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Measurement of the Kinetics of Photo-Oxidation of Self-Assembled Monolayers Using Friction Force Microscopy Karen S. L. Chong, Shuqing Sun, and Graham J. Leggett* Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. Received January 4, 2005. In Final Form: February 9, 2005 The advancement of molecular nanotechnology requires new tools for the characterization of surface chemical reactivity with nanometer spatial resolution. While spectroscopy on sub-100 nm length scales remains challenging, friction force microscopy (FFM) is a promising tool for the characterization of molecular materials, although to date it has been little used in studies of surface reactivity. Here we report the use of FFM to measure the kinetics of photo-oxidation of self-assembled monolayers (SAMs) of alkanethiols adsorbed on gold surfaces. Two alternative approaches (analysis of friction-load plots and the use of line sections through images of patterned materials) are compared and found to yield data in very good agreement, with rate constants being found to be in good agreement despite being carried out on different microscopes. The use of line-section analysis provides a convenient method for the quantification of the extent of reaction in nanometer-scale patterns created in SAMs by the novel approach of scanning near-field photolithography.
Introduction The creation of novel nanostructured molecular devices and materials demands the development of methods for the transformation of chemical structure and function on nanometer length scales.1 Such systems include nanostructured arrays of biological molecules for ultrahighsensitivity detection of analytes,2 polymeric light-emitting diodes3 and photovoltaic cells,4 smart polymer systems for sensing and other applications,5 molecular electronics,6 and DNA nanotechnology.7 To facilitate the development of such methods, tools are required that are capable of yielding quantitative information on molecular structure and bonding. Ideally, these would be based on spectroscopic methods. However, there is currently a lack of suitable techniques. Conventional surface spectroscopic tools lack the spatial resolution to address truly nanoscale molecular structures (i.e., smaller than 100 nm), although recent developments in secondary ion mass spectrometry are promising in this respect.8 The only approach that has thus far yielded genuine spectroscopic information on nanometer length scales is near-field scanning optical microscopy (NSOM). Recently, some impressive strides have been made in the use of apertureless Raman microscopy.9-11 However, these techniques remain tech* Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Li, X.-M.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2004, 14, 2954. (2) Lee, K.-B.; Kim, E.-Y.; Mirkin, C. A.; Wolinsky, S. M. Nano Lett. 2004, 4, 1869. (3) Boroumand, F. A.; Fry, P. W.; Lidzey, D. G. Nano Lett. 2005, 5, 67. (4) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (5) Ahn, S. J.; Kaholek, M.; Lee, W.-K.; LaMattina, B.; LaBean, T. H.; Zauscher, S. Adv. Mater. 2004, 16, 2141. (6) Wang, W.; Lee, T.; Reed, M. A. J. Phys. Chem. B 2004, 108, 18398. (7) Le, J. D.; Pinto, Y.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. Nano Lett. 2004, 4, 2343. Voicu, R.; Boukherroub, R.; Bartzoka, V.; Ward, T.; Wojtyk, J. T. C.; Wayner, D. D. M. Langmuir 2004, 20, 11713. (8) Ding, Y.; Micheletto, R.; Okazaki, S.; Otsuka, K. Appl. Surf. Sci. 2003, 211, 82. (9) Hartschuh, A.; Sanchez, E. J.; Xie, X. S.; Novotny, L. Phys. Rev. Lett. 2003, 90, 095503.
