Langmuir 1997, 13, 4357-4368
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Scanning Force Microscopy with Chemical Specificity: An Extensive Study of Chemically Specific Tip-Surface Interactions and the Chemical Imaging of Surface Functional Groups Eric W. van der Vegte and Georges Hadziioannou* Department of Polymer Chemistry and Materials Science Centre, University of Groningen, Nijenborgh 4, NL-9747 AG, The Netherlands Received ‡ January 8, 1997. In Final Form: April 28, 1997X An extensive and systematic scanning force microscopy (SFM) study is presented. The observations are based on hydrogen bonding, van der Waals, and Coulombic interactions between the scanning probe (tip) and the substrate and provide the basis for scanning force microscopy with chemical specificity (SFMC).1 The self-assembly (SA) of ω-functional n-alkanethiol compounds was used to chemically modify standard SFM probes and substrates with a variety of functional groups (CH3, OH, NH2, COOH, and CONH2). The Johnson-Kendall-Roberts (JKR) theory of adhesion mechanics provided the theoretical background for our measured data and enabled the calculation of surface free energies, the number of interacting molecules, and single-bond forces. Furthermore, a recently developed statistical analysis of the force distribution was applied to evaluate single-bond strengths and the number of molecules interacting between scanning probe and substrate. Good agreement was found between this analysis and the JKR theory. Furthermore, from the lateral force measurements the friction coefficients were determined and a direct relationship between friction and the measured adhesion forces was demonstrated. This relationship was used as the basis for chemically specific imaging of functional groups with predictable frictional force contrast. Imaging of up to three different functional groups on the same surface was demonstrated on chemically patterned surfaces. Experimentally, a chemical resolution of 100 nm in the lateral dimensions was achieved. pHdependent adhesion force measurements were performed to study acid-base properties of surface-bound functional groups and were found to be a unique way to determine surface pK values. Finally, a new method was developed to differentiate and image the surface-bound functional groups on the basis of their acid-base properties.
Introduction Since the invention of the scanning tunneling microscope in 1982,2 the family of scanning probe microscopes (SPM) has experienced a massive growth. The scanning force microscope (SFM)3 is probably the most successful and applied SPM imaging method. It has proven to be useful in imaging a variety of conducting and nonconducting surfaces in various fields within chemistry, physics, and biology.4 Not only has the SFM been used as an analytical method to characterize surfaces, it has become a research tool in tribology5 and in structural research in biological chemistry.6 However, the major drawback of the SFM is its lack of chemical specificity; i.e., it does not allow the identification of chemical functionalities at the surface or the mapping of the spatial distribution of these chemical functional groups on the surface. Information about the chemical nature of the sample surface is of great importance in tribology, catalysis, (bio)compatibility, chemical recognition, semiconductor devices, adhesion between polymers and metals, and the chemical modification of polymer surfaces. ‡ This manuscript was originally submitted to J. Am. Chem. Soc. on June 19, 1996. X Abstract published in Advance ACS Abstracts, July 15, 1997.
(1) Although the term scanning chemical force microscopy (SCFM) was advanced by others (see: Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C .M. Science 1994, 265, 2071), we believe that “scanning force microscopy with chemical sensitivity” (SFMC) is a more appropriate terminology. (2) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (3) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (4) For reviews see: (a) Rugar, D.; Hansma. P. K. Phys. Today 1990, 43, 23. (b) Frommer, J.; Meyer, E. J. Phys.: Condens. Matter 1991, 3, S1. (c) Smith, I.; Howland, R. Solid State Technol. 1990, 53. (d) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298.
S0743-7463(97)00025-5 CCC: $14.00
Our approach to introducing chemical specificity into SFM is to provide a specific interaction (e.g., a hydrogen bond, Coulombic, or π-stacking interaction) between the scanning probe and the surface which bears the chemical functionality of interest (Y). We achieve this by chemical modification of the SFM probe with functional groups (X) that provide the specific interaction, as shown in Figure 1. We have chosen self-assembling monolayers (SAM) of ω-functional n-alkanethiol compounds on gold(111) surfaces to chemically modify standard SFM probes. Robust, highly ordered, crystalline-like monolayers are formed in which the molecules are covalently pinned to the surface, with the ω-terminal functional group exposed at the air interface.7 SAM’s of thiol compounds are well studied and widely used in a variety of applications8 to chemically modify surfaces. In recent literature,9-11 researchers have successfully proven the use of SAM’s of alkanethiols for chemical modification of SFM probes to introduce chemical specificity. (5) (a) Salmeron, M. B. MRS Bull. 1993, 5, 20. (b) Overney, R.; Meyer, E. MRS Bull. 1993, 5, 26. (c) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 58, 133. (d) Overney, R. M.; Bonner, T.; Meyer, E.; Ruetschi, M.; Lu¨thi, R.; Howald, L.; Frommer, J.; Fujihira, M.; Takano, H. J. Vac. Sci. Technol. B 1994, 12, 1973. (e) Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. (f) Erlandsson, R.; Hadziioannou, G.; Mate, C. M.; McClelland, G. M.; Chiang, S. J. Chem. Phys. 1988, 89, 5190. (g) Bhushan, B.; Blackman, B. ASME J. Tribol. 1991, 113, 452. (6) (a) Drake, B.; Prater, C. B.; Weisenhorn, A. L.; Gould, S. A. C.; Albrecht, T. R.; Quate, C. F.; Cannell, P. S.; Hansma, H. G.; Hansma, P. K. Science 1989, 243, 1586. (b) Radmacher, M.; Tillmann, R. W.; Fritz, M.; Gaub, H. E. Science 1992, 257, 1900. (c) Hansma, H. G.; Hoh, J. H. Annu. Rev. Biophys. Chem. 1994, 23, 115. (d) Schabert, F. A.; Engel, A. Forces in Scanning Probe Methods; NATO Advanced Study Institute Series E, Appl. Sci.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; Vol. 287, p 607. (e) Schabert, F. A.; Henn, C.; Engel, A. Science 1995, 268, 92.
© 1997 American Chemical Society
4358 Langmuir, Vol. 13, No. 16, 1997
Figure 1. Schematic drawing of a chemically modified tip with functional groups X, which provide a specific interaction with functional groups of interest Y on the sample surface.
