Friction Force Microscopy of Alkylphosphonic Acid and Carboxylic

Sep 30, 2006 - UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K., ... UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., a...
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Langmuir 2006, 22, 9254-9259

Friction Force Microscopy of Alkylphosphonic Acid and Carboxylic Acids Adsorbed on the Native Oxide of Aluminum Trevor T. Foster,† Morgan R. Alexander,*,‡ Graham J. Leggett,*,§ and Eoghan McAlpine| Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K., School of Pharmacy, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K., Department of Chemistry, UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., and InnoVal Technology, Beaumont Close, Banbury OX16 1TQ, U.K. ReceiVed April 21, 2006. In Final Form: July 15, 2006 Monolayers of alkylphosphonic acids (APA) and alkylcarboxylic acids (ACA) on magnetron-sputtered aluminum films have been investigated by friction force microscopy (FFM), contact angle measurement, and polarizationmodulation infrared reflection-absorption spectroscopy (PM-IRRAS). Clear evidence has been provided from PMIRRAS that friction coefficients, determined from FFM data, may be correlated directly with variations in adsorbate molecular structure. The friction coefficient increased with the length of the adsorbate molecule, but reached a limiting value when the alkyl chain of the adsorbate contained eight carbons in the case of APA or 12 carbons in the case of ACA. For a given alkyl chain length, APA monolayers yielded coefficients of friction that were similar to those of monolayers of alkylthiols of the same length, but smaller than those of ACA. These data indicate that APA monolayers are better ordered than ACA monolayers. These inferences were supported by PM-IRRAS data, which enabled the density of gauche defects to be estimated and correlated with variations in the coefficient of friction.

Introduction (FFM)1

Friction force microscopy is a powerful tool for the quantitative measurement of nanoscale tribological phenomena.2 It relies upon measurement of the lateral deflection of an atomic force microscope (AFM) cantilever as it traverses the sample surface. This is influenced strongly by friction forces acting on the tip as it contacts the sample, and careful analysis of the surface friction yields information on a diverse range of phenomena. Self-assembled monolayers (SAMs) have been extensively studied by FFM, which has yielded information on their surface chemical compositions,3 molecular organization,4 mechanical properties,5 and acid-base characteristics.6 Because of its extremely high spatial resolution, combined with sensitivity * Corresponding authors. E-mail: [email protected]; [email protected]. † UMIST. ‡ University of Nottingham. § University of Sheffield. | Innoval Technology. (1) Overney, R.; Meyer, E. MRS Bull. 1993, 18, 26. (2) (a) Carpick, R. W.; Salmeron, M. Chem. ReV. 1997, 97, 1163. (b) Gnecco, E.; Bennewitz, R.; Gyalog, T.; Meyer, E. J. Phys.: Condens. Matter 2001, 13, R619. (c) Tsukurk, V. V. AdV. Mater. 2001, 13, 95. (d) Leggett, G. J.; Brewer, N. J.; Chong, K. S. L. Phys. Chem. Chem. Phys. 2005, 7, 1107. (3) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Akari, S.; Horn, D.; Keller, H.; Schrepp, W. AdV. Mater. 1995, 7, 549. (c) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (d) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (e) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345. (f) Clear, S. C.; Nealey, P. F. J. Colloid Interface Sci. 1999, 213, 238. (g) Zhang, L.; Li, L.; Chen, S.; Jiang, S. Langmuir 2002, 18, 5448. (h) Li, L.; Chen, S.; Jiang, S. Langmuir 2004, 19, 666. (i) Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109. (j) Kim, H. I.; Houston, J. E. J. Am. Chem. Soc. 2000, 122, 12045. (4) (a) Beake, B. D.; Leggett, G. J. Langmuir 2000, 16, 735. (b) Clear, S. C.; Nealey, P. F. J. Chem. Phys. 2001, 114, 2802. (c) Brewer, N. J.; Foster, T. T.; Leggett, G. J.; Alexander, M. R.; McAlpine, E. J. Phys. Chem. B 2004, 108, 4723. (d) Yang, X.; Perry, S. S. Langmuir 2003, 19, 6135. (e) Li, S.; Cao, P.; Colorado, R.; Yan, X.; Wenzl, I.; Schmakova, O. E.; Graupe, M.; Lee, T. R.; Perry, S. S. Langmuir 2005, 21, 933. (5) (a) Hammerschmidt, J. A.; Moasser, B.; Gladfelter, W.; Haugstad, G.; Jones, R. R. Macromolecules 1996, 29, 8996. (b) Hammerschmidt, J. A.; Gladfelter, W. A.; Haugstad, G. Macromolecules 1999, 32, 3360. (c) Dinelli, F.; Buenviaje, C.; Overney, R. M. J. Chem. Phys. 2000, 113, 2043. (6) (a) Marti, A.; Hahner, G.; Spencer, N. D. Langmuir 1995, 11, 4632. (b) Vezenov, D.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (c) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446.

