Langmuir 1997, 13, 3761-3768
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Measurements of Single-Molecule Bond-Rupture Forces between Self-Assembled Monolayers of Organosilanes with the Atomic Force Microscope L. A. Wenzler, G. L. Moyes, G. N. Raikar, R. L. Hansen, J. M. Harris, and T. P. Beebe, Jr.* Department of Chemistry, Center for Biopolymers at Interfaces, and Facility for Optical Spectroscopy & Surface and Interface Laboratory, University of Utah, Salt Lake City, Utah 84112
L. L. Wood and S. S. Saavedra Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received December 10, 1996X Atomic force microscopy (AFM) tips and glass surfaces were modified with various organosilanes to determine magnitude and dispersion information about single-molecule bond-rupture forces. X-ray photoelectron spectroscopy (XPS) and contact-angle measurements were used to study and quantify organosilane adsorption on the glass surface and on SiO2-coated AFM tips. Hydrogen bond interactions between hydroxyl- and thiol-terminated groups on the tip and surface were detected and measured. Differentiation between the functionalities of the acetate- and thioacetate-terminated silanes and their reduced forms produced by on-surface reduction (the alcohol and thiol, respectively) was also accomplished. The experiments demonstrate the complementary information that can be obtained from AFM and XPS and illustrate how they can be used to determine the nature of the surface after an organic transformation has occurred to the functional groups present. They also represent a first step in detecting chemical reactions on a localized scale and in measuring the dispersion in the single-molecule bond-rupture force when it exists.
Introduction The formation of organized monolayer films on a surface by the spontaneous adsorption of molecules from solution or the gas phase has become known as self-assembly. Two types of self-assembled monolayers have been extensively studied and have shown great promise as a means of controlling the chemical structure of organic surfaces: adsorption of organosulfur compounds onto gold surfaces1-6 and adsorption of alkylsilanes onto silicon or glass surfaces.7-10 The use of alkylsilanes, or more specifically, alkyltrichlorosilanes, results in monolayers which are durable, thermally stable,11 and resistant to degradation.12,13 The * Author to whom correspondence should be addressed. Phone: (801) 581-5383. Fax: (801) 581-8433. E-mail: beebe@ chemistry.chem.utah.edu. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (3) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (4) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (c) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (e) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (5) Ulman, A. Characterization of Organic Thin Films; ButterworthHeinemann: Boston, 1995. (6) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (7) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (8) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (9) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 90, 235. (10) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. (11) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (12) Maoz, R.; Sagiv, J. Thin Solid Films 1985, 132, 135. (13) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239.
S0743-7463(96)02086-0 CCC: $14.00
trichlorosilyl head group forms a covalent bond to hydroxyl groups on the hydrophilic surface; it can also form crosslinks to adjacent molecules via Si-O bonds created upon hydrolysis with trace water.14 By designing monolayers containing terminal functional groups, in situ or on-surface transformations can be performed and studied. These groups can also be viewed as protecting groups for transformation to other groups for later study. Two such transformations were carried out in this study: an ester to an alcohol14 and a thioacetate to a thiol.15,16 Force-Distance Measurements. The statistical method used in this study is described in detail in previously published papers.17-19 An AFM pull-off event represents the rupturing of several individual bonds, and a sampling of many of these events will produce a mean measured pull-off force µm and a pull-off force variance, σm2. The analysis method makes use of the properties of the Poisson distribution, which describes the sum of discrete bond forces. If the mean number of chemical bonds formed in a given contact area is µn, then the variance of the number of bonds (σn2) will also be µn.20 The measured pull-off force in one force-distance measurement m is related to the number of bonds n ruptured during a pull-off event by
m ) nF
(1)
(14) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (15) Wasserman, S. R.; Siebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886. (16) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (17) Williams, J. M.; Han, T.; Beebe, T. P., Jr. Langmuir 1996, 12, 1291. (18) Han, T.; Williams, J. M.; Beebe, T. P., Jr. Anal. Chim. Acta 1995, 307, 365. (19) Wenzler, L. A.; Moyes, G. L.; Olson, L. G.; Harris, J. M.; Beebe, T. P., Jr. Adhesion Force Analysis of Interactions Between AFM Tips and Substrates Modified with Organosilanes. Anal. Chem., in press. (20) Barlow, R. Statistics; John Wiley & Sons, Inc.: New York, 1988.