In this paper we describe an extensive and systematic SFM study of hydrogen bonding, van der Waals, and Coulombic interactions between tip and substrate, both of which have been modified with a variety of functional groups (CH3, OH, NH2, COOH, CONH2). The force interactions are quantified in terms of their adhesion forces. The Johnson-Kendall-Roberts theory of adhesion mechanics and a recently developed statistical analysis have been applied to deduce single-bond forces, the number of interacting molecules, and the surface and interfacial free energies. Both approaches are found to be in good agreement. Lateral force measurements and the determination of the absolute friction coefficients show a direct relationship between the measured adhesion forces and the frictional forces, which immediately indicates the potential of the lateral force mode to reveal the chemical nature of the well-defined functional patterns fabricated with the microcontact printing (µCP)14 technique. pHdependent adhesion force measurements allow the study of the acid/base behavior and the determination surface pKa values of surface-bound ionizable groups. The observed acid-base properties constitute the basis for a method to differentiate surface bound ionizable groups via pH-dependent imaging, which is demonstrated on a µCP patterned substrate, containing acidic and basic functional groups. Experimental Section Materials. Dodecanethiol and n-heptane p.a., both from Janssen Chimica, and ethanol p.a. and KCl p.a., both from Merck, were used as received. Water used in the experiments was deionized (18 MΩ cm resistivity) with a Millipore Milli-Q filtration system. Gold wire (Scho¨ne Edelmetalen) was 99.99% pure. Solutions with different pH values were made from 10 mM KCl aqueous electrolyte solutions by adjusting the pH values with (7) For example: (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (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) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (e) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (f) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (g) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (h) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (i) Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994, 10, 4103. (8) (a) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (b) Spinke, J.; Liley, M.; Schmitt, F.-J.; Guder, H.-J.; Angermaier, L.; Knoll, W. J. Chem. Phys. 1993, 99, 7012. (c) Moore, L. W.; Springer, K. N.; Shi, J.-X.; Yang, X.; Swanson, B. I.; Li, D. Adv. Mater. 1995, 7, 729. (d) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehnig, M. C. Langmuir 1994, 10, 619. (e) Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672. (9) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (10) (a) Green, J-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (b) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (11) Sinniah, S. K.; Steel, A. B.; Miller, J. C.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925.
van der Vegte and Hadziioannou HCl in the acidic region and KOH in the basic region, choosing the proper concentration to maintain a (nearly) constant ionic strength. Mica (clear ruby grade 2) was obtained from Mica New York Corp., and Sylgard silicon elastomer 184 was a gift from Mavom b.v. (Alphen a/d Rijn, The Netherlands). 2-Aminoethanethiol (Aldrich) was purified by sublimation prior to use. 12-Mercapto-1-dodecanol and 11-mercapto-1-undecanoic acid were synthesized according to methods described in the literature.7c,12 11-Mercapto-1-undecanamide was synthesized using a modified procedure and is described briefly below.7d Synthesis of 11-Mercapto-1-undecanamide.7d A 10 g sample of 11-bromoundecanoic acid was placed in a three-necked bottle and heated to melting. Slowly 5.4 mL of redistilled thionyl chloride was added dropwise. The solution was stirred for 3 h at reflux. Excess thionyl chloride was distilled off. The crude 11-bromoundecanoyl chloride obtained was dissolved in dry CH2Cl2 and added dropwise to a vigorously stirred, ice-cooled solution of concentrated aqueous ammonia (150 mL). The 11-bromoundecanoyl chloride solution was added in such a rate that no white fumes were lost. The amide was filtered off as a white solid and recrystallized from ethanol (yield 90%). The 11-bromoundecanamide was converted into 11-mercaptoundecanamide using the same procedure as for the synthesis of 11-mercaptoundecanoic acid12 starting from 11-bromoundecanamide. 1H NMR (CDCl3) δ: 1.2-1.8 (m, 18 H, CH2-(CH2)9-CH2), 2.2 (t, 2 H, CH2CONH2), 2.5 (m, 2 H, CH2-SH), 5.4 (s, 2 H, NH2). Au Coating of Probe and Sample Substrates. Commercial Si3N4 cantilevers with integrated Si3N4 tip (Topometrix GmbH) were gold-coated, using a diffusion-pumped thermal evaporator (Edwards Auto 306). First, 2 nm of chromium was deposited to promote the adhesion of gold, followed by a 75 nm thick gold layer. To avoid bending of cantilevers,9 the backside of the cantilevers was also coated with a 40 nm thick gold layer. All probe coating procedures were performed at room temperature. Gold surfaces used to create the model substrates were formed by thermal evaporation of gold on mica. Freshly cleaved mica was immediately placed in the vacuum chamber of the evaporator. The mica sheet was heated to 400 °C at a pressure below 1 × 10-4 mbar and kept 6 h at this temperature before evaporation. The gold evaporation was performed at a speed of 0.1 nm/s until a thickness of 50 nm was reached.13 The gold substrates were subsequently annealed for 2 h at 400 °C and then cooled to room temperature in vacuum. The gold substrates used to create patterns with the µCP technique were made from 1 × 1 cm2 cut Si wafers, by thermal evaporation of 2 nm Cr as an adhesion promoter followed by a 100 nm thick gold layer. Substrates were kept at room temperature during evaporation. Chemical Modification of Tips and Model Samples. After the gold deposition, the probes and gold substrates were immediately placed in a 1-3 mM solution of the desired thiol in ethanol for at least 12 h. Just prior to use, the substrates and probes were taken out of the solution and rinsed with ethanol and dried in a stream of prepurified nitrogen gas. Patterning of Substrates: Microcontact Printing (µCP).14 PDMS (Sylgard silicon elastomer 184) stamps were created from photolithography patterns (masters) exhibiting either 20 × 20 µm2 squares or 20 µm lines, separated by 20 µm. The masters were placed in a Petri dish, and a 10:1 mixture of the Sylgard 184 silicon elastomer and curing agent was poured over it. Curing was performed after setting overnight at room temperature, in an oven for 1-2 h at 70 °C. After cooling to room temperature, the PDMS stamp was peeled away from the master. Before use, the stamps were rinsed with ethanol and n-heptane several times and then dried with prepurified nitrogen gas. The stamping was performed immediately after the gold deposition. 1-3 mM solutions of the desired thiol in ethanol were used as ink and placed on the stamp as a droplet after which the bulk of the liquid was evaporated with prepurified nitrogen. To avoid spreading of the ink, the µCP procedure was performed under water. The pattern containing three different functional groups (12) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (13) (a) Inukai, I.; Mizutani, W.; Saito, K.; Shimizu, H.; Iwasawa, Y. Jpn. J. Appl. Phys. 1991, 30, 3496. (b) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (c) Zheng, X.-Y.; Ding, Y.; Bottomley, L. A.; Allison, D. P.; Warmack, R. J. J. Vac. Sci. Technol. B 1995, 13, 1320.
Scanning Force Microscopy with Chemical Specificity was prepared, using a line stamp, in two steps. First, parallel lines of the desired thiol were stamped, followed by stamping of another line pattern of the second thiol, oriented perpendicular to the first pattern. The remaining bare gold areas were derivatized by washing the substrate with a solution containing the third thiol. After printing, the patterned substrates were washed with ethanol and blown dry in a stream of prepurified nitrogen. The quality of the patterns obtained was checked with SEM,14b,15 water condensation patterns,16 or SFM (lateral force in air, Si3N4 tip).17 The patterned substrates were used immediately in the SFMC experiments after they had been made. Scanning Force Microscopy with Chemical Specificity (SFMC). The adhesion force measurements were performed on a Topometrix Explorer (TMX1010) AFM with an open liquid cell design. Lateral force mode images of the patterned substrates were recorded with a Topometrix Discoverer (TMX2010) SFM equipped with a closed liquid cell. The measurements were performed immediately after mounting of the chemically modified scanning probes. All SFMC measurements were performed in ethanol unless stated otherwise. Cantilever Calibration. The normal spring constant kc of every cantilever used in the experiment was individually determined, using a nondestructive method described by Hutter and Bechhoeffer,18 which is implemented in the data-acquisition software of Topometrix. This method is restricted to lowfrequency cantilevers with a resonance frequency below 20 kHz. The normal spring constant of the cantilevers used for the adhesion force measurements and imaging were ∼0.2 and ∼0.06 N/m, respectively. Variations between cantilevers up to 20% were found in the measurements. The radiuses of curvature of the gold-coated tips were obtained individually from HRSEM (Jeol 6420F) measurements. Radiuses of gold-coated tips were all found in the range of 30-40 nm, with no exceptions. To measure the friction coefficients, we used a method described by Ruan and Bhushan,19 which does not require the calculation of the lateral spring constant and calibration of the cantilever torsion.20 The friction signal was recorded as a function of different applied loads. The slope of the curve obtained is the friction coefficient, whereas the x-axis cutoff gives the adhesion force.
