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Nanometer-Scale Patterning of Surfaces Using Self-Assembly Chemistry. 2. Preparation, Characterization, and Electrochemical Behavior of Two-Component Organothiol Monolayers on Gold Surfaces William A. Hayes,† Hyun Kim,‡ Xiaohui Yue,† Scott S. Perry,‡ and Curtis Shannon*,† Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312, and Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received December 5, 1996. In Final Form: March 6, 1997X We report a study of the physical and electrochemical properties of two-component self-assembled monolayers (SAMs) composed of both electroactive (4-aminothiophenol, 4-ATP) and electroinactive (noctadecanethiol, ODT) species. In all of the experiments reported here, relatively short (3 h) assembly times were used to prepare the mixed SAMs. We have characterized the macroscopic composition and the microscopic structure of these SAMs using Auger electron spectroscopy (AES), coulometry, grazing angle Fourier transform infrared spectroscopy, and lateral force microscopy (LFM). The adsorption isotherms determined by AES and coulometry show significant deviation from Langmuirian behavior and are suggestive of phase separation. LFM images obtained at three points near the critical region of the isotherm ([4ATP]/[ODT] ∼ 4) indicate that these two-component SAMs display complex phase behavior: At relatively low 4-ATP coverages, the surface consists of small islands of 4-ATP imbedded in an ordered film of ODT. At higher coverages of 4-ATP, however, we find evidence of both separation into distinct phases and mixing of the two components. In a second series of experiments, we demonstrate that phase domains of 4-ATP are electroactive and can be used to carry out localized electrochemistry. That is, the islands of 4-ATP, which are randomly distributed in the ODT matrix, behave as an array of ultramicroelectrodes. Surfaceconfined 4-ATP molecules can be used to nucleate the growth of polyaniline selectively from the phase separated domains of 4-ATP. We find that if a 4-ATP/ODT mixed monolayer is oxidized in the presence of aniline, nanometer scale polyaniline features are formed. The size and distribution of these features have been characterized by AFM and can be controlled through a combination of monolayer composition and the concentration of aniline in solution.
Introduction Intermolecular interactions between coadsorbed species play an important role in determining the structure and long-range order of two-component monolayers. For example, it is well-known that the coadsorption of carbon monoxide can induce order in many organic monolayers formed in ultrahigh vacuum.1 In a classic study, Mate et al.2 showed that the tendency of CO to induce order in organic adlayers was correlated to the magnitude of the dipole moment of the coadsorbed species. In cases where the coadsorbate dipole moment was oriented antiparallel to that of CO, well-ordered, intermixed monolayers were formed. On the other hand, if the dipole moment was oriented parallel to that of the adsorbed CO, a disordered or phase-separated monolayer resulted. In these relatively simple systems, dipole-dipole interactions dominated the thermodynamics of monolayer formation. Coadsorption also has been studied in more complex two-dimensional systems, such as Langmuir monolayers formed at the air-water interface and Langmuir-Blodgett (LB) films on solid surfaces. However, there appears to be no detailed general understanding of phase behavior in these systems, due primarily to the difficulty of obtaining information on thin film microstructure. In cases where the two components are chemically dissimilar, * Address correspondence to this author: tel, 334.844.6964; fax, 334.844.6959; e-mail,
[email protected]. † Auburn University. ‡ University of Houston. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley Interscience: New York, 1994; p 426. (2) Mate, C. M.; Kao, C.-t.; Somorjai, G. A. Surf. Sci. 1988, 206, 145.
