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Surface Patterns of Supramolecular Materials Janelle Gunther and Samuel I. Stupp* Department of Materials Science, Department of Chemistry, and Medical School, Northwestern University, Evanston, Illinois 60208 Received February 19, 2001
We report here on the two-dimensional patterns formed by supramolecular materials deposited from solution on oxidized silicon substrates. The supramolecular materials studied are composed of mushroomshaped nanostructures measuring 2-5 nm in cross-section and approximately 7-8 nm in height. Two different materials were studied, one containing nanostructures with a hydrophilic phenolic base surface and the other containing a hydrophobic one with trifluoromethyl groups. The substrates were exposed to solutions of these materials for a set induction time at a series of concentrations using a motorized dipping apparatus. Samples were characterized by contact-angle measurements and tapping-mode atomic force microscopy. We observed distinct patterns as a function of concentration in phenolic supramolecular materials that interact favorably with the oxidized silicon surface. At low concentrations (0.01 wt %), the nanostructures form islands with uniform size of approximately 0.02 µm, which have the height of a single nanostructure (7.2 nm). As concentration increases, a string-like morphology with uniform width is observed first, followed by a percolating texture. At yet higher concentrations, the film transforms to a honeycomb morphology, but its height still remains equal to that of a single nanostructure. When interactions between the nanostructure and the surface are not favorable (i.e., between trifluoromethyl end groups and oxidized silicon), uniform height patterns are not observed. The distinct geometries are possibly the result of strong material-substrate interactions balanced by a repulsive force that could have electrostatic origin. The extremely uniform thickness of the two-dimensional patterns may originate in the hydrophobic and hydrophilic nature of opposite poles of the nanostructures, thus suppressing vertical growth of the film.
Introduction The formation of nanoscale supramolecular structures by self-assembly has been an area of interest in chemistry over the past few years.1-6 There are obvious similarities between this approach and the way biology creates its highly functional units and materials through proteins and their complex aggregates. Our laboratory has been interested in supramolecular chemistry as a strategy to create materials containing constituent units with interesting dimensionality. For example, we have studied the formation of two-dimensional polymers using supramolecular precursors,7,8 one-dimensional structures based on rodcoil polymers,9-11 and more recently zero-dimensional nanostructures using the preprogrammed finite aggregation of triblock molecules.12-16 All of these selfassembled structures order over several length scales to * To whom correspondence should be addressed. Most of this work was carried out at the Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801. (1) Lehn, J. M. Science 1993, 260, 1762-1763. (2) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312-1319. (3) Zimmerman, S. C.; Zeng, F. W.; Reichert, D. E. C.; Kolotuchin, S. V. Science 1996, 271, 1095-1098. (4) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 285, 785-788. (5) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J. W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199-8208. (6) Hudson, S. D.; Jung, H. T.; Percec, V.; Cho, W. D.; Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449-452. (7) Stupp, S. I.; Son, S.; Lin, H. C.; Li, L. S. Science 1993, 259, 59-63. (8) Stupp, S. I.; Son, S.; Li, L. S.; Lin, H. C.; Keser, M. J. Am. Chem. Soc. 1995, 117 (19), 5212-5227. (9) Razilowski, L. H.; Wu, J. L.; Stupp, S. I. Macromolecules 1993, 26 (4), 879-882. (10) Radzilowski, L. H.; Stupp, S. I. Macromolecules 1994, 27 (26), 7747-7753. (11) Radzilowski, L. H.; Carragher, B. O.; Stupp, S. I. Macromolecules 1997, 30 (7), 2110-2119.
