Compositional Mapping of Micropatterned, Mixed Self-Assembled

The patterns consisted of areas of gold modified by SAMs formed from .... Hajime Takano, Jeremy R. Kenseth, Sze-Shun Wong, Janese C. O'Brien, and Marc...
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Langmuir 1998, 14, 660-666

Compositional Mapping of Micropatterned, Mixed Self-Assembled Monolayers by Lateral Force Microscopy Yuqing Zhou,† Hongyou Fan,† Tommy Fong,† and Gabriel P. Lopez*,†,‡ Department of Chemical and Nuclear Engineering and Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 87131 Received June 3, 1997. In Final Form: November 4, 1997 Lateral force microscopy (LFM) was used to image patterned organic surfaces and showed consistent changes in image contrast with surface composition. Self-assembled monolayers (SAMs) of ω-substituted alkanethiolates were patterned on surfaces of gold films coated on silicon wafers. The patterns consisted of areas of gold modified by SAMs formed from dodecanethiol (HS(CH2)11CH3) and areas derivatized by mixed SAMs formed by exposure to solutions of mixtures of HS(CH2)11CH3 and hexaethylene glycolterminated alkanethiol [HS(CH2)11(OCH2CH2)6OH]. Several types of patterned SAMs were investigated, each of which differed in the relative mole fractions of methyl- and hexaethylene glycol-terminated alkanethiolates in the mixed SAMs. Analysis of the relative friction measured for the different mixed SAMs suggests that LFM can be used as a semiquantitative technique for compositional mapping of mixed SAMs. Correlation between relative friction and the composition (as estimated by X-ray photoelectron spectroscopy) and the wettability of mixed monolayers are presented for the types of SAMs investigated. The friction is directly proportional to the fraction of hexaethylene glycol-terminated alkanethiolates present in the mixed SAMs. The correlations can be applied in the semiquantitative estimation of the compositional and wetting properties of microscopic patterns in the mixed SAMs. As an example, we have used the compositional correlation together with LFM imaging to estimate the degree of displacement of an hexaethylene glycol-terminated SAM by a microdroplet of HS(CH2)11CH3.

Introduction Patterned self-assembled monolayers (SAMs) incorporating different ω-substituted alkanethiolates have been used to create patterned surfaces that have gradients in chemical properties, such as composition and wettability, and to study many interfacial phenomena, including etching,1 biorecognition,2 heterogeneous nucleation,3 and protein4 and cellular adhesion.5 A number of techniques have been used to characterize patterned SAMs, including X-ray photoelectron spectroscopy (XPS),6 scanning electron microscopy (SEM),7 secondary ion mass spectroscopy (SIMS),8 and analysis of condensation figures.9 Lateral force microscopy (LFM) has provided images of patterned SAMs,10-13 chemically-distinct domains,14 and composite organic films.15 LFM has also been used for measuring * Author to whom correspondence should be addressed: phone, (505) 277-4939; fax, (505) 277-5433; e-mail, [email protected]. † Department of Chemical and Nuclear Engineering. ‡ Department of Chemistry. (1) Kumar, A.; Whitesides, H. A. Appl. Phys. Lett. 1993, 63, 2002. (2) Lea, A. S.; Pungor, A.; Hlady, V.; Andrade, J. D.; Herron, J. N.; Voss, E. W., Jr. Langmuir 1992, 8, 68. (3) Kumar, A.; Whitesides, G. M. Science 1994, 236, 60. (4) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (5) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopouos, G. N.; Wang, D. I. C.; Whitesides, G. M. Ingber, D. E. Science 1994, 264, 696. (6) Lopez, G. P.; Biebuyck, H. A.; Harte, P.; Kumar, A.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10774. (7) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (8) Frisbie, C. D.; Martin, J. R.; Duff, R. R. J.; Wrighton, M. S. J. Am. Chem. Soc. 1992, 114, 7142. (9) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Science 1993, 260, 647. (10) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 827 (11) Bar, G.; Rubin, S.; Parikh, A. N.; Swanson, B. I.; Zawodzinski, T. A.; Whangbo, M. H. Langmuir 1997, 13, 373. (12) Bar, G.; Rubin, S.; Taylor, T. N.; Swanson, B. I.; Zawodzinski, T. A.; Chow, J. T.; Ferraris, J. P. J. Vac. Sci. Technol., A 1996, 14, 1794. (13) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511.