to chemical structure, FFM offers unique capabilities for the measurement of reaction kinetics on nanometer-length scales.7 Alkylphosphonic and alkylcarboxylic acids (APA and ACA) are classes of adsorbates that have been shown to form ordered monolayers on aluminum oxide surfaces8,9 and have also been proposed as alternatives to the chromium-based pretreatments used to prepare metallic artifacts for adhesive bonding and painting in many applications.10 Chromate (Cr6+) conversion coatings have traditionally been used as pretreatments to increase underpaint corrosion resistance and to ensure good adhesion of organic coatings.11 This treatment provides excellent protection, but developing environmental legislation on the use of chromate makes it increasingly environmentally unacceptable, with chromium-free treatmentssincluding poly(acrylic acids)sbeing increasingly employed in some product applications. The use of such acids is rationalized by at least two of their known characteristics: adhesion promotion through interfacial coupling and corrosion inhibition. ACAs can adsorb onto the oxide surface of aluminum to form close-packed and highly orientated molecular assemblies.9 Allara and Nuzzo have carried out a thorough investigation into their structures and formation kinetics using contact angle measurements, infrared reflection-absorption spectroscopy (IRRAS), ellipsometry, and X-ray photoelectron spectroscopy (XPS).9 They have demonstrated that the adsorption process involves dissociation of the carboxylic acid group to form carboxylate species with bidentate bridge bonded structures. Above C16, the alkyl chains form all-trans orientated chains tilted from the surface normal by ∼10°. APAs have also been shown to readily form monolayers on aluminum oxide surfaces.12,13 Ramsier et al. demonstrated, using inelastic tunneling spectroscopy (IETS), that phosphonic acids chemisorb to the aluminum (7) Chong, K. S. L.; Sun, S.; Leggett, G. J. Langmuir 2005, 21, 2903. (8) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (9) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52, 52-65. (10) (a) Schmidt-Hansberg, T.; Schubach, P. Aluminium Surface Science and Technology, Bonn, Germany, May 18-21, 2003; pp 9-14. (b) Bram, C.; Jung, C.; Stratmann, M. Fresenius’ J. Anal. Chem. 1997, 358, 108. (11) Critchlow, G.; Brewis, D. Int. J. Adhes. Adhes. 1996, 16, 255. (12) Liakos, I. L.; Newman, R. C.; McAlpine, E.; Alexander, M. R. Surf. Interface Anal. 2004, 36, 347. (13) Taylor, C. E.; Schwartz, D. K. Langmuir 2003, 19, 2665.

10.1021/la061082t CCC: $33.50 © 2006 American Chemical Society Published on Web 09/30/2006