nically challenging. There is no generally applicable technique for the characterization of surface chemistry on nanometer length scales. Friction force microscopy (FFM)12-17 has attracted significant interest as a tool for the characterization of molecular materials. A growing body of literature has begun to accumulate on the characterization of selfassembled monolayers (SAMs) of alkanethiols adsorbed onto gold surfaces by FFM. While studies of SAMs by FFM offer considerable promise as a means of understanding molecular tribology at a fundamental level, FFM is also a tool that is proving useful for the characterization of SAM structure and bonding. Early studies examined the influence of adsorbate terminal-group chemistry on tip-sample frictional interactions.18-25 FFM has proved to be a very convenient means of differentiating between adsorbates with contrasting terminal group chemistries but which are in all other respects identical. Indeed, for nanopatterned materials composed of contrasting alkanethiols, FFM is perhaps uniquely capable of providing characterization.26,27 Variations in adsorbate order due (10) Hartschuh, A.; Pedrosa, H. N.; Novotny, L.; Krauss, T. D. Science 2003, 301, 1354. (11) Bachelot, R.; D’Hili, F.; Barchiesi, D.; Lerondel, G.; Fikri, R.; Royer, P.; Landraud, N.; Peretti, J.; Chaput, F.; Lampel, G.; Boilot, J.-P.; Lahil, K. J. Appl. Phys. 2003, 94, 2060. (12) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1990, 57, 2089. (13) Marti, O.; Colchero, J.; Mylnek, J. Nanotechnology 1990, 1, 141. (14) Overney, R.; Meyer, E. MRS Bull. 1993, May, p 26. (15) Carpick R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (16) Gnecco, E.; Bennewitz, R.; Gyalog, T.; Meyer, E. J. Phys.: Condens. Matter 2001, 13, R619. (17) Leggett, G. J. Anal. Chim. Acta 2003, 479, 17. (18) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (19) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (20) Green, J.-B.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (21) van der Wegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (22) Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925. (23) Burns, A. R.; Houston, J. E.; Carpick, R. W.; Michalske, T. A. Phys. Rev. Lett. 1999, 82, 1181. (24) Burns, A. R.; Houston, J. E.; Carpick, R. W.; Michalske, T. A. Langmuir 1999, 15, 2922. (25) Houston, J. E.; Kim, H. I. Acc. Chem. Res. 2002, 35, 547.
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to changes in adsorbate alkyl chain length, molecular packing density, and adsorbate-substrate registry have been characterized by FFM.28-41 Because of the capability of the technique for use in fluid media, solid-liquid interfacial interactions (for example, acid-base interactions) may be investigated.21,42 FFM offers considerable potential as a tool for the measurement of reaction rates in nanometer-scale systems,43 but to date, this has been little utilized. We have recently developed a novel lithographic tool, scanning near-field photolithography (SNP),26,27,44 in which an NSOM coupled to a UV laser is used to selectively oxidize alkanethiolates. The weakly bound alkylsulfonate oxidation products may either be replaced by contrasting thiols to yield patterned structures as small as 20 nm27 or be used as resist for the etching of three-dimensional structures into the underlying substrate.44 The macroscopic kinetics of oxidation of alkanethiols have previously been studied using spectroscopic tools such as static SIMS, X-ray photoelectron spectroscopy, Raman spectroscopy, SERS, RAIRS, and surface plasmon spectroscopy.45-56 However, to develop methods for the fabrication of molecular nanostructures using SNP and related techniques, there is an urgent need for methods capable of characterizing nanometer-scale reactions in a quantitative (26) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414. (27) Sun, S.; Leggett, G. J. Nano Lett. 2004, 4, 1381. (28) Joyce, S. A.; Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Phys. Rev. Lett. 1992, 68, 2790. (29) Liu, Y.; Evans, D. F.; Song, Q.; Grainger, D. W. Langmuir 1996, 12, 1235. (30) McDermott, M. T.; Green, J.-B. D.; Porter, M. D. Langmuir 1997, 13, 2504. (31) Wong, S.-S.; Takano, H.; Porter, M. D. Anal. Chem. 1998, 70, 5209. (32) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880. (33) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 1999, 111, 9797. (34) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2000, 113, 2413. (35) Salmeron, M. Tribol. Lett. 2001, 10, 69. (36) Barrena, E.; Palacios-Lidon, E.; Munuera, C.; Torrelles, X.; Ferrer, S.; Jonas, U.; Salmeron, M.; C. Ocal, C. J. Am. Chem. Soc. 2004, 126, 385. (37) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192. (38) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaluddin, S.; Lee, T. R.; Perry, S. S. Langmuir 1999, 15, 3179. (39) Shon, Y.-S.; Lee, S.; Colorado, R.; Perry, S. S.; T. R. Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (40) van der Vegte, E. W.; Subbotin, A.; Hadziioannou, G.; Ashton, P. R.; Preece, J. A. Langmuir 2000, 16, 3249. (41) Beake, B. D.; Leggett, G. J. Langmuir 2000, 16, 735. (42) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (43) Clear, S. C.; Nealey, P. F. J. Colloid Interface Sci. 1999, 213, 238. (44) Sun, S.; Leggett, G. J. Nano Lett. 2002, 2, 1223. (45) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5304. (46) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F. Anal. Chem. 1994, 66, 2170. (47) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (48) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (49) Lewis, M.; Tarlov, M. J.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574. (50) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (51) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174. (52) Cooper, E.; Leggett, G. J. Langmuir 1998, 14, 4795. (53) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089. (54) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (55) Horn, A. B.; Russell, D. A.; Shorthouse, L. J.; Simpson, T. E. J. Chem. Soc., Faraday Trans. 1996, 92, 4759. (56) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Chem. Mater. 1999, 11, 2191.
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fashion. In the present paper, we examine the potential of using FFM to explore the kinetics of SAM photooxidation in a quantitative fashion. We have compared two alternative approaches to the quantification of photooxidation reactions in patterned SAMs: first, the measurement of friction-load plots and, second, the use of line sections through images of patterned materials. We have found exceptionally good quantitative agreement between the two approaches, with the latter being readily applicable to nanostructured materials and providing a convenient and widely accessible tool for the evaluation of rates of reaction in nanoscale regions of a surface. Experimental Section Preparation of SAMs. Polycrystalline gold films were prepared by evaporating 25 nm of Au (Goodfellow, Cambridge, UK) onto Cr-primed glass microscope slides (no. 2 thickness, Chance Proper, UK). For easier location of the nanopatterns generated by SNP using FFM on a separate atomic force microscope (AFM, ThermMicroscopes Explorer, Veeco, Cambridge, UK), polycrystalline gold was evaporated through a copper grid to form microscopic relief features that could be visualized through an optical microscope. Alkanethiols were purchased from Fluka and were used as received. SAMs were formed by immersion of the substrate in a dilute solution of the appropriate thiol in ethanol for 18 h. After exposure to UV light, samples were immersed in a solution of hexadecanethiol (C15CH3) for 2 h to displace the oxidized species. Photo-Oxidation. During far-field illumination, samples were exposed to UV light with a wavelength of 244 nm from a frequency-doubled argon ion laser (Coherent Innova FreD 300C) emitting at a power of 100 mW. The power was fixed throughout. The beam diameter was ca. 1 mm. With one exception (the image data shown in Figure 2 below), the beam was passed through a convex lens prior to interaction with the sample in order to irradiate a larger area (diameter ≈ 24 mm) to facilitate measurement of contact angles and enable acquisition of friction data at multiple different locations. The exposure time was controlled using a shutter. Micropatterning was accomplished by exposing the sample to UV light through an electron microscope grid (2000 mesh, Agar, Cambridge, UK). Separate samples were used for each time point. Nanopatterning using near-field exposure was carried out using a ThermoMicroscopes Aurora 3 near-field scanning optical microscope (Veeco, Cambridge, UK) operating in shear-force mode. Tuning fork NSOM probes made from fused silica fibers, with a nominal aperture diameter of 50 nm, were obtained from Veeco (Cambridge, UK). During near-field exposure, the laser power was reduced to smaller levels to prevent damage to the NSOM probe. Estimation of the power during near-field exposure was difficult because of the small area of illumination. Empirically, it was found that the quality of contrast in the near-field patterned samples was variable for laser powers up to 6 mW but was unchanged at higher powers. FFM. Measurements of friction forces were made on two different instruments. The sample was immersed in ethanol throughout the acquisition of data for both instruments. Unless otherwise stated, error bars are given as the standard deviation of at least three different measurements made on different samples. Friction-load measurements were made on a Digital Instruments Nanoscope Multimode IIIa atomic force microscope (Digital Instruments, Cambridge, UK). The probes used were silicon nitride Nanoprobes (Digital Instruments, Cambridge, UK). The nominal force constants of these probes were 0.06 or 0.12 N m-1. Force constants of individual cantilevers were measured using a routine contained within the microscope software and based upon the measurement of the thermal oscillations of the cantilever.57 No attempt was made to calibrate the torsional force constants of the cantilevers. Although a number of methods for accomplishing this have been published, we used instead an (57) Hutter, J. L.; Bechhoeffer, J. Rev. Sci. Instrum. 1993, 64, 1868, 3342.