friction, shear, and adhesion properties of monolayers,16-19 which are important in many applications including lubrication, tribology, and recognition in biological systems. The purpose of this work was to develop the LFM method for semiquatitative characterization of organic monolayers by correlation of surface friction and image contrast to the composition and surface wettability of mixed SAMs formed from two different ω-substituted alkanthiols. The LFM technique is relatively cheap and simple when compared to XPS, SEM, and SIMS, making it convenient for the investigation of chemical features of patterned organic surfaces. LFM, as a local probe technique, also has the potential for imaging surfaces with high lateral resolution and, as shown below, with good resolution in detecting variations in the chemical composition of patterned surfaces. LFM works by a principle similar to that of normal atomic force microscopy (NAFM or height mode): It measures the interaction between a probe tip and the features on the surface of a substrate. As shown in Figure 1, a silicon nitride tip is placed in contact with a sample. A piezoelectric scanner moves the sample beneath the tip in the x, y, and z directions. A beam from a laser diode is focused onto the top of the cantilever. The beam reflects off the top of the cantilever onto a four-quadrant photodetector. Variations in interactions of the tip with the surface deflect the cantilever, which in turn changes the position of the laser beam on the photodetector. A feedback loop uses the signal differences from the top and bottom (14) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (15) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791. (16) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241. (17) Liu, Y.; Evans, D. F.; Song, Q.; Grainger, D. W. Langmuir 1996, 12, 1235. (18) Ohno, H.; Motomatzu, M.; Mizutani, W.; Tokumato, H. Jpn. J. Appl. Phys. 1995, 34, 1381. (19) McDermott, M. T.; Green, J.-B. D.; Porter, M. D. Langmuir 1997, 13, 2504.

S0743-7463(97)00577-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/16/1998

Mapping of Patterned SAMs

Figure 1. Schematic of the sensing system for both normal atomic force microscopy (NAFM) and lateral force microscopy (LFM). In the NAFM mode, the tip scans in the y-direction. In the LFM mode, the tip scans in the x-direction and a torsional force is produced due to its interaction with the sample surface. (Figure adapted courtesy of Digital Instruments, Santa Barbara, CA.)

halves of the photodetector to adjust the piezoelectric scanner and maintain constant normal force between the sample and the tip. Twisting of the cantilever can occur due to lateral forces arising from interaction between the tip and surface features as the tip scans in the x direction, which is perpendicular to the major axis of the cantilever (the y direction). The twisting of the cantilever is reflected in the signal difference between the left and right photodiodes and is displayed in the LFM image quantitatively as differences in pixel brightness. Experimental Section Dodecanethiol, HS(CH2)11CH3 (referred to herein as C12), was purchased from Aldrich Chemical Co. and was purified by chromatography through silica gel prior to use. HS(CH2)11(OCH2CH2)6OH (EG6) was synthesized in our laboratory according to the protocol of Pale-Grosdemange et al.20 Gold films (∼1000 Å thick) were prepared by vacuum evaporation of gold (99.999%) onto silicon wafers that had been precoated with ∼30 Å of chromium. The silicon wafers coated with gold were cleaved into 1 cm × 2 cm square slides. Half of each slide was used to form a patterned SAM by microcontact printing,21 and unpatterned SAMs were formed on the other half which was used for contact angle measurements. Similar unpatterned SAMs were used for XPS analysis. The fresh, gold-coated slide was first washed with heptane, deionized water, and dehydrated ethanol, and dried with N2 gas. A microstamp made of cross-linked poly(dimethylsiloxane) was used to pattern half of each slide by forming a patterned SAM from C12.21 The slides were then rinsed with ethanol and dried with N2. Each slide was then immersed in an ethanolic solution containing EG6 and C12 in a specific mole ratio to form a mixed SAM on the rest of the surface of slide (i.e., on both the patterned and the unpatterned halves of the slide). After 2 min, the slides were removed from solution, washed with ethanol, dried, and analyzed. A Rame-Hart Model 100-00 contact angle goniometer was used to measure the wettability of the SAMs formed on the unpatterned halves of the slides. Advancing contact angles of deionized water were recorded. A commercially-available instrument (Nanoscope III, Digital Instrument Co.) was used for LFM and NAFM imaging. A 200 mm narrow-legged standard triangle silicon nitride cantilever was chosen to provide a large lateral deflection signal. The scan angle was set to 90° in order to make the scan direction perpendicular to the major axis of the cantilever. Friction forces (20) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12. (21) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498.