FFM of Organic Monolayers on Al

oxide surface via hydroxyl groups, forming phosphonate functionalities at the surface in a symmetrical tridentate structure.14 FFM offers significant potential for the characterization of these important materials, particularly in view of its sensitivity to adsorbate molecular organization, established in the earlier studies of alkylthiol systems. To date, there have been few published studies of the behavior of SAMs on aluminum oxide or, indeed, any other engineering metals using FFM. Comparison of the behavior of ACA and APA monolayers with the much more extensively studied alkylthiol systems may offer important insights into the influence of adsorbate bonding on the nanoscale friction of molecular materials. For example, while ACA monolayers are expected to exhibit more open packing than alkylthiols, APA monolayers are strongly adsorbed and may be expected to exhibit high packing densities. In the present study, the frictional behavior of ACA and PA monolayers formed from adsorbates with different lengths has been investigated. The objective was to examine the capability of FFM as a tool for probing molecular order and organization in these systems. An additional objective was to test deductions about molecular organization by comparison with spectroscopic measurements. Previously, there have been few direct attempts to compare deductions drawn from FFM data with structural data obtained from spectroscopic characterization. Here, polarization modulation (PM) IRRAS has been used to study alkyl chain organization, enabling the direct testing of hypotheses relating to alkyl chain organization deduced from FFM data. XPS has been used to confirm the compositions of the samples. Experimental Section Monolayer Formation. Glass microscope slides (22 mm × 64 mm and No. 2 thickness) were obtained from Chance Proper Ltd. All glassware used was cleaned with Piranha solution. Piranha solution consists of a 3:7 mixture of 30% hydrogen peroxide and concentrated sulfuric acid. This mixture is a Very strong oxidizing agent and has been known to detonate spontaneously when brought into contact with organic material, and therefore great care should be employed. An Edwards bell jar vacuum coating system was used to evaporate first chromium (1 nm) onto the slides as an adhesion promoter. This was followed by the evaporation of gold to form a polycrystalline layer of 25-nm thickness, measured using a vibration quartz crystal. A base pressure of 6 × 10-7 mbar was obtained prior to evaporation. The gold-coated slides were immediately immersed in 1mM ethanolic solutions of decanethiol (Fluka). The SAMs were kept in the decanethiol solution until removal just before use. Magnetron sputtered aluminum (MS-Al) samples were prepared using an aluminum target (99.999%) with an argon plasma onto the cleaned glass slides. A base pressure of better than 4.0 × 10-7 mbar was obtained prior to the introduction of argon (flow ) 10 sccm and pressure ) 5.2 × 10-3 mbar) within which a DC plasma was sustained using an unbalanced magnetron for 10 min to give an aluminum deposit of ∼30-nm thickness. The sputter unit was vented to ambient air, and the MS-Al samples were either stored until required or used immediately. Stored samples were etched prior to use, by immersion in concentrated (5 M) nitric acid for 10 min, after which they were rinsed in copious amounts of deionized water. Failure to use sufficient water resulted in the retention of nitrate species, as determined by XPS. Both etched and freshly prepared Al films were left in the atmosphere for 40 min prior to use to allow the oxide surface to become hydroxylated.15 The substrates were then immersed in 5 mM solutions of APA or ACA in ethanol and hexadecane, respectively. Hexyl-, decyl-, dodecyl-, and hecadecyl phosphonic acid and methanoic-, ethanolic-, butanoic-, octanoic-, decaoic-, and (14) Ramsier, R. D.; Henriksen, P. N.; Gent, A. N. Surf. Sci. 1988, 203, 72. (15) Alexander, M. R.; Lewington, T. A.; Foster, T. T.; Thompson, G. E.; Leggett, G. J.; McAlpine, E. Aluminium Surface Science and Technology, Brussels, 2004; Terryn, H., Ed.; pp 60-65.

