Single-Molecule Bond-Rupture Force Analysis of Interactions between

Netzer, L.; Sagiv, J. J. Am. Chem. .... S. A. C.; Albrecht, T. R.; Quate, C. F.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. ..... Treavor A. Kendall...
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Anal. Chem. 1997, 69, 2855-2861

Single-Molecule Bond-Rupture Force Analysis of Interactions between AFM Tips and Substrates Modified with Organosilanes L. A. Wenzler,†,‡ G. L. Moyes,§ L. G. Olson,† J. M. Harris,† and T. P. Beebe, Jr.*,†

Department of Chemistry and Center for Biopolymers at Interfaces, University of Utah, Salt Lake City, Utah 84112

Recent atomic force microscopy studies have probed specific chemical interactions between the tip and the surface. A statistical method that produces information about the magnitude of single-molecule bond-rupture forces has been developed in our laboratories. This paper describes the extension of this technique to the study of single-molecule bond-rupture forces for organosilane coupling agents covalently attached to hydroxyl-bearing surfaces. Since both the tip and the surface can be modified by functionalized organosilanes, studies of a variety of tip-surface interactions have been possible. We were able to verify the adsorption of a variety of organosilanes on both the tip and surface using X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Interactions that were studied and quantified include van der Waals interactions between bromineterminated silanes and the hydrophobic interactions between methyl-terminated silanes on the tip and surface. Sagiv and co-workers1-4 opened the door to the study of new self-assembled organic monolayer and multilayer films. These films are similar in organization and packing to LangmuirBlodgett (LB) films5 but are anchored to hydroxyl-bearing surfaces by a cross-linked siloxane network. The ease of preparation of these self-assembled films and their relatively robust chemical and physical properties have allowed the study of such organized assemblies to be extended into different areas. Organosilane coupling agents are widely used with hydroxylated surfaces, primarily silica-based, as “bonded phases” in liquid and gas chromatography and in enzyme immobilization. They generally have the formula Y(CH2)nSiX3, where X is an alkoxy, acetoxy, or chloro function, Y is the outer functionality to be chemically compatible with the system under study,6 and n is the chain length of the molecule to promote interchain interactions. Over the past few years there have been several atomic force microscopy (AFM) studies that have probed specific chemical and mechanical interactions between the tip and the surface.7-15 These studies have included hydrogen and van der Waals bonding †

Center for Biopolymers at Interfaces. Present address: Department of Chemistry, 31 Washington Place, 1066 Waverly, New York University, New York, NY 10003. § Department of Chemistry. (1) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674-680. (2) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (3) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 90, 235-241. (4) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67-76. (5) Ulman, A. In An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (6) Plueddemann, E. P. In Silane Coupling Agents; Plenum: New York, 1982. ‡

S0003-2700(96)01065-7 CCC: $14.00

© 1997 American Chemical Society

between functionalized self-assembled thiols,9,10 ligand-receptor interactions,11-13 ligand-protein interactions,14 and interactions between single strands of DNA.15 When the AFM tip withdraws from contact with a sample surface, an adhesion force develops between the tip and the sample. When AFM force measurements are made “in air”, a capillary meniscus probably dominates the pull-off force, obscuring any localized chemical information with the physics of the meniscus. When the tip and sample are immersed in liquid, capillary forces are no longer present and the adhesion force arises from interactions between the tip and sample surfaces. Analysis of these adhesion forces can produce localized chemical information about the tip-surface interface in liquid medium, such as the bonding characteristics of the interface9,10 or the local effects of the liquid medium.10 A statistical method that can determine the magnitude of single-molecule bond-rupture forces, taken from the distribution of adhesive force measurements, was developed in our laboratory.8,10 This method is based on the assumption that the adhesive force (i.e., the pull-off force) in the force-distance curve is composed of discrete individual forces, or interactions, as has been described in recent literature.7 The distribution of the number of chemical bonds formed at the pull-off point of multiple measurements will follow a Poisson distribution for a discrete bond-rupture process. If the mean number (µn) of chemical bonds formed in that fixed contact area is n, the variance of the number of bonds (σ2n) will also be n.16 The physical quantities obtained from the adhesive force measurements in the force-distance curves are related to n by the following equations: µpull-off force ) nF, and (σ2pull-off force ) nF2, where F represents the magnitude of the single-molecule bond-rupture force, and µpull-off force and σ2pull-off force represent the mean and the variance of the total adhesive or pull-off force, respectively. Therefore, the singlemolecule bond-rupture force F can be calculated from the distribution of pull-off force measurements: F ) σ2pull-off force/ µpull-off force. This is indicated by the solid line in Figure 1. In practice this involves measuring ∼50 pull-off events in the AFM (7) Hoh, J. H.; Cleveland, J. P.; Prater, C. B.; Revel, J.-P.; Hansma, P. K. J. Am. Chem. Soc. 1992, 114, 4917-4918. (8) Williams, J. M.; Han, T.; Beebe, T. P., Jr. Langmuir 1996, 12, 1291-1295. (9) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830-3834. (10) Han, T.; Williams, J. M.; Beebe, T. P., Jr. Anal. Chim. Acta 1995, 307, 365-376. (11) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 354-357. (12) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415-417. (13) Hinterdorfor, P.; Baumgartner, W.; Gruber, H. J.; Schilcher, K.; Schindler, H. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3477-3481. (14) Stuart, J. K.; Hlady, V. Langmuir 1995, 11, 1368-1374. (15) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 266, 771-773. (16) Barlow, R. Statistics; John Wiley & Sons, Inc.: New York, 1988.