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approach based upon the normalization of friction coefficient data to internal standards previously reported by a number of workers.17,37-39 A series of measurements of friction-load behavior was made using a single cantilever. Friction force measurements were made from friction loops, acquired in the scope mode, at 10 separate locations on the sample surface. The friction coefficients were then determined from the gradients of the detector response-load plots, determined using linear regression, and normalized by dividing each by the magnitude of the largest coefficient. Although this means that it is difficult to compare the absolute friction coefficients reported here with those acquired on other instruments, the relative magnitudes of the coefficients determined for the different SAMs are highly repeatable. By utilizing this approach, it was possible to carry out multiple repeat determinations of the friction coefficient and average data recorded on different days using different cantilevers. The data presented here are the result of five repetitions, and the error bars shown are the standard error in the mean for each data point. FFM measurements were made on patterned SAMs using a ThermoMicroscopes Explorer AFM (Veeco, Cambridge, UK). Cantilevers with nominal spring constants of 0.12 N m-1 were used. Contact Angles. Advancing water contact angles were measured at room temperature (21 °C) using the sessile drop method on a Rame-Hart model 100-00 contact angle goniometer. Results shown are the means of at least three different measurements. Separate samples were used for each time point.
Results and Discussion Contact Angle Data. Exposure of SAMs to UV light in the presence of oxygen leads to the conversion of strongly bound alkanethiolate species to weakly adsorbed alkylsulfonates:
Au-SR + 3/2O2 + e- f Au0 + RSO3The alkylsulfonates may be readily displaced by rinsing or by the adsorption of unoxidized alkanethiols on exposure to an appropriate solution. This process forms the basis of the photopatterning of SAMs using both mask-based and near-field methods. We have previously studied this process extensively using spectroscopic methods. In the present study, however, contact angle measurement has been used as a convenient measure of the extent of oxidation of the SAM. Partial oxidation of a carboxylic acid-terminated monolayer leads to the conversion of a fraction of the adsorbate molecules to sulfonates; immersion of the monolayer in a solution of a methyl-terminated adsorbate will result in the displacement of these oxidation products and the formation of a mixed monolayer containing both methyl- and carboxylic acid-terminated thiols. In contrast to mixed SAMs formed by the coadsorption of contrasting thiols, which tend to phase-separate,58 this process should lead to a randomly mixed layer, with the position of insertion of the methyl-terminated thiol depending upon which sites have been subject to oxidation. As oxidation progresses to completion, the advancing contact angle of water will increase until it reaches a limiting value that corresponds to the complete oxidation and displacement of the adsorbate molecules.59 For a methyl-terminated SAM, the trend will be reversed, with the water contact angle decreasing to a low limiting value on complete oxidation and displacement of the adsorbate molecules. Figure 1 shows the variation in the advancing water contact angles of carboxylic acid- and methyl-terminated adsorbates with short and long alkyl chains following (58) Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109. (59) Brewer, N. J. PhD Thesis, UMIST, 2001.