Langmuir, Vol. 14, No. 3, 1998 661 were measured by sliding the tip repeatedly across the sample surface in the x direction with the y scan disabled. Both image mode and scope mode were used for acquiring data. All measurements were made in air at room temperature. Scan rates of 2.0 Hz for 100 µm scan sizes were chosen on all samples. Before LFM imaging of each SAM, the procedure of force calibration for AFM mode was used to minimize the contact forces (setpoint ∼-0.6 V) of the tip on samples to prevent the samples from damage, and the forces were kept the same for all the samples. X-ray photoelectron spectra of unpatterned mixed SAMs were acquired using a Physical Electronics 5400 ESCA system with monochromatized Al KR radiation and the detector at an angle of 45° as described previously.22,23 Binding energies in the spectra were referenced to the Au 4F7/2 peak at 84.0 eV. An estimate of the relative mole fraction of EG6 in the mixed SAMs (χEG6surf) was obtained by dividing the intensity of the O 1s X-ray photoelectron peak obtained from the mixed SAM by that of a SAM containing only the EG6 component and by assuming that this normalized intensity is directly proportional to the number of oxygen atoms in the SAM.4

Results and Discussion Figure 2 shows LFM images and normal atomic force microscopy (NAFM) images of a patterned (not mixed) SAM comprised of areas with different surface properties that are primarily determined by the identity of the terminal functional groups (CH3 and EG6OH). The contrast in the LFM image is obviously better than that in the NAFM image. The contrast in the LFM image reproduced the pattern formed by the different chemical functionalities with sharp boundaries. Difference in pixel brightness arose from differences in the interaction between the probe tip and the chemical functionalities in each region. Changes in friction between the tip and surface result in changes in the torsion of the cantilever in LFM imaging. The greater the friction, the brighter the pixels in the LFM image. Thus, the tip experienced lower friction in the regions terminated by CH3 than in regions terminated by EG6OH. The friction between the tip and a region in the surface may be related to the surface free energy and the composition of the surface.10,24-26 For example, the difference in brightness of LFM images for patterned SAMs terminated by various functional groups such as CH3, OH, and COOH has been reported.10 Chemical force microscopy (CFM), which uses chemicallymodified tips to measure the adhesion and friction forces between tips and patterned SAMs terminated with distinct functional groups, has also been reported.11,28,29 These studies have indicated that the image contrast of LFM is sensitive to chemical functional groups that give rise to different solid-vapor interfacial tension (γSV) and that the friction forces increase with an increase of the surface free energies of the SAMs and the tip.28,29 In this work, we used LFM imaging to investigate a series of patterned SAMs in which one component of the patterns varies in the ratio of hexaethylene glycol to methyl moieties and, hence, varies in interfacial energy. (22) Fan, H.; Lopez, G. P. Langmuir 1997, 13, 119. (23) Vaidya, R.; Simonson, R. J.; Cesarano, J., III; Dimos, D.; Lopez, G. P. Langmuir 1996, 12, 2830. (24) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luthi, R.; Rowwald, L.; Guntherodt, H. J.; Fujihira, M. Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (25) Meyer, E.; Overney, R. M.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H. Phys. Rev. Lett. 1992, 69, 1777. (26) Overney, R. M.; Meyer, E.; Frommer, J.; Guntherodt, H. J. Langmuir 1994, 10, 1281. (27) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (28) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (29) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943.

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Figure 2. Images obtained by lateral force microscopy (LFM) and normal atomic force microscopy (NAFM) reveal contrast between regions of a patterned SAM terminated with CH3 groups (dark stripes, lower torsion in LFM, lower topography in NAFM) and those containing terminal EG6OH groups (bright stripes, higher torsion, and higher topography). The contrast between the regions in the LFM image is much higher than that in the NAFM images. The micropatterned SAMs were formed by microcontact printing with dodecanethiol on gold, followed by exposure of the sample to an ethanolic solution of hexaethylene glycol-terminated alkanethiol to derivatize the remainder of the gold surface. Note: defects in the definition of the stripes in the LFM image are due to defects in the micropatterned stamp used to form the patterned SAM.