produce materials with interesting properties. These include thermal stability, second harmonic generation, piezoelectricity, and contrasting surface properties on parallel macroscopic surfaces, to name a few. In this work, we have studied the surface patterns formed by supramolecular materials developed previously in our laboratory.12 These materials are composed of mushroom-shaped nanostructures lacking a center of inversion with dimensions on the order of 5 nm. We describe below details of their molecular and microstructural features as well as the properties that were known prior to the beginning of this study. The synthesis of mushroom-shaped nanostructures utilizes triblock molecules, as recently reported by Stupp et al.12 These triblock molecules consist of a stiff, rod-like segment covalently linked to a diblock segment in which the terminal block has the largest cross-section. These rodcoil block molecules form finite aggregates on the order of a few nanometers as revealed by transmission electron microscopy as well as small-angle X-ray scattering.14 Their finite nature is believed to result from repulsive forces among large cross-section segments and consequent entropic factors.17 It is still not clear what is the nature of the short-range forces that drive parallel vs antiparallel aggregation of molecules, but simple empirical force fields have identified parallel alignment as the most favorable arrangement. This is also supported by the fact that films composed of these nanostructures exhibit a layered (12) Stupp, S. I.; LeBonheur, V.; Walter, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384-389. (13) Tew, G. N.; Pralle, M. U.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 9852-9866. (14) Pralle, M. U.; Whitaker, C. M.; Braun, P. V.; Stupp, S. I. Macromolecules 2000, 33, 3550-3556. (15) Tew, G. N.; Pralle, M. U.; Stupp, S. I. Angew. Chem., Int. Ed., in press. (16) Pralle, M. U.; Urayama, K.; Tew, G. N.; Neher, D.; Wegner, G.; Stupp, S. I. Angew. Chem., Int. Ed., in press. (17) Sayar, M.; Stupp, S. I. Unpublished results.
10.1021/la0102511 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001
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Chart 2
morphology in which nanostructures appear to be parallel to each other. Most interestingly, surface properties, piezoelectricity, and second harmonic generation suggest that polar stacking of layers is one of the features in the microstructure. The opposite termini of the triblock molecules (1; Chart 1) studied here have contrasting chemistry, since one is a hydrophobic CH3 group and the other is a hydrophilic phenolic group. The macroscopic films fabricated from 1 reveal contact angles on the hydrophobic top surface of 97°. This value is consistent with that observed for oxidized silicon surfaces with chemisorbed monolayers of alkylsilanes that are terminated with methyl groups.18 Monolayers of hydroxyl-terminated surfaces on gold reveal contact angles of less than 10°.19 The hydrophilic surface of 1 (phenolic terminal groups) had a contact angle of approximately 40°. One possible reason explaining the difference between the hydroxyl-terminated alkylsilanes and the phenolicterminated surface of the nanostructured films is that surface reconstruction may take place in the latter. Alternatively, the order parameter of the surface in these bulk films may not be as high as that of monolayers. Reconstruction is supported by the fact that the measured contact angle increases to that of the hydrophobic surface after several days of storage in air. The polar stacking of nanostructures with hydrophobic and hydrophilic opposite poles on a hydrophilic substrate would certainly yield the observed results on surface properties. Of course, it is clear that other mechanisms such as end group migration driven by interfacial surface energies or entropy could (18) Ruhe, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 9, 2383-2388. (19) Bain, C. D.; Troughton, E. B.; Tao, Yu-Tai; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.
result in similar behavior.20-22 In the materials studied here, the supramolecular nature of the nano-objects generates surfaces consisting of clusters of about 100 hydrophilic or hydrophobic end groups. The clustering of molecules in the objects coupled to self-assembling behavior could offer kinetic advantages in the formation of bulk materials with defined surface properties. Also, we had previously found evidence for the ability of these films to adhere tenaciously to oxide surfaces.23 This observation could be explained by the collective action of adhesive phenolic clusters in each supramolecular unit on the substrate. This paper reports on the evolution of surface patterns formed by supramolecular materials of the type described above. We have also compared their behavior with that of other materials that are chemically similar but not supramolecular in structure. Specifically, it has been of interest here to understand how the nanostructured nature of these self-assembling materials affects twodimensional development of the patterns. The effect of terminal group chemistry in supramolecular materials has also been investigated. The main technique used in the study was atomic force microscopy (AFM) in the tapping mode. Materials and Methods In addition to 1, we have used in this study 2-5 (Chart 2). The syntheses of 1-4 have been described elsewhere (1,12 2,24 and (20) Jalbert, C.; Koberstein, J. T.; Hariharan, A.; Kumar, S. K. Macromolecules 1997, 30, 4481-4490. (21) Tead, S. F.; Kramer, E. J.; Russell, T. P.; Volksen, W. Polymer 1992, 33 (16), 3382-3387. (22) Walton, D. G.; Mayes, A. M. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 54 (3), 2811-2815. (23) LeBonheur, V. Ph.D. Thesis, University of Illinois at UrbanaChampaign, Urbana, IL, May 1996.