Figure 1. Variation in the advancing water contact angle of SAMs following exposure to UV light and immersion in a solution of a contrasting thiol. (a) Photo-oxidation of polar SAMs followed by immersion in a solution of C11CH3. (b) Photooxidation of methyl-terminated SAMs followed by immersion in a solution of C10COOH. The error bars (calculated as the standard deviations of at least three measurements on different samples) were similar in magnitude to the dimensions of the symbols shown in the graphs.
exposure to UV light for varying periods of time and immersion in an ethanolic solution of a contrasting adsorbate. Figure 1 shows that the expected behavior was observed in the present study. Previous studies by static SIMS suggest that the complete oxidation of the monolayer occurs at the point where no further change occurs in the contact angle following immersion in a solution of a contrasting thiol.59 For both carboxylic acid- and methylterminal groups, the short-chain adsorbates were found to oxidize faster than the long-chain adsorbates because of the greater mobility of their alkyl chains, which allows comparatively more ready access to the metal-sulfur bond by oxygen species than is the case for the long-chain adsorbates. Carboxylic acid-terminated SAMs oxidize much faster than methyl-terminated adsorbates on exposure to light with a wavelength of 244 nm. This is the reverse of behavior previously reported using a mercury arc lamp52 but is in agreement with behavior reported previously using a UV lamp fitted with a filter to eliminate short-wavelength UV light emission.53 Using a frequencydoubled argon ion laser, we were able to selectively excite only one oxidation pathway, probably involving the generation of hot electrons, which go on to initiate reaction between oxygen and the adsorbate headgroup, while the arc lamps used in earlier studies probably excited more than one process, including, possibly, ozonolysis. For exposure at around 250 nm, we have showed elsewhere that differences in the kinetics of oxidation of SAMs correlate both with differences in the alkyl chain length of the adsorbate and with differences in the adsorbate terminal group, attributed to differences in the dipole moment and, hence, the work function (as determined by contact potential measurements) of the monolayer.
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measurements and, second, the analysis of line sections through images of patterned materials. These are described separately below. Friction-Load Relationships. A number of different approaches have been explored to the analysis of contact mechanics in FFM. In the literature relating to SAMs, many authors, including ourselves, have previously used Amonton’s law
FF ) µFN
Figure 2. Friction force microscopy images of C10COOH SAMs exposed to UV light for 2, 4, and 8 s through a mask and immersed in a solution of C11CH3.
Friction Force Microscopy. The frictional characteristics of photo-oxidized SAMs were explored using FFM. Qualitatively, FFM provides evidence for the formation of photopatterned materials and enables the examination of feature sizes and edge resolution. Figure 2 shows FFM images of C10COOH SAMs that have been exposed to UV light through a mask and immersed in a solution of C11CH3. In regions where the carboxylic acid-terminated adsorbates have been masked, the contrast is bright because there is a strong frictional interaction. This is because the surface of the tip is composed of a layer of polar silicon dioxide and this interacts strongly with the polar, acid-terminated regions of the surface. In the exposed regions (the squares), oxidation of the adsorbates is followed by their displacement by methyl-terminated thiols. This leads to the observation of darker contrast in the FFM images because the polar tip interacts less strongly with these regions than with the acid-terminated ones. Qualitatively, it is clear that the magnitude of the contrast difference between the masked and exposed areas increases with the time of exposure to the UV light. After 8 s, little further change was observed, suggesting that the reaction had come to completion. It should be noted that while acquiring the data in Figure 2 the laser beam was fully focused; it was defocused in all of the experiments described below. Two approaches to the quantification of such data were explored: first, the use of friction-load
(1)