S0743-7463(96)02074-4 CCC: $14.00
micrometer-sized phase domains form which can be imaged using fluorescence microscopy.3 With the development of scanning probe microscopies, much more detailed information has become available on the local structure within organic thin films, including information on the phase behavior of single- as well as two-component systems. For example, Zasadzinski et al.4 have identified three coexisting lattice structures in LB multilayers of barium arachidate using atomic force microscopy (AFM). Guntherodt et al.5 have employed frictional force microscopy (FFM), a variant of AFM in which the lateral as well as normal tip forces are measured simultaneously, to study phase separated domains in two-component LB monolayers. These workers demonstrated that local frictional (lateral) forces could be used to distinguish between the two coadsorbates, allowing resolution of phase-separated domains in the monolayer. Two-component systems are obvious choices for the modification of surface properties using self-assembly methods6 and have been used to control surface functionality in a variety of applications.7 For example, mixed monolayers that incorporate molecular “gate” sites are being developed as a way of designing monolayers with molecular recognition properties.8 The factors governing the formation of mixed self-assembled monolayers currently are not well understood. What is generally accepted is that long-chain alkanethiols are always preferentially adsorbed compared to short-chain alkanethiols and that (3) Birdi, K. S. Lipid and Biopolymer Monolayers at Liquid Interfaces; Plenum Press: New York, 1989. (4) Schwartz, D. K.; Viswanathan, R.; Zasadinski, J. A. N. Langmuir 1993, 9, 1384. (5) Overney, R.; Meyer, E.; Frommer, J.; Brodbeck, D.; Howald, L.; Guntherodt, H.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133.
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monolayer composition appears to be a function of thermodynamic, as opposed to kinetic, considerations.9 For example, Chidsey et al.10 showed that two-component self-assembled monolayers (SAMs) can be enriched slightly in one component by equilibrating the monolayer in a solution containing the desired component. On the basis of this result, the authors concluded that molecules adsorbed at defect sites are labile and can exchange with solution phase species, while molecules adsorbed at terraces were stable and did not exchange to any significant extent. This finding has been confirmed by Creager11 and Whitesides,12 who have observed similar behavior in different two-component systems. Finally, in a study of the electrochemistry of two-component SAMs, Crooks et al.13 found that if a gold electrode was incubated in a solution containing both a long-chain and a short-chain thiol, the mole fraction of the long-chain component increased monotonically with soaking time. In the limit of very long exposures (90 h), the resulting SAM contained only the long-chain component. Stranick et al. have reported the most thorough study to date of phase separation in two-component self-assembled systems.14 These workers used scanning tunneling microscopy (STM) to determine the structure of a SAM composed of a polar and a nonpolar alkanethiol of identical chain lengths and found that the monolayer was not intermixed, but contained nanometer-sized molecular domains. We are interested in exploiting phase separation phenomena as a way to prepare arrays of electroactive domains of variable size on electrode surfaces.15 In this paper, we continue and extend our previous study of the 4-aminothiophenol/octadecanethiol (4-ATP/ODT) system.16 In the first section, we discuss three independent techniques to measure the adsorption isotherm for this system and show that there is a significant deviation from simple Langmuir-type behavior. A discontinuity in the isotherm is one indication of a two-phase coexistence region. Next, we describe the use of lateral force micros(6) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (c) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (d) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (e) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (f) Laibinis, P. E.;Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (g) Folkers, J. P; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (h) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (i) Laibinis, P. E.; Fox, M. A. ; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (j) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (k) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (l) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (m) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (7) (a) Whitesides, G. M.; Gorman, C. B. In Handbook of Surface Imaging Visualization; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; pp 713-32. (b) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771. (c) Ulman, A. MRS Bull. 1995, 20, 46. (d) Buess-Herman, C. Prog. Surf. Sci. 1994, 46, 335. (e) Whitesides, G. M.; Ferguson, G. S.; Allara, D.; Scherson, D.; Speaker, L.; Ulman, A. Crit. Rev. Surf. Chem. 1993, 3, 49. (8) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1991, 113, 5464. (c) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884. (9) (a) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (c) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (10) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (11) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186. (12) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (13) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (14) Stranick, S. J.; Parikh, A. N.; Tao, Y.-t.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (15) Hayes, W. A.; Shannon, C. Langmuir, submitted for publication.