Results and Discussion I. Uncharged Functional Groups. Adhesion Force Measurements. The force interaction between a functionalized tip and a chemical functionality on the surface is probed via force-distance curves.21 The attractive force in the unloading part of the force-distance curve reflects the adhesion between tip and sample. All adhesion measurements were performed in a liquid (ethanol or water)22 to avoid the appearance of a meniscus between tip and sample due to adsorbed water, which gives rise (14) (a) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (b) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (c) Lo´pez, G.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1513. (d) Wilbur, J. L.; Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 649. (e) Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 471. (15) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993, 9, 1517. (16) Lo´pez, G. P.; Biebuyck, H. A.; Frisbie, C. D.; Whitesides, G. M. Science 1993, 260, 647. (17) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825. (18) (a) Hutter, J. L.; Bechhoeffer, J. Rev. Sci. Instrum. 1993, 64, 1868.(b) Hutter, J. L.; Bechhoeffer, J. Rev. Sci. Instrum. 1993, 64, 3342 (Erratum). We used the corrected method by Butt et al. which accounts for the spatial distribution of thermal energy along the entire length of the cantilever. Furthermore, a correction factor of x4/3 is required in the most common case of a rectangular beam cantilever [Butt, H.-J.; Jaschke, M. Nanotechnology 1995, 6, 1]. (19) Ruan, J.-A.; Bhushan, B. Trans. ASME 1994, 116, 378. (20) O’Shea, S. J.; Welland, M. E.; Rayment, T. Appl. Phys. Lett. 1992, 61, 2240. (21) (a) Burnham, N. A.; Colton, R. J.; Pollock, H. M. Nanotechnology 1993, 4, 64. (b) Weisenhorn, A. L.; Maiveld, P.; Butt, H.-J.; Hansma, P. K. Phys. Rev. B 1992, 45, 11226.
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Figure 2. Representative force-distance curves for a CONH2functionalized tip on the various model substrates: (A) CH3functional SAM, (B) NH2-functional SAM, (C) CONH2-functional SAM. All measurements were recorded in ethanol.
to uncontrolled capillary forces that mask the true introduced tip-sample interaction.23 The self-assembly (SA) method of ω-functionalized n-alkanethiols on gold surface was used to chemically modify the probes and substrates in a well-controllable manner.7 Thus, well-defined and highly reproducible model substrates and probes are prepared, using COOH, OH, NH2, CONH2, and CH3 functional groups. The forcedistance interactions were measured for the various tipsample combinations, reflecting for the uncharged functional groups, the hydrogen bonding (e.g., COOH-CONH2, OH-NH2, etc.) and the purely van der Waals interactions (e.g., CH3-OH, CH3-CH3, etc.). As an example, typical force-distance curves for an amide (CONH2) modified tip on the different model substrates, functionalized with CH3, NH2, and CONH2 groups, are presented in Figure 2 and indicate the progressive increase of the adhesive forces. To quantify the adhesive forces, a number of at least 50 force-distance curves was acquired for every tip-sample combination, while maintaining a fixed contact position. (22) The role of the medium in regulating tip-sample interactions is rather complex and not completely understood. Therefore, one has to be extremely careful in comparing results from measurements in air and in various solvents. All measurements performed in this study were carried out in ethanol and compared as such (see also: ref 11 and: Hutter, J. L.; Bechhoefer, J. J. Appl. Phys. 1993, 73, 4123). (23) (a) Binggeli, M.; Mate, C. M. Appl. Phys. Lett. 1994, 65, 415. (b) Grigg, D. A.; Russel, P. E.; Griffith, J. E. J. Vac. Sci. Technol. A 1992, 10, 680. (c) Weisenhorn, A. L.; Hansma, P. K., Albrecht, T. R.; Quate, C. F. Appl. Phys. Lett. 1989, 2651.
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van der Vegte and Hadziioannou Table 1. Mean Adhesive Forces (in nN) for Every Tip-Substrate Combinationa substrate tip
CH3
OH
NH2
COOH
CONH2
CH3 OH NH2 COOH CONH2
0.9 0.1 0.2 0.7 0.3
0.3 0.9 0.5 1.2 1.3
0.3 1.2 0.8 1.2 1.2
0.3 1.2 0.7 1.3 2.2
0.3 1.4 0.9 2.2 1.8
a All measurements were performed in ethanol. The values represent the average adhesive forces out of 50 f/d curves. The experimental error in the measurements is approximately 20%.
expected,24 the cohesive CH3-CH3 van der Waals interaction is found to be larger than the van der Waals forces for dissimilar pairs, e.g. CH3-COOH. In the case of an introduced hydrogen bond, e.g. for a COOH or NH2 tip, again the pure van der Waals interaction with a CH3model surface is found to give the lowest adhesive force, whereas the observed trend in hydrogen bond formation for OH-, NH2-,COOH-, and CONH2-model surfaces is found to coincide with the trend in hydrogen bond energies,25 as expected. At this stage, quantitative comparisons can only be made for one particular tip on different substrates. Comparing different tips with each other is not possible, because of the fact that not every tip has the same radius of curvature; i.e., the contact area is different and hence the number of interacting molecules contributing to the measured adhesive force is not the same for every tip. The single chemical bond force is a value independent of the radius of curvature and contact area and should allow comparison of the different tip-sample interactions. Lieber and co-workers9a,b have shown that the JohnsonKendall-Roberts (JKR) theory of adhesion mechanics26 can be applied to these systems and adequately describes the measured data. The use of a variety of functionalized tips should verify their findings to a larger degree and should give more insight into the applicability of the JKR theory. The JKR theory of adhesion mechanics27 describes the adhesion between a spherical tip 1 and a flat surface 2 in a medium 3. The adhesion force Fadh is given by the following formula:
Fadh ) -(3/2)πRW12
Figure 3. Histograms presenting the adhesion force distribution for a CONH2-functionalized tip on the various model substrates: (A) CH3-functional SAM, (B) NH2-functional SAM, (C) OH-functional SAM, (D) CONH2-functional SAM, (E) COOH-functional SAM. Each histogram was constructed from at least 50 force-distance measurements in ethanol.