© 1997 American Chemical Society
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Figure 1. Schematic representation of monolayer deposition of acetate- and thioacetate-terminated surfaces and transformation to hydroxyl- and thiol-terminated surfaces using an on-surface reduction.
where F represents the average single-molecule bondrupture force in the sample. From a large number of pulloff force measurements, the relationship between the measured mean pull-off force µm and its variance σm2 (dominated by the sample n) can be used to determine the quantity that we seek, the magnitude of the singlemolecule bond-rupture force F.
F ) σm2/µm
(2)
The measured pull-off force variance is
σm2 ) µn〈F2〉
(3)
where the angular brackets (〈 〉) represent expectation values and µn represents the mean number of bonds ruptured in the set of many measurements. It is important to emphasize that the force F calculated in this manner is the single-molecule bond-rupture force, not the total adhesive force resulting from the entire tip-surface contact area. Relative comparisons of total pull-off forces can provide qualitative information for a single tip used on different samples only when the tip shape does not change. The method used here provides quantitative single-molecule bond-rupture force information with any number of tips and without the need for assumptions about tip shape and contact area. The functionalized surfaces discussed in this study are schematically represented in Figure 1. They were prepared by placing SiO2-coated AFM tips and clean glass slides in a solution containing an alkyltrichlorosilane (XSiCl3). Initially formed monolayers (X-terminated in the figure) include acetate- and thioacetate-terminated silanes. On-surface reduction of these monolayers allowed their transformation into the various Y-terminated silanes (hydroxyl- and thiol-terminated silanes). Significant information about each organosilane system can be obtained if different functional groups can be discriminated on the basis of their differing chemical properties. Over the past few years there have been several atomic force microscopy (AFM) studies which have probed specific chemical and mechanical interactions between the tip and the surface.17-19,21-31 Organosilane (21) Hoh, J. H.; Cleveland, J. P.; Prater, C. B.; Revel, J.-P.; Hansma, P. K. J. Am. Chem. Soc. 1992, 114, 4917. (22) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (23) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 354. (24) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415. Moy, V. T.; Florin, E.-L.; Gaub, H. E. Science 1994, 266, 257. (25) Hinterdorfor, P.; Baumgartner, W.; Gruber, H. J.; Schilcher, K.; Schindler, H. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3477. (26) Stuart, J. K.; Hlady, V. Langmuir 1995, 11, 1368.
self-assembled monolayer surfaces have broad importance both industrially and fundamentally because of their use in chromatography as bonded phases. Since both the tip and the surface can be modified by organosilanes bearing a variety of chemical functionalities, these “Sagiv-type” self-assembled monolayers have allowed us to measure hydrogen bonding, van der Waals, and hydrophobic interactions on a localized scale with AFM.17-19 This work was undertaken to examine simple organic reactions occurring to acetate- and thioacetate-terminated monolayers and to use these reactions to characterize the newly formed chemical nature of the surfaces. Changes in bond interactions were also examined. With a statistical method of obtaining the individual or single-molecule bond-rupture force from the adhesion force measured in force-distance curves,17-19 as described above, we have determined the hydrogen bond strength between hydroxyland thiol-terminated groups. Changes in the bond strengths in various liquid media have been detected. We also present a new method by which to determine the dispersion or variation, if any, in the single-molecule bondrupture force. To characterize the functionalized tips and surfaces, we present results from X-ray photoelectron spectroscopy (XPS) analyses of organosilane-modified SiO2-coated AFM tips and similarly modified surfaces. Experimental Section Reagents. Methanol (Optima Grade, Fisher), 1-propanol (Fisher), chloroform (EM Science), acetone (EM Science), dimethyl sulfoxide (Aldrich), and LiAlH4 (Aldrich) were used as received. Dicyclohexyl (Aldrich) was vacuum distilled. 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), which resulted in a resistivity of 18 MΩ‚cm. Silanes. Undecylenyl acetate was prepared by combining ω-undecylenyl alcohol (3.5 mL, 0.0187 mol) with triethylamine (3.9 mL, 0.0187 mol) and cooling the stirred mixture to 0 °C under nitrogen. Acetyl chloride (1.23 mL, 0.0224 mol) was mixed with 50 mL of dichloromethane, and this solution was added dropwise to the stirred ω-undecylenyl alcohol mixture over a 30 min. period. The solution was stirred for an additional 1.5-2 h, then transferred to a separatory funnel, and washed sequentially with cold water, 5% HCl, 20% NaHCO3, and saturated aqueous NaCl. The resulting solution was dried with MgSO4, filtered, (27) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 266, 771. (28) Biggs, S. Langmuir 1995, 11, 156. Biggs, S.; Mulvaney, P. J. Chem. Phys. 1994, 100, 8501. (29) Rabinovich, Y. I.; Yoon, R.-H. Langmuir 1994, 10, 1903. Rabinovich, Y. I.; Yoon, R.-H. Colloids Surf. 1994, 93, 263. (30) Mantel, M.; Rabinovich, Y. I.; Wightman, J. P.; Yoon, R.-H. J. Colloid Interface Sci. 1995, 170, 203. (31) Noy, A.; Frisbie, D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943.