Langmuir, Vol. 22, No. 22, 2006 9255 octadecanoic acid were all obtained from Sigma and used as received. All substrates were immersed in the solution for at least 18 h before being removed and rinsed with copious amounts of degassed absolute ethanol (or hexadecane). Samples rinsed in ethanol were dried in a stream of oxygen-free nitrogen gas, whereas samples rinsed in hexadecane were placed on a spin-drying device and spun until all fluid was removed from the surface. Contact Angle Measurement. Advancing contact angles were measured on a CAM 200 (KSV Ltd supplied by LOT Oriel) sessile drop video capture instrument. All angles are averaged from measurements made on at least nine drops. Atomic Force Microscopy. Atomic Force Microscopy was performed in ethanol and in air using a Digital Instruments Nanoscope IIIa Multimode atomic force microscope. The nominal force constants of the silicon nitride cantilevers (Nanoprobes Ltd) were 0.12 N m-1. The calibration of normal forces involved two steps. First, the photodetector sensitivity was calibrated by measuring a force curve for a very stiff sample. Mica was used, because relative to the very flexible lever, the stiffness of the mica is sufficiently large that it may be assumed that all deflection during the force measurement will be in the lever. Under these circumstances, the photodetector sensitivity is the gradient of a plot of photodetector signal versus displacement while measuring repulsive forces. Second, the spring constants of the levers were determined from their thermal spectra using a routine implemented within the microscope software and based on the method of Hutter and Bechhoefer.16 This approximates the cantilever as a harmonic oscillator, the motion of which is driven by thermal noise. Applying the equipartition theorem, Hutter and Bechhoefer derived a relationship between the spring constant and the power spectrum of the cantilever response. Experimentally, the laser spot was focused on the apex of the cantilever, and the thermal fluctuations of the cantilever were measured and used to derive the power spectrum. Friction force measurements were recorded in scope mode by obtaining friction loops from four separate areas on the monolayer surfaces with the scan velocity fixed at 3 µm s-1. By varying the normal load applied to the tip, it is possible to construct friction load plots. Friction coefficient data were standardized using an internal normalization procedure previously described elsewhere.17,18 A single tip was used to acquire a complete set of data for an entire series of samples, and the gradients of the friction-load plots were used to obtain friction coefficients. The coefficients of friction for this data set were then normalized by being divided by the highest friction coefficient value obtained. It is difficult to compare the absolute friction coefficients obtained in this study to values obtained by other authors, but comparisons made between the SAMs in the present investigation are highly reliable and excellent reproducibility was observed from day to day. X-ray Photoelectron Spectroscopy. XPS analysis was carried out using a Kratos Axis Ultra. The base pressure was approximately 10-9 mbar. The monochromatic Al KR X-ray source was operated at a current of 10 mA and 15 kV, and the samples were analyzed using a takeoff angle normal to the surface. A charge compensating electron flood gun was not required. All core levels were acquired using a pass energy of 20 eV, while a pass energy of 160 eV was used for the survey spectra. Samples were screwed onto a stainless steel holder maintaining them in electrical contact with the earthed body of the spectrometer. Quantification and curve fitting were carried out using the CASA XPS processing software (Casa Software Ltd, http://www.casaxps.com). Sensitivity factors, determined from Schofield cross sections, have been used to quantify the surface elemental composition, after correction of spectra for the transmission function of the instrument following the method of Seah.19 The sensitivity factors have been corrected for the X-ray beam analyzer plane angle of 60°.20 (16) Hutter, J. L.; Bechhoefer, J. ReV. Sci. Instrum. 1993, 64, 1868, 3342. (17) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 17, 1970. (18) Shon, Y.-S.; Lee, S.; Colorado, R.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (19) Seah, M. P. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 19.

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Foster et al.

Figure 1. 1 × 1 µm2 AFM topographical images of films of (a) MS-Al, (b) thermally evaporated Au, and (c) thermally evaporated Ag. Polarization-Modulation Infrared Reflection-Absorption Spectroscopy. PM-IRRAS data in the present study was acquired using a FTS 6000 spectrometer (Digilab Ltd) at a resolution of 4 cm-1 using an MCT detector. The PM-IRRAS setup has been described in detail elsewhere.21 The PM-IRRAS technique utilizes a linear polarizer followed by a photoelastic modulator to modulate the polarization of the infrared radiation from p to s states relative to the sample surface, thus eliminating the requirement for a “background” sample. The Digilab instrument used digital signal processing to “decode” the signal detected in place of an analogue lock in amplifier traditionally used. The sample was mounted at an incidence angle of 85°. An FT-IR purge gas generator (Parker Filtration) was used to reduce the water vapor in the spectrometer during measurements. All spectroscopic data was processed using Win IR Pro version 3.1 (Biorad).

Results and Discussion Preparation of Al Films. Analysis of freshly prepared Al films by XPS confirmed that they were coated with a thin (2 to 3 nm) oxide film onto which carbonaceous material had adsorbed from ambient atmosphere exposure (see Supporting Information). The contamination material could readily be removed by a brief immersion in nitric acid, a process that regenerated the oxide surface in a form indistinguishable from that of the freshly deposited film (see Supporting Information). Figure 1a shows an AFM topographical image of a MS-Al film. For comparative purposes, images are also shown for gold and silver films. The Al oxide film is amorphous and exhibits grains that are slightly larger than those observed in the image of the Au film, but similar to those observed for Ag. This is reflected in the rms roughness values measured. For Al films, an rms roughness of 3-6 nm was measured over a 1 × 1 µm2 area, while for Au and Ag, values in the range 1-4 nm were measured. There was some variation from sample to sample because of small random variations in the deposition rate. Preparation of Monolayers on Al. Monolayers of ACA and APA with a range of alkyl chain lengths were formed on aluminum (20) Reilman, R. F.; Msezane, A.; Manson, S. T. J. Electron Spectrosc. 1976, 8, 389. (21) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 3, 380.

