NANO LETTERS
Measuring the Adhesion Forces between Alkanethiol-Modified AFM Cantilevers and Single Walled Carbon Nanotubes
2004 Vol. 4, No. 1 61-64
Mark A. Poggi and Lawrence A. Bottomley* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
Peter T. Lillehei AdVanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681-2199 Received October 6, 2003; Revised Manuscript Received November 7, 2003
ABSTRACT Nanotube/polymer composite interfaces are of interest for next generation composites. We have examined the adhesion between thiolated AFM cantilever tips and single walled carbon nanotube paper using chemical force microscopy. We have observed a direct correlation of adhesion force with respect to the thiol terminal group (NH2 > CH3 > OH). Our findings demonstrate that the interfacial interactions between single walled carbon nanotubes and terminally functionalized hydrocarbons can be evaluated with an atomic force microscope, provided that one accounts for variations in contact area caused by tip shape and sample topology.
Single walled carbon nanotubes (SWNTs) display a unique combination of properties that may be of great utility in the production of next-generation high strength, lightweight polymer composites. SWNTs have a large tensile modulus and high thermal and electrical conductivity.1-4 Incorporation of SWNTs into polymeric composites is being actively pursued by a number of groups.5,6 A key challenge in fabrication of these composite materials involves alignment and orientation of SWNTs. The mechanical integrity of the composite material will be optimal with uniform dispersal of the nanotubes within the polymer matrix and strong adhesion of the polymer to the sidewall of the nanotube. Dispersion of nanotubes in polymer composites has been achieved following chemical functionalization of the nanotube backbone7-9 and grafting of polymers to the nanotube through covalent attachments.10 While chemical modification of the nanotube sidewall promotes dispersion,11,12 it also alters the electronic properties of the nanotube.13-16 Noncovalent modifications of the carbon nanotube backbone have been used to enhance nanotube solubility in organic17,18 and aqueous19 solutions. Our goal is to obtain homogeneous dispersion of SWNTs through molecular design of the polymer matrix.20,21 We seek to identify chemical moieties * Corresponding author. E-mail:
[email protected]. Phone: (404) 894-4014. Fax: (404)-894-7452. 10.1021/nl0348701 CCC: $27.50 Published on Web 12/02/2003
© 2004 American Chemical Society
that bind strongly to the sidewall of the nanotube. Polymers incorporating these moieties should, in principle, inhibit selfassociation of the nanotube and promote their dispersion within the matrix. The integrity of the nanotube/polymer interface contributes significantly to the strength of the composite. However, this interface has proven difficult to characterize.22-24 The atomic force microscope, when used as a surface force apparatus, enables measurement of the adhesion between specific functional groups and the outer surface of a SWNT. We present herein evidence that adhesive interactions between the sidewall of SWNTs and self-assembled alkanethiol monolayers (SAM) depend on the identity of the terminal group on the SAM. We have examined the interfacial adhesion between a ω-substituted undecanethiol monolayer and the outer surface of a SWNT. The terminal groups investigated herein were NH2, OH, and CH3. A NanoScope IIIa multimode scanning probe microscope (Veeco Instruments) was used to acquire topographical and force volume images of SWNTs confined in a carbon nanotube paper (obtained from the Advanced Materials and Processing Branch, NASA Langley Research Center).25 The piezo scanner was calibrated in x, y, and z using NIST certified calibration gratings (MikroMasch). Force constants of rectangular cantilevers (MikroMasch) with
Figure 2. Topographical (left) and force volume (right) images of the SWNT paper acquired using a hydroxyl-terminated, alkanethiol-functionalized gold-coated tip. The horizontal scale bars in both images are 12 nm. Scan domain is 50 nm × 50 nm.
Figure 1. AFM topographic image of the SWNT paper acquired in contact mode using a clean, gold-coated tip. The horizontal scale bar in the image is 250 nm; scan domain is 1.0 × 1.0 µm.
gold-coated tips were determined using the thermal resonance technique and ranged from 0.2 to 0.7 N/m.26-29 Tip radii for each cantilever were determined with tip-deconvolution software (SPIP by Image Metrology). All experiments were performed under a nitrogen atmosphere to maintain the relative humidity below 2%. Force volume experiments were conducted in relative triggering mode. The same loading rate was used in all force spectroscopic measurements (200 nN/s, scanner z-velocity ) 400 nm/s). Cantilever tips were coated with alkanethiols that self-assembled into a closepacked monolayer on the gold-coated tip during immersion in ethanolic solution. Tips were modified with either bis(11-hydroxyundecyl) disulfide,30 11-amino-undecanethiol (Dojindo Chemicals), or 11-dodecanethiol (Sigma-Aldrich). After several hours, each cantilever was removed from the derivatizing solution, washed with ethanol, and dried under a stream of nitrogen. The assembly of the alkanethiol onto gold surfaces was verified using microcantilever-based sensors31 and lateral force microscopy.32 In the former method, cantilever deflection was monitored during their exposure to ethanolic alkanethiol solutions. Increases in cantilever deflection were proportional to the mass loading of the alkanethiol onto the gold-coated cantilever. In the latter method, thiols were patterned onto template-stripped gold surfaces33 using microcontact printing and imaged via lateral force microscopy. The changes in friction force observed were similar to those reported previously.32 An AFM topographical image acquired in contact mode using an underivatized cantilever (Figure 1) reveals the presence of individual tubes and bundles of tubes throughout the paper. Figure 2 (left panel) is a topographical image acquired in force volume mode at higher magnification with a cantilever derivatized with a hydroxyl-terminated thiol. Figure 2 (right panel) is the force volume image acquired in parallel with the topographical image. Each pixel corresponds to an individual force measurement. Force-separation curves were extracted from force volume images (256 force measurements per image) with a custom-designed data extraction program. Adhesion forces were calculated from the point of maximum cantilever deflection. For example, with the hydroxyl functionalized cantilever tip, the average 62
Figure 3. Adhesion force map of the data presented in Figure 2. Blue represents an adhesive force from 0 to 4 nN, maroon 4-8 nN, yellow 8-12 nN, and green 12-16 nN.