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secondary ion mass spectrometry (TOF-SIMS) analysis of organosilane-modified SiO2-coated AFM tips and glass surfaces. Both the tip and the surface can be modified by organosilanes bearing a variety of chemical functionalities; this has allowed us to measure single-molecule interaction forces in AFM for a variety of tipsurface chemical interactions.

Figure 1. Hypothetical plots of variance σ2pull-off force vs mean force µpull-off force demonstrating the differences between data for singlemolecule bond-rupture force Fi, a nonspecific long-range force Fo, and possible force variations due to bond position and orientation.

and applying the above analysis to that population of pull-off forces. After moving to a new area on the surface, changing tips, or both, additional points are generated to construct curves such as those in Figure 1. This method does not require any assumptions about tip radius or knowledge of the tip-surface contact area, which often are fraught with problems due to tip and surface asperities which confound estimates of the actual contact area. We have also considered the possibility of nonspecific, longrange forces Fo and force variations between the surface and the tip, as shown hypothetically in Figure 1 (dashed and dotted curves, respectively). Nonspecific forces are long-range interactions which are normally observed between macroscopic particles and surfaces in liquids (for example, as encountered between colloidal particles) and for biomolecule systems. It was shown in a previous paper8 that the action of a nonspecific force would significantly modify the variance vs mean force plots, giving a negative intercept where µpull-off force ) nF + Fo and σ2pull-off force ) µF - FFo (dashed line in Figure 1). In the second case (dotted line in Figure 1), if the discrete bond force F is not constant, it would alter the statistics of the force measurement. A variance in the singlemolecule bond-rupture force σ2F (not to be confused with the variance in the number of bonds σ2n or in the pull-off force σ2pull-off force) would manifest as a curvature in the variance vs mean force plots.8 If multiple types of interactions took place in a significant fraction of the force measurements, the adhesion force distribution would be single and broad. Neither of these trends was seen in these data,17 which demonstrates that single-molecule bond-rupture forces, measured by this statistical method, are discrete and homogeneous. In this paper we will apply these single-molecule bond-rupture force methods to the analysis of several tip-sample pairs in liquid media and discuss the method for chemical modification of the AFM tips and glass surfaces. We will also present results from X-ray photoelectron spectroscopy (XPS) and time-of-flight static (17) A new manuscript is in press (Wenzler, L. A.; Moyes, G. L.; Raikar, G. N.; Hansen, R. L.; Harris, J. M. Beebe, Jr., T. P.; Wood, L. L.; Saavedra, S. S. Langmuir, in press) in which a curvature in the plots of variance (σ2 vs mean force µ was observed for the systems 1-aceto-11(trichlorosilyl)undecane and 1-(thioacetato)-11-(trichlorosilyl)undecane. Our analysis of the curvature, observed for the first time here, will allow us to set an upper bound on the degree to which the single-molecule bond-rupture force is described by its own variance σ2force.