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copy to probe the distribution of the two components across the surface. We show that the frictional properties of the two adsorbates are sufficiently different to allow chemically resolved imaging of the mixed monolayers and go on to provide evidence of nanometer-scale phase separation in these systems. We also find evidence for more complex phase behavior at the highest 4-ATP coverages we investigated. Finally, we use the phase-separated monolayers to carry out localized electropolymerization of aniline from the 4-ATP islands. AFM data show that the polymer grows selectively from the adsorbed 4-ATP. Experimental Section Chemicals. 4-Aminothiophenol (Aldrich) was recrystallized two to three times from methanol (Fisher) and stored in the dark at 0 °C. Aniline (Fisher) was distilled over zinc metal and stored for short periods of time in the dark at 0 °C. Ethanol (200 proof, Florida Distillers), 70% HClO4 (Fisher) and n-octadecanethiol (97%, Aldrich) were used as received. Substrate Preparation. Substrates suitable for atomic force microscopy experiments were prepared by melting and annealing a polycrystalline gold wire (99.999% Johnson Matthey) to produce a 1-2 mm diameter bead containing macroscopic atomically flat Au(111) single crystal facets. The details of this procedure have been previously described.17 For Auger electron and Fourier transform infrared (FTIR) spectroscopy experiments, glass microscope slides were cleaned in hot piranha solution for about 1 h and were then sonicated for at least 10 min in Millipore H2O and absolute ethanol. The slides were then rinsed with copious amounts of ethanol and blown dry in a stream of ultrahigh purity (UHP) Ar. A thin layer of Ni/Cr followed by an approximately 200 nm thick layer of Au was then deposited onto these slides using a Denton DV-502A vacuum evaporator. After deposition, samples were rinsed with ethanol and dried in a stream of UHP Ar. Monolayer Assembly. The thin film gold substrates and 1-2 mm gold beads were rinsed with ethanol and immersed for 3 h in an ethanol solution containing a total thiol concentration of 1 mM. Upon emersion, the samples were rinsed with copious amounts of ethanol and dried in a stream of UHP Ar. Auger Electron and FTIR Spectroscopy. Grazing angle FTIR experiments were performed using a Mattson RS-1 spectrometer equipped with a Graseby Specac surface reflectance attachment. In all cases, 500 scans were coadded and the instrumental resolution was 4 cm-1. The Auger electron spectroscopy apparatus has been previously described.18 Electrochemistry. All electrochemistry experiments were performed using a Pine AFRDE-5 potentiostat and HewlettPackard 7015B X-Y recorder in a conventional three-electrode configuration, with the gold bead as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. We refer all potentials to this reference electrode. All cell components are either Teflon or Kel-F. When electrochemical measurements were made, no attempt was made to mask the polycrystalline portions of the bead electrodes; therefore, all electrochemistry is characteristic of polycrystalline Au surfaces. Unless otherwise noted, the deposition of aniline was carried out from a 0.1 M aniline solution in 0.5 M HClO4. The potential was cycled between 0.0 and 0.775 V five times at 100 mV s-1 and held at 0.0 V while the working electrode was emersed from the electrochemical cell. The electrode was then rinsed with a gentle stream of Millipore H2O and dried in a stream of UHP Ar. (16) We find that the precision of the isotherm data is significantly higher when we use the C/S ratio compared to any absolute Auger intensity and believe this is related to the fact that the relative measurement is not as sensitive as an absolute intensity to run-to-run variations in primary beam energy, current, etc. Further, we believe the data indicate that there is a negligible amount of advantitious C on the surface. See also: Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (17) Hsu, T. Ultramicroscopy 1988, 11, 167. Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (18) Bozack, M. J.; Williams, J. R.; Ferraro, J. M.; Feng, Z. C.; Jones, R. E. J. Electrochem. Soc. 1995, 142, 485.