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3-425). 5 was obtained from Aldrich (Milwaukee, WI) with a M hw ) 8000 Da and used as received. Surfaces of silicon [100] were used as substrates in all experiments. The wafers (supplied by Silicon Quest, Santa Clara, CA) were first treated with the RCA standard clean procedure. First, any insoluble organic contaminants were removed by placing the wafers in a solution of 5:1:1 H2O:H2O2:NH4OH held between 75 and 80 °C for 10 min. After a thorough rinsing in deionized water filtered using a Millipore filtration system (Millipore, Bedford, MA), the wafers were placed in a dilute solution of HF (50:1 H2O:HF) for 15 s to remove the thin silicon dioxide layer where any metallic contaminants could have accumulated. Following an additional rinse, the wafers were placed in a solution of 6:1:1 H2O:H2O2:HCl for 10 min to remove any ionic or heavy metal contaminants. After a final rinse, the wafers were spin-dried. The oxidation step consists of a dry oxidation step, a steam oxidation step, and a final dry oxidation step. The dry oxide is higher quality, but the steam oxide grows more quickly. To create a high-quality oxide layer, the wafers were placed in an oxidation furnace where a 100-Å oxide layer was grown under O2 (dry oxide) followed by a 1000-Å layer grown under O2 and H2 (steam oxide) and then a top layer of 100 Å grown under O2 (dry oxide). The dry oxide grows slowly but provides a high-quality interface layer. The thick oxide layer was produced so that the same substrates could later be used for microcontact printing schemes, following the method of Whitesides et al.26 The organic films were deposited on small pieces of silicon substrate using a home-built dipping apparatus. This consisted of a motor that was mounted on a bracket and placed on a vibration isolation table. The sample was then lowered into the solution with a motor speed selected such that the dipping process would be complete within approximately 30 min. The dipping process used 0.01-2 wt % solutions of the various materials in ethyl acetate. One exception was the fluorinated 2, which was dissolved in tetrahydrofuran. In the final preparation step, the samples were allowed to dry slowly in air under a glass dish before imaging. Imaging of the surface morphology was accomplished via AFM. A Digital Instruments Multimode AFM and Nanoscope III controller were used to acquire height and phase-contrast scans of the materials. The AFM was operated in the tapping mode using 125-µm etched silicon probes. Typical forces were a few nanonewtons with scan rates of 0.5-1 Hz. The NIH Image software (a public domain image processing and analysis program available from the NIH) was used to calculate area per particle statistics as well as to determine if a particular texture was percolating or not.
Results and Discussion The solid structures formed when small sections of silicon wafers were dipped in solutions of 1 of various concentrations under identical conditions are shown in the 20 × 20 µm images of Figures 1-4. The first image (Figure 1) reveals an island texture formed using solutions with the lowest concentration (0.01 wt %). These islands have an average diameter of 0.12 µm and are distributed evenly on the substrate. The islands shown in Figure 1 not only are fairly monodisperse in size but also may have some additional hierarchical structure in the way that they align with each other (islands seem positioned in a string-like arrangement). The average height of these islands is approximately 7.2 nm (measured over 1000 particles), corresponding to the height of a single nanostructure. Given the known structural nature of 1, there should be approximately 400 supramolecular nanostructures in each island. At the slightly higher concentration of 0.025 wt %, the solid structures form a string-like morphology using identical substrate-dipping conditions. (24) Zubarev, E. R.; Pralle, M. U.; Li, L. M.; Stupp, S. I. Science 1999, 283, 523-526. (25) Gresham, K. Ph.D. Thesis, University of Illinois at UrbanaChampaign, Urbana, IL, May 1998. (26) Kumar, A.; Whitesides, G. M. Appl. Rev. Lett. 1993, 63 (14), 2002-2004.
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Figure 1. Three-dimensional rendering of the island morphology observed by AFM when 1 was deposited on an oxidized silicon substrate from a solution of 0.01 wt % (10 × 10 µm image).
Figure 2. Three-dimensional rendering of the string morphology observed by AFM when 1 was deposited on an oxidized silicon substrate from a solution of 0.025 wt % (10 × 10 µm image).