AFM Measurements of Bond-Rupture Forces and concentrated by rotary evaporation. Purification was performed by flash chromatography using 5% ethyl acetate/95% hexane as the eluent. The product, undecylenyl acetate, was then used to prepare 11-(trichlorosilyl)-1-undecyl acetate.32 11(Trichlorosilyl)-1-undecyl thioacetate was prepared according to ref 16, except the chain-lengthening step was eliminated (the ω-hexadecenyl step). The purity of the products was determined by 1H and 13C NMR analysis. Preparation of Tips and Surfaces. Glass microscope slides (Fisher Scientific) were soaked in concentrated NH4OH for 1 h and washed five times by ultrasonication in 18 MΩ‚cm water. The slides were then dried for 1 h at 140 °C and placed directly into methanol to protect the surfaces 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) 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 SiO2.33-35 The silicon monoxide was vaporized by passing an electrical current of 65 A through the refractory boat. The thickness of the deposited films was monitored using a quartz-crystal microbalance gauge (Edwards FTM5), by which the nominal rate of deposition for SiO236 was measured to be 1.0 Å‚s-1. SiO2 films were deposited to a typical thickness of 60-80 Å. The SiO2-coated cantilevers were removed from the vacuum chamber and placed directly into dry methanol. To produce the desired functionality on the AFM tips or surfaces, glass slides and SiO2-coated cantilevers were immersed in a 3 mM silane solution in dicyclohexyl for 4 h (see Figure 1). Upon removal from the silane solution, the silanated tips and surfaces were washed with chloroform, then acetone, and then 18 MΩ‚cm water to remove unreacted silane materials. The treated surfaces and tips were then dried carefully with pure nitrogen and used immediately for AFM, contact-angle, or XPS analysis or carried through the reduction (vide infra). To perform on-surface reduction of surface-bound functionalities, a solution of LiAlH4 in ether was prepared by stirring a mixture of 1 g of LiAlH4 in 100 mL of ether and allowing it to stand until a clear supernatant was formed. The supernatant was withdrawn into a clean beaker, and the silanated substrates were immersed in this solution for 1 min. The substrates were then washed sequentially in 4% HCl, chloroform, acetone, and 18 MΩ‚cm water and dried under a stream of dry nitrogen. The reduction process was repeated two times to ensure complete conversion of the acetate to the alcohol and the thioacetate to the thiol, producing a self-assembled monolayer composed of covalently bound “-C12OH” and “-C12SH” moieties, respectively. These monolayers will be referred to as hydroxyl- and thiolterminated monolayers, respectively. Force Measurements. A commercial AFM system (Topometrix) was used to obtain force-distance curves; it employed commercial cantilevers (Park Scientific) that were modified as described above. The force constants of the cantilevers were calculated using their measured unloaded resonance frequencies.37 The average resonance frequencies of unmodified cantilevers for the three size tips employed here were (5.79 ( 0.22 kHz, n ) 13), (13.09 ( 0.34 kHz, n ) 15), and (30.16 ( 0.82 kHz, n ) 11). These frequencies corresponded to force constants of 0.0078 ( 0.0009, 0.029 ( 0.002, and 0.075 ( 0.006 N‚m-1, respectively. Individually measured force constants of the modified cantilevers (not average or nominal manufacturers’ (32) Edmiston, P. L.; Lee, J. E.; Cheng, S.-S.; Saavedra, S. S. Molecular Orientation Distributions in Protein Films. I. Cytochrome c Adsorbed to Substrates of Variable Surface Chemistry. J. Am. Chem. Soc., in press. (33) Holland, L. Vacuum Deposition of Thin Films; John Wiley & Sons, Inc.: New York, 1956; pp 449-450. (34) George, J. Preparation of Thin Films; Marcel Dekker: New York, 1992. (35) Vacuum Deposition Chemicals & Evaporation Materials, 3rd ed.; CERAC, Inc.: Milwaukee, WI, 1988; pp 11-12. (36) Lacy, W. B.; Williams, J. M.; Wenzler, L. A.; Beebe, T. P., Jr.; Harris, J. M. Anal. Chem. 1996, 68, 1003. (37) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403.