Nanometer-Scale Patterning of Surfaces AFM. Atomic force microscopy was used to study the formation and phase separation of each of the mixed monolayers prepared by the methods described above. Measurements were performed using a beam-deflection microscope in which light from a laser diode was focused on to the back of a V-shaped microfabricated cantilever (Park Scientific Instruments) and detected by a fourquadrant photodiode. The laser diode and processing of the photodiode signals was controlled by RHK AFM 100 electronics. Sample positioning, data collection, and data analysis were performed using RHK STM 1000 control electronics and software. In all of the experiments described here, the sample was mounted on a piezo tube scanner (1.0 in long × 0.5 in o.d.) and moved relative to the tip position. The maximum scan range of the microscope was 1.4 µm × 1.4 µm. The four quadrant photodetector allowed simultaneous measurement of the normal deflection and lateral torsion of the V-shaped cantilever. Normal deflections of the cantilever were used to measure the topography the surface region and the total normal load applied to the system (distance × force constant ) force). Lateral torsions of the cantilever which occurred as the sample was scanned beneath it were ascribed to frictional forces between the tip and the sample. A cantilever with a relatively weak normal force constant (0.03 N/m, Park Scientific) was used for imaging these films, while a stiffer cantilever (0.5 N/m normal force constant, Park Scientific) was used in the collection of friction-load maps. Due to the uncertainties in lever dimensions, the manufacture’s value of the normal force constant was used without further calibration and lateral forces were measured in terms of relative voltage changes. As a result, the data presented here can only be interpreted in terms of relative differences between different films and phases of the two-component films; nevertheless, by using the same cantilever for investigation of a series of films, we are assured of precision within a given set of measurements. Furthermore, normal and lateral motions of the cantilever were decoupled by an alignment procedure performed before each set of experiments. Simultaneous topographic and lateral force images were collected19 for each composition of the mixed monolayer films under a normal applied load of 0 nN held constant by a feedback set point. Adhesive forces between the tip and the sample resulted in an effective total load greater than 0 nN at this applied load. Lateral force contrasts between different regions of the films were measured by performing spatial averages over different phases of the films and were further investigated by studying the frictional properties of the compositionally pure films. In studies of the pure films, friction-load maps were collected to establish fundamental differences in the frictional properties of the different monolayers. In this approach, frictional forces were measured as a function of load by scanning the sample in a lateral direction while first increasing and then decreasing the applied load. The lateral torsion of the lever during sliding was averaged over each cycle of the lateral scan and taken as a measure of the average kinetic friction. The slope of the plot of average kinetic friction versus normal load is defined as the coefficient of friction and is commonly used to describe the frictional properties of a specific interface. All AFM characterization of electrodeposited polyaniline nanostructures was performed in air on a Park Scientific Instruments Autoprobe CP scanning probe microscope operated in intermittent-contact mode. The tips used were commercially available Model APUL-20AU-25 2µm Ultralevers (Park Scientific). The resonant frequency of the tips was consistently between 175 and 225 kHz and the drive amplitude was varied as necessary to obtain high signal-to-noise images. The scan rates used were 3-4 Hz on a 5 µm piezoelectric scanner (scan sizes are given in the figures).
Results Macroscopic Characterization of the Mixed Composition Monolayers. Critical phenomena in interfacial systems have been studied actively for over fifty years. (19) Images were typically collected within an hour after emersion of the sample from the assembly solution. The phase domains were found to be stable on the order of 1 week (Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir, manuscript in preparation).
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For example, Onsager20 was the first to demonstrate that a positive nearest-neighbor interaction energy between like molecules is sufficient to cause the system to split into two phases below a certain critical temperature, leading to a discontinuity in the corresponding adsorption isotherm. Thus, deviation of the adsorption isotherm from simple Langmuir-type behavior can be an indication of the coexistence of two phases on the surface, although this evidence is not always conclusive.6d We used three independent macroscopic surface characterization techniques to determine the surface composition of the twocomponent SAMs as a function of the makeup of the assembly solution: Auger electron spectroscopy, coulometry, and grazing angle FTIR spectroscopy. Previous AES studies have shown16 that the absolute intensities of the S and C Auger peaks can be used to calculate the mole fraction of 4-ATP in a two-component film due to the fact