To investigate the relationship between image contrast (and surface friction) and wettability (contact angle) of SAMs, we patterned gold surfaces with C12 first and then derivatized the remainder of each surface with a mixed SAM formed from a mixture of C12 and EG6, each with a different mole fraction of EG6. The areas of the sample with patterned SAMs were imaged by LFM, while the other areas containing the unpatterned mixed SAMs were used to measure contact angles. Images obtained for four representative samples are shown in Figure 3. Comparing the regions having only C12 and regions having mixed SAMs with different fractions of EG6, a decrease in image contrast is obvious as the contact angle of the mixed SAMs is increased and as the mole fraction of EG6 is decreased. The increase in the amount of the CH3 functional groups on the surface of the mixed SAMs lowered the interfacial tension (γSV) in that region. We have estimated γSV for the various mixed SAMs from contact angle measurements by the method of Li and Neumann30 (see Appendix). In order to correlate image contrast to contact angles, γSV, and the fraction of EG6 of unpatterned mixed monolayers, we used the scope mode of LFM. The photodiode output signal in the scope mode is displayed as a voltage. Frictional force (f) between tip and surface can be estimated from the voltage signal in the scope loop of LFM by the equation17

f ) R(V - V0)

(1)

where V is the voltage signal due to deflection of the tip in the lateral direction by the frictional force between the (30) Li, D.; Neumann, A. W. J. Colloid Interface Sci. 1990, 137, 304. This method has produced good estimates of γSV for the case of large difference of |γLV - γSV|.

tip and surface, V0 is the voltage corresponding to the undeflected tip with no frictional force exerted, and R is a conversion factor that depends on experimental conditions such as the laser power, the detector, the tip used, and the ambient conditions. We may assume that R is a constant by maintaining constant experimental conditions. In this way, the difference in the frictional force (∆f) of an area with high friction (i.e., the mixed SAMs) vs an area with low friction (i.e., the areas prepared from C12 by microcontact printing) is directly proportional to the difference of the corresponding voltage signals between the two areas

F)

∆f R

) V - VC12

(2)

where V and VC12 are the voltage signals corresponding to high and low frictional forces as mentioned above, respectively, between the tip and the features on the surface. The difference of the voltages between the highfriction area and low-friction area can be used as an indication of their relative friction force, and we define the relative friction, F (in voltage), by eq 2. The relative friction can be normalized for our materials as follows:

Fn )

V - VC12 F ) FEG6 - FC12 VEG6 - VC12

(3)

The normalized relative friction (Fn) between the areas with mixed SAMs formed from different mole fractions of EG6 and those with SAMs formed from only C12 was thus obtained from the scope mode LFM of the patterned SAMs. Representative scope traces are presented in Figure 4. The differences in the voltage signals between

Mapping of Patterned SAMs

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Figure 3. Change in the contrast of LFM images between regions with SAMs formed from dodecanethiol and regions containing mixed SAMs formed by exposure of the sample to mixtures of dodecanethiol and hexaethylene glycol-terminated alkanethiol in various relative mole fractions. Contact angles of the mixed SAMs (brighter regions in the LFM images) estimated by goniometer measurement of unpatterned halves of the samples on which only the mixed SAMs were formed (see text for details) are (A) 39°, (B) 50°, (C) 92°, (D) 113°. The mole fractions of EG6 (χEG6surf) in the mixed SAMs are estimated as (A) 1.0, (B) 0.58, (C) 0.29, and (D) 0.0.

the regions terminated by CH3 (VC12) and by mixtures of CH3 and EG6OH (V) (and, thus Fn) increase as the fraction of EG6OH in the mixed SAMs is increased and as contact angle of the unpatterned SAMs decreased. The scope traces presented in Figure 4 were obtained using only simple vibration-isolation techniques based on suspending a platform from the ceiling with elastic cords. It is possible that further efforts to reduce the noise (i.e., vibrational, electronic) may result in an improvement in compositional mapping capabilities for micropatterned samples. The greater the mole fraction of EG6OH in the SAM, the lower the contact angle.22 The relationship between the relative friction and contact angle (θ) of water and the calculated interfacial tension (γSV) of the mixed SAMs with different wettabilities is given in Figure 5. We observe a monotonic decrease in the relative friction between mixed SAMs of EG6 and C12 and unmixed SAMs of C12 as the contact angle of the mixed SAMs increases. The monotonic increase in relative friction is also consistent with the increase in solid-vapor interfacial energy of the mixed SAMs as the surface concentration of CH3 is decreased. It is important to note that other properties of the SAMs may influence the relative friction measured. These include differences in the modulus of the SAM arising from differences in chain packing and chain elasticity.19

