratio of the monomers. Copolymers where 5% and 50% of the repeat unit side-chain was substituted (3) were thus obtained. Single-walled carbon nanotubes (SWCNTs) could be dispersed in water by sonication in a solution of polymer 1. Centrifugation was used to remove insoluble CNT bundles, and excess polymer in the supernatant was removed by filtration of the CNT dispersion through a 0.22 μm cellulose acetate filter and washing with excess water. Excess polymer in the filtrate solution could be easily detected by UV−vis spectroscopy. CNT/polymer 1 adducts in the filter were resonicated in water, and the resulting aqueous dispersion was

Figure 1. CNT surface modification using poly(thiophene)s by selective deposition from solution. Interaction between the poly(thiophene) side-chains and desired parts of the substrate leads to selective deposition of the CNTs only on those parts of the substrate.

Figure 2. Poly(thiophene)s used for the directed assembly of carbon nanotubes. through a cellulose acetate filter (0.22 μm pore size). The filter was washed with 100 mL of miliQ water to remove excess polymer and tetramethylammonium hydroxide, and the residue was resonicated into 10 mL of miliQ water with a bath sonicator. The CNT dispersion was sonicated using a probe sonicator for 45 min, followed by centrifugation for 30 min to remove CNT bundles. The resulting supernatant was used for the directed assembly of the CNTs. Patterned substrates of HfO2 on SiO2 were plasma-cleaned for 5 min and dipped in a 100:1 HF/H2O solution followed by thorough rinsing with water and drying under nitrogen. The HfO2 parts of the substrate were selectively coated with a monolayer of positively charged molecules, NMPI, (4-(N-hydroxycarboxamido)-1-methylpyridinium iodide), by immersion for 2 h in an NMPI solution (24 mg of NMPI in 9 mL of EtOH, 3 mL of H2O), followed by rinsing with water. Exposure of these substrates to the CNT/polymer 1 solution for 2 h led to the self-assembly of thin films of CNTs with very high density and selectivity in the HfO2 parts of the substrate. Directed Assembly of CNT/Polymer 2 from THF. For the deposition of CNTs wrapped in polymer 2, 3 mg of the thioester version of polymer 2 were dissolved in 10 mL of THF, to which 2 mg of CNTs was added. The resulting mixture was sonicated using a probe sonicator for 45 min. The CNT dispersion was centrifuged for 30 min to remove CNT bundles, and the supernatant was filtered through a PTFE filter (0.22 μm pore size). The filter was washed with 100 mL of THF to remove excess polymer, and the residue was resonicated into 10 mL of THF with a bath sonicator. The CNT dispersion was sonicated using a probe sonicator for 45 min, followed by centrifugation for 30 min to remove CNT bundles. The resulting B

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Scheme 1. Synthesis of Side-Chain Functionalized Poly(thiophene)s

Figure 3. (A) Dispersion of CNTs in H2O using polymer 1. (A) Dispersion of CNTs in THF using the thioester precursor of polymer 2.

stable over a period of months. Concentrations of up to several milligrams per milliliter of SWCNTs in water could be obtained this way (Figure 3A). Dispersions of SWCNTs in organic solvents could be obtained using the protected thioacetate version of polymer 2 using an analogous procedure. Polymer 7, the thioacetate precursor of polymer 2 was used for CNT dispersion instead of the thiol for stability purposes and hydrolyzed in situ at the moment of CNT deposition by DA. The CNT/polymer 2 adducts were soluble in organic solvents such as THF and CHCl3 (Figure 3B). The electrostatic directed assembly of CNT/polymer 1 on positively charged surfaces was studied using patterned substrates of hafnium oxide (HfO2) and silicon dioxide (SiO2; Figure 4A). Hydroxamic acids have been reported to selectively interact with certain metal oxides over silicon dioxide,23 and this phenomenon has been used to selectively coat HfO2 with monolayers of specific compounds over SiO2.14

Figure 4. Selective deposition of CNTs using electrostatic directed assembly. (A) Selective DA of CNTs on positively charged HfO2 via Coulombic interactions. (B) SEM image of patterned substrates of HfO2 on SiO2 after DA of CNT/polymer 1. HfO2 areas are completely covered by CNTs, whereas SiO2 areas show no CNT deposition. Scale bar = 10 μm. (C) SEM image of HfO2 area of the substrate. Areas with CNTs have a texture, and CNTs are indicated with a red arrow. Scale bar = 1 μm. (D) SEM image of SiO2 area of the substrate. Scale bar = 1 μm. All SEM images taken at 1 kV.