Langmuir, Vol. 13, No. 14, 1997 3763 values) were used in the calculation of forces and the calibration of the instrument. The force-distance curves were obtained in a liquid cell of a design similar to that of Drake et al.38 All force-distance curve measurements were performed at a vertical scan rate of 5 µm‚s-1 (the instrument was not scanned in the lateral dimension). This rate is in the same range as that 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 by Hoh to cause oscillations at the pull-off point (195 µm‚s-1 in air).39 We did not attempt a systematic study of the loading rate in this work but plan to investigate it in the future. XPS Surface Characterization. To independently measure the adsorption of the organosilanes, X-ray photoelectron spectroscopy was performed using an ESCALab 220i-XL (VG Scientific, East Grinstead, U.K.). 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), Cl 2p (199.9 eV), S 2s (228.0 eV), and S 2p (164.1 eV). High-resolution, multiplex spectra of the individual elements were acquired from a ∼150 µm diameter area using a 20 eV pass energy and were signal averaged for 50 scans, requiring ∼100 s per spectral region. Peak positions were assigned by referencing the methylene component of the C 1s peak to an energy of 284.6 eV and linearly shifting all other peaks by an equal amount, as is customary. Although not shown here, XPS images were acquired in parallel using a monochromated, microfocused Al KR X-ray source operating at 150 W anode power. The nominal spatial resolution of this imaging XPS instrument on a gold knife edge is 1 µm. Contact-Angle Measurements. A standard contact-angle goniometer (Model A-100, Rame-Hart) was used to make contactangle measurements on each modified surface. Water (18 MΩ‚cm) was used as the sessile drop for all samples.
Results and Discussion XPS Measurements. XPS was used to characterize and confirm the presence of the organosilanes on the glass substrates and SiO2-coated AFM tips. Table 1 shows integrated peak areas and binding energies for each of the major elements from the various organosilanemodified glass substrates. XPS allowed us to differentiate between the functional groups of the acetate- and thioacetate-terminated silanes and their reduced forms. The middle column in each element group of Table 1 represents the atomic concentration ratio of major elements calculated from normalized photoelectron counts and compared to molecular stoichiometries in the third column of each element group.40 The normalized photoelectron counts are not corrected for photoelectron attenuation through the organic film. In layered monolayers, as opposed to homogeneously distributed materials, attenuation tends to underestimate the elements buried deeper in the monolayer relative to those at the outer interface. Operating in the opposite direction to this trend, it can be seen that the oxygen and silicon ratios were larger than the values expected from stoichiometry, while the carbon and sulfur ratios were smaller. This is due to the fact that there are more -SiOH groups on the surface than can react with the much larger silane group, so that there remain some unreacted -SiOH groups.41 This elevates the Si and O signals as measured by XPS. (38) 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. (39) Hoh, J. H.; Engel, A. Langmuir 1993, 9, 3310. (40) The equation Cx ) (Ix/Sx)/(∑iIi/Si) is used for the calculation of the atomic concentration ratio of the three major elements in thiols. C, I, and S represent the atomic concentration ratio, XPS intensity, and XPS sensitivity factor, respectively. S values for the XPS data taken with the ESCALab 220i-XL are as follows: carbon 1.0, oxygen 2.93, silicon 0.82, chlorine 2.29, and sulfur 1.68. Wagner, C. D. Surf. Interface Anal. 1981, 3, 211. (41) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979; Chapter 6.
Wenzler et al.
-Si(CH2)11OCdO(CH3) -Si(CH2)11OH -Si(CH2)11SCdO(CH3) -Si(CH2)11SH
a XPS atomic percentages were calculated as described in ref 40. Only C, O, Si, and S (when present) were used in the calculation of atomic %. to calculate % atomic stoichiometry, based on the precursor molecule used.