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EXPERIMENTAL SECTION Reagents. 11-Bromoundecyltrimethoxysilane, (3-chloropropyl)trimethoxysilane, (4-aminobutyl)trimethoxysilane, n-octadecyltrimethoxysilane (Gelest or United Chemical), methanol (Optima Grade, Fisher), and heptane (Optima Grade, Fisher) were used as received. Silicon monoxide (99.99% purity) was obtained from Cerac Coating Materials. Water was deionized, distilled over quartz, and filtered by a Milli-Q reagent water system (Millipore Co.), which resulted in a resistivity of 18 MΩ‚cm. Preparation of Coated Surfaces. Glass microscope slides (Fisher Scientific; rms surface roughness measured by AFM was 1.5 ( 0.2 Å21) were soaked in concentrated NH4OH for 1 h and washed 5 times by ultrasonication in 18 MΩ‚cm water. The surfaces were then dried for 1 h at 140 °C and placed directly in methanol to keep the surface free from water vapor and other unwanted adsorbates. Si3N4 AFM tips (Park Scientific) were prepared as follows: in a conventional vacuum deposition chamber (Edwards, Model E306A), the tips were placed 20 cm above a tantalum boat (ME6A-0.005Ta, R.D. Mathis Co.) containing silicon monoxide. The chamber was partially evacuated to a pressure of 5 × 10-4 Torr (or 1 × 10-4 Torr oxygen partial pressure) to assure complete oxidation of SiO to SiO2.18-20 The SiO was vaporized by passing electrical current of 65 A through the refractory boat. The thickness of deposited film was monitored using a quartz-crystal microbalance gauge (Edwards FTM5), by which the rate of deposition for SiO221 was nominally 1.0 Å‚s-1. SiO2 films were deposited at a typical thickness of 60-80 Å. The SiO2-coated cantilevers were removed from the vacuum chamber and placed directly into methanol. Glass surfaces and SiO2-coated cantilevers were silanized by immersion in a 5 mM silane solution in heptane for 24 h. The silanized tips and surfaces were washed 3 times with heptane to remove unreacted silane materials. The treated surfaces and tips were then heated for 1 h at 70 °C. After being coated, they were rinsed with acetone to remove any unreacted silane and then stored in methanol until used. Force Measurements. A Topometrix AFM system was used to obtain force-distance curves in liquid media at room temperature (T ) 297 ( 3 K). The system employs commercial cantilevers (Park Scientific) which were modified as described above. The force constants of the cantilevers were calculated using their unloaded resonance frequencies.22 The average resonance frequencies of unmodified cantilevers for two size tips were 13.35 ( 0.78 (n ) 13) and 30.13 ( 0.98 kHz (n ) 12). These frequencies correspond to force constants of 0.031 ( 0.005 and 0.075 ( 0.007 N‚m-1. Individually measured force constants (as (18) Holland, L. Vacuum Deposition of Thin Films; John Wiley & Sons, Inc.: New York, 1956, pp 449-450. (19) George, J. Preparation of Thin Films; Marcel Dekker: New York, 1992. (20) Vacuum Deposition Chemicals & Evaporation Materials, 3rd ed.; CERAC, Inc.: Milwaukee, WI, 1988; pp 11-12. (21) Lacy, W. B.; Williams, J. M.; Wenzler, L. A.; Beebe, Jr., T. P.; Harris, J. M. Anal. Chem. 1996, 68, 1003-1011. (22) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403-405.

Table 1. Single-Molecule Bond-Rupture Forces (pN) for Chemically Functionalized AFM Tips and Surfaces in Various Media liquid medium tip chemistry “Si3N4”a)

OH (unmodified (OCH3)3(CH2)10CH2Br (OCH3)3(CH2)17CH3 (OCH3)3(CH2)10CH2Br

surface chemistry ”a )

OH (unmodified “SiO2 (OCH3)3(CH2)10CH2Br (OCH3)3(CH2)17CH3 OH (unmodified “SiO2”a)

type of interaction

water ( ) 78.5)

methanol ( ) 34.3)

hydrogen bond van der Waals hydrophobic hydrogen bond

236 ( 20 31 ( 5 658 ( 68 152 ( 21

b 75 ( 11 b b

1-propanol ( ) 20.2) b 101 ( 3 281 ( 35 b

a Si N hydrolyzes in the presence of water8 so that the near-surface region of this material is a silicon oxynitride Si O N of varying stoichiometry. 3 4 x y z More importantly, this material and the unmodified “SiO2” express surface OH groups that can participate in hydrogen bonding. b Not all tipsurface-media combinations were measured in the present studies. Results for short-chain systems are discussed in the text.