We have previously published a correlation between the contact angle of mixed SAMs formed from EG6 and C12 and their composition, as determined by XPS.22 Using this correlation, the relationship between the relative friction and the composition of the mixed SAMs is summarized in Figure 6. The value of χEG6surf in the abscissa is the estimated mole fraction of EG6 in mixed SAMs. As χEG6surf increases from 0.0 to 1.0, the relative friction is found to increase steadily. It is interesting to note that this dependence is approximately a straight line (R2 ) 0.94). Thus, the friction between the tip and the surface appears to be directly proportional to the number of ethylene glycol units in the SAM. A better fit (R2 ) 0.98) for the data can be obtained using the polynomial in Figure 6. Using this equation, we can semiquantitatively estimate the composition of micropatterned mixed SAMs formed from EG6 and C12 from the value of its relative friction as determined by LFM. LFM Compositional Analysis of Microscopic Patterned Monolayer Prepared by SAM Displacement. There have been several efforts recently to develop new methods for creating microscopic patterns of SAMs for a variety of applications.3,5,6,21,31 The above demonstration (31) Carlvert, J. M. J. Vac. Sci. Technol. 1993, 11, 2155.

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Figure 4. Scope traces of the samples imaged by LFM. The top and bottom curves in each panel are obtained by the signal differences for the left and right photodiodes of the four quadrant detector, when the tip scans in trace (x) and retrace (-x) directions, respectively (see Figure 1). The contact angles of the mixed SAMs in each sample are (A) 39°, (B) 50°, (C) 92°, and (D) 113°. (χEG6surf in each mixed SAM as estimated as (A) 1.0, (B) 0.58, (C) 0.29, and (D) 0.0.)

that LFM can be used for semiquantitative compositional mapping of such patterned SAMs has implications toward these efforts because of the general lack of analytical techniques with the capability to examine the composition of microscopically-patterned surfaces. The traditional quantitative surface analytical technique, X-ray photoelectron spectroscopy, has a mapping resolution on the order of 10 µm.32 Other microscopic imaging techniques such as secondary ion mass spectrometry are generally not capable of quantitative compositional mapping because of matrix effects.33 In this study, we demonstrate the use of LFM for the compositional analysis of a micropatterned SAM formed by a new method of fabrication: displacement of SAMs by microdroplets of alkanethiols. This technique for micropatterning is based on the fact that SAMs of alkanethiolates on gold are known to be susceptible to displacement by alkanethiols in solution.34 Figure 7 is an LFM image of a micropatterned SAM formed by partial displacement of an EG6OH-terminated SAM (bright areas) with C12. This method for forming micropatterns may be useful for patterning adsorbed proteins for applications in biosensing35 or controlled cell culture,5 because the EG6 is known to form SAMs that (32) Sherwood, P. M. A. In The Handbook of Surface Imaging and Visulization; Hubbard, A. T., Eds.; CRC Press: Boca Raton, FL, 1995; p 877. (33) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398. (34) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (35) Tender, L.; Worley, R. L.; Fan, H.; Lopez, G. P. Langmuir 1996, 12, 5515.

Zhou et al.

Figure 5. Normalized relative friction (Fn) of mixed SAMs formed from HS(CH2)11CH3 and HS(CH2)11EG6OH relative to those of pure SAMs formed from HS(CH2)11CH3 only, as a function of the advancing water contact angles (the error bars represent the standard deviations of three separate measurements) of the unpatterned, mixed SAMs (top), and as a function of γSV, the solid-vapor interfacial tension calculated from the measured contact angle data for the unpatterned mixed SAMs (bottom) by eq 7 (see Appendix for details). The normalized relative frictions were calculated by eq 3 from the difference of voltages (Z range) obtained from scope mode LFM (see Figure 4) between the areas of mixed SAMs and the areas of the SAM formed from C12.