(where µ is the coefficient of friction) or one of its modified forms to analyze friction-load data. The drawback with this approach is that the interpretation of Amonton’s Law is dependent upon a macroscopic model for friction that involves the contact between multiple asperities on the sliding surfaces. Intuitively, one might expect the situation in FFM to be much better modeled by a single asperity contact mechanics model, such as the Johnson-KendallRoberts (JKR) model or the Derjaguin-Muller-Toporov (DMT) model. The JKR model predicts that the friction force will be proportional to the contact area between the tip and the surface and that there will be a nonlinear relationship between the friction force and the load. This behavior has been observed previously in studies using the interfacial force microscope (IFM) but has not been widely reported in studies of monolayer friction using atomic force microscopes, where linear behavior has typically been reported. There remain significant uncertainties about the contact mechanics in FFM of monolayer systems. For instance, we recently showed that good fits could be obtained to the measured friction-load behavior using both Amonton’s Law and the JKR model in the limited range of force typically used in FFM experiments.55 Some workers have alternatively proposed that pressuredependent contributions to the frictional force in monolayer systems may alter the form of the friction-load behavior for a single asperity contact such that linearity is observed.15 Under such circumstances, the surface shear stress, τ, is given by
τ ) τ0 + RP
(2)
where P is the pressure and R is a constant. The friction force is given by
FF ) τ0A + RFN
(3)
where A is the contact area.15 Such pressure-dependent phenomena may explain the apparent linearity of frictionload behavior in FFM of molecular monolayers. In such circumstances, the “apparent friction coefficient” would be R. It is beyond the scope of the present paper to resolve these issues. For present purposes, we note that both eqs 1 and 3 may yield linear friction-load behavior, and this is supported empirically. Figure 3 shows the friction force as a function of the applied load for short- and long-chain carboxylic acid-terminated SAMs following exposure to UV light for varying periods of time and immersion in solutions of C11CH3. The relationship between the friction force and the load appears to be linear for both adsorbates, and fitting by linear regression yielded a straight line whose gradient decreased with increasing exposure of the sample to UV light, reflecting the progressive replacement of polar adsorbates with nonpolar ones. The gradient of the friction-load plot may thus be used as a measure of the strength of the frictional interaction between the tip and the sample. We shall hereafter refer to the gradient
Kinetics of SAM Photo-Oxidation
Figure 3. Friction-load plots for (a) C10COOH and (b) C2COOH SAMs exposed to UV light for varying periods of time and immersed in a solution of C11CH3.
of the friction-load plot as the coefficient of friction, µ, but without attempting to distinguish between Amontontype behavior and the pressure-dependent single asperity contact in eq 3. In the remainder of the paper, we explore the value of this analysis as an empirical means of quantifying the extent of photo-oxidation. Coefficients of friction, µt, were measured for both adsorbates as a function of the time of exposure to UV light (in each case followed by immersion in a solution of C11CH3) and are shown in Figure 4a. Measurements were also made for methyl-terminated SAMs, following exposure to UV light for varying time periods and subsequent immersion in a C10COOH solution, and these are shown in Figure 4b. For the carboxylic acid-terminated SAMs, µt is initially large but declines with increasing exposure to UV light because, as the monolayer becomes progressively oxidized, an increasing fraction of the acidterminated adsorbates are replaced by methyl-terminated thiols that exhibit reduced frictional interactions with the tip. For the methyl-terminated SAMs, the reverse is true, with increasing oxidation and displacement of the adsorbate molecules leading to a progressive increase in µt. Comparison of Figures 1 and 4 shows that there is a clear correlation between µt and the variation in the water contact angle as a function of exposure. To make the correlation more explicit, Figure 5 shows the value of µt as a function of cos θ for the carboxylic acid-terminated thiols. In a recent paper, we reported that the coefficient of friction of mixed SAMs formed by the coadsorption of alkanethiols with methyl- and hydroxyl-terminal groups varied linearly with cos θ. To a first approximation, the value of cos θ may be considered to be proportional to the fraction of polar-terminated adsorbates in a mixed SAM, using Cassie’s equation.60 Figure 5 indicates that a similar relationship applies for SAMs that have been subjected (60) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990.
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Figure 4. Variation in the coefficient of friction, µt, as a function of exposure time for (a) carboxylic acid-terminated SAMs following immersion in a solution of C11CH3 and (b) methylterminated SAMs following immersion in a solution of C10COOH.
