NANO LETTERS
Long-Range Periodicity in Carbon Nanotube Sidewall Functionalization
2004 Vol. 4, No. 8 1541-1546
Kimberly A. Worsley, Kevin R. Moonoosawmy, and Peter Kruse* Department of Chemistry, McMaster UniVersity, 1280 Main St. W., Hamilton, Ontario, L8S 4M1, Canada Received May 7, 2004; Revised Manuscript Received July 2, 2004
ABSTRACT Using the Bingel reaction as a model for side-wall functionalization of single-walled carbon nanotubes, we report the discovery of highly regular, long-distance (several nanometer) patterns and examine the conditions for the occurrence of such patterns, possibly due to longrange induced reactivity. Varying periodicities of the patterns have been observed via scanning tunneling microscopy and are attributed to nanotube geometry. Patterns are most prominent on medium heavy functionalized nanotubes and likely tied to a nucleophilic reaction mechanism.
Since their first characterization in 1993,1,2 single-walled carbon nanotubes (SWCNTs) have become a staple in materials research based on their remarkable electronic, chemical, and mechanical properties due to the strong sp2 carbon network of tubular graphene-like sheets.3 Applications extend to various fields ranging from microelectronics to microbiology. Their electronic properties are directly linked to their geometry (diameter, chirality), the presence of defects in the structure, and the degree of chemical functionalization. Unfunctionalized SWCNTs can be metallic or semiconducting, depending on the diameter and helicity of the tubes.4 Covalently functionalized sites on the nanotube can act as defects and disturb the periodicity,5 resulting in a larger band gap. Alternatively, SWCNTs can be doped or intercalated with electron donors or acceptors in order to enhance the electrical conductivity.6 Because of these characteristics, it has been possible to construct devices such as field-effect transistors,7 single-electron transistors,8 and diodes9 from SWCNTs. Similar to the cohesion between layers seen in graphite, SWCNTs prefer to form bundles due to weak π-π interactions. Over extended structures, such as the typical length of a carbon nanotube or in graphite, the interactions add up to considerable strength. For the purposes of handling and processing nanotubes, it is desirable to break up the nanotube bundles in order to suspend or dissolve the individual nanotubes. Unfunctionalized nanotubes are insoluble, therefore a substantial body of work has focused on the goal of improving their solubility through a variety of functionalization procedures.10-15 Generally, two effects of functionalization can be postulated that lead to increased solubility. First, steric hindrance due to the bulky groups on side-wall * Corresponding author: Phone: (905) 525-9140 x23480, Fax: (905) 522-2509. E-mail:
[email protected]. 10.1021/nl0493141 CCC: $27.50 Published on Web 07/17/2004
© 2004 American Chemical Society
functionalized SWCNTs may prevent close π-π interactions. Second, the tails of the functional groups (in particular in the case of attached polymers) are more easily solvated, thus suspending the nanotubes or nanotube bundles in the solvent. While the second mechanism is likely responsible for the good solubility of polymer functionalized nanotubes (both side-wall and end functionalized), the first premise is more questionable since it relies on the nanotubes being kept apart during the functionalization process. Only the freely accessible surface area of nanotubes or bundles under the particular reaction conditions will permit the attachment of functional groups. Vigorous agitation of the reaction mixture (e.g., through sonication) should therefore be required for even functionalization. Most functionalization procedures only provide for sonication before rather than during the reaction. In this paper, we offer a tentative comparison of the effects of stirring vs sonicating the nanotubes during the reaction. Despite the significance of functionalization to the future of carbon nanotube research and application, little attention has been paid to the functionalization process itself. More recently, it has become accepted that covalent functionalization of carbon nanotubes will affect their electronic properties.16,17 While a number of atomically resolved images of unfunctionalized SWCNTs have been published,18-21 only a fluorination study by Halas and co-workers12 using scanning tunneling microscopy (STM) addresses the spatial distribution of functional groups resulting from common sidewall functionalization processes. Given that each functional group is covalently bonded to the sidewall thus converting a planar sp2-hybridized carbon atom into a strained tetrahedral sp3-hybridized atom, the rather high degrees of covalent functionalization occasionally reported in the literature would likely lead to severe damage on the nanotubes and loss of their exceptional properties. In this