This surface string texture is shown in Figure 2. The average width of the strings is nearly 3× greater than the diameter of the islands observed at lower concentration. Interestingly, islands and strings have the same height as measured by AFM. Further increases in concentration (to 0.05 wt %) lead to a lace-like percolating surface texture (Figure 3), which has a feature height that remains unchanged with respect to that of the island and string morphology. At a higher concentration (0.075 wt %), a honeycomb-like texture is observed (see Figure 4) with a characteristic width that is 2× larger than that of the percolating texture. Again, a change in morphology is
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Figure 3. Three-dimensional rendering of the percolating (lace) morphology observed by AFM when 1 was deposited on an oxidized silicon substrate from a solution of 0.05 wt % (10 × 10 µm image).
Figure 4. Three-dimensional rendering of the honeycomb morphology observed by AFM when 1 was deposited on an oxidized silicon substrate from a solution of 0.075 wt % (10 × 10 µm image).
observed but with no change in feature height. In other words, up to this point, all features have a height equal to that of a single supramolecular nanostructure. Eventually at high enough concentrations (0.1 wt %), the material forms a single monolayer with a thickness of a single nanostructure. The change in morphology is summarized in Figure 5 using a two-dimensional representation. We measured the contact angle of water on substrates having the surface textures discussed previously. The data are shown in Figure 6 as a function of the solution concentration used to prepare the samples. The advancing and receding contact angles of water on the unexposed
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substrate are both 15° and then rise sharply on substrates dipped in solutions of rising concentration. This is consistent with values obtained for silicon substrates with a surface oxide layer.27 A plateau in the contact angle for water is observed at 97°, which is the same value measured for bulk films of the supramolecular material. This is close to the value of 100-108° that is observed for alkylsilane films chemisorbed onto oxidized silicon.27 Surface coverage, as measured by AFM, also increases at a similar rate to the contact angle. The plateau value for the contact angle is observed at the same starting solution concentration that is necessary to obtain 100% coverage of the surface. On the basis of our earlier work,12 these self-assembling films generate hydrophobic surfaces when cast on hydrophilic substrates such as glass. Furthermore, we know that these films are made up of nanostructures that have a hydrophobic and hydrophilic surface and a height of 7.2 nm. Thus, our observation here implies that nanostructures orient in a specific way on the substrate. In addition, the hydrophobic surface presentation seems to not only occur in the submonolayer textures but also persist in multilayered films. Similar experiments to those described above were carried out using 3-5, and a plot of feature height as a function of the starting solution concentration for all these materials is shown in Figure 7. Each data point represents an average measurement obtained from 200 to 500 sites. The plot clearly shows that 1 is the only material for which vertical growth does not occur below complete substrate coverage. In 3-5, an increase in the concentration of the starting solution leads to patterns that increase in height. All of these materials form island, string-like, and percolating structures but not necessarily at the same threshold concentrations. The difference in behavior between 1 and 3 as well as between 1 and 5 is most easily appreciated by overlaying three-dimensional renderings of the 5 × 5 µm AFM images.28 Figure 8 shows two AFM images of 3, the carboxylated polystyrene, deposited on silicon from two different starting solution concentrations. Figure 9 shows a similar image for 5, the polyvinylphenol. The gold features represent islands formed from the most dilute solution (0.01 wt %), while the black structures represent the features formed from a 0.025 wt % starting solution. In both cases, significant vertical growth is observed with increasing concentration. This is in great contrast to what is observed for 1 for which Figure 10 shows an overlay plot of AFM images for island and honeycomb textures, demonstrating that they both have identical feature heights (starting solutions of 0.01 and 0.075 wt %, respectively). The height of these two very different features corresponds to the height of a single nanostructure. Thus, the behavior of 1 up to monolayer coverage is similar to that of a self-assembled monolayer in that a well-defined thickness can be obtained by dipping substrates into solutions. There may be several factors behind the observed surface patterns and the fact that the feature height remains unchanged for submonolayer coverage. The island and percolating textures are very similar to those observed previously in end-grafted polystyrene chains exposed to (27) Ruhe, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 9, 2383-2388. (28) The “SoftImage”-rendering animation software package (SoftImage, Montreal, CA) was used to create “bumpmaps” from twodimensional AFM images. These were rendered in three-dimensions and overlayed in order to further compare changes or similarities between AFM images.