opposed to nominal or average values) were used in the calculation of forces. The force-distance curves were obtained in a liquid cell, which is a design similar to that of Drake et al.23 All force-distance curve measurements were performed when the tip was not being scanned in the lateral direction, with a vertical scan rate of 5 µm‚s-1. This rate is in the same range as the scan rate used by Hoh and Engel (2 µm‚s-1 in water and 5 µm‚s-1 in air) and is much less than the scan rate reported to cause oscillation at the pull-off point (195 µm‚s-1 in air).24 XPS Surface Characterization. To determine adsorption of the organosilanes, X-ray photoelectron spectroscopy was performed using an ESCALab 220i-XL (Fisons). Data were collected in the binding energy regions of Si 2s (153.0 eV), Si 2p (103.4 eV), C 1s (284.6 eV), O 1s (531.6 eV), N 1s (397.9 eV), Cl 2p (199.9 eV), and Br 3d (68.5 eV). High-resolution multiplex spectra of the individual elements were acquired from 200-µm-diameter areas, using a 20-eV pass energy. XPS images were acquired in parallel, also using a monochromated, microfocused Al KR X-ray source operating at 150-W anode power. TOF-SIMS Characterization. Static TOF-SIMS was performed using a prototype commercial instrument (ESCA-TOF, VG Scientific) which contains an integrated XPS hemispherical analyzer and TOF drift tube on the same UHV system. The spectra were acquired with a 30-keV Ga+ liquid metal ion gun using a pulse width of ∼15 ns. The irradiated area was approximately 55 µm × 55 µm and the ion dose was 1.5 × 1012 ions‚cm-2, conforming to the static limit of ion dosage. The sample was biased to a potential of +3.5 kV for positive-ion analysis and -3.5 kV for negative-ion spectra. The analyzer was then set to a constant retard ratio (CRR) of 0.92 to provide first-order energy compensation. Charge compensation was achieved using a pulsed electron gun operating at 14 eV. Contact-Angle Measurements. A standard contact-angle goniometer (Model A-100, Rame-Hart, Inc.) was used to make contact-angle measurements on each modified surface; 18 MΩ‚ cm water was used as the sessile drop for all samples. RESULTS AND DISCUSSION Force-Distance Measurements. Table 1 shows the summary of force-distance measurements with various tips, surfaces, and media, to be discussed below. Each sample had multiple sets of pull-off force measurements performed. These were carried (23) Drake, B.; Prater, C. B.; Weisenhorn, A. L.; Gould, S. A. C.; Albrecht, T. R.; Quate, C. F.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. Science 1989, 243, 1586-1589. (24) Hoh, J. H.; Engel, A. Langmuir 1993, 9, 3310-3312.