that the packing densities of 4-ATP and ODT on the Au surface are significantly different. The analysis also requires Auger data from the two singlecomponent monolayers. The mole fraction of 4-ATP on the surface can also be determined coulometrically because adsorbed 4-ATP is electroactive. In this experiment, the mole fraction of 4-ATP in the monolayer is determined by stepping the electrode potential through the one-electron oxidation wave of 4-ATP and integrating the resulting current-time trace. The total faradaic charge passed is proportional to the packing density of 4-ATP in the monolayer. A plot of the mole fraction of 4-ATP in the monolayer (χ4-ATP,surf) as a function of the composition of the assembly solution is given in Figure 1 for both the AES and coulometric determinations. In both cases, we observe a distinctly non-Langmuirian isotherm, with a sharp break occurring as the concentration fraction of 4-ATP in the assembly solution approaches 80%. Another macroscopic, but more molecularly sensitive, surface probe is grazing angle FTIR spectroscopy. In Figure 2A we show a series of surface IR spectra in the aliphatic CH stretching region. The top trace was obtained for a pure monolayer of ODT and is identical to what has been previously observed for well-ordered monolayers of ODT, indicating the presence of a well-ordered, densely packed monolayer on the surface. As the mole fraction of ODT in the monolayer decreases (i.e., as the mole fraction of 4-ATP increases), several trends can be observed in the FTIR data. First, and most strikingly, there is a pronounced shift in the asymmetric methylene stretch to higher frequencies. A similar, although more subtle, frequency shift can be seen in the asymmetric CH3 stretch at ∼2850 cm-1. Similar changes in ν(CH2) have been observed in temperature dependent studies of SAMs and have been correlated to the percent gauche defects in disordered monolayers.21 Second, the full-width halfmaximum of each of these peaks increases monotonically as χ4-ATP, surf increases. This finding is also consistent with an increase in the disorder (i.e., an effective decrease in the density) of the ODT film as χ4-ATP, surf increases. Finally, it is important to note that the FTIR peak intensities in Figure 2 do not appear to follow Beer’s law. We attribute this to a change in orientation of the principle axis of the ODT as the monolayer becomes disordered, which complicates application of Beer’s law to this system. If, for example, the average orientation of the ODT principle axis becomes more closely normal to the surface as more 4-ATP is doped into the film, the intensity of the ν(CH2) band should display a positive deviation from Beer’s law, as observed. These findings indicate that the position (20) Onsager, L. Phys. Rev. 1944, 65, 117. (21) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437 and references therein.
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Figure 1. Adsorption isotherms for 4-aminothiophenol/octadecanethiol mixed monolayers. The mole fraction of 4-ATP on the surface determined by AES and coulometry is plotted versus the composition of the assembly solution. (A) Isotherm determined using Auger spectroscopy. (B) Isotherm determined using coulometry. Notes: Points 1-3 indicate composition of SAMs investigated using atomic force microscopy. The dashed line shows the expected isotherm in the case of no preferential adsorption; the solid line is a guide to the eye.
of the ν(CH2) band is a reasonable measure of the film order parameter. Indeed, this is borne out in Figure 2B, where the frequency shift for the asymmetric ν(CH2) stretch is plotted as a function of the assembly solution composition for a complete data set. Interestingly, the FTIR data display a somewhat sharper break near the critical composition (80% 4-ATP) than was observed in the Auger electron spectroscopy (AES) and coulometry isotherms. Considering the isotherm and FTIR data together suggests the following picture for the structure of 4-ATP/ ODT two-component SAMs. The long-chain ODT molecules, which tend to form a well-ordered two-dimensional crystalline film in the absence of a coadsorbate, disorders in the presence of the much shorter 4-ATP. At low 4-ATP concentrations within the film, the surface appears to consist of regions of ODT that are still well-ordered, as evidenced by the relatively small shift in the methylene asymmetric stretch. As the concentration of 4-ATP in the monolayer increases, the surface appears to consist of 4-ATP imbedded in a disordered matrix of ODT. The large shift in the methylene asymmetric stretch at the highest 4-ATP coverages is suggestive of more complex phase behavior at the highest surface concentrations of 4-ATP. Microscopic Characterization of the Mixed Composition Monolayers. Our initial attempts at the microscopic characterization of these mixed monolayer systems by conventional AFM were unsuccessful. Experimentally, no height differences could be observed in the AFM images of the mixed component monolayers
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Figure 2. Grazing angle FTIR spectroscopy of mixed monolayers. Shown in the upper panel is the aliphatic CH stretching region. The composition of the assembly solution is indicated next to each spectrum. In the lower panel the methylene asymmetric stretching frequency (ν(CH2)) is plotted as a function of solution composition for a complete data set.