As a result, distributions of adhesion forces were obtained, which are depicted in the histograms of Figure 3 for the CONH2-modified tip. From this distribution, the numerical average and standard deviation were calculated. All possible tip-sample interactions were characterized by measuring their adhesive force distributions. The mean adhesive force values in nN for the various tip-sample combinations are summarized in Table 1. For all modified tips, the trend expected on basis of the nature of the introduced interactions in the adhesion forces is qualitatively observed: for a CH3-modified tip, for which only van der Waals interactions play a role, small adhesive forces are found, compared to the hydrogen-bondingcapable tips (e.g. OH, NH2, COOH, and CONH2 tip). As
(1)
where W12 is the work of adhesion to pull the tip off the sample and R is the radius of curvature of the tip. W12 can be expressed in surface free energies: W12 ) γ13 + γ23 - γ12, where γ13 is the tip surface free energy in equilibrium with the medium (in our measurements ethanol), γ23 the sample surface free energy in equilibrium with the medium (ethanol), and γ12 the interfacial free energy of the tipsample contact interface. For identically functionalized tip-sample combinations, e.g. OH-OH or CH3-CH3, the (24) Israelachvili, J. N. Intermolecular & Surface Forces; Academic Press: New York, 1992. (25) Armstrong, F. B. Biochemistry; Oxford Press: London, 1983. In general, care should be taken in comparing bond energies with bond forces as a measure of the bond strength. Bond energy is a thermodynamic quantity describing an equilibrium interaction averaged over space and time, whereas force is an instantaneous, nonequilibrium interaction along a certain direction over a finite time and distance. When a system is subjected to a sudden pull, breakage will occur at the link having the smallest bond force, not energy. However, if the pulling force is increased slowly, breakage will occur at the link of lowest energy [Israelachvili, J. N.; Berman, A. Isr. J. Chem. 1995, 35, 85]. (26) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London A 1971, 324, 301. (27) Although the JKR theory is an approximation and uses macroscopic quantities to describe the adhesion, it has proven to be valid even at the microscopic level.
Scanning Force Microscopy with Chemical Specificity Table 2. Surface and Interfacial Free Energies (in mJ/m2) (γ13 against Ethanol) for All Possible Tip-Substrate Combinations, Calculated from the Measured Adhesion Forces Using the JKR Theory of Adhesion Mechanics surface free energy γ13 CH3 OH NH2 COOH CONH2 CH3-OH CH3-NH2 CH3-COOH CH3-CONH2 OH-NH2 OH-COOH OH-CONH2 NH2-COOH NH2-CONH2 COOH-CONH2
interfacial free energy γ12
2.5 3.0 2.8 4.5 5.3 4.4 3.8 4.8 6.1 -1.9 -1.7 -2.0 -2.4 0.6 -6.0
work of cohesion is W12 ) 2γ, where γ ) γ13 ) γ23 is the surface free energy of the particular surface functionality against the medium (ethanol), the interfacial free energy γ12 being zero. To check the validity of the JKR theory for our experiments, we calculated on the basis of formula 1 an expected adhesion force of Fadh ) 0.82 nN for a CH3-CH3 tip-sample pair, using a known value of γ ) 2.5 mJ/m2 for a CH3-terminated SAM in contact with ethanol7c,28 and a measured tip radius of 35 nm. This value is in good agreement with the measured value of Fadh ) 0.9 ( 0.3 nN. We have performed a complementary check by using water instead of ethanol as a medium with the same tipsample combination. An adhesive force of 18 ( 2 nN was measured. Applying formula 1 and using the measured values of Fadh ) 18 nN and R ) 35 nm, we calculated the value of the surface free energy γ of a CH3-terminated SAM in contact with water and found it to be 51.5 mJ/m2. This value is in very good agreement with other experimental values (γ ∼ 52 mJ/m2) from contact angle measurements with water on hydrophobic alkane surfaces.24 Both checks validate the applicability of the JKR theory9a,b to our adhesion force measurements with chemical specificity. We have applied the JKR theory to calculate the surface free energies and the interfacial free energies of all possible tip-sample combinations and the results are summarized in Table 2. From the values for γ in Table 2, it can be clearly seen that hydrogen bond interactions (e.g. CONH2COOH) have small or even negative29 and thus favorable interfacial energies, which lead to high adhesive forces, whereas purely van der Waals interactions such as CH3CONH2 exhibit high and unfavorable interfacial free energies, which account for small adhesive forces. As mentioned before, surfaces with high surface free energies are completely wet by liquids like ethanol, making contact (28) Values of the surface free energies of the functional groups used (CH3, COOH, OH, NH2, CONH2) in equilibrium with ethanol (the medium in which the force-distance curves are acquired) are very rare, because surfaces with high surface free energies (COOH, OH, NH2, CONH2) are completely wet by ethanol and make contact angle measurements, from which surface free energies are usually determined, difficult or even impossible. A value of 2.5 mJ/m2 for CH3-CH3 is known from contact angle measurements (see ref 7c). (29) The interfacial free energy is defined for two coexisting phases being at equilibrium. A value of the interfacial free energy less than zero means that these phases would mix. Thus, in practice, γ values smaller than zero are not found. However, in our measurement the two phases, tip and sample, cannot mix, which results in the measurement of negative γ values [Safran, S. A. Statistical Thermodynamics of Surfaces, Interfaces, and Membranes; Addison-Wesley Publishing Co.: New York, 1994].
Langmuir, Vol. 13, No. 16, 1997 4361 Table 3. Single Chemical Bond Forces (in pN) for Every Tip-Substrate Combination, Calculated on the Basis of the JKR Theory of Adhesion Mechanicsa substrate b
tip
CH3
OH
NH2
COOH
CONH2
CH3b OH NH2 COOH CONH2
81 50 54 95 62
57 101 88 109 110
59 113 98 105 102
61 112 95 114 125
60 117 100 137 120
a All bond strengths apply to measurements in ethanol. b Although the van der Waals interaction is not a “two-center” bond interaction in this experimental setup, the calculated values for Fsingle presented in this table represent an effective binding force experienced by the molecular pairs and are listed for comparison reasons.