Figure 5. Variation in the coefficient of friction of carboxylic acid-terminated SAMs, following oxidation for variable time periods and immersion in a solution of C11CH3, as a function of cos θ.
to photo-oxidation and immersion in a solution of a contrasting thiol. This indicates that the coefficient of friction provides a very convenient measure of the extent of photo-oxidation in these systems. It is clear from these data that, at the point where the coefficient of friction reaches a limiting value, the contact angle of the SAM following immersion in a solution of a contrasting thiol will also have reached a limiting value and it may be concluded that the monolayer has been fully oxidized. We may extend the quantitative analysis further to estimate the rate constant for the photo-oxidation reaction. For simplicity, we focus on C10COOH, although very similar findings were made for other SAMs. Previously, we have shown that the photo-oxidation of alkanethiolon-gold SAMs may be analyzed quantitatively using firstorder kinetics.50,51 For the oxidation of carboxylic acidterminated SAMs, we may write the following integrated rate equation:
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ln
[COOH]0 [COOH]t
) kt
Chong et al.
(4)
Here [COOH]t is the concentration of carboxylic acidterminated adsorbates at time t, and k is the rate constant. From Figure 5 it is clear that µ is proportional to the value of cos θ. At completion of the photo-oxidation reaction, µt is equal to the coefficient of a C11CH3 SAM, µCH3. At t ) 0, it is equal to the coefficient of friction of a C10COOH SAM. The fraction of unreacted carboxylic acid-terminated adsorbates at time t is related to (µt - µCH3). Hence, eq 4 may be modified thus:
ln
µCOOH - µCH3 µt - µCH3
) kt
(5)
Hence, the rate constant may be determined. For C10COOH, a value of 0.78 s-1 was obtained. This value could then be compared with a value obtained using the alternative approach described below. Line Section Analysis. Although the preceding method is rigorous and yields accurate values with small errors, it suffers the limitation that it is necessary to acquire a trace-retrace line profile at a number of loads in order to determine the friction coefficient. In many situations, it may be advantageous to acquire data for a patterned sample exhibiting variations in composition on small length scales. This may render repeated measurement of trace-retrace cycles difficult. Moreover, in conjunction with the investigation of a process for modification of surface structure on small length scales, it may be advantageous to image a region of surface and extract information about surface reactions retrospectively. As an alternative to the direct measurement of the coefficient of friction, we examined the analysis of line sections through FFM images of patterned materials. It is important to note that a different microscope, as well as different cantilevers, was used for these studies. A representative example of the resulting data is shown in Figure 6. A sample of C10COOH was exposed to UV light through a mask and immersed in a solution of C11CH3, leading to displacement of the oxidation products and the formation of darker contrast in the exposed areas. A line section across masked and exposed areas yields mean friction forces, FCOOH, in the masked areas and Ft in regions exposed for a time period t to UV light. Now if the friction force is proportional to the load,
FCOOH ) µCOOHFN
(6)
Ft ) µtFN
(7)
and
Hence,
µt µCOOH
)
Ft FCOOH
(8)
Thus, a plot of the photodetector signal in the exposed areas divided by that in the masked areas should reproduce the behavior shown in Figure 4. Figure 7 shows relative friction coefficients determined in this way together with the values of µt for C10COOH shown in Figure 4. It shows that a good fit is indeed achieved. These data can, like those obtained from the frictionload plots, be used to determine the fraction of carboxylic acid-terminated adsorbates remaining at the surface as
Figure 6. Schematic diagram showing the quantification of the extent of oxidation from a line section through an FFM image.