b
Only C, O, Si, and S (when present) were used
6.3 7.7 164.6 163.6 6.3 7.7 6.3 7.7 284.6 284.6 284.6 284.6
monolayer studied
16.9 12.1 8.7 9.7
81.3 84.6 81.3 84.6
102.6 101.6 102.6 102.6
4.2 5.7 5.2 5.3 12.5 7.7 6.3 0 78.9 82.2 85.9 84.6 531.6 530.6 531.6 531.6
S 2p
0.2 0.4
chemical shift (eV) atomic % stoichiometryb Si 2p
XPS atomic %a chemical shift (eV) atomic % stoichiometryb
O 1s
XPS atomic %a chemical shift (eV) atomic % stoichiometryb
C 1s
XPS atomic %a chemical shift (eV)
Table 1. XPS Data for Organosilane Self-Assembled Monolayers on Glass Surfaces
XPS atomic %a
atomic % stoichiometryb
3764 Langmuir, Vol. 13, No. 14, 1997
Figure 2. XPS survey scan of acetate-terminated silane deposited on a glass surface. Inelastic scattering observed in the background is indicative of the layered nature of the selfassembled monolayer.
Figure 3. XPS spectra of thioacetate- and thiol-terminated silane monolayers covalently bonded to SiO2-covered glass surfaces. Overlay of both spectra representing a high-resolution scan of the S 2p region. Dotted peaks represent Gaussian peak fits to the data, with the thioacetate sulfur at 163.5 eV and the thiol sulfur at 162.9 eV.
Figure 2 shows a survey scan of acetate-terminated silane deposited on a glass surface. As expected, the major surface species are carbon, oxygen, and silicon. The observed trends in the background to the left of each primary XPS peak are meaningful for assessing the selfassembled or layered nature of the monolayers in the dimension normal to the surface plane. The rising background to higher binding energies (lower kinetic energies) for O 1s, Si 2s, and Si 2p can also be used as a qualitative, nondestructive indication of the depth of an element in the sample.42 The rising-and-then-falling background observed above the O 1s, Si 2s, and Si 2p peaks indicates more inelastic electron scattering from those elements buried deeper in the adsorbate layer. Electron emission from further out in the adsorbate layer results in less inelastic electron scattering, which is observed as a continuous decline or relatively constant background. These trends are seen in the C 1s peak of Figure 2 and in the S 2p peaks of Figure 3 for the thiolterminated monolayers (vide infra). These trends therefore give some indication of the layered structure of the (42) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Muhlenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1978.
AFM Measurements of Bond-Rupture Forces
Figure 4. XPS spectra of acetate- and hydroxyl-terminated silane monolayers on a SiO2-covered glass surface. (A) Highresolution scan of the C 1s region of acetate-terminated silane. Dotted peaks represent Gaussian peak fits to the data, with the acetate carbon at 288.6 eV, the ether carbon at 286.6 eV, and the methylene carbon at 284.6 eV. (B) High-resolution scan of the C 1s region of reduced acetate-terminated silane on a clean glass surface. Dotted peaks represent Gaussian peak fits to the data, with the hydroxyl carbon at 286.8 eV and the methylene carbon at 284.6 eV.
sample, and hence the self-assembled nature of the sample in the direction normal to the surface plane, as has been surmised from other studies of these “Sagiv-type” selfassembled monolayer systems.6-12 The presence of the thioacetate and the reduced thioacetate (thiol-terminated silane) on the surface can be verified by the S 2p signal. In Figure 3 we analyzed the S 2p signal for both surfaces and observed, when the two S 2p regions are overlayed, the thioacetate-terminated silane XPS peak is shifted to higher binding energies than the thiol-terminated silane. This shift to higher S 2p binding energy is due to the thioacetate functionality, which was also observed in the C 1s region (data not shown for thioacetate and thiol). Analysis of the C 1s peak was used to distinguish between the acetate-terminated silane and the reduced acetate (hydroxyl-terminated) silane monolayers. Figure 4A shows the analysis of the acetate-terminated silane, for which peak fitting revealed three carbon states corresponding to the methylene carbons (284.6 eV), the carbon attached to the ester oxygen (286.6 eV), and the acetate carbonyl carbon (288.6 eV), which is consistent with previous research.43 For the hydroxyl-terminated (43) Sun, F.; Grainger, D. W.; Castner, D. G. J. Vac. Sci. Technol. 1994, 12, 2499.