Figure 3. Frictional forces measured as a function of load for monolayers of pure 4-aminothiophenol (triangles) and octadecanethiol (circles). The frictional difference between these two materials provides the physical basis for distinguishing the two components of a mixed monolayer by lateral force mocroscopy. The vertical dashed line indicates the load at which LFM images were collected.
despite a significant expected height difference in the two molecules based on the known orientation in pure films. The reasons for this are not entirely clear at the present time. Nevertheless, we were confident that a suitable alternative image contrast mechanism could be found due to the obvious chemical differences between the two molecules. We reasoned that there should be significant
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Figure 5. Lateral force microscopy image (5.0 × 5.0 nm2) of the low friction ODT region within the film formed from 65% 4-ATP and 35% ODT solution. The lattice parameter of the ordered region of the film is 4.9 ( 0.1 Å and is consitent with a (x3×x3) overlayer on the Au(111) surface.
Figure 4. Lateral force microscopy images of phase-separated two-component monolayers: (A, top) 65% 4-ATP:35% ODT (point 1 in Figure 1); (B, middle) 80% 4-ATP:20% ODT (point 2 in Figure 1); (C, bottom) 90% 4-ATP:10% ODT (point 3 in Figure 1). Each image shows a 1 × 1 µm2 image of the SAM acquired at 0 nN load. Light regions correspond to areas of high friction and are assigned to islands of 4-ATP, while darker regions correspond to low friction and are are assigned as predominantly ODT.
differences between the frictional properties of the two molecules. To experimentally verify frictional differences between the molecules, friction-load maps were measured for pure monolayers of 4-ATP and ODT. In this experiment, the friction (i.e., lateral force) was monitored as the load (i.e., normal force) on the tip was varied (Figure 3). The data show that the frictional properties of the two molecules differ by roughly an order of magnitude: friction increases at a rate of 3.3 mV/nN for ODT (circles) and 26 mV/nN for 4-ATP (triangles). From these data, it is also evident that there is about a factor of 2 difference in the frictional properties of the two molecules even at zero load. This difference is sufficient to allow molecular contrast imaging under zero load, ensuring minimal perturbation of the films by the scanning probe tip during image acquisition. Lateral force images (1 × 1 µm2) of the mixed monolayers were obtained at 0 nN load for SAMs formed from solutions containing 65%, 80%, and 90% 4-ATP (Figure 4). These compositions are indicated for reference on the isotherms in Figure 1. Multiple regions of each sample were investigated, and the data presented here are representative of the entire surface of these mixed monolayers. In the first image (formed from the 65% 4-ATP solution), a few phase-separated regions can be easily identified, specifically, the two bright spots located in the lower central and upper central portions of the image. These two domains are approximately 50 and 70 nm in diameter, respectively. The average friction (250 mV) within these phase-separated domains, obtained statistically by averaging the friction values associated with each pixel, is identical to the average friction measured for the pure 4-ATP SAM at 0 nN load with tips of similar radii of curvature. These data indicate therefore that the 4-ATP component, present at the lowest concentration in the SAM, forms islands within the ODT monolayer. In Figure 4B, which was obtained for the 80% 4-ATP SAM, clear phase separated regions are seen across the entire surface. The average diameter of the phase-separated domains is about 100 nm, although there is a significant dispersion in domain size. As in the previous image, each domain exhibited frictional properties identical to 4-ATP. Finally,
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Figure 6. Electrochemistry. (A) Electrochemical behavior of a 4-ATP monolayer in 0.1 M aniline showing the oxidation potentials of each molecule. (B) Formation of polyaniline at 4-ATP by scanning between 0.0 and 0.750 V. (C) No polyaniline is formed under the same conditions in pure electrolyte at a naked Au electrode.