Figure 7. Comparison of the variation in the relative magnitude of the friction coefficient as a function of the exposure time determined from friction-load plots (squares) and from line sections through images (diamonds).
a function of time, χt. Data obtained from line sections were compared with samples treated in the same way but analyzed using friction-load plots in Figure 8. It may be seen that the two approaches agree very closely. To demonstrate this more clearly, we determined the rate constant for the photo-oxidation reaction from the linesection data. A value of 0.69 s-1 was obtained. This is in remarkably good agreement with the value obtained by analysis of the friction-load data. However, the method of line-section analysis offers the significant advantage that, because the contrast in the exposed areas is plotted as a fraction of the contrast in the masked areas, no calibration steps are required because the system effectively utilizes its own internal reference. Furthermore, the acquisition of large numbers of trace-retrace cycles is unnecessary. Analysis of Nanopatterned Materials. To explore the utility of the approach based on line sections, we applied it to a problem of current interest in our laboratory, the characterization of nanopatterned SAMs fabricated using scanning near-field photolithography (SNP). In particular, we consider here the case where a line is drawn
Kinetics of SAM Photo-Oxidation
Figure 8. Comparison of values for the fraction of carboxylic acid-terminated adsorbates remaining in the monolayer determined from friction coefficient measurements with those determined from line sections through FFM images.
Langmuir, Vol. 21, No. 9, 2005 3909
Figure 10. Variation in the relative friction force as a function of the laser power determined from line sections such as those shown in Figure 9.
region. However, although the lines were observed to be darker after exposure at 6 mW, it was not clear how close the line was to full oxidation and replacement by C11CH3. Analysis of a line section through the sample enabled this to be investigated quantitatively (Figure 10). Using eq 8, we determined the relative coefficient of friction as a function of the laser power, µr. Mean values from experiments performed using the same probe on five separate samples are shown in Figure 10. It may be seen that the relative value of the coefficient of friction decreases with increasing laser power. At a power of 6 mW, the mean value acquired was 0.28. Within experimental error, this is in good agreement with the values acquired at complete oxidation using the other methods (3.0), suggesting that complete photo-oxidation had occurred when using this power. Conclusions
Figure 9. FFM image of C11CH3 features written into a C10COOH SAM by SNP (top) and section (bottom) along line marked on image. The power of the laser, measured before coupling to the NSOM probe, was varied from 1 to 6 mW.
in a SAM using an NSOM and the oxidation products along this line displaced by immersion of the sample in a solution of a contrasting thiol. A major problem in this type of work is to determine the extent of oxidation along the line because for many applications it would be useful to know whether complete, or only partial, oxidation has occurred. The method of line-section analysis above seems promising in this respect. Figure 9 shows a series of lines written into a C10COOH SAM using SNP. Each line was written using a different laser power. The sample was then immersed in a solution of C11CH3. The lines written at the highest powers in this example have widths of ca. 100 nm (full width at halfmaximum (fwhm)), while those written at lower powers are narrower. As the laser power was increased, from 1 to 6 mW, the lines became darker, indicating that at 1 mW the photo-oxidation reaction was incomplete, leading only to partial oxidation of the adsorbates in the exposed
FFM provides valuable data on surface composition that may be used to monitor surface chemical reactions. On exposure to UV light, carboxylic acid-terminated SAMs are photo-oxidized to yield alkylsulfonates that are displaced by methyl-terminated thiols to yield surfaces whose coefficients of friction correlate closely with their compositions. Analysis of line sections through images of samples exposed to UV light through a mask provides an alternative route to the measurement of the kinetics of SAM photo-oxidation and yields data that are in good agreement with those acquired from friction-load plots, despite being acquired on a different instrument. Significantly, this close agreement is achieved despite the fact that these contrasting methodologies were implemented on different microscopes. The latter approach provides a convenient means of characterizing reactivity in nanometer-scale regions of surface because there is no need for cantilever calibration or for the acquisition of multiple friction-load plots. It has been successfully applied to the measurement of the extent of reaction in nanopatterns fabricated using scanning near-field photolithography. Clearly, this kind of approach will not be applicable to all surface reaction systems because it does depend on the reactant and product having distinctive frictional characteristics. However, for such systems that do, FFM provides a straightforward and readily accessible route to the measurement of the kinetics of surface chemical reactions. Acknowledgment. The authors are grateful to the EPSRC (Grant No. GR/N82197/01) for financial support. G.J.L. thanks the EPSRC and the RSC Analytical Chemistry Trust Fund for their support. LA0500169