Figure 2. Contact angles of APA (top) and ACA (bottom) monolayers as a function of the number of carbon atoms in the alkyl chain.

oxide layers on MS-Al. XPS was used to confirm the purity of the monolayers (see Supporting Information). Water contact angle measurements were also made (Figure 2). The contact angles of both ACA and APA monolayers increased with adsorbate chain length. However, the contact angles of APA monolayers rose faster, reaching a limiting value of ca. 115° for octylphosphonic acid. For the ACA monolayers, a similar limiting contact angle was reached but was not observed for adsorbates with fewer than 12 carbon atoms in their alkyl chains. These variations in the contact angle may be attributed to differences in the packing density of the adsorbate molecules, in agreement with previous studies. Short-chain adsorbates are thought to exist in a state in which the positions of the headgroups remain fixed but the alkyl chains possess significant mobility because the effect of dispersion forces between short chains is modest. As the alkyl chain length increases, the strength of the intermolecular dispersion forces increases, leading to a reduction in alkyl chain mobility until, at the point where the limiting contact angle is observed, the structure is probably a two-dimensional crystal phase. FFM of Monolayers on Al. Friction force measurements were made for samples immersed in ethanol using FFM. Previous studies have demonstrated that the coefficients of friction of SAMs on Au measured in air and in ethanol are, within experimental error, identical. Moreover, a recent study has suggested that in air frictional interactions may be dominated by adhesive interactions; working in ethanol thus simplifies the contact mechanics.22 Figure 3a,b shows representative friction load plots. The relationship between the lateral deflection and the load remained linear up to loads of 80 nN. Repetition of friction load measurements yielded identical results, indicating that under these conditions there was no modification to the probe. There was no topographical evidence of any damage to the sample, as determined by subsequent examination of the same area at low load by AFM, at these loads. The linearity of (22) Hurley, C. R.; Leggett, G. J. Langmuir 2006, 22, 4179.

FFM of Organic Monolayers on Al

Langmuir, Vol. 22, No. 22, 2006 9257

Figure 3. (Left) Representative friction load plots for APA (a) and ACA (b) acquired with samples immersed in ethanol. Here, n is the number of carbon atoms in the alkyl chain. (Right) variation in the normalized coefficient of friction with alkyl chain length for APA (c) and ACA (d).

the friction load relationship suggested that the data were in agreement with Amontons’ law, although not all of the friction load plots passed through the origin, suggesting that Deraguin’s modification of Amontons’ law

FF ) F0 + µFN should be used, where F0 is a finite friction force measured at zero load for adhering surfaces. According to this analysis, the gradient of a plot of the friction force against the load is equal to the coefficient of friction, µ. In the data shown here, the photodetector response has not been converted to a lateral force. To compare friction coefficients from different systems, a normalization procedure was used as described above. Friction coefficients were determined for ACA and APA monolayers and normalized, for each headgroup, to the largest value. The results are shown in Figure 3c,d. The coefficient of friction varied with the adsorbate alkyl chain length in a way that reflected closely the variations in the contact angle as a function of chain length for both APA and ACA monolayers. The coefficient of friction was found to be large for short-chain adsorbates and smaller for longer adsorbates. The value of µ was observed to decline with increasing chain length, eventually reaching a limiting value at a length that correlated with the length at which a limiting contact angle was observed: eight carbon atoms for APA and 12 for ACA. A number of authors have previously reported correlations between adsorbate alkyl chain length and frictional behavior measured by FFM. For example, Xiao et al. concluded that friction decreased with alkyl chain length for monolayers of alkylsilanes on mica23 and attributed this to the different densities of chain defects in the monolayers. A number of subsequent studies of SAMs of both alkylsilanes and alkanethiols have supported this (23) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235.