angle measurements to determine γ difficult or even impossible. However, high surface free energies lead to high adhesive forces (see formula 1), which are easily measured with the SFM. This shows that by using SFMC to determine surface free energies, important thermodynamic data are obtained and that SFMC is extremely useful in cases where contact angle measurements fail. Moreover, SFMC allows the determination of γ under various environmental conditions and on a microscopic scale, not attainable with other methods. To calculate single-bond forces, the JKR theory was used to estimate the number of interacting molecules between modified tip and sample. The number of interacting molecules can be calculated from the radius a of a spherical contact area at pull-off, for which JKR theory gives
as )
[
]
3πW12R2 2K
1/3
(2)
in which K is the elasticity modulus of tip and sample,30 R is the radius of curvature of the tip, and W12 is the work of adhesion to separate tip and sample. Once the radius of the contact area is known, the surface area of the contact region can be calculated. From other experiments,7 it is known that one alkanethiol molecule occupies about 20 Å2. From these values the number of interacting molecules n between modified tip and sample are calculated for every tip-sample combination. The calculated values of n vary from 2 to 18, supporting the validity of the JKR theory down to molecular level and the fact that the SFMC provides access to direct measurement of local force interactions between a small ensemble of molecules or even between single molecular pairs.31-34 Single chemical bond forces are calculated from the determined number of interacting molecules using the average adhesion forces for every tip-sample combination and are summarized in Table 3. The trend observed in the average adhesive forces, depending on the nature of the interaction between and sample, is exactly reflected (30) We used the elasticity modulus of gold, K ) 64 GPa, in our measurements, assuming that the SAM does not influence the elastic behavior of the tip-substrate ensemble. The E-modulus for a SAM is 1 GPa, whereas the E-modulus for gold is 64 GPa. Since the layer thickness of the SAM is very small compared to the tip radius and its elasticity is one order of magnitude smaller than that of gold, the highly elastic SAM will recover much faster under negative load (pull-off force). As a result, the gold determines the contact area (see also ref 9). (31) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 354. (32) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 661, 771. (33) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415. (34) Hoh, J. H.; Cleveland, J. P.; Prater, C. B.; Revel, J.-P.; Hansma, P. K. J. Am. Chem. Soc. 1992, 114, 4917.
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in the single bond forces.25,35 Comparing single-bond forces,36 which are not contact area dependent, allows the comparison of different tips with each other, which is supported by the fact that Table 3 is, within the experimental error, nearly symmetrical around the diagonal, which means that single-bond forces of the tip-sample combinations agree reasonably with those of the reverse tip-sample combinations, e.g. CONH2-CH3 and CH3CONH2. Although the JKR theory provides a theoretical description of the measured force interactions and allows the calculation of (interfacial) surface free energies, the number of interacting molecules, and single-bond forces, it requires supplementary information about the physical properties, shape, and size of the tip. The radius of curvature is especially difficult to measure routinely. Recently Han and co-workers37 developed a new statistical analysis of the distribution of adhesion forces, measured for a particular tip-sample combination, to calculate single-bond forces and the number of interacting molecules. Their method circumvents the requirement of measuring tip shape and size. Their statistical analysis is based on the assumption that the adhesive force in a force-distance curve is composed of a discrete number of individual chemical bonds. The validity of this assumption has been demonstrated in recent literature.31-34 Then, the number of the chemical bonds will follow a Poisson distribution if a fixed contact area is maintained during the measurement. From the fact that a Poisson distribution is obtained, it can be derived directly that, if the average number of bonds is n, the variance σ2 is also equal to n. However, from the force distance curves one obtains the distribution of the adhesion forces rather than the distribution of the number of chemical bonds. The adhesive force Fadh is related to the number of bonds by Fadh ) nFsingle, whereas σ2 ) nFsingle2. Hence, the singlebond force Fsingle can easily be calculated from the distribution of the adhesive forces for a particular tipsample combination from
Fsingle ) σ2/Fadh
(3)
where Fadh is the average adhesive force. We have applied this statistical analysis to measurements of a CONH2 tip on the various model substrates (CH3, OH, NH2, COOH, CONH2) by measuring 50 forcedistance curves for each tip-substrate combination. From the adhesion force distribution obtained, the average Fadh and variance σ2 were calculated. The results of this statistical analysis are given in Table 4 together with values calculated with the JKR theory for the same tip-substrate combinations. The calculated (35) Until now, no other comparable experimental data about the strength of single hydrogen bond forces are known; rather, values of bond energies emerge from other studies through indirect measurements (see, for example: Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker Inc.: New York, 1974). The development of SPM has introduced a whole new research area and has made direct measurements of single-bond forces of various intermolecular interactions possible (see for example refs 32 and 33). Recently, computer simulations have been performed to calculate bond forces as measured with SFM to provide comparison data (Grubmu¨ller, H.; Heymann, B.; Tavan, P. Science 1996, 271, 997. Berendsen, H. J. C. Science 1996, 271, 954). (36) The calculated single-bond forces for the hydrogen bonding tipsubstrate pairs reflect the total force, i.e., the sum of the van der Waals force and the hydrogen bond force, acting between the single molecular pair. Since the van der Waals contribution is small and not affected by the functional groups (see ref 10b), the trend observed in Fsingle, according to the nature of the interaction, is not influenced. (37) (a) Han, T.; Williams, J. M.; Beebe Jr., T. P. Anal. Chim. Acta 1995, 307, 361. A more accurate analysis can be performed by constructing a variance versus mean plot, using measurements from a series of sets of measurements; see: (b) Williams, J. M.; Han, T.; Beebe, T. P., Jr. Langmuir 1996, 12, 1291.
Table 4. Comparison of the Number of Interacting Molecules n Calculated with the Statistical Analysis and the JKR Theory of Adhesion Mechanicsa n
tip-substrate combination
Fadh (nN)
σ
Poisson
JKR
CONH2-CH3 CONH2-OH CONH2-COOH CONH2-CONH2
0.3 1.3 2.2 1.8
0.1 0.4 0.5 0.4
9 9 16 17
7 12 18 15
a As an example the results of a CONH functional tip on the 2 various model substrates are presented.
average numbers of interacting molecules n are in excellent agreement with the values calculated with the JKR theory. This good agreement between this statistical analysis and the JKR theory proves again the validity of both approaches.38 Friction Force Measurements. On the basis of surface force apparatus (SFA) measurements it has been proposed by Israelachvili39-43 that friction correlates with the adhesion hysteresis (dissipated energy). However, Lieber et al.9 propose that friction and adhesion forces correlate directly with each other, because both forces originate from the breaking of (chemical) bonds.44 Lieber et al.9 based their conclusion on a frictional force study involving only two chemically distinct functional groups (CH3 and COOH). We performed a more detailed study of the direct relationship between the friction and adhesion force. Several models make predictions about the functional dependence of friction as a function of the normal load.45 Amonton’s law states that the friction force is linearly proportional to the normal load. This empirical law, Ffric ) µFload, holds for macroscopic contacts where multiple asperity contacts statistics are involved. However, in the single asperity contact measurements, like SFM and SFA, the friction force is proportional to the contact area, which is depending on the exact contact mechanics (e.g. JKR) and thus a complex function of the normal load.46 Generally, one would expect non-Amonton’s law like friction-load behavior in single asperity friction force measurements.47 We have used a new method, developed by Bhushan et al.,19,42 to experimentally determine the friction behavior (38) Han et al. (ref 37) claim that the magnitude of the total adhesive force is largely independent of the magnitude of the single-bond force and that the magnitude of Fadh is mainly determined by local geometrical factors of tip and sample; i.e., a small total adhesive force does not necessarily give small single-bond forces and vice versa. However, from our measurements it is clear that there is a direct correspondence between the magnitude of the total adhesive force and the single-bond force. This discrepancy between our findings and the measurements performed by Han et al. can be explained by the fact that we used a well-defined tip-substrate geometry. In our experiment the gold substrates exhibit large, atomically flat domains13 which excludes strong contributions of the local geometry of tip and substrate to the total adhesive force. (39) Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. N. J. Phys. Chem. 1993, 97, 4128. (40) Chaudhury, M. K.; Owen, M. J. Langmuir 1993, 9, 29. (41) Chen, Y.-L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736. (42) Bhushan, B. Handbook of Micro/Nanotribology; CRC Press: Boca Raton, FL, 1995; p 298. (43) Israelachvili, J. N.; Chen, Y.-L.; Yoshizawa, H. J. Adh. Sci. Technol. 1994, 8, 1231. (44) The discrepancy between the experiments of Lieber et al.9 and the SFA measurements might be explained by the fact that the SFA experiments were performed on structurally dissimilar species, whereas Lieber et al.9 used structurally similar though chemically different SAM’s in their measurements. (45) Bowden, F. P.; Tabor, D. Friction and Lubrication of Solids: Part II; Oxford University Press: London, 1964. (46) Krim, J. Comments Condens. Matter Phys. 1995, 17, 263. (47) (a) Meyer, E.; Luthi, N.; Howald, L.; Bammerlin, M.; Guggisberg, M.; Guntherodt, H.-J. J. Vac. Sci. Technol. B 1996, 14, 1285. (b) Carpick, R.; Aigrait, N.; Ogletree, D. F.; Salmeron, M. J. Vac. Sci. Technol. B 1995, 14, 1289. (c) Carpick, R.; Aigrait, N.; Ogletree, D. F.; Salmeron, M. Langmuir 1996, 12, 3334.