For this study, we exposed substrates with patterns of HfO2 on SiO2 to a positively charged pyridinium salt bearing a hydroxamic acid moiety, NMPI (4-(N-hydroxycarboxamido)1-methylpyridinium iodide), which resulted in the selective coating of HfO2 with a positively charged monolayer. The substrates were then exposed to a solution of CNT/polymer 1 for 2 h, followed by rinsing with water to remove excess solution, and CNT deposition was confirmed by SEM (Figure C

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Figure 5. (A) SEM image showing the deposition of CNT/polymer 1 on HfO2/SiO2 patterned substrates where no monolayer treatment has been applied. CNTs are indicated with a red arrow. Scale bar: 2 μm. (B) SEM image showing the deposition of CNT/P3HT on HfO2/ SiO2 patterned substrates. No selective placement is observed. Scale bar: 200 nm.

Figure 6. SEM image showing the deposition of CNT/polymer 1 on HfO2/SiO2 patterned substrates, where the polymer has (A) 5% content of phosphonic acids in the side chain, no selective placement observed, and (B) 50% content of phosphonic acids in the side chain. Selective placement was observed, but the CNT density was very low, and the CNTs were aggregated in bundles. Scale bar: 1 μm.

4B). CNTs were self-assembled only on the monolayer-coated HfO2 areas of the substrate with very high selectivity, due to the Coulombic attraction between the positively charged substrate and the negatively charged CNT/polymer 1. The deposition density on HfO2 was very high (tens of CNTs per square micrometer) as shown in the SEM images, which is desired for assembly purposes (Figure 4C). No CNTs were deposited on the areas of the substrate where the surface composition is SiO2 (Figure 4D). This was further confirmed by repeating the same process on two separate substrates, one being blank HfO2 and the other SiO2. Only the HfO2 substrate showed P and S on XPS (see Supporting Info, Figure S1). When substrates that had not been covered with a positively charged monolayer were exposed to the CNT/polymer 1 solution, the CNTs were selectively deposited on the HfO2 as observed by SEM, but the density was comparatively low, as shown in the SEM images (Figure 5A), and the CNTs could be detached by sonication. This is due to the fact that phosphonates are weaker ligands for HfO2 than hydroxamic acids. When poly(3-hexylthiophene) (P3HT) was used instead of polymer 1 for the DA of CNTs, then no selectivity was observed (Figure 5B), which highlights the importance of the specific functional group in the side chain of the polymer. The effect of the degree of functionalization in the polymer side chain was also studied for polymer 1. Copolymers with 5% of phosphonic acid in the side chain (polymer 9, 100b/(a + b) = 5) were only soluble in organic solvents such as CHCl3, and copolymers with 50% phosphonic acid in the side chain were only slightly soluble in water in their deprotonated form (polymer 3, 100b/(a + b) = 50). Directed assembly of CNTs was tested using polymers containing 5% phosphonate side chains on the HfO2/SiO2 substrates where the HfO2 was coated with a positively charged monolayer, which resulted in nonselective placement of the CNTs (Figure 6A). This is probably due to the fact that such a low content of phosphonic acid functional groups is not enough to direct the assembly of the nanotubes, since most of the CNT wall will be coated with hydrophobic groups. On the other hand, when copolymers that contained 50% of phosphonate substitution in the side chain were used for the DA of CNTs from water, selective placement was observed on the HfO2/SiO2 substrates. However, the nanotubes were aggregated, and the placement density was very