study, we have attempted to shed light on the true degree of functionalization under previously reported conditions.11 Apart from finding a comparatively low degree of functionalization, we report that functional groups can arrange in very regular patterns that may be explained by postulating induced reactivity. This effect would be consistent with reports of modifications of the electronic structure of carbon nanotubes upon functionalization,16 except that we also propose a spatially periodic distortion in the electron density. One possible pathway to covalent functionalization is provided by the Bingel reaction, which is historically based on the cyclopropanation of fullerenes,22 and more recently has been reported for SWCNT’s.11 The reaction mechanism was postulated by Bingel to be nucleophilic addition of the deprotonated species of diethyl bromomalonate followed by an intramolecular substitution of the halogen in a [2+1] cycloaddition. Despite several hundred citations of the original paper,22 we could not find any evidence in the literature of a validation or dispute of this mechanism. A carbene mechanism or electrophilic addition may also be conceivable, depending on the postulated sequence of events (deprotonation, bromine abstraction, attachment). Although it is known that the Bingel reaction can form up to a hexasubstituted product on the fullerene structure,22-24 the degree of functionalization relative to the type of nanotube and spatial distribution on a carbon nanotube is unknown. As reported by Coleman and co-workers, the malonated SWCNTs were reacted further and tagged with gold colloids to allow for indirect detection of the functional groups using atomic force microscopy (AFM).11 Since this methodology does not permit a quantitative evaluation of the functionalized nanotubes, we have chosen the route of direct imaging of the functionalized nanotubes by STM. Even though we do not presently have the capability to resolve the atomic lattice along the carbon nanotubes, the resolution of our images is sufficiently high for identification of single functional groups and their position along the nanotube. Our results were obtained with SWCNTs purchased from Carbon Nanotechnologies Inc., TX. The functionalization of SWCNTs via the Bingel reaction was previously published.11 Unshortened SWCNTs were added to 15 mL of o-dichlorobenzene in a 50 mL flame dried round-bottom flask. Sonication for 20 min allowed the black solid to disperse. After sonication, 0.5 mL of 1,8-diazabicyclo[5.4.0]undec7-ene (DBU) and 0.3 mL of diethylbromomalonate were added to the flask. The reaction was allowed to proceed for 19 h, either in a sonicator or with stirring. The reaction was carried out without heating, however, the sonicated mixture temporarily reached 33 °C during the reaction. The reaction was quenched with trifluoroacetic acid and was allowed to sit with stirring for 5 min. This was then filtered using a 0.2 µ PTFE paper and washed extensively with ethanol. The paper was then allowed to dry at room temperature in air overnight. A small amount of each sample was suspended in ethanol or DMSO by further sonication for a minimum of 30 min. Each suspension was then blotted on a freshly peeled highly oriented pyrolytic graphite (HOPG) surface, and the solvent 1542
Figure 1. Spreading of a nanotube bundle (19 h stirred) under ambient imaging conditions (IT ) 1.5 nA, USB ) +20 mV). (a) Upon initial imaging; (b) 8 image scans later; (c) schematics of bundle spreading process.
was allowed to evaporate off in air. The surfaces were imaged both under ambient conditions (DI Nanoscope II, type D head) using cut Pt/Ir wire tips and under ultrahigh vacuum (RHK UHV 300) using an electrochemically etched tungsten tip. Typical imaging conditions in the ambient system were a tunneling current of IT ) 1.5 nA and a sample bias of USB ) +20 mV. The images were taken over the course of several months providing reproducible results in both instruments. The data are unprocessed except for background subtraction. Fixation of nanotubes to the graphite surface is aided by the interaction between the top graphene sheet and the nanotubes.25 Initial imaging of a freshly prepared sample in ambient conditions was rather difficult, resulting in blurred, fuzzy images due to the rolling of the nanotubes (e.g., Figure 1a). With time, the nanotubes settled down on the surface (possibly aligning themselves with the graphene lattice), maximizing the interaction and allowing imaging by STM with little or no rolling. Figure 1 demonstrates the typical process of flattening out a nanotube bundle due to interaction with a swiftly scanned tip. In Figure 1a, a nanotube bundle from a 19 h stirred sample is first scanned with the tip at a line frequency of approximately 4 Hz (400 lines per image). As the tip scans the surface, the interaction with the sample causes the bundles to spread apart. Figure 1b is the eighth frame out of a series of images taken over the same area. The bundle has spread out with a greater number of individual tubes being distinguishable next to each other on the surface. The fact that this process (illustrated in Figure 1c) is observed suggests that the functionalization reaction (19 h stirred) did not succeed in completely breaking up the bundles, leading to varying degrees of functionalization on SWCNTs throughout the bundles. However, even along a single nanotube, areas of functionalization may alternate with unfunctionalized stretches. A similar phenomenon has been reported during the fluorination of nanotubes by Halas and co-workers.12 Figure 2 shows Nano Lett., Vol. 4, No. 8, 2004
Figure 2. Areas of functionalization alternate with unfunctionalized (black arrows) stretches: (19 h stirred) USB ) +20 mV, IT ) 1.5 nA.