description. For example, McDermott et al. studied the frictional properties of a range of SAMs of methyl-terminated alkanethiols with differing chain lengths on gold24 and found that the slope of the friction load plot increased as the alkyl chain length decreased. A variety of properties of SAMs have been reported to depend on the alkyl chain length. As the adsorbate chain length increases, chain mobility decreases and the packing of the adsorbates becomes more rigid. Short-chain SAMs have mobile chain structures containing larger numbers of gauche defects than long-chain SAMs, which may (for greater than approximately 12 carbon atoms) adopt two-dimensional crystalline structures. Because of their more mobile chain structures, short-chain monolayers may dissipate energy via pathways that are less readily accessible to the more rigid long-chain monolayers. For example, the alteration of alkyl chain conformation to create gauche defects may present an important pathway for energy dissipation.25 The data in Figure 3 appear to be consistent with such explanations. Monolayers of short-chain ACA and APA exhibit large coefficients of friction compared, respectively, to monolayers of long-chain ACA and APA, because the shorter alkyl chains are conformationally less well-ordered and more susceptible to deformation under loading beneath the AFM probe. As the chain length of the adsorbate increases, the degree of disorder decreases and the chains become increasingly less readily deformed under loading. This is reflected in a decrease in the coefficient of friction. However, Figure 3 suggests that the coefficient of friction declines more rapidly for APA monolayers and reaches a limiting value at a shorter alkyl chain length. This suggests that, at a given length, APA monolayers are better ordered and/or exhibit lower chain mobility. (24) McDermott, M. T.; Green, J.-D. B.; Porter, M. D. Langmuir 1997, 13, 2504. (25) (a) Tutein, A. B.; Stuart, S. J.; Harrison, J. A. Langmuir 2000, 16, 291. (b) Mikulski, P. T.; Harrison, J. A. J. Am. Chem. Soc. 2001, 123, 6873. (c) Mikulski, P. T.; Herman, L. A.; Harrison, J. A. Langmuir 2005, 21, 12197.

9258 Langmuir, Vol. 22, No. 22, 2006

Foster et al. Table 1. Peak Positions for ACA, APA, and AT Adsorbates of Varying Alkyl Chain Lengths Obtained Using PM-IRRASa Alkylphosphonic Acids on Aluminum wavenumber/cm-1 alkyl chain length

νaCH2

νaCH3

νsCH2

νsCH3

1 2 4 10 18

2917 2917 2916 2917 2918

2955 2950 2968 2968 2966

2848 2847 2848 2849 2849

2862 2864 2881 2881 2879

Alkylcarboxylic Acids on Aluminum wavenumber/cm-1 alkyl chain length

νaCH2

νaCH3

νsCH2

νsCH3

6 10 12 16

2929 2930 2926 2924

2965 2965 2967 2967

2859 2859 2856 2853

2892 2874 2881 2879

Figure 4. Histogram showing the coefficients of friction of three SAMs formed from adsorbates of equal length on Au and Al.

To compare the frictional behavior of ACA and APA better, measurements were made using the same probe for monolayers of adsorbates of the two types with equal chain lengths. This enables the relative magnitudes of the coefficients of friction to be established with some accuracy. The data were normalized to the largest value measured. Figure 4 illustrates the results of such measurements, for monolayers of decylcarboxylic acid and decylphosphonic acid. Because alkylthiol SAM systems have been much more extensively studied, measurements were also made for a monolayer of mercaptodecane adsorbed onto gold for comparative purposes. There was a significant difference between the coefficient of friction measured for decylcarboxylic acid and decylphosphonic acid. However, the coefficient of friction of the APA monolayer was very similar to that of the thiol monolayer. While the rms roughness of the Al films was slightly larger than that of the Au films, the difference was small and was not expected to have a significant influence on the magnitude of the coefficient of friction. These data can most readily be explained by the thiol and phosphonic acid monolayers having similar packing and ordering, while the carboxylic acid monolayer exhibits lower order and less dense packing. Such a conclusion is hard to test by comparison with the literature, for there have been few detailed structural studies of APA monolayers on aluminum oxide surfaces. However, it is consistent with the previous work of Mallouk et al.,26 who determined the headgroup separation of 0.48 nm for phosphonate salts adsorbed on small divalent metal ions, similar to the 0.499 nm widely reported for alkanethiols adsorbed on Au. Moreover, Byrd and co-workers reported a contact angle of 110° and a tilt angle of ca. 22° for octadecylphosphonic acid monolayers adsorbed on a zirconated Langmuir Blodgett template,27 similar to the values reported for monolayers of alkanethiols on gold and silver. To further test the hypothesis that the variations in the coefficient of friction may be attributed to changes in alkyl chain ordering, PM-IRRAS measurements were made. Spectroscopic methods have previously been little used in conjunction with FFM. However, PM-IRRAS is a particularly valuable technique in the present context because of its well-established sensitivity to the conformations of adsorbed molecules with alkyl chains. It thus provides an excellent means of testing whether there is a correlation between frictional data acquired by FFM and alkyl chain order. Representative spectra are reproduced in the Supporting Information. Table 1 shows the positions of the symmetric and asymmetric stretching bands for the methylene and methyl groups. For ACA, the position of the νaCH2 peak (26) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (27) Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295.