Langmuir, Vol. 13, No. 14, 1997 3765
silane, shown in Figure 4B, only two peaks resulted from peak fitting, representing the C-O and methylene carbons observed at 286.8 and 284.6 eV, respectively. XPS Images. In addition to spectral XPS, spatially resolved XPS images of the individual elements present for each organosilane were acquired for several of the functionalized AFM tips. Although not shown here,44 the XPS images were collected for C 1s, O 1s, Si 2p, and S 2p images from the thiol-terminated silane deposited on a SiO2-coated Si3N4 AFM tip. The images demonstrated uniform coverage of the silane on the AFM tip and cantilever underside. Contact-Angle Measurements. Advancing contact angles were measured on all samples in order to correlate adhesion force measurements with the hydrophobicity of the silane monolayer. Before immersing in silane, clean glass surfaces had contact angles of 12.3 ( 2.1°, indicating the hydrophilic nature of the surface before silanization. Table 2 gives the measured contact angles for all organosilane systems studied. All systems, except the hydroxyl-terminated silane, exhibited relatively high contact angles, consistent with the well-packed, hydrophobic nature expected for these films. The lower contact angle on the hydroxyl-terminated silane is indicative of the surface’s hydrophilic groups. Force Measurements. Figure 5 shows a plot of the measured force variance σm2 versus the measured mean force µm for an SiO2-coated AFM tip and glass surface, measured in H2O. Both tip and surface bear thiolterminated silane monolayers produced by the on-surface reduction of the thioacetate, as described above. In general, one notes a linear relationship between the mean force and its variance, as expected for Poisson statistics, and an ordinate intercept not significantly different from zero, indicating negligible nonspecific binding.17-19 More specifically, these results indicate a single-molecule bondrupture force of 60 ( 5 pN. The lower right inset of the figure is an actual force-distance curve obtained for this system. Repetitive sets of measurements were performed at one position of the sample surface, and a mean pull-off force µm and pull-off-force variance σm2 were calculated per set to generate each point on the plot. Different values along the abscissa were achieved by using several different tips, each having a natural variability in the tip radius and hence contact area. This natural tip-to-tip variability leads fortuitously to variations in the measured mean pull-off force values µm. Since the method does not require any assumptions about the tip radius or contact area, this natural tip variability is an added advantage of this method. Table 3 shows the summary of force-distance measurements with various tip and surface functional groups, in different liquid media. As expected, the measured single-molecule bond-rupture forces show a dependence on the surrounding medium, which we note follows a phenomenological dependence on the medium’s dielectric constant. The tabulated single-molecule bond-rupture forces, which we presume are the result of hydrogen bond interactions, decrease with an increase in the dielectric constant of the medium, for both -OH‚‚‚HO- and -SH‚‚‚HS- interactions. These trends are depicted graphically in Figure 6. Since a hydrogen bond is predominantly an electrostatic interaction,45 its electrostatic (Coulombic) field can be effectively shielded by polar solvents such as water. However, the observed decrease (44) XPS images of AFM cantilevers modified with some similar “Sagiv-type” monolayers have been presented in another work from this group (see ref 19). (45) Israelachvili, J. Intermolecular & Surface Forces; Academic Press Inc.: San Diego, 1992.
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Table 2. Advancing Contact Angles of Functionalized Monolayer Surfaces alkylsilane self-assembled monolayer -Si(CH2)11OCdO(CH3) acetate-terminated monolayer -Si(CH2)11OH hydroxyl-terminated monolayer -Si(CH2)11SCdO(CH3) thioacetate-terminated monolayer -Si(CH2)11SH thiol-terminated monolayer a
contact angle (deg)
reagent used to form self-assembled monolayera 11-(trichlorosilyl)-1-undecyl acetate
67.0 ( 2.0
formed from on-surface reduction of 11-(trichlorosilyl)-1-undecyl acetate
41.0 ( 1.7
11-(trichlorosilyl)-1-undecyl thioacetate
77.7 ( 1.5
formed from on-surface reduction of 11-(trichlorosilyl)-1-undecyl thioacetate
78.3 ( 3.0
See text for details of self-assembled monolayer production. Table 3. Tip-Surface Bond-Rupture Forces (pN) Measured by AFM liquid medium (dielectric constant) tip-surface interaction OH HO SH HS
DMSO, 1-propanol, self-assembled monolayer water, H2O (78.5) (CH3)2SdO (49.1) CH3(CH2)2OH (20.1) present on tip and surface 119 ( 16 60 ( 5
Figure 5. Plot of measured bond-rupture force variance, σm2, vs measured mean bond-rupture force µm for a SiO2-coated tip and glass surface in H2O modified with thiol-terminated silane. The top left-hand inset schematically depicts the tip and the surface. The bottom right-hand inset is an actual forcedistance curve indicating the pull-off force for this system.