Figure 6. Normalized relative friction (Fn) of mixed SAMs formed from HS(CH2)11CH3 and HS(CH2)11EG6OH relative to those of pure SAMs formed from HS(CH2)11CH3 only, as a function of χEG6surf, the estimated mole fraction of alkanethiolates with EG6OH on the surface of the mixed SAMs. χEG6surf was estimated from a standard calibration curve relating advancing water contact angles to XPS O1s peak intensities obtained for unpatterned, mixed SAMs (see ref 22).

are resistant to protein adsorption, while monolayers containing CH3 terminal groups may promote protein adsortion.6 Thus, by forming micropatterns such as the one imaged in Figure 7 and serially exposing them to different protein solutions, it should be possible to form arrays of adsorbed proteins. As seen by examination of the evenness of the contrast between light and dark areas across the microscopic area in Figure 7, the microdis-

Mapping of Patterned SAMs

Langmuir, Vol. 14, No. 3, 1998 665

Fn,X:C12 )

FX:C12 FEG6:C12

)

VX - VC12 VEG6 - VC12

)

(VEG6 - VC12) - (VEG6 - VX) VEG6 - VC12

)1-

Figure 7. LFM image for micropatterned SAMs formed by partial displacement of a microscopic region of an EG6OHterminated SAM by exposure to a microdroplet of C12 under water. The uniformity of the image contrast in the LFM image suggests that the EG6OH-terminated SAM was partially displaced in a uniform manner over the entire contact area of the microdroplet. Quantification of the relative friction in scope mode of the micropattern suggests that the degree of displacement of EG6OH-terminated SAM by C12 corresponds to 62% (see text for details).

placement procedure results in a relatively sharp boundary between the mixed SAM and the EG6OH-terminated SAM. The micropatterned SAM imaged in Figure 7 was prepared by microinjection of C12 under water onto a SAM formed from EG6. To form the sample, a gold film (500 Å, with a 15 Å Cr adhesion layer) was evaporated onto a plasma-cleaned glass slide and then overcoated with a SAM by exposure to an ethanolic solution of EG6 as described in the Experimental Section. C12 was microinjected using a Nikon Diaphot 300 inverted microscope adapted with a 1 pL resolution, computer-controlled, precision microsyringe pump (Cell Selector, Cell Robotics Inc., Albuquerque, NM), and a high speed, computercontrolled X-Y-Z stage (SmartStage, Cell Robotics Inc., Albuquerque, NM). The microsyringe pump drives a 10 µL glass syringe equipped with a Teflon adapter to a pulled borosilicate capillary tube tip. The tips were cleaned with ethanol and dried with nitrogen gas and then plasma cleaned with argon gas at a pressure of 100 mTorr for 2 min. In this way, the hydrophobic C12 is prevented from adhering to the clean hydrophilic borosilicate tips when in water. The glass slide and EG6OH-terminated monolayer were submerged in deionized water in a custombuilt dish which is mounted to the microscope stage. The capillary tips were also submerged in the water in the dish. Due to the hydrophilicity of the EG6 SAM and the hydrophobicity of the neat C12, the spreading of the C12 was restricted when it was microinjected on to the surface of the SAM. By allowing the C12 microdrop to react with the SAM surface for 2 h at room temperature, a microscopic spot containing a mixed SAM (CH3/EG6OH-terminated) was created (diameter ∼50 µm, as shown in Figure 7) within the EG6 monolayer by partial displacement of EG6OH-terminated SAM by C12 at the droplet/surface interface. The average relative friction between the microscopic spot and EG6OH-terminated SAM measured by scope mode LFM was about 0.090 V. To use the correlation given by the equation in Figure 6 to get an estimate of the degree of displacement in the microscopic drop, we calculate the normalized relative friction as follows:

FX:EG6 FEG6:C12

(4)