Scanning Force Microscopy with Chemical Specificity Table 5. Friction Coefficients for All Possible Tip-Substrate Combinationsa substrate tip
CH3
OH
NH2
COOH
CONH2
CH3 OH NH2 COOH CONH2
0.7 0.4 0.3 0.4 0.4
0.4 0.6 0.7 0.8 0.7
0.3 0.7 0.8 0.7 0.7
0.3 0.8 0.8 1.1 1.6
0.5 0.6 0.8 1.8 1.3
a All measurements were performed in ethanol. The values were obtained from 10 µm forward and reverse scan lines with a scan speed of 30 µm/s. The experimental error in the measurements is (10%.
in a simple and accurate way. This so-called “height mode with parallel scans” method makes more accurate measurements possible since it requires neither the calculation of the lateral spring constant nor the calibration of the cantilever’s torsional deflection.20 For every combination of tip and substrate, modified with a variety of functional groups (CH3, NH2, OH, COOH, CONH2), we observed a linear load-friction relationship, indicating Amonton-lawlike behavior, which is identical to observations made by other researchers9,10a on the same system. The reason for this rather unexpected finding is believed to be the nanometer scale roughness of the tip, making it behave like a multiple asperity contact.48 Since a linear friction-load dependence is observed, we determined the absolute friction coefficient, µ, for all possible tip-substrate combinations. The results of the absolute friction coefficient measurements are presented in Table 5. For hydrogen bond interacting tip-substrate combinations, high friction coefficients are found, whereas purely van der Waals interacting pairs exhibit low friction coefficients. The trend in friction coefficients for the different tip-substrate combinations (Table 5) compares well to the trend in the mean adhesive forces (Table 1). This direct one-to-one correspondence between the absolute friction coefficient and adhesion force provides a proof for the direct correlation of the adhesion force and friction. Chemical Specific Imaging of Uncharged Functional Groups. The basis for the SPM with chemical specificity, in lateral imaging mode, is founded on the differences of the friction coefficient µ for various functionalized tip-substrate combinations (see Table 5). These differences make it possible to predict the frictional force contrast between various chemical functional areas on a surface. With a CONH2-functionalized tip the friction force contrast will be higher on a surface area with COOHgroups than on the areas with CH3-groups, since µCONH2-COOH > µCONH2-CH3. When a CH3-functionalized tip is be used, the opposite frictional force contrast will be observed since µCH3-CH3 > µCH3-COOH. We have used the microcontact printing (µCP) method developed by Whitesides and co-workers14,17 to create chemically patterned gold substrates using ω-functionalized alkanethiols. This µCP technique provides a convenient and simple way to create patterns with features down to 200 nm.14e We have created a chemically patterned COOH/CH3 substrate as presented schematically in Figure 4A. Since alkanethiols of equal length (12 C atoms) are used, no height differences are expected between both areas. (48) Although nonlinear (JKR or extended JKR) type friction loads have indeed been observed in ultrahigh vacuum in SFM measurements, there are cases of where also a linear dependence is observed. (a) Mate, C. M. Handbook of Micro/Nano Tribology; CRC Press: New York, 1995; p 167. (b) Putman, C. A. J.; Igarshi; Kaneko, R. Appl. Phys. Lett. 1995, 66, 3221.
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The topography image presented in Figure 4B shows, as expected, no height features at the boundaries between the CH3- and COOH-patterned areas; it only displays some dirt particles accumulated on the surface. No information is revealed about the chemical composition of the surface. When this same surface is scanned in lateral force mode49 with a CONH2-functionalized tip, the chemical contrast is clearly visible (Figure 4C). Due to the hydrogen bond interaction between the CONH2-functionalized tip and the COOH groups at the patterned surface, the frictional force is higher in the COOH-rich areas (brighter image) than in the CH3-terminated areas (darker image) in which only van der Waals interactions between the CONH2-tip and CH3-substrate groups play a role. For the lateral force image recorded with the CH3-functionalized tip (Figure 4D), the reverse contrast is found, i.e., higher friction force in the CH3-terminated areas (bright) and a lower frictional force in the COOH-terminated areas (dark), as expected on the basis of the differences in µ-values (Table 5). The imaging can be extended to the mapping of the spatial distribution of three different chemical groups on the substrate. Using the µCP technique in two steps with a line pattern stamp, the chemically patterned surface, as schematically presented in Figure 5A, was created. The topography images showed no height features or chemical contrast, as did the previous topography measurements (Figure 4B) on patterned substrates. As in the previous case, the CH3-modified tip shows the highest friction (Figure 5B) the CH3-patterned areas as predicted on the basis of differences in the value of the friction coefficient, µCH3-CH3 > µCH3-COOH ≈ µCH3-OH. The CH3 areas are visible in image 5B as the bright 20 µm wide lines. The narrow parallel bright lines on both sides of the broad line are also CH3-stamped areas, caused by a defect in the master from which the stamp was prepared. If the same sample is scanned with a CONH2-modified tip (Figure 5C), the COOH and OH areas are chemically imaged, based on the hydrogen-bonding capabilities of the CONH2 tip. The bright 20 µm line now apparent in Figure 5C corresponds to the COOH/OH-stamped line. The brighter sections on this line correspond to the COOH-functional area, whereas the somewhat less bright sections correspond to the OH-functional area. The dark line in this image corresponds to the CH3-terminated area. This trend in contrast is expected on the basis of the trend in friction coefficients, µCONH2-COOH > µCONH2-OH > µCONH2-CH3. This example demonstrates the ability of SFMC to image in a chemically specific way more than two different functional groups on the substrate surface, with predictable frictional force contrast. As mentioned before, small defects in the stamped pattern of Figure 5 can be seen as thin lines on both sides of the 20 µm lines, caused by imperfections on the master from which the stamp was created. A zoom on such a defect in a CH3/COOH region of the CH3/OH/COOH pattern with a CONH2 tip is presented in Figure 5D,E. The topography (E) of the zoomed area shows again no height features at the boundaries between the COOH and CH3 regions. The contrast in the chemical image (D) is determined in the same way as in Figure 5C; i.e., dark regions correspond to CH3-functional areas, whereas the bright areas correspond to COOH-functional regions. The corrugation visible in the image is due to the crystalline structure of the gold substrate (gold crystals ∼ 20-30 nm). The narrow bright line (COOH) exhibits a measured width of 100 nm. This observation demonstrates im(49) Normal forces during lateral force imaging never exceeded the threshold load for monolayer damage, recently reported by: (a) Liu, G.; Salmeron, M. B. Langmuir 1994, 10, 367. (b) Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Science 1993, 259, 1883.