Figure 6. Single hydrogen bond bond-rupture forces in various liquid media: (0) thiol-terminated tip and sample surface; (O) hydroxyl-terminated tip and sample surface.
in the measured discrete bond strength with dielectric constant does not follow a simple -1 functional form, as would be expected from an ideal adherence to the Coulomb law for isolated point charges;45 instead it follows a near-
202 ( 8 84 ( 11
302 ( 40
-Si(CH2)11OH
118 ( 11
-Si(CH2)11SH
linear one. This has been observed in previous studies18 as well. Interactions that take place in a solvent medium involve polarization effects.45 These polarization forces arise from the dipole moment induced in atoms and molecules by the electric fields of nearby charges and permanent dipoles. Two or more interactions that occur simultaneously, plus their second-order effects, cause difficulty in applying a simple Coulomb function to calculate the single-molecule bond-rupture force dependence on . In order to convert bond-rupture forces to bond energies, a functional form for the force must first be assumed, as well as the limits of the required integration. There is no straightforward procedure for this process that we are aware of,24 and so the ensuing discussion should be viewed as a speculative, zero-order attempt to estimate bond energies from bond-rupture forces, rather than a conclusion or finding of this work. To compare these energies with tabulated energies, a force relevant to ) 1 (vacuum) must be used. Thus, we begin by extrapolating the F() versus plots to ) 1 using the phenomenological linear trend observed, and then use this value of F( ) 1) in the integration to obtain a bond energy [see ref 46]. For the hydroxylterminated silane, the value obtained is 362 ( 10 pN, from which a bond energy of 19.5 ( 2.1 kJ‚mol-1 is calculated. For the thiol-terminated silane the value obtained is 136 ( 6 pN, from which a bond energy of 7.5 ( 1.4 kJ‚mol-1 is calculated. Reported errors are from propagation of the error on the force and may well be less than errors associated with the assumed functional form of the potential and limits of the integration.45,46 A typical hydrogen bond for an oxygen-containing system has an energy of 10-40 kJ‚mol-1;45 our hydroxyl-terminated silane measurements fall within this range, whereas the thiol-terminated silane falls below it, consistent with the known weaker hydrogen bonding in thiols. (46) The equation E ) ∫∞ro - NF(ro/r)3 dr is described in ref 45 as relevant to hydrogen bonding and was used for the hydrogen bond energy calculation in this study. N, F, and ro reprsent Avogadro’s number, the single hydrogen bond force in a vacuum (i.e. hydroxyl-terminated silane ) 362 pN), and a typical hydrogen bond length (1.76 Å) in water, respectively. The integral used in these experiments was used only as an estimate. The assumption was made where the minimum energy of the potential occurs at ro. The repulsive portion of V(r) is neglected, therefore giving an overestimation of the true energy. See also ref 45. Converting bond-rupture forces to bond energies thus involves several assumptions which were described above. The bond-rupture magnitudes are not limited by these assumptions.
AFM Measurements of Bond-Rupture Forces
Langmuir, Vol. 13, No. 14, 1997 3767
Dispersion in Bond-Rupture Forces. The foregoing analysis of bond-rupture forces, and previously published analyses,17-19 have relied on data for which the dominant source of variance in the measurement has been n, the number of bonds ruptured. An additional source of variance can derive from dispersion in the bond-rupture force F itself. This dispersion could result, for example, from a distribution of orientations of interacting groups. If the discrete bond-rupture force is not a constant, it would alter the statistics of bond rupture and introduce nonlinearity into the σm2 versus µm plots. This can be seen by examining the full propagation of error analysis for the measured mean force variance, where both F and n contribute to the observed variance of m.