out in different spots on the same surface and with several different surfaces. This produced a distribution of adhesion forces. Although samples were prepared in the same way, the radius of curvature of the Si3N4 tips can vary substantially. This variation causes set-to-set changes in the number of tip-surface bond interactions taking place between different tips and surfaces and is advantageous for our methods because it allows one to probe a wide range of mean force µ values along the x-axis. One cannot generally control or select the number of tip-surface interactions, and so the natural variations caused by different tip-sample combinations is welcomed. Figure 2 is an example of a plot of the force variance σ2pull-off force vs the mean force µpull-off force for three tips and surfaces (different symbols) in 1-propanol; tips and surfaces were modified with 11bromoundecyltrimethoxysilane. This plot depicts points taken with different surfaces and tips to demonstrate the change in adhesion force due to variations between tip geometries. Note, however, that all points lie on the same line, indicating the same bond-rupture force for similarly modified tips and surfaces, which we measure to be 101 ( 3 pN. In the case of the asterisk data points (representing a third tip-sample combination, with each asterisk representing measurements at different positions on the sample), the total number of bonds being ruptured in the one case (3700 pN) is 37 bonds, while in the other case (∼4700 pN) it is ∼47 bonds. These differences reflect local tip asperities interacting with the local curvature of the sample surface.8 As discussed above, these natural variations provide a fortuitous if passive method of varying the mean tip-sample adhesive force over a substantial range (without the need to make assumptions about tip-sample contact area or tip geometry) and increase the precision and accuracy with which we can determine the slope and hence F. Bond-rupture forces in the two long-chain organosilane systems that we studied show a dependence on the surrounding medium (Table 1). The measured single-molecule bond-rupture force for 11-bromoundecyltrimethoxysilane-modified tip and sample surface increases with a decrease in the dielectric constant and a decrease in polarizability of the medium. This is a trend that has previously been reported for hydrogen bonding between a carboxylic acidmodified tip and sample surface.10 Since bromine-terminated organosilane surfaces in different media should exhibit no tipsurface hydrogen bonding, the explanation for the trend is a van der Waals interaction caused by dipole-induced-dipole interactions. The time scale of the force-distance measurements (microseconds) is much slower than that of electronic polarizability changes or molecular motions. As the surfaces are pulled Analytical Chemistry, Vol. 69, No. 14, July 15, 1997

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Figure 2. Force variance σ2pull-off force vs mean force µpull-off force for SiO2-coated tip and glass surface, both modified with 11-bromoundecyltrimethoxysilane in 1-propanol medium. Points demonstrate differences in tips and samples: 9 represents four different areas with the same tip and surface, O represents a new tip and surface in two different spots, and * represents another new tip and surface in two different spots. Although different tips and surfaces result in different numbers of bonds being ruptured, all points lie on the same line, indicating that the same type of bond-rupture force, 101 ( 3 pN, is being measured in all tip-surface combinations. Each point is derived from ∼50 pull-off measurements.

apart on a time scale that is slow compared to molecular motions, solvent molecules have plenty of time to make their way into the emerging tip-surface gap between the bromine-terminated groups. The dipole moment and polarizability of the solvent molecules can interact with the dipole moment of the bromosilane, lowering the energy of the separated tip and surface (compared to its unsolvated energy) and reducing the force required to pull them apart. As the solvent is changed from water to methanol to 1-propanol (decreasing the dielectric constant and polarizability), the final state energy is raised, which in turn increases the measured bond-rupture force. Size-based arguments for solvent molecules and their ability to diffuse and solvate the emerging gap could only explain the observed trends if force measurements were made on a much faster time scale. Film mechanical response can also have an effect on the measurement. It is implicitly included in the effective bond-rupture force, although not in any explicit manner. Just as for any nonspecific contribution to the pull-off force Fo, we would expect for this to manifest as a nonzero intercept, which was not observed. The calculated single-molecule bond-rupture force obtained with n-octadecyltrimethoxysilane-modified tip and sample surface is greatest in water (Table 1). The higher single-molecule bondrupture force in water is due to the hydrophobic nature of both the tip and the surface; this is also seen in contact-angle measurements (see below). A hydrophobic surface cannot interact with water molecules by means of ionic or hydrogen bonds; the entropic influence of the hydrophobic effect probably imposes order or structuring of the water near the surfaces.25 Other studies26,27 have also found that the hydrophobic force can be far stronger than the van der Waals attraction, especially between hydrocarbon surfaces for which the Hamaker constant is quite small. These experiments have found evidence for discrete structuring of water over short length scales. (25) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: San Diego, 1992.