low as evidenced by SEM (Figure 6B). The fact that the polymer has only 50% of hydrophilic side chains probably causes hydrophobic microdomains to form, which will interact with each other and not with the solvent or the substrate, leading to CNT aggregation. This is not the case when using polymers with 100% phosphonate groups in the side chain (polymer 1), where the CNT/polymer solution was completely stable over time and no large-scale CNT aggregation could be observed from the SEM pictures (Figure 4B). This is due to the fact that the CNT surface is completely decorated with negative charges from the polymer wrapping it, which makes the CNT completely water-soluble and prevents aggregation. It is evident from these experiments that variations in the degree of polymer side-chain substitution can affect CNT solubility and the dynamics of CNT aggregation in different solvents, as well as the deposition process and its selectivity. A different directed assembly strategy based on metal−ligand interactions was studied using CNT/polymer 2. The assembly on gold substrates was studied using CNT dispersions in THF with polymer 7, the thioacetate precursor of polymer 2, to avoid stability issues. A base was added to these dispersions in the presence of the substrate to induce in situ thioester hydrolysis and thiol formation during the deposition. Different bases were tested for this purpose: ethanolamine, triethylamine, and pyridine. Thioester hydrolysis occurred faster when using ethanolamine as evidenced by the fact that after 2 h of deposition in the presence of this base, the highest density of CNTs on the gold surface could be observed for all three bases (Figure S2). In order to study the selectivity and efficiency of this DA, plasma-cleaned HfO2 substrates patterned with gold dots were used for the assembly (Figure 7A). DA of CNT/ polymer 2 was observed selectively only on the areas of the substrate covered with gold (Figure 7B), where the CNT density was very high as evidenced by SEM (Figure 7C). When plasma cleaning of the substrates was not performed prior to the deposition, the density of CNTs being deposited on the gold patterns was much lower, even though the selectivity was still really high (Figure S3). This shows the importance of surface cleaning and treatment for assembly purposes. When no base was added to induce the thioester hydrolysis during the directed assembly, CNT deposition occurred with very low CNT density and no selectivity, and the CNTs were easily D

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS CNTs, carbon nanotubes; DA, directed assembly; P3HT, poly(3-hexylthiophene)

Figure 7. Selective deposition of CNTs using metal−ligand directed assembly. (A) Selective DA of CNTs on gold via metal−ligand interactions. (B) SEM image of patterned substrates of Au dots on HfO2 after DA of CNT/polymer 2. Scale bar = 200 μm. (C) SEM image of Au area of the substrate. Areas with CNTs have a texture, and CNTs are indicated by a red arrow. Scale bar = 100 nm. (D) SEM image of HfO2 area of the substrate. Scale bar = 100 nm. All SEM images taken at 1 kV.

detached by sonication. This is explained by the fact that in this latter case there was no thiol−gold interaction to direct the directed assembly and to keep the CNTs on the surface.

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CONCLUSION In summary, we have shown a new methodology for the directed assembly of carbon nanotubes from water or organic solvents that can be used for the selective coating of desired surfaces. Modular chemical modifications can be performed in the side chain of the polymers used to mediate the directed assembly, which leads to different types of directed assembly based on electrostatic or metal−ligand interactions. Thin films of carbon nanotubes can be easily obtained from solution on specific areas of the substrate with very high selectivity and very high CNT density. The influence of the degree of side-chain functionalization in the directed assembly process was also studied, and control experiments using different conditions for the directed assembly supported the proposed interactions between the CNTs and the substrates. This methodology is a reproducible, industrially feasible process that can be used for the selective coating of patterned substrates with homogeneous thin films of CNTs from water or organic solvents, with numerous industrial applications. Other possible directedassembly schemes based on hydrogen bonding or other covalent reactions (Diels−Alder, click chemistry) can be envisioned by further chemical modifications on the side chains of these polymers.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Instruments and materials, conditions for the synthesis of sidechain functionalized polymers, and additional SEM micrographs (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. E

dx.doi.org/10.1021/cm401881w | Chem. Mater. XXXX, XXX, XXX−XXX