nanotubes from a 19 h stirred sample. The alternation of functionalized areas (thick black shadow to the left of a bright ridge) with unfunctionalized areas (thin light line between) is visible on the nanotube in the center. Traces of other unfunctionalized or partially functionalized nanotubes are visible throughout the image. The coexistence of densely functionalized areas on individual tubes with unfunctionalized stretches, seen in Figure 2, seems to suggest together with data we obtained from 1 h stirred samples (almost indistinguishable from unfunctionalized nanotubes, see Supporting Information) that the reaction is difficult to start, but once a defect has been created by functionalization of one site, further reaction readily occurs in the vicinity. Nanotubes can be distinguished by the degree of functionalization. In order for a nanotube to be visible by STM, tunneling must be possible. Because unfunctionalized nanotubes are metallic or semiconducting, STM is able to image them. When functionalized, however, the formation of the sp3 cyclopropane structure results in a disruption in the sp2 lattice, altering the electronic properties of the nanotube and resulting in a larger band gap depending on the degree of functionalization.5 The nanotubes that are sparsely functionalized have a smaller band gap and are more easily visible in the images. Heavily functionalized nanotubes become invisible. As seen in Figure 2, when a high degree of functionalization has occurred, only the functional groups are imaged under our typical conditions. At a sample bias of +20 mV, only the functional groups and graphite surface are imaged. Tunneling into the functional groups is aided by the electron affinity of the oxygen atoms in the malonate groups. In images with higher magnification, a long range order (periodicity) is frequently observed in the arrangement of the functional groups. This is seen within the nanotube as Nano Lett., Vol. 4, No. 8, 2004
well as between nanotubes. A number of examples are shown in Figure 3. Figure 3a shows a very clear image of functional groups being spaced out evenly by approximately 4.6 nm (i.e., approximately 20 carbon rings) along a number of tubes. A sequence of images from the same area taken on the same day is provided as Supporting Information. Figure 3e is magnified from this image for ease of viewing. Figure 3b shows an earlier image of another bundle with periodic patterns (approximately 5 nm) within the nanotubes; the periodicity here was disregarded by us at the time as being possibly due to imaging artifacts. However, after observing periodicity as clearly as in Figure 3a we went back over old images and found that periodicity had been present all along. Another periodicity (different sample, different day) is evident in Figure 3c. The periodicity along the three parallel tubes running vertically in this image is rather large with 12.2 nm. However, a certain degree of correlation between tubes is most apparent in this image. We do not presently have a sound explanation for this cross-correlation. It could be speculated that tubes of similar chirality tend to accumulate next to each other when bundles are agitated or that interactions between the tubes override the effects of chirality within the tube. Finally, we also found examples of periodicity (2.3 nm) during preliminary studies with an ultrahigh vacuum instrument (RHK UHV 300 STM) under different conditions (electrochemically etched W tip, USB ) -100 mV, IT ) 155 pA), as show in Figure 3d. The variety of situations in which periodicity was observed as well as the quality in particular of the image in Figure 3a and 3e (see also Supporting Information) lead us to conclude that we are observing a real phenomenon rather than an imaging artifact. Figure 3e in particular highlights irregularities in the periodic structure and a crossing of two functionalized nanotubes, hence excluding noise or multiple tips as possible origins. From experience in imaging various physisorbed and chemisorbed systems with STM, it can further be concluded that a covalent bond (or similarly strong interaction, which in this particular system could hardly be conceived of) between functional groups and nanotubes indeed exists. Physisorbates would be sheared of by interaction with the STM tip. The considerably large distance between the functional groups in the patterns (e.g., 4.6 nm vs 0.142 nm26 for a Cd C bond) leads us to conclude that long-range electronic effects must play a role rather than steric effects or simple chemical bonding arguments. If the Bingel reaction indeed, as proposed, involves a nucleophilic attack on the SWCNT sidewall by a negatively charged diethyl malonate group (after deprotonation by DBU22), this nucleophilic attack would be eased in areas of lower electron density at the sidewall. Initially, when the first deprotonated (i.e., negatively charged) malonate group is brought in close proximity with the metallic surface (i.e., delocalized electrons) of an unfunctionalized, defect-free nanotube, a positive image charge may be formed, easing the nucleophilic attack. In a defect-free nanotube, this process is met with a significant activation barrier. However, spatial fluctuation of electron density in a flawed nanotube would aid the process and steer 1543
Figure 3. Examples of periodicity on 19 h stirred samples. USB ) +20 mV, IT ) 1.5 nA. (a) Well-resolved image of individual functional groups on a nanotube bundle, showing clear periodicity of approximately 4.6 nm. (b) Earlier example of periodicity in functionalization. (c) Example of three parallel nanotubes with larger periodicity and correlation of functional group location between the tubes. (d) Image obtained with UHV STM, USB ) -100 mV, IT ) 155 pA, showing a periodicity of approximately 2.3 nm. (e) Enlargement of section from (a) showing a crossing of two nanotubes.