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Since any long-range structuring of the water should involve many molecules,27 it was surprising to see discrete forces for the present case of hydrophobic forces. Recently, groups have begun to address the AFM-measured interaction force with thermodynamic parameters.28,29 Although still controversial, these works have correlated the force of interaction with both the enthalpic and free energy parameters and suggest that the AFM is insensitive to entropic changes during pull-off measurements. Other studies aimed at understanding the retention processes in reversed-phase chromatography have proposed that two driving forces dominate the retention process.30,31 One is the difference in the free energy attributable to contact interactions of the solute with surrounding molecular neighbors. The other is the partial ordering of the stationary-phase chains, which at sufficiently high bonded-phase density leads to an entropic expulsion of solute from the stationary phase. It is thus possible that entropic contributions, such as those proposed here and observed for the first time in AFM force measurements, contribute to the trends observed for different solvents. Still other studies have observed a discrete nature to water structuring between metal electrodes in close proximity.32 Convergence. Figure 3 shows the deviation of measured values for single-molecule bond-rupture forces as a function of the number of measurements made for various chemical interactions. The repetitive measurements depicted in each point were performed at one position on the sample surface. Some data sets (9, O, 0) converge to a final stable value within 15 repetitions, while others ([, b, *, +) approach the final value more slowly. Previous experiments on a different chemical system10 showed convergence of all data sets after ∼50 repetitions with little variation after that. Experiments carried out with the organosilanes indicated similar convergence at ∼50 repetitions. This allowed for reduction in measurement time and sample damage due to repetitive contact. Analysis of Data. In addition to observing how the number of repetitions affected the deviation of single-molecule bondrupture forces for each system, we also binned each set of data in groups of various sizes. By using smaller sets, more data points can be plotted for each system studied; each point represents fewer measurements. This also helps to determine how many measurements are needed to accurately analyze adhesion force measurements. Each system studied had several sets of data, each derived from ∼50 points, representing a measured singlemolecule bond-rupture force. We took each data set and divided it into two groups representing the first half of the points and the last half of points, thereby giving twice the number of data sets. We carried this one step further, generating 4 times the number of data sets with one-quarter of the points in each bin. For example, when both the tip and the glass surface were coated with 11-bromoundecyltrimethoxysilane and measurements taken in methanol, a 75 ( 11 pN single-molecule bond-rupture force was measured. If each data set is divided in half or a quarter, 58 ( 20 and 52 ( 23 pN were measured, respectively. It can be seen that some systems asymptotically approached their final (26) Israelachivili, J. N.; Pashley, R. M. Nature 1983, 306, 249-250. (27) Israelachivili, J. N.; Pashley, R. M. Nature 1982, 300, 341-342. (28) Moy, V. T.; Florin, E.-L.; Gaub, H. E. Science 1994, 266, 257-259. (29) Chilkoti, A.; Boland, T.; Ratner, B. D.; Stayton, P. S. Biophys, J. 1995, 69, 2125-2130. (30) Dill, K. A. J. Phys. Chem. 1987, 91, 1980-1985. (31) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A-866A. (32) Porter, J. D.; Zinn, A. S. J. Phys Chem. 1993, 97, 1190-1203.

Figure 3. Convergence of calculated single-molecule bond-rupture forces as the number of repetitions increases. From plots similar to this one, the optimum number of repetitions was determined to be ∼50. Note how the overall envelope of relative errors (value calculated with fewer repetitions minus asymptotic value for 50 repetitions) asymptotically approaches the mean, regardless of the system studied. The inset describes each system studied. SiO2/SiO2 represents a SiO2-coated tip and clean glass surface, each of which bears surface silanol groups,8 Bromo/ Bromo represents a SiO2-coated tip and clean glass slide, both coated with 11-bromoundecyltrimethoxysilane, and Methyl/Methyl represents a SiO2-coated tip and clean glass slide, both coated with n-octadecyltrimethoxysilane.

Figure 4. Several force-distance curves showing how the adhesive force between the tip and the sample decreases with repetitive measurements. This set of curves was obtained at the same spot on the sample, in a water medium using a tip and sample surface modified with (3-chloropropyl)trimethoxysilane. The systematic decrease was only observed for the short-chain systems studied and is indicative of the inability of these systems to withstand repetitive tipsample contact at the lowest forces possible in this instrument. If the tip is moved to a different spot, the same trend is observed, indicating that the tip is able to “heal”.

values from below, while others approached it from above. There was no systematic direction of approach. The common trend is that the error in F, determined by the variance in the population of adhesive force measurements, converges to less than a few percent of the mean after ∼50 measurements regardless of the system. Short-Chain vs Long-Chain Systems. Short-chain organosilanes, (3-chloropropyl)trimethoxysilane and (4-aminobutyl)tri-