σm2 )
2
2
∂m σ +( ) σ (∂m ∂n ) ∂F 2
n
2
F
∂m σ (∂m ∂n )( ∂F )
+2
2
nF
(4)
) 〈F2〉σn2 + 〈n2〉σF2 + 2〈F〉〈n〉σnF2
(5)
) (σF2 + 〈F〉2)n + (σn2 + 〈n〉2)σF2 + 2µmσnF2
(6)
) 2σF2
(
µm 〈F〉
) 〈F〉 +
+ Fµm +
2σF2 〈F〉
µm2
σF2 + 2µmσnF2 〈F〉2
)
+ 2σnF2 µm +
σF2 〈F〉
2
µ 2 m
(7)
(8)
The new terms σF2 and σnF2 in eq 4-8 are the variance in the bond-rupture force and the covariance of that bondrupture force with the number of bonds ruptured, respectively. The variance of the measured quantity, σm2, is now a linear plus quadratic function of the mean force µm. The coefficient of the µm2 term in eq 8 is the square of the relative standard deviation (σF/F) of the singlemolecule bond-rupture force, and it can in principle give some indication of the dispersion in that bond-rupture force for the first time. Until now we have had no reason to consider it because of all σm2 versus µm plots have been well fit by a linear equation. In the case of little or no dispersion in the discrete bondrupture force, the plot of σm2 against µm is dominated by the linear term, as can be seen in Figure 5, and as was observed here for the hydroxyl- and thiol-terminated silane monolayers. However, both the acetate- and thioacetateterminated silanes were also analyzed before on-surface reduction. These are the first systems for which the singlemolecule bond-rupture forces could not be determined using only the linear term in the above statistical method. Plots of the variance σm2 against the mean of the total force µm for the acetate-terminated silane were not linear, but instead exhibited considerable curvature, as can be seen in Figure 7. Figure 7 depicts different data sets (represented by different symbols) acquired with different surfaces and tips having the same surface chemistry. When the data in Figure 7 are fit to a model containing a linear plus quadratic term, the quadratic coefficient gives relative standard deviations in the discrete bond-rupture force ranging from 4 to 16%. This fit also returns linear coefficients that are negative, although estimated errors from the quality of this fit are larger than the linear coefficient itself at the 95% confidence level, due to scatter in the data. A negative value for the linear coefficient is not, however, unreasonable given the covariance term σnF2 within the linear coefficient in eq 8. This term can assume negative values; if the measured discrete bond-rupture force decreases as the number of bonds ruptured increases, the covariance between these variables would be negative.
Figure 7. Plot of measured bond-rupture force variance σm2 vs measured mean bond-rupture force µm for a SiO2-coated tip and glass surface in H2O, each modified with acetate-terminated silane monolayers. The curvature exhibited in this system was used, along with a new analysis method presented in the text, to estimate the dispersion, or relative variation, in the magnitude of the single-molecule bond-rupture force F. Different symbols represent different tips and samples: (9) six different areas with the same tip and surface; (*) a new tip and surface in three different spots; (O) another tip and surface in seven different spots.
Negative linear coefficients resulting from a two-component fit imply that the covariance is large. The acetate-terminated silane tips and surfaces described above were expected to exhibit a van der Waals interaction. The nonlinear geometry of the terminating acetate group and the possibility for free or hindered rotation about the C-O bond of the acetate could allow this terminating group, unlike all others studied by this method so far, to present a wider variety of interaction geometries, leading to the observed variance in F as well as in n. During the acquisition of force-distance data for the acetate and thioacetate systems in water, frequently the total adhesion force increased with the number of repetitive measurements,47 whereas no adhesion was observed in other solvents. The higher adhesion force in water could be due to the hydrophobic nature of both the tip and the surface; this is also seen in contact-angle measurements (see Table 2). The possibility of both van der Waals and hydrophobic interactions between the tip and the surface would result in varying contact forces. Conclusion We have shown that bond-rupture forces between individual functional groups terminating the self-assembled monolayers on chemically modified tips and surfaces can be measured for “Sagiv-type” monolayer systems. It was possible to detect changes in the interaction force between a modified tip and surface by varying the liquid medium or chemical environment. Single-molecule hydrogen bond bond-rupture forces between tips and surfaces covered with hydroxyl- and thiol-terminated silane groups were measured by following the on-surface reduction of acetateand thioacetate-terminated silanes. In each system studied, XPS was used to characterize the organosilane monolayer on the modified SiO2-coated AFM tips and glass surfaces. Using high-resolution XPS scans of the C 1s region and S 2p region, distinct and expected differences were observed between the acetate- and thioacetate(47) Both the acetate- and thioacetate-terminated silanes were analyzed in DMSO and 1-propanol in addition to water. Unlike in water, these solvents exhibited no observable pull-off force and could therefore not be analyzed.
3768 Langmuir, Vol. 13, No. 14, 1997
terminated silanes and their reduced counterparts, hydroxyl and thiol. A new analysis method has allowed us to report preliminary estimates of the dispersion or variability in the magnitude of the discrete bond-rupture force for the acetate-terminated silane monolayers. Acknowledgment. The authors wish to thank Carrie Brockway for help with the on-surface reduction steps
Wenzler et al.
and Lydia Olson for help with SiO2 vacuum deposition. This work was supported by grants from the National Science Foundation (CHE-9357188 to T.P.B., CHE9510312 to J.M.H.) and 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. LA9620869