adhesion force obtained from 2560 force-separation events acquired at random locations on the nanotube paper is 8.7 ( 6.7 nN. Figure 3 depicts a mapping of the adhesion force with the position of the tip over the nanotube paper. The figure was generated by point-by-point multiplication of the spring constant of the cantilever times its maximal deflection. A direct correlation exists between the topographical and adhesion force images. The highest adhesion forces were observed in areas that are low in the topographical image (in the “valley” between nanotubes), whereas the lowest adhesion forces were observed on the highest regions in the topographical image (along the backbone of a nanotube or bundle). This correlation is not without precedent. McKendry and co-workers34 found a 50% narrower distribution of adhesion forces between carboxylic-acid-terminated monolayers on gold obtained at a single point compared to those obtained over a 1 mm2 area and attributed this distribution narrowing to differences in tip-surface contact area. Eaton and co-workers35 have observed that adhesive forces between an uncoated AFM tip and the surface of a poly(methyl methacrylate) composite material scaled with area of contact. Figure 4 depicts histograms summarizing the adhesion forces obtained for each terminal group on the alkanethiol. Clearly, the arithmetic mean of adhesion forces obtained is an inappropriate measure of the interaction between the Nano Lett., Vol. 4, No. 1, 2004
Figure 4. Histograms showing the distribution of forces required to detach a chemically modified cantilever from the side of a SWNT. (a) OH-terminated thiol. (b) CH3-terminated thiol. (c) NH2terminated thiol. Table 1. Adhesion Forces Normalized for Tip Radius and Sample Topography thiol endgroup
mean adhesion force (nN)
F/R (pN/nm)
No. force curves
-OH -CH3 -NH2
4.1 ( 1.1 1.6 ( 0.4 6.3 ( 1.1
88 ( 23 147 ( 40 344 ( 61
696 142 1029
terminally substituted alkanethiol and the SWNT. The magnitude and range of adhesion forces is dependent upon contact area; the contact area is related to both sample and tip topology (see Table 1). A relative triggering threshold of 3 nm was employed in collecting the data presented in Figures 2-4. Additional experiments conducted at gradually increasing thresholds gave no observable differences in the adhesion force (data not shown), suggesting that over the force range of 0.5-8.0 nN there is no buckling of the nanotubes. This observation is consistent with the high in- and out-of-plane strength of SWNTs.36-38 Nano Lett., Vol. 4, No. 1, 2004
The latter observation suggests a straightforward means for determining the dispersion of adhesion force devoid of variances in contact area. Since the SWNT does not appear to be buckling under the force regime of our experiments, then adhesive force measurements obtained along the top of the cylindrical tube (or nanotube bundle) should exhibit minimal variation in contact area. Selection of adhesion force data acquired only along the tops of ridges yields average adhesion forces listed in Table 1. The range in adhesion force values used to compute this area corrected value is commensurate with the first peaks in the histograms presented in Figure 4. A common method for normalizing the impact of tip topology on AFM-based adhesion force measurements involves division of the mean adhesive force by the radius of the cantilever tip.39,40 Tip radii for each cantilever were determined with tip-deconvolution software (SPIPTM by Image Metrology). Using this method, our force per unit area values (Table 1) depend on the alkanethiol terminal group (OH- > CH3 > NH2). This observation is consistent with the known affinity of single-walled carbon nanotubes for amine-functionalized surfaces, whereas methyl-functionalized substrates yielded substrates almost totally devoid of nanotubes.41 The affinity for amine groups has been exploited in the construction of nanotube-based structures.42 Thus, our findings demonstrate that the interfacial interactions between SWNTs and terminally substituted hydrocarbons can be evaluated with an AFM, provided that one accounts for variations in contact area caused by tip shape and sample topology. In a broader context, our findings suggest that surface adhesive force measurements based on multiple force curves taken at a single point on the surface or based on the interpretation of single force curves acquired at different locations on the surface may be subject to a systematic error resulting from variations in contact area. Second, surface adhesive force measurements acquired using multiple force curves acquired at several points on the surface without consideration of the compliance of the surfaces may also be subject to errors resulting from variations in contact area. Contact area assessment requires characterization of tip shape, consideration of sample topography, and careful examination of the compliance of both surfaces. We are presently determining the strength of adhesive interactions between SWNTs and other alkanethiol-coated tips. Our intent is to elucidate structure-adhesive force correlations as the length of the thiol and identity of the thiol terminus is systematically varied. We are also modeling the contact area between the chemically modified cantilever tip and the SWNT to elucidate rupture forces at the single molecule level. This correlation should be of value in enhancing dispersal of SWNTs within polymer composite systems and promoting strong adhesion of the polymer to the nanotube. Acknowledgment. This work was supported by the NASA-sponsored Graduate Student Researchers Program (NGT-1-02002), the National Institutes of Health (EB000767), and the Polymer Education and Research Center at Georgia Tech. 63
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NL0348701
Nano Lett., Vol. 4, No. 1, 2004