methoxysilane, were also investigated. It was found that singlemolecule bond-rupture forces could not be determined accurately with the present statistical method. A plot of the variance σ2 against the mean of the total force µ was not always linear, as it was for long-chain systems.17 During these measurements, the total adhesion force decreased with the number of repetitive measurements, as is evident in Figure 4. This leads us to conclude that as the molecules on the tip and surface interact repetitively, the short-chain organosilanes are undergoing tip and sample deformation and are perhaps permanently changed through desorption or gross deformation. If information from 10 or fewer measurements in one location is analyzed for (3-chloropropyl)trimethoxysilane, the plot of variance σ2pull-off force vs the mean µpull-off force is linear, with negligible intercept and a slope of 63 ( 3 pN for the single-molecule bond-rupture force in H2O (data not shown). But after 10 measurements, the systematic decrease in adhesion forces becomes too large to determine a single-molecule bond-rupture force. Similar trends were observed in methanol and 1-propanol. A possible explanation for the declining adhesion forces during repetitive measurements in water is that hydrolysis of the monolayer (irrespective of AFM measurements) could be taking place. Contact-angle measurements were performed on samples immersed in water for varying amounts of time. After being immersed for 2 h, neither long- nor short-chain organosilanes showed any measurable differences in contact angle from those measured on freshly prepared surfaces. This demonstrates that prolonged exposure to solvents such as water during forcedistance measurements is not disrupting the monolayer in areas away from the AFM tip. The mechanical action of the AFM tip could, however, locally increase the rate of hydrolysis. ContactAnalytical Chemistry, Vol. 69, No. 14, July 15, 1997

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Figure 5. XPS survey spectrum of 11-bromoundecyltrimethoxysilane deposited on the SiO2-coated AFM tip. Inset: high-resolution spectrum of the Br 3d region. Trends in the baselines on the high-binding-energy sides of the various peaks are consistent with a monolayer possessing a high degree of order in the direction perpendicular to the surface (see text).

angle measurements would be insensitive to this highly localized phenomenon. A body of evidence has emerged in the selfassembled monolayer community5 concerning the robustness and order in self-assembled monolayers as a function of the chain length, and the present data for short-chain systems are consistent with this evidence. XPS Measurements. Examination of each of the silane films by XPS confirmed that the monolayer formed reflected the adsorbates intended, and not contaminants. As an example, Figure 5 shows a survey scan of 11-bromoundecyltrimethoxysilane deposited on a SiO2-coated Si3N4 cantilever. It shows each of the elements present, as well as a high-resolution core-level spectrum of the Br 3d region. The trends seen in the baseline of the survey scan on the high binding energy sides of the various peaks are instructive and contain important information about the ordering of the monolayer into layers parallel to the substrate. For the present system, the outermost layer consists of a bromine, followed by a thicker layer of carbon and hydrogen, followed deeper by a layer of oxygen and then silicon. If this system is indeed well ordered, then this should be reflected in the electron scattering through these layers. The rising-and-then-falling background (as for the O 1s), the falling background (as for the C 1s), or the constant background (as for the Br 3d) to higher binding energies (lower kinetic energies) of each element can be used in a qualitative approach to obtaining information on the depth of an element in the layered selfassembled monolayer sample.33 The rising-and-then-falling backgrounds observed above the O 1s, Si 2s, and Si 2p peaks indicate more electron scattering from those elements buried deeper in the adsorbate layer. Electron emission from further out in the adsorbate layer (as for C) results in less electron scattering, which (33) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1978.

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is observed as a continuous decline in the background. The background is relatively constant for those elements at the outside interface (in this case Br). These trends indicated the existence of a layered monolayer, implying an ordered monolayer. Taken together with contact-angle data and molecular fragments observed in TOF-SIMS experiments (described below), all evidence is consistent with a contaminant-free Sagiv-type silane selfassembled monolayer.1 In addition to XPS spectroscopy, spatially resolved images of the individual elements present for each organosilane were acquired. As an example, Figure 6 shows the Br 3d, C 1s, O 1s, and Si 2p regions from 11-bromoundecyltrimethoxysilane deposited on a SiO2-coated Si3N4 cantilever. The perspective of the image is looking down on the triangular AFM cantilever from the functionalized (pyramid) side. On the micrometer scale, the images depict a uniformly deposited film and adsorbate monolayer. TOF-SIMS Measurements. Static TOF-SIMS was also performed on both long-chain organosilane samples, n-octadecyltrimethoxysilane and 11-bromoundecyltrimethoxysilane. In each sample, both positive and negative secondary ion spectra were collected (not shown). The observed molecular fragments reflect the adsorbates present, as expected, on the surfaces. In the bromosilane sample, a doublet peak at 79 and 81 amu was present in the negative secondary ion spectrum, which indicates the presence of the bromine-terminated group on the surface. Other mass fragments in the positive secondary ion spectrum were Si+, CH3Si+, and consistent fragments of the alkyl chain (i.e., C4H7+, C4H9+, and C5H9+). Contact-Angle Measurements. Advancing contact angles were measured on all samples in order to correlate our adhesion force measurements with the hydrophobicity of the silane monolayer.34 Before immersing in silane solutions, clean glass surfaces had contact angles of 12.3 ( 2.1°, indicating the hydrophilic nature of the surface before silanization. The length of the alkyl chain