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Figure 4. (A) Schematic representation of the chemically patterned CH3/COOH substrate. The 20 × 20 µm2 squares are COOHfunctionalized (gray), whereas the surrounding area is CH3-functionalized (white), (B) topography SFM image (35 × 35 µm2) of the COOH/CH3-patterned gold substrate; the black-to-white height scale is 13 nm, (C) simultaneously recorded lateral force image with a CONH2-functionalized tip, (D) lateral force image recorded with a CH3-functionalized tip. Areas exhibiting high friction appear bright, low-friction areas appear dark. All images were recorded in ethanol.
mediately the potential of SFMC to chemically image functional groups from the micrometer scale down to the nanometer range.50 II. Charged Functional Groups. pH-Dependent Adhesion Force Measurements. In this section we present the study on electrostatic (Coulombic) interactions between charged surfaces (e.g., COO--COO-, NH3+-COO-, (50) Although recently Akari et al. claimed molecular resolution on single polymer molecules with chemical force measurements, their results should be interpreted with care, since (a) their measurements were carried out in air, hence ill-defined capillary forces mask the true tip-substrate interaction and (b) topography differences make a major contribution to the lateral force signal [Akari, S.; Schrepp, W.; Horn, D. Langmuir 1996, 12, 857].
etc.). Knowledge of these Coulombic interactions and the acid-base properties of surfaces is important for understanding phenomena like colloidal stability, protein folding, enzymatic catalysis, etc. The use of ionizable functional groups like COOH, NH2, OH, and CONH2 in adhesion force measurements has allowed the direct evaluation of charge-charge interactions and the determination of the acid-base behavior of surface acid and base groups.51 The ionization behavior of surface-bound carboxylic acid (COOH) groups was directly measured by performing pH-dependent adhesion (51) van der Vegte, E. W.; Hadziioannou, G., submitted to J. Phys. Chem. 1997.
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Figure 5. (A) Schematic representation of a chemically patterned SAM containing three different functional groups (COOH/ OH/CH3). The pattern was created on a gold substrate by microcontact printing of dodecanethiol (CH3) in a series of 20 µm wide parallel lines (20 µm separation distance), followed by stamping of another 20 µm line pattern using 12-mercapto-1-dodecanol (OH) as the ink, oriented perpendicular to the first pattern. The substrate was then washed with a solution of 11-mercaptoundecanoic acid (COOH), derivatizing the gold areas that remained bare after the first two steps. (B) Lateral force image (70 × 70 µm2) of the COOH/OH/CH3-patterned gold substrate recorded with a CH3-functionalized tip. (C) Lateral force image recorded with a CONH2-functionalized tip. (D) 4 × 4 µm2 lateral force image of a line defect in a CH3/COOH-patterned region of (C) recorded with a CONH2-functionalized tip; the apparent width of the narrow bright line is approximately 100 nm. (E) Topography image of the 4 × 4 µm2 zoom; the black-to-white height scale is 1 nm.
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Figure 6. Adhesion forces as a function of the pH at constant ionic strength: (A) COOH-COOH tip-substrate combination, (B) NH2-NH2 tip-substrate combination, (C) OH-OH tipsubstrate combination.
force measurements of the COOH-COOH tip-substrate combination, while maintaining a constant ionic strength. The measured adhesion force as a function of the pH of the medium is presented in Figure 6A. The plot shows a sharp decrease in adhesive force when the pH of the medium is increased and data points follow a curve that resembles a general titration curve for acids as was obtained from contact angle measurements.52 From Figure 6A the surface pKa is estimated to be approximately pH ) 4.8, which is equal53 to the pKa of carboxylic acids in water [pKa ) 4-5].54 The pH-dependence can be understood considering the nature of the interaction between tip and substrate COOH groups. At low pH levels (pH ) 1-4), the acids groups are still protonated and hydrogen bonding is essentially responsible for the magnitude of the measured adhesion force. As the pH of the medium increases, the COOH groups gradually get deprotonated and become negatively charged. Because tip and sample bear the same functionality COOH, both tip and sample become negatively charged, which results in a repulsive Coulombic contribution to the total adhesion force acting between tip and sample. The more the COOH groups get deprotonated with increasing pH, the larger this repulsive contribution will be, resulting in a decreasing adhesive force. At pH ) 10, the total adhesive force becomes zero and only Coulombic repulsion can be measured at still higher pH. The same pH-dependent measurements were performed to determine the base behavior of surface-bound amine (52) (a) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (b) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741.