Alkanethiols on Gold wavenumber/cm-1 alkyl chain length

νaCH2

νaCH3

νsCH2

νsCH3

10

2919

2966

2851

2878

a

Each peak position is the average of three spectra acquired from separate samples.

shifted from 2929 to 2924 cm-1 and the νsCH2 peak shifted from 2859 to 2853 cm-1 as the number of carbon atoms in the alkyl chain increased from six to 16. These changes are consistent with an increase in the conformational order in SAMs formed from adsorbates with longer alkyl chains (i.e., a decrease in the concentration of gauche defects). Comparisons of the position of the νaCH2 and νsCH2 stretches from ACA SAMs with those from alkanes adsorbed on silica indicated that they have a disordered structure even at the longest chain length.28 For APA, in contrast, the νaCH2 and νsCH2 peak positions were invariant with chain length at 2917 ( 1 and 2848 ( 1 cm-1, respectively. Similar peak positions have previously been assigned to a high degree of conformational order with only 1-4% gauche defect concentration for an alkanethiolate monolayer on gold.29 These data support the explanation above that the differences in the values of the friction coefficient measured for ACA and APA of the same length may be attributed to differences in alkyl chain organization. However, the apparent invariance of the positions of these bands for the APA implies that, apparently at odds with the FFM and contact angle data, order does not change significantly with length for these adsorbates. This apparent contradiction requires further investigation. To facilitate comparison of different chain lengths, the ratio of the νaCH2 to νaCH3 intensity was normalized to the number of methylene units by division of the ratio by the number of CH2 units in the molecule (νaCH2/νaCH3). This is presented as a function of alkyl chain length in Figure 5. As the alkyl chain length increased, the νaCH2/νaCH3 ratio decreased for both APA and ACA adsorbates, reflecting the decrease in the absorption of the CH2 groups in the p-plane sampled in the IRRAS experiment. The ratio approached a limiting value as the alkyl chain length increased. For the APAs, the constant and low gauche defect concentration inferred from the νaCH2 position (2917 ( 1 cm-1) may be taken in conjunction with the data in Figure 5 to imply that the tilt angle increases toward perpendicular as the chain length increases, leading to a concomitant decrease in the magnitude of the νaCH2/νaCH3 ratio. For the ACAs, the decrease in the density of gauche defects inferred from the change in the (28) Allara, D. L.; Parikh, A. N.; Judge, E. J. Chem. Phys. 1994, 100, 1761. (29) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437.

FFM of Organic Monolayers on Al

Figure 5. Variation in νaCH2/νaCH3 ratio with alkyl chain length for APA ([) and ACA (4) monolayers on MS-Al surfaces.