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Figure 5. AFM micrographs of surface patterns formed by the phenolic supramolecular material on oxidized silicon substrates as a function of solution concentration. (a) 0.01, (b) 0.025, (c) 0.05, (d) 0.075, and (e) 0.1 wt %. Each image is a 20 × 20 µm tapping-mode AFM scan.
silicon with a native oxide layer.29 In this earlier work, Karim et al. observed a transition from an island to a percolating texture for an increase in induction time. For short induction times, they observed the formation of an inhomogeneous island-like texture, with the chains grafting to the surface in a random deposition process. For longer induction times, when the grafting density was sufficiently high, a percolating texture was observed. Eventually, for very long induction times, a homogeneous layer formed. The transitions observed in that work are (29) Karim, A.; Tsukruk, V. V.; Douglas, J. F.; Satija, S. K.; Fetters, L. J.; Reneker, D. H.; Foster, M. D. J. Phys. II 1995, 5, 1441-1456.
similar to those in 1 as the concentration of the starting solution is increased. A change in solution concentration of 1 has a similar effect to a change in induction time for end-grafted polystyrene chains on silicon. However, some significant differences are observed between these materials. On the basis of AFM images, the island texture in the end-grafted material is inhomogeneous with a broad distribution of island sizes. In contrast, Figure 1 indicates that for 1 the islands have a rather uniform size and are distributed evenly. A histogram of area per island for this material is shown in Figure 11 and clearly indicates that there is a preferred island size. The grafted material studied by Karim et al. forms “clumps” as adsorption
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Figure 6. Plots of advancing and receding contact angle as a function of concentration of the starting solution used in dipcoating experiments. A plot of surface coverage reaches a maximum at the plateau value for the contact angle, confirming formation of a continuous film.
Figure 8. Overlay of AFM images (20 × 20 µm scan) for 3. The gold features are islands formed from a 0.01 wt % starting solution. The black features are islands formed from a 0.025 wt % starting solution. This clearly shows the increase in feature height that occurs as the starting solution becomes more concentrated.
Figure 7. Plot of feature height for 1 and 3-5 as a function of the concentration of the starting solution used for dip coating. Supramolecular 1 is the only sample in that series that maintained a consistent feature height for submonolayer coverage.
Figure 9. Overlay of AFM images (20 × 20 µm scan) for 5. The gold and black features represent islands formed from 0.01 and 0.025 wt % starting solutions, respectively. An increase in feature height is evident in the overlay plot.
occurs, perhaps reflecting its relative tendencies to stick to itself vs the substrate. In the material studied here, there seems to be a strong preference for substrate adsorption judging from the uniform distribution of fairly monodisperse islands. This would be consistent with observations in inorganic materials that exhibit a more even surface coverage without a tendency toward “clumping”.30 This type of model has also been used to describe deposition of polymer chains from a good solvent, where the chains repel each other from their molecular domains.29 In our system, it is reasonable to suggest that uniform distribution of islands (absence of clumping) also results from a favorable interaction with the substrate balanced by some other force that prevents coalescence. On the basis of recent work in our laboratory,31,32 it would be reasonable to suggest that this repulsive force has (30) Garboczi, E. J.; Bentz, D. P. J. Mater. Res. 1991, 6, 196-201. (31) Sayar, M.; Solis, F. J.; Olvera de la Cruz, M.; Stupp, S. I. Macromolecules 2000, 33 (20), 7226-7228. (32) Pralle, M. U.; Urayama, K.; Tew, G. N.; Neher, D.; Wegner, G.; Stupp, S. I. Angew. Chem. 2000, 39 (8), 1486-1489.
electrostatic origin. With regard to the constant height of the patterns, the nature of the mushroom nanostructures of 1 with hydrophobic caps and hydrophilic stems could be an important factor. That is, nanostructures with these dissimilar chemistries on opposite poles would not have a strong tendency to stack on top of each other. Island structures are also observed for many other materials but with slightly different geometries depending on several factors, including the interaction between the adsorbed molecules and the substrate, the intermolecular interactions, and the mobility of the adsorbed material. A fractal-like island structure has been observed in cases where the surface mobility is relatively high, for example, in octadecyltrichlorosilane (OTS) on mica.33 This is typical of cases where there is considerable mobility for the molecules to diffuse on the surface until they encounter other adsorbed ones. In contrast, the surface coverage is more uniform for OTS on silicon, where the surface binding is much stronger.34 This type of growth is characteristic of a random deposition process where there is volume (33) Schwartz, D. K.; Steinberg, S.; Israelachvilli, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354.