that we studied were 11-bromoundecyltrimethoxysilane and noctadecyltrimethoxysilane. Contact angles measured were 74.3 ( 1.5 and 90.3 ( 2.9°, respectively, which demonstrates the wellpacked, hydrophobic nature of these films.38 The contact angles for (3-chloropropyl)trimethoxysilane and (4-aminobutyl)trimethoxysilane were 37.0 ( 1.7 and 40.0 ( 2.2°, respectively.

Figure 6. XPS image of 11-bromoundecyltrimethoxysilane on SiO2coated AFM tip. (A) Br 3d image, (B) C 1s image, (C) O 1s image, and (D) Si 2p image. Images all measure 200 µm on a side.

plays a significant role in determining monolayer formation, density, and the success of our statistical method for measuring single-molecule bond-rupture forces. For long-chain organosilanes, the van der Waals interactions between the chains help form a densely packed, well-ordered film with fully extended alkyl chains.35,36 Short-chain organosilanes with fewer interactions between the chains are more mobile and were more easily deformed.37 The trend is also consistent with our observations that the short-chain silanes are less robust after multiple measurements in the same contact area. The long-chain organosilanes (34) Contact-angle measurements were also attempted with each of the other solvents used for force-distance measurements. Each of the fluids will have different interfacial tensions due to different components of their respective surface energies. Both alcohol solvents were wetting to the surface, and only water gave a measurable contact angle. (35) Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1068-1077. (36) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (37) van Damme, H. S.; Hogt, A. H.; Feijen, J. J. Colloid Interface Sci. 1966, 22, 165-172. (38) Go¨lander, C.-G.; Lin, Y.-S.; Hlady, V.; Andrade, J. D. Colloids Surf. 1990, 49, 289-302.

CONCLUSIONS The adhesion forces between various AFM tips, surfaces, and media can be used to measure and detect changes in the magnitudes of the single-molecule bond-rupture forces. AFM tips and glass surfaces modified with bromo- and methyl-terminated silanes were used to determine the magnitudes of van der Waals and hydrophobic interactions, respectively. Silane carbon chain length significantly affects the success of the statistical method. Short chains do not form a monolayer robust enough to withstand multiple interactions during force measurements, whereas longchain silanes form well-packed films which successfully support functional groups at the interface during measurements. In each system studied, contact-angle measurements were performed to study the hydrophobic nature of the surface. XPS and static TOF-SIMS were also used to verify the presence of each organosilane on the modified SiO2-coated AFM tips and clean glass surfaces and to infer the ordering of the monolayers. Qualitative XPS depth-relative scattering showed bromine at the outside interface, with oxygen buried below carbon and bromine, supporting the conventional view of silane-based self-assembled monolayers. ACKNOWLEDGMENT The authors thank Ganesh Raikar and Andrew Vogt for help in obtaining XPS data. Bruce McIntosh and John Wolstenholme of VG Scientific are acknowledged for collaborations involving the TOF-SIMS data. This work was supported by grants from the National Science Foundation (CHE-9357188 to T.P.B.), from the U.S. Department of Energy (to J.M.H.), and by the ACCESS Program at the University of Utah (G.L.M.). T.P.B. is a Camille Dreyfus Teacher-Scholar and an Alfred P. Sloan Research Fellow.

Received for review October 15, 1996. Accepted April 9, 1997.X AC961065G X

Abstract published in Advance ACS Abstracts, May 15, 1997.

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