νaCH2 position suggests that the change in the νaCH2/νaCH3 ratio results from both a decrease in the concentration of gauche defects and possibly also a decrease in tilt from the normal. These data aid significantly in the interpretation of the FFM data. They confirm that there are structural changes in the APA monolayers that correlate closely with the variations measured in the friction coefficient. Comparison of Figure 5 with Figure 3c,d reveals a striking similarity in the variation in the νaCH2/ νaCH3 ratio and the variation in µ as a function of chain length for the two different headgroups. For the alkylthiol systems, it is thought that the fully dense phases (as opposed to the “striped”, or low coverage phases) exhibit a uniform tilt angle. However, for APA, a variety of types of coordination to the substrate have been proposed, including mono-, bi-, and tridentate ones, and these are associated with differences in the tilt angle.30 The tilt angle is known to increase with time for APA monolayers,31 and this has been reported to be correlated with changes in advancing water contact angles, too. Therefore, the data in Figure 5 are not inconsistent with our knowledge about APA monolayers. There are few previous reports of changes in frictional properties with adsorbate tilt angle in molecular monolayers. However, in one study,32 Barrena et al. reported a load-dependent transformation of alkylthiol monolayers from the equilibrium phase, in which the tilt angle is ca. 30°, to a metastable phase33 in which the tilt angle is increased to 50° and the coefficient of friction is also increased. It is possible that a similar effect applies here: that as the alkyl chain tilt angle decreases, the ease of deformation increases, leading to an increase in the coefficient of friction. The close correlation between the variations in the contact angles (Figure 2) and the coefficient of friction (Figure 3) suggests a common origin for the trends observed. The closeness of the correlation between the FFM data and the PM-IRRAS data provides further strong evidence for the utility of FFM as a tool for surface characterization. In contrast to PM-IRRAS, however, FFM has nanometer spatial resolution, and this, combined with the high sensitivity to molecular structure demonstrated here, makes it a powerful tool for addressing surface structure problems beyond the resolution range of most conventional surface spectroscopic techniques. While previous studies of alkylthiol monolayers led to close correlations between FFM data and changes in molecular organization, the application of this approach to a new system, and the corroboration by (30) Pedllerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Word, E. J. J. Phys. Chem. B 2003, 107, 11726. (31) Bram, C.; Jung, C.; Stratman, M. Fresenius’ J. Anal. Chem. 1997, 358, 108. (32) Barrena, E.; Ocal, C.; Salmeron, M. Surf. Sci. 2001, 482-485, 1216. (33) Barrena, E.; Palacios-Lidon, E.; Manuera, C.; Torrelles, X.; Ferrer, S.; Junas, U.; Salieron, M.; Ocal, C. J. Am. Chem. Soc. 2004, 126, 385.

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spectroscopic methods, represent strong support. The fact that changes in orientation (i.e., tilt angle) may be measured using PM-IRRAS, but not directly by FFM, also emphasizes the importance of utilizing complementary spectroscopic techniques to support FFM characterization. The data presented in this article have provided significant insights into the structures of APA monolayers. Clearly they exhibit extremely high degrees of order and a packing density greater than that of alkylthiol SAMs. These findings not only add support to the hypothesis that APAs may be valuable in surface coating technology, but also point to a potential role for APA monolayers in fundamental surface chemistry investigations. Their close packing and high degree of order, combined with a likely stability under ambient conditions superior to that of alkylthiol SAMs (which undergo ready oxidation), make them attractive systems for future utilization.

Conclusions The characterization of surface structure on length scales smaller than 100 nm remains a major challenge. FFM is a simple technique that is widely accessible. The data presented here support a growing body of evidence that FFM provides valuable information on surface structure with high spatial resolution. Analysis of friction coefficients provides a very convenient method for the quantification of such data. Despite the growing FFM literature, there have been few studies that have sought to correlate friction measurements with spectroscopic data. Here, however, clear evidence has been provided that differences in the friction coefficients of friction measured for APA and ACA by FFM correlate with differences in the densities of gauche defects and, by inference, with adsorbate order. Studies of APA, which have been found by PM-IRRAS to exhibit little lengthdependent change in order, indicate that changes in the tilt angle of the adsorbates may also lead to changes in the coefficient of friction. While ACA monolayers on aluminum have been extensively studied, APA monolayers are less well-studied. Here, we demonstrate that the phosphonic acids are significantly better ordered and more closely packed than the corresponding carboxylic acids. The coefficients of friction of APA monolayers are smaller than those formed from ACA, and PM-IRRAS indicates that the densities of gauche defects are lower. Packing in APA monolayers is similar to, or closer than, that in monolayers of alkylthiols, while FFM and PM-IRRAS both suggest that packing is more open in ACA monolayers. For chains shorter than 12 carbon atoms, the coefficients of friction and defect densities of APA monolayers decrease more rapidly than those of ACA. In contrast, order increases more slowly for ACA, reaching a plateau for longer alkyl chains. PM-IRRAS suggests that ACA does not achieve the limiting close packing of APA at any length in the range studied here. These results emphasize the utility of FFM as a surface characterization tool and suggest that close-packed, ordered monolayers of APA may also have significant merits for fundamental studies of surface chemistry. Acknowledgment. T.T.F. thanks the EPSRC and Alcan for an Industrial CASE Studentship. G.J.L. thanks the EPSRC and the RSC Analytical Division for financial support. Supporting Information Available: XPS and FFM data on the effect of exposure to the laboratory atmosphere on the compositions of MS-Al films. This material is available free of charge via the Internet at http://pubs.acs.org. LA061082T