Here X refers to the SAM with unknown composition and FEG6:C12 ) 0.12 V. Thus, through use of eq 4 and the equation in Figure 6, the displacement reaction can be estimated to yield a microscopically-patterned mixed SAM with an average composition of alkanethiolates corresponding to a value of χEG6surf of 0.38, which corresponds to a degree of displacement of EG6OH-terminated SAM by C12 at the droplet/surface interface of 62%. It should be noted that the displacement reaction may result in a molecular structure that is different from that formed by codeposition of the alkanethiols used to create the patterned mixed SAMs for which the correlation in Figure 6 was obtained. If this is the case, the different molecular structures may influence the accuracy of the estimated average composition. We attempted to verify the result of the LFM compositional mapping of the micropatterned SAM prepared by displacement by XPS mapping.36 Because of damage of the X-ray beam to the SAM during the relatively long acquisition time necessary for small spot analysis, quantitative mapping of the micropattern was not possible using XPS. The damage was evident in the XPS spectra (taken during and after mapping) and by subsequent imaging of the mapped area using LFM. In LFM images, the damaged area was evident both in the apprearance of nonuniformities in the surface and in a decrease in contrast between the area formed from EG6 and the partially displaced region. Thus, the LFM method as developed herein, and similar local probe based methodologies (e.g., based on phase measurements by force modulation microscopy (FMM)11) may be the methods of choice for nondestructive compositional mapping of micropatterned SAMs of alkanethiolates on gold. As an alternative method for verifying the degree of displacement obtained by LFM measurement and use of the calibration curve in Figure 6, we used XPS to estimate the degree of displacement of EG6OH-terminated SAMs by a bigger droplet of C12 (∼4 mm in diameter). The second displacement reaction (using the bigger drop of C12) was performed under similar reaction conditions to that using the microdroplet. By collecting XPS spectra within the displaced region and outside of the displaced region, we estimated a degree of displacement by the bigger droplet corresponding to a χEG6surf of 0.4. This result indicates that the LFM compositional mapping technique described herein is capable of semiquantitative analysis of micropatterned SAMs. Conclusion LFM was used to image micropatterned SAMs that contained areas terminated with CH3 groups and areas (36) The XPS mapping was done using a Kratos Analytical AXISHSi system with monochromatized Al KR radiation and the detector at an angle of 45°. Binding energies were referenced to the Ag 3d5/2 peak at 368.3 eV.

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terminated by mixtures of C12 and EG6OH groups. The contrast in LFM images between areas corresponding to these two types of SAMs correlated with the mole fraction of EG6OH groups in the monolayers. A semiquantitative correlation was developed between the relative friction of the mixed monolayers and the CH3-terminated monolayers and the composition of the mixed monolayers. The correlation was then used to estimate the composition of a micropatterned SAM prepared by microscopic displacement of an EG6OH-terminated SAM by C12. These demonstrations suggest that LFM can be used as a sensitive tool for compositional mapping of patterned, welldefined surfaces at the micrometer and, perhaps, at the submicrometer level. Other potential applications of this technique include the semiquantitative monitoring of surface reactions and the analysis of surfaces incorporating covalently-bound or physically-adsorbed biomolecules. Appendix For the estimation of γSV, an equation of state for interfacial tensions has been formulated by Li and Neumann30

and liquid-vapor interfacial tensions, respectively, and β is a parameter determined by experiment data to be equal to 0.000115 (m2/mJ)2.37 In conjunction with Young’s equation

γSV - γSL ) γLV cos θ

(6)

where θ represents the contact angle of the liquid on the solid, eq 5 becomes

x

cos θ ) -1 + 2

γSV -β(γLV-γSV)2 e γLV

(7)

By use of eq 7, γSV can be estimated by Newton’s method38 from contact angle data of water (γLV ) 72.8 mJ/m2) for the SAMs with different fractions of C12 and EG6. Acknowledgment. This research was supported by the Office of Naval Research (Grant Nos. N00014-95-10901, N00014-95-1-1315, N00014-95-1-0255, N00014-961-1126) and the National Science Foundation (Grant No. HRD-9450475). We thank Paolina Atanassova for technical assistance. LA9705773

-β(γLV-γSV)2

γSL ) γLV + γSV - 2xγLVγSV e

(5)

where γSV, γSL, and γLV are the solid-vapor, solid-liquid,

(37) Neumann, A. W.; Good, R. J.; Hope, C. J.; Sejpal, M. J. Colloid Interface Sci. 1974, 49, 291. (38) Swokowski, E. W. Calculus with Analytic Geometry, 2ed. ed.; Prindle, Weber & Schimit Press: Boston, MA, 1979.