Figure 4C shows the lateral force image of a 90% 4-ATP SAM. As before, clear evidence of separated islands of 4-ATP can clearly be seen, although the islands are significantly smaller in size than in the case of the 80% 4-ATP sample. Further insight into the phase separation of these twocomponent SAMs was gained by measuring the structural and frictional properties of the ODT matrix into which the 4-ATP was added. In the SAM formed from the 65% 4-ATP solution, the average friction in regions away from the 4-ATP islands corresponded to what was measured for the pure ODT monolayer (15 mV). In addition, lateral force images obtained from 5.0 × 5.0 nm2 areas within these regions exhibited a lattice resolution identical to that observed in the pure ODT film (Figure 5) with a lattice constant (0.49 ( 0.01 nm) corresponding to a (x3×x3)
overlayer structure on Au(111).22 No molecular features could be detected within the 4-ATP islands or in the pure 4-ATP monolayer. In the SAM formed from the 80% 4-ATP solution, a modest increase in the frictional force was observed within the ODT phase and lattice resolution of the well-ordered ODT could still be obtained. Finally, in the SAM formed from the 90% solution, the friction at 0 nN load in all regions outside the 4-ATP islands increased by at least a factor of 2 with respect to the pure ODT film. In addition, no molecular-level order was observed, in contrast to what was seen for the two previous films. As the 4-ATP concentration within the film reaches 0.8, significant mixing of 4-ATP and ODT occurs, in addition to the discrete phase separation evidenced by the high (22) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367.
Nanometer-Scale Patterning of Surfaces
friction islands. Mixing of the two components disrupts the ordered ODT structure leading to a loss of lattice resolution away from 4-ATP islands. This interpretation is corroborated by the changes in the FTIR data discussed above. In interpreting these images, it is important to note that the average (statistical) friction of these surface, as noted in each image, increases as the mole fraction of 4-ATP increases, indicating that the total amount of 4-ATP on the surface is increasing in accordance with the previously measured isotherms. The tendency of a two-component SAM to phase separate depends on the free energy of mixing, which contains two terms, an entropy term that drives the system toward disorder and an interaction energy term that drives the system toward order. When the concentration of 4-ATP within the film is low, the monolayers resemble pure ODT monolayers much more than they resemble pure 4-ATP monolayers. In this limit, discrete phase separation is favored because the relatively large cohesive energy of ODT is large enough to overcome the entropy of mixing. As the mole fraction of 4-ATP in the film increases, however, the ODT matrix becomes increasingly disordered, as evidenced by changes in the FTIR spectra and the AFM images discussed previously. Under these conditions, lateral self-interaction energies become relatively less important and the entropy term dominates the free energy of mixing. This leads to films containing regions where ODT and 4-ATP are well-mixed as well as distinct phase separated domains. This argument also suggests that the size of the phase-separated domains should depend on the degree of mixing. In particular, as the degree of mixing increases, the size of the 4-ATP domains should decrease. This expectation is borne out by our AFM data on the film formed from a 90% 4-ATP solution which shows both a decrease in the size of the phase separated islands and an increase in the amount of 4-ATP in regions away from the islands. Electrochemistry of Mixed Monolayers. The voltammetric response of a 4-ATP monolayer adsorbed on a Au electrode and immersed in 0.1 M aniline is shown in Figure 6A. The surface-confined wave at 0.730 V has been shown previously to be due to the oxidation of adsorbed 4-ATP, while the wave at about 1.00 V corresponds to the oxidation of bulk aniline.23 The initial step in the electropolymerization of aniline proceeds by formation of the aniline cation radical.24 We have shown that the electrooxidation of a 4-ATP monolayer involves a similar first step, that is, formation of the cation radical.16 The oxidation potential of adsorbed 4-ATP is about 260 mV more negative than the oxidation potential of aniline itself, indicating that it should be possible to initiate the growth of polyaniline preferentially at an adsorbed 4-ATP molecule if the electrode is held at the 4-ATP oxidation potential. In addition, at this potential there will be no precipitation of polyaniline from solution or growth of the polymer from adventitious defects within the 4-ATP SAM because the potential is not sufficiently oxidizing to produce the aniline cation radical. The remaining peaks have been assigned and discussed in our earlier work and will not be considered further here. Figure 6B shows that (23) Hayes, W. A.; Shannon, C. Template-Directed Growth of Polyaniline Nanostructures on Organothiol SAMS on Gold In Scanning Tunneling Microscopy/Atomic Force Microscopy 3; Cohen, S. H., Ed.; Plenum Press: New York; in press. (24) (a) Syed, A. A.; Dinesan, M. K. Talanta 1991, 38, 815. (b) Ginder, J. M.; Epstein, A. S.; MacDiarmid, A. G. Synth. Met. 1989, 29, E395. (c) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 2491. (d) Bacon, J.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 6596. (e) Sabatini, E.; Gafni, Y.; Rubinstein, I. J. Phys. Chem. 1995, 99, 12305. (f) Rubinstein, I.; Rishpon, J.; Sabatini, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135.