van der Vegte and Hadziioannou
groups, using a NH2-NH2 tip-substrate combination. The results are given in Figure 6B. For the NH2 group, the expected pH-dependent behavior is found analogous, but opposite to, the behavior of acidic COOH groups. An increase in adhesive force with increasing pH is found, i.e., at low pH levels, both tip and substrate are positively charged (NH3+ groups) resulting in electrostatic repulsion, whereas at higher pH levels, the NH2 groups are recovered, changing the tip-substrate interaction into an attractive hydrogen-bonding interaction. The exact determination of the pKa from the obtained titration curve is not possible, since reproducible adhesive force measurements were difficult to obtain. The fluctuations in the measured adhesive forces were caused by (a) the fact that the pH of the solutions used was not completely stable due to the acidizing effect of CO2 absorption from the air (unbuffered solutions were used to maintain constant ionic strength), and (b) the fact that NH2 groups are reactive surface species.55 In order to assess the acid behavior of surface hydroxyl (OH) groups, a OH-OH tip-sample combination was employed in the pH-dependent adhesion force measurements. The results are given in Figure 6C. A nearly pHindependent adhesion force was obtained, which indicates that there is no detectable acid behavior (ionization) of surface OH groups in the pH regime studied.56 Chemical Specific Imaging of Charged Functional Groups. On the basis of the differences found in the pHdependent behavior of the above-described functional groups, we propose a method to differentiate surface-bound functional groups through pH-dependent imaging.57 Carboxyl and amine groups show a change in ionization around their pKa when the pH of the medium is changed (Figure 6, A and B). This means that by varying the pH and thereby varying the surface charges, the attractive and/or repulsive contributions to the adhesion forces (and hence to the friction forces) are changed. This observation suggests that two (or more) groups with different pKa’s can be identified and chemically distinguished in pHdependent imaging. In order to demonstrate the method proposed here, we used a patterned substrate consisting of carboxylic acids and amine groups, which have very different pKa values of 4.8 and 10, respectively, to identify and distinguish these groups on the substrate. To be able to predict the contrast in the lateral force measurements on the basis of the magnitude of the adhesion force, we avoided a pHdependent contribution of the tip functionality to the tipsubstrate interaction, by using a OH-tip functionality which does not show pH-dependent behavior (Figure 6C). This OH-functional tip will give the highest adhesive force (strong hydrogen bonding) and hence frictional force (53) Acid and base properties of ω-functionalized alkanethiol SAM’s have been the subject of several studies (see ref 52) and have been characterized through indirect measurements, e.g. wetting in the contact angle method. Our finding is contradictory to what was found from pH-dependent contact angle measurements performed on the same COOH-terminated SAM, where surface pKa’s are found to be shifted upwards by approximately 2 pH units. This discrepancy between the adhesion force with SFMC and contact angle measurements can be explained by the fact that the SFMC provides a microscopic and local measurement of the sample nature, whereas contact angle measurements are macroscopic measurements which average over a large area. Furthermore, contact angle measurements are prone to “reactive spreading” (see ref 52a). (54) Allinger, N. L.; Cava, M. P.; de Jongh, D. C.; Johnson, C. P.; Lebel, N. A.; Stevens, C. L. Organic Chemistry; Worth Publishers Inc.: New York, 1980; p 260. (55) Amine groups are known to react with atmospheric CO2 under the formation of carbamates. This may affect the surface density of amine groups in the SAM. See: Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, H.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116. The pKa of primary amine groups is about 10-11 (ref 54). (56) The pKa of organic alcohols (ref 54) lies in the range of 15.5-19, far above the pH range (1-14) studied in our SFMC measurements. (57) Marti, A.; Ha¨hner, G.; Spencer, N. D. Langmuir 1996, 11, 4632.
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Scheme 1
on ionized functional groups and a smaller adhesive force (and frictional force) on uncharged (protonated) functional groups.24,58 Thus, at a pH below the pKa of the carboxylic acid (pKa ) 4.8), where the amine is fully ionized (NH3+) and the carboxyl groups are fully protonated (COOH), the OH tip will show the highest friction on the amine terminated areas due to the stronger interaction between OH‚‚‚+H3N, compared to the OH‚‚‚HOOC hydrogen bond. Above the pKa of the amine group (pH >10), where the carboxyl groups are fully ionized (COO-) and the amine groups uncharged (NH2), the OH tip will show the highest friction on the carboxyl-terminated areas, where stronger hydrogen bonding between OH‚‚‚-OOC is experienced compared to the OH‚‚‚H2N interaction. Microcontact printing was used to create a chemically patterned surface, consisting of 20 × 20 µm2 NH2terminated squares surrounded by a COOH-terminated area. Figure 7A shows the topography image of the NH2/ COOH pattern at pH ) 2.5 recorded with an OHfunctionalized tip. No height features or chemical contrast is apparent in this topography image. In the lateral force images, at pH ) 2.5, below the pKa of the carboxylic acid, the highest frictional force was found in the amine (NH3+) areas of the pattern (Figure 7B), as expected. When the pH was increased to 11.5, above the pKa of the amine (Figure 7C), the carboxyl (COO-) areas exhibit the highest friction, as expected. This example verifies our proposed mechanism for pHdependent chemical imaging, based on the acid-base properties of the substrate groups. This method does not only provide a means to identify and distinguish acidic from basic groups; it can be used to differentiate between any functional groups within an ensemble on the basis of different pKa values. Conclusions We have presented an extensive scanning force microscopy with chemical specificity (SFMC) study, using chemically modified SFM probes. We have used selfassembling monolayer’s (SAM’s) of ω-functional alkanethiols with different functionalities (CH3, OH, NH2, COOH, CONH2) to modify the standard SFM probes. A variety of intermolecular force interactions, including hydrogen bonding, van der Waals interactions, and Coulombic interactions, have been studied between CH3, OH, NH2, COOH, and CONH2 functional groups. The Johnson-Kendall-Roberts theory was found to provide a very good theoretical background for analyzing the adhesive force data and allowing thus the evaluation of single-bond forces, the number of interacting molecules, and (interfacial) surface free energies. Furthermore, a recently developed statistical analysis was applied to (58) Although a COOH- or NH2-functional tip would provide a higher contrast difference, an OH-functional tip is able to distinguish (i.e., show a contrast difference) and identify the nature of surface bound acidic and basic groups (i.e., show contrast disappearance or reversal upon pH change) by only changing the pH. A COOH-functional tip will always show the highest friction contrast on the amine groups at both high and low pH values, whereas a NH2-tip will always show the highest frictional force on the acidic groups at both high and low pH values as indicated in Scheme 1.
Figure 7. pH-dependent imaging of a NH2/COOH-patterned SAM using a OH-functional tip: (A) topography at pH ) 2.5; the black-to-white height scale is 6 nm; (B) lateral force image at pH ) 2.5; (C) lateral force image recorded at pH ) 11.5. The images represent 35 × 35 µm2 scan areas.
derive single-bond strengths and the number of interacting molecules. The results of the JKR analysis and those
4368 Langmuir, Vol. 13, No. 16, 1997
from the statistical analysis were found to be in excellent agreement. For a variety of functional tip-substrate combinations the frictional force was measured providing the absolute friction coefficient, and a one-to-one direct relationship between friction and adhesive forces was demonstrated. The differences in friction coefficient µ between the various tip-substrate combinations provide the basis for the frictional force contrast, and thus for the SPM with chemical specificity. Chemically specific imaging has been demonstrated on chemically patterned SAM’s obtained with the microcontact printing (µCP) technique. Patterns containing up to three different functional groups were successfully imaged with predictable frictional force contrast. Chemical sensitivity down to 100 nm lateral resolution has been achieved. pH-dependent adhesion force measurements provide a unique way to determine acid-base properties of surface functional groups and to determine surface pK values. A new method is presented to differentiate surface-bound functional groups on the basis of their acid-base properties through pH-dependent imaging. This method was verified by demonstration of the pH dependence of the imaging
van der Vegte and Hadziioannou
characteristics of amine and carboxyl groups in a NH2/ COOH-patterned substrate. We have shown that SFMC is a versatile and simple technique, which could become an important research tool for approaching existing or new problems in various fields within chemistry, physics, and biology. Acknowledgment. The authors thank Dr. P. F. van Hutten and Dr. P. C. M. Grim for careful reading of this manuscript and V. Koutsos for useful discussions. J. L. Wilbur (Department of Chemistry, Harvard University, Cambridge, MA) is gratefully acknowledged for help and advice concerning the µCP technique and S. Bakker (Department of Applied Physics, University of Groningen, The Netherlands) for supplying the photolithography patterns used as masters to create the µCP stamps. Mavom b.v. (Alphen a.d. Rijn, The Netherlands) is thanked for their generous gift of Sylgard 184. This work was financially supported by the Foundation for Fundamental Research of Matter (FOM) and the Netherlands Foundation for Chemical Research (SON). LA970025K