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Figure 10. Overlay of AFM images (20 × 20 µm scan) for the island and honeycomb textures in 1. The identical height of each morphology is clearly seen here.
Figure 11. Histogram of area per particle in 1, calculated using NIH Image. The Gaussian distribution shown here indicates that there is relatively narrow size distribution of islands and also a preferred size.
exclusion between the deposited structures and also little rearrangement after surface binding. Another type of structure, compact islands, is observed for materials such as butanethiols on gold where there is some surface mobility of the molecules but also an intermolecular (34) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852.
interaction that is weaker than that of OTS on silicon. This type of interaction produces islands that coarsen into compact structures. Clearly, the variety in the types of islands and their resulting surface arrangements and size distribution in different materials is a complex interplay between intermolecular and molecule-substrate interactions. We studied here the effect of molecule-substrate interactions by using 2, and these results are described below. Similar experiments studying surface patterns prepared by dip coating from solutions of increasing concentration were carried out on 2. This material is nearly identical to 1, except that it has a CF3 terminal group that is highly hydrophobic, as indicated by contact angles of water that exceed 100°.24 It can also form nanostructures but is not expected to have a favorable interaction with the oxidized silicon substrate. This material also forms similar surface patterns as 1, but very important differences are observed. As shown in Figure 12, at a concentration of 0.01 wt %, the fluorinated material forms islands that are very irregular in both shape and size as compared with 1. This is supported by the data shown in Figure 13, where a histogram of area per particle is shown for 2. The inset shows what the distribution looks like for the total range that was shown in Figure 11 for 1. Although for this material there are many particles within the same size range as for 1, there are also a number of particles that are well into the micrometer range. The inset as well as the entire distribution show that there is no preferred size for adsorbed islands of 2. Another interesting feature
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Figure 12. 20 × 20 µm image of an inhomogeneous island texture in fluorinated material 2 from a starting solution of 0.01 wt %. The islands are irregular in size and shape and also have many interior holes, which did not occur in phenolic 1.
Figure 13. Histogram of area per particle for fluorinated 2. There is a broad distribution of particle areas, with some well into the micrometer range. In addition, there is no preferred size, which is further illustrated by the inset that covers the same size range of particles as the histogram for 1 (see Figure 11).
is the formation of interior holes within some of the larger clumps of material. This was not observed in 1, which always formed islands. An additional feature of interest is the behavior of the two materials formed from starting solutions of high concentration. A 20 × 20 µm image of a honeycomb-like texture formed from a starting solution of 0.075 wt % is shown in Figure 14. The main feature to note here is the fact that the patterns formed have many
interior holes in contrast to the honeycomb texture of 1 (Figure 4), which again formed solid objects. In fact, at concentrations where 1 forms a single monolayer, 2 still forms a percolating texture with a feature height that is greater than those at lower concentrations. The random covering process shown in Figure 14 is characteristic of the island-like morphologies associated with random sequential adsorption without volume exclusion. This model leads to situations where the adsorbed material tends to form clumps of varying size and arrangement.29 The intermolecular interactions for 2 are expected to be strong since these materials also form finite-sized nanostructures. Consequently, the change in the surface patterns is a result of the unfavorable nature of the interaction between the fluorinated end groups and the substrate. The lack of hydrogen bonding between the stem portion of the mushroom-shaped nanostructure and the substrate gives this material potential for additional migration and mobility as compared with 1. In other materials, a transition between random deposition with and without overlap is mediated by the solvent, with poor solvents producing systems characterized by overlap and good solvents producing systems without overlap. In this case, however, the solvents used in each case were good solvents for both 1 and 2, so the resulting differences in their surface patterns are most likely based on differences in molecule-substrate interactions. As mentioned previously, a very intriguing observation is the uniform height of all patterns in supramolecular 1 and not in the chemically related 3-5. We concluded this on the basis of hundreds of measurements at various sites
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Figure 14. 20 × 20 µm AFM image of a texture formed from a starting solution of 0.075 wt % in 2. The pattern reveals a random covering process with many interior holes unlike 1, which only formed solid objects.