the site of attack toward areas along the tube with a lower electron density. If the attachment of the first functional group was to lead to regular spatial fluctuations, a pattern for subsequent attack would be prescribed. It is the pattern that we have observed on a large number of nanotubes. The process is illustrated in Figure 4. The varying distances between groups between different tubes may be due to the varying diameter and helicity of the nanotubes, leading to a 1544
different electronic structure and hence a different periodicity of electron-deficient locals. Electronic structure calculations will be required to verify this hypothesis. Not all samples exhibit periodicity, however. Rather, it appears that a only a certain set of parameters favors the observed periodicity. One hour stirred samples, for example, exhibit an extremely low degree of functionalization (see Supporting Information), and no examples of periodic Nano Lett., Vol. 4, No. 8, 2004
Figure 4. Schematics illustrating the process of induced reactivity by the nucleophile.
Figure 5. Example of irregular functionalization (19 h, sonicated) on two parallel nanotubes, USB ) +20 mV, IT ) 1.5 nA.
functionalization have been found. Samples sonicated for 19 h, on the other extreme, tend to be very heavily functionalized such as shown in Figure 5. Note that (a) the density of functional groups is higher than in samples exhibiting periodicity and (b) there is 2-fold evidence of these tubes being functionalized more evenly around the circumfence. First, the functional groups no longer line up neatly on top of the tubes as in all images in Figure 3. They form a more frayed pattern, indicative of more even functionalization around the circumference. Second, the image is more fuzzy than the images in Figure 3, due to more tip-induced movement of the tube during imaging. This observation is rather difficult to quantify, but we did note that sonicated samples are more difficult to image than stirred samples. Tubes appear to interact less with the HOPG lattice and move around more during imaging at comparable conditions. Our observations can be explained by the following two postulates. (1) Sonication increases the energy of the reactants, hence increasing the probability of attachment of the malonate group to the SWCNT’s. Even if a periodic pattern was formed at some point in the process, crossing the reaction barrier to further functionalization becomes more likely. (2) Sonication during the reaction leads to in situ separation of the bundles, leading to more uniform functionalization around the circumference. As a result, the Nano Lett., Vol. 4, No. 8, 2004
nanotubes can no longer efficiently form π-π interactions with the graphite substrate, complicating the imaging process. We did not perform any shortening procedure on our nanotubes prior to functionalization, hence our nanotube bundles were still attached to each other in a loose network. Postulate (2) would predict a falling apart of the nanotube bundles after functionalization, which was not clearly observed in our samples. It should also be noted that sonication has been reported to lead to the polymerization of o-dichlorobenzene.27 No sonopolymer was found to adhere to our CNTs during imaging, likely due to subsequent processing (washing, filtering, sonication in DMSO). An influence of the polymer on the reaction itself cannot be excluded, however. This may account for different reaction patterns under stirred vs. sonicated conditions. In this paper, we have reported the discovery of longrange periodicity on sidewall functionalized SWCNTs. This phenomenon is likely tied to the electronic structure and the geometry of the nanotube. It has been observed in the specific case of medium-heavy functionalization conditions for the nucleophilic Bingel reaction. At the onset of the reaction, an incubation period of several hours is needed before regular patterns can be detected on the nanotubes. Sonication of the reaction mixture appears to accelerate the reaction and increase the degree of functionalization past the point where long range periodicity is exhibited. We are continuing to study the nature of the observed long-range induced reactivity, in particular the behavior of other nucleophilic, electrophilic, radical, and carbene reactions. Our observations are of relevance both for the understanding of previously reported reactions and for the development of synthetic strategies for purposefully patterned carbon nanotubes with one or more types of functional groups. Acknowledgment. We thank Alex Adronov, Paul Ayers, and Bob Haddon for fruitful discussions and the Brockhouse Institute for Materials Research for the use of their Nanoscope II. The Adronov group also assisted us in the setup of our synthesis. This work was made possible by the financial assistance of McMaster University and the Natural Sciences and Engineering Research Council of Canada. The UHV STM was purchased with funds from the Canadian Foundation for Innovation and the Ontario Innovation Trust. Supporting Information Available: Images of unfunctionalized and 1 h stirred nanotubes. Further images of longrange periodicity. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603-605. (2) Bethune, D. S.; Klang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605-607. (3) Avouris, Ph. Acc. Chem. Res. 2002, 35, 1026-1034. (4) Louie, S. G., Carbon Nanotubes. In Carbon Nanotubes - Synthesis, Structure, Properties, and Applications; Dresselhaus, M. S., Dresselhaus, G., Avouris, Ph., Eds.; Springer-Verlag: Heidelberg, 2001; Vol. 80, p 113-145. (5) Kamaras, K.; Itkis, M. E.; Hu, H.; Zhao, B.; Haddon, R. C. Science 2003, 301, 1501. 1545
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Nano Lett., Vol. 4, No. 8, 2004