Langmuir, Vol. 13, No. 9, 1997 2517
Figure 7. Atomic force microscopy of polyaniline nanostructures formed at phase separated mixed monolayers. Topographic images of (A, top) 65% 4-ATP: 35% ODT cycled 10 times between 0 and 0.750 V in 0.1 M aniline. (B, bottom) 65% 4-ATP:35% ODT cycled 10 times between 0 and 0.750 V in pure electrolyte.
polyaniline can be grown at a 4-ATP modified Au electrode with potential excursions between 0.00 and 0.775 V. The voltammetric features at 0.150 and 0.750 V are characteristic of polyaniline and are well developed after about ten cycles. The cyclic voltammogram in Figure 5C demonstrates that the growth of polyaniline does not occur when a naked Au electrode is subjected to the same treatment. Extrapolating from the behavior of a pure monolayer, we reasoned that it should be possible to effect selective deposition of polyaniline at 4-ATP molecules imbedded within an electrochemically blocking SAM, resulting in the production of small polymer features covalently anchored to the surface. In the following experiment, a SAM consisting of 65% 4-ATP and 35% n-octadecanethiol (ODT) was subjected to 10 cycles between 0 and 0.775 V at 100 mV s-1. The specimen was removed from solution under potential control at 0.00 V and AFM was used to detect the presence of polyaniline features on the electrolyzed surface. In Figure 7A we show a micrometer scale image of a 65% 4-ATP/ODT mixed SAM after electrochemical cycling. This image is characterized by a high density of randomly dispersed features averaging
2518 Langmuir, Vol. 13, No. 9, 1997
120 ( 20 nm in diameter and 7.0 ( 1.6 nm in height. The average feature density is 35 ( 3 µm-2 and is extremely reproducible. To demonstrate that these features are due to the formation of polymer, we repeated this experiment in the absence of solution phase aniline. In Figure 7B we show a micrometer scale AFM image of a 65% 4-ATP/ ODT SAM cycled 10 times in pure electrolyte. No features are formed under these conditions; the surface appears atomically flat, reflecting the topography of the underlying Au(111) substrate. Conclusion In summary, we have characterized two-component SAMs containing 4-ATP and ODT using macroscopic surface analytical techniques as well as atomic force microscopy. These monolayers exhibit a complex phase behavior: When [4-ATP]/[4-ATP] + [ODT] ∼ 0.6, phaseseparated domains of 4-ATP form. The surface consists of small islands of 4-ATP ranging in size from about 10
Hayes et al.
to 100 nm surrounded by a well-ordered film of ODT. When the concentration fraction of 4-ATP increases above this value, we observe evidence of phase separation and mixing in the same SAM. The islands of 4-ATP are electroactive and can be used to carry out localized electrochemistry. Surface-confined 4-ATP molecules template the growth of polyaniline selectively from the phase-separated domains. Acknowledgment. W.A.H., X.Y., and C.S. gratefully acknowledge the financial support of the National Science Foundation (OSR-9553348), the Society of Analytical Chemists of Pittsburgh, and Auburn University. H.K. and S.P. wish to acknowledge the generous financial support of the University of Houston and the Energy Laboratory of the University of Houston. We are grateful to Professor M. J. Bozack (Auburn University) for carrying out the Auger analysis of the mixed monolayer samples. LA962074N