for a particular pattern, indicating that molecules consistently align themselves in a particular orientation on the substrate. Most importantly, however, we believe that the nature of self-assembly in this material suppresses vertical growth. As mentioned before, the polar nature of the nanostructures (mushroom-shaped as opposed to dumbbell-shaped) and their uniform orientation on the surface would lead to very weak interactions with other polar nanostructures not interacting with the substrate. In great contrast, the supramolecular nanostructures of 2 have hydrophobic caps and stems as well; therefore, relative to 1, these nanostructures will have a greater tendency to aggregate and thus exhibit vertical growth. This is also the case for the other materials, especially 3, which has a terminal carboxyl group attached to polystyrene chains and most likely deposits in bilayer organization thus justifying the observation of vertical growth. Carboxyl groups are well-known to dimerize, and thus, bilayer formation is a reasonable suggestion. Similar arguments would apply to 4 (without the hydrophilic terminal group), which is likely to favor moleculemolecule interactions rather than molecule-substrate interactions. With a higher molecular weight material such as 5 that has hydrophilic side groups, there are not only possibilities for physical entanglement but also possibilities for forming inter- and intrachain bonding interactions. Physical entanglement will naturally lead to an increased likelihood for vertical growth, since this
will provide a kinetic barrier to the mobility that might have otherwise allowed the molecules to align themselves with the substrate in a particular way. In addition, because these molecules can form hydrogen bonds with themselves, there is not as strong a preference to form hydrogen bonds with the substrate. In summary, it is clear that details of supramolecular organization are important in the patterning behavior exhibited by 1 when deposited on the substrate studied. The observed self-patterning behavior may also be relevant to device fabrication on templating substrates. In this context, we tested phenolic 1 with the patterning technique known as microcontact printing.26 A microcontact printing scheme was used to stamp oxidized silicon substrates prepared as described in the Materials and Methods. Monolayers of OTS were stamped onto the substrate to create a pattern with hydrophilic and hydrophobic contrast. The wafer was then sectioned and exposed to a relatively concentrated solution of 1 (0.5 wt %) using the same dipping method as described earlier. After the wafer was removed from the solution, the material remained adsorbed on the hydrophilic portions of the pattern but completely dewetted from the hydrophobic OTS-coated regions. An example depicting this is shown in the AFM image of Figure 15. This is a 40-µm image showing 1 surrounded by an OTS matrix. The height of the feature is 260 µm, which corresponds to about four layers of nanostructures. Additional experiments have
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to distinct surface patterns at submonolayer coverage. These two-dimensional patterns are always one nanostructure thick and have geometrically well-defined features. As a function of surface coverage on oxidized silicon, they evolve from regular islands to string-like features of regular width and finally to a honeycomb morphology, which is the inverse of the islands. The distinct geometries are possibly the result of strong material-substrate interactions balanced by a repulsive force that could have electrostatic origin. The extremely uniform thickness of the two-dimensional patterns may originate in the hydrophobic and hydrophilic nature of opposite poles of the nanostructures, thus suppressing vertical growth of the film.
Figure 15. 40 × 40 µm AFM image depicting a multilayer film of islands deposited on a substrate that was prepared by microcontact printing to create a substrate with hydrophilic/ hydrophobic contrast. 1 adheres to the hydrophilic parts of the substrate (exposed silicon oxide) but dewets from the hydrophobic regions of the substrate (OTS monolayer).
shown that by dipping nanostructures in solutions of increasing concentration one can create structures with increasing height above the substrate. Conclusions Polar supramolecular nanostructures that interact strongly with substrates only in one orientation give rise
Acknowledgment. This work was supported by a grant from the Office of Naval Research (Grant N0001496-1-0515), and the Beckman Institute at the University of Illinois at Urbana-Champaign provided a postdoctoral fellowship to J.G. The authors are also grateful to Dr. Kyle Gresham, Dr. Eugene Zubarev, and Ken Walker for the synthesis of materials used in this study and to Dr. Martin Pralle for assistance with contact-angle measurements. The authors also acknowledge Martin Erhardt and Dr. Ralph Nuzzo at the University of Illinois at UrbanaChampaign for assistance with microcontact printing procedures. LA0102511
