J. Phys. Chem. 1996, 100, 9093-9097
9093
Pattern Formation in a Substrate-Induced Phase Transition during Langmuir-Blodgett Transfer H. D. Sikes, J. T. Woodward, IV, and D. K. Schwartz* Department of Chemistry, Tulane UniVersity, New Orleans, Lousiana 70118 ReceiVed: March 4, 1996X
An atomic force microscope study of Langmuir-Blodgett films deposited from the liquid expanded phase was undertaken to explore self-organizing methods of creating patterned surfaces. Although tetradecanoic (TDA), pentadecanoic (PDA), and hexadecanoic acid (HDA) monolayers deposited on silicon oxide at pH ) 5.8 are uniform flat films, when the same monolayers are deposited on silicon oxide at pH ) 9.3 or on mica substrates, the molecules organize into close-packed “islands”, indicative of a two-dimensional condensation. The substrate dependence of this islanding behavior suggests that surface charge plays an important role. Small islands have compact morphology, while large islands are dendritic. The length scale for the fingering instability changes by 2 orders of magnitude between TDA and HDA, from 60 nm to 6 µm.
Introduction Self-organizing ultrathin organic films like self-assembled monolayers and Langmuir-Blodgett (LB) films have been studied extensively over the last decade because they illustrate fundamental properties of molecular organization and of twodimensional statistical mechanics,1 but also because they represent an extremely versatile method of modifying the physical and chemical properties of surfaces. Certainly, such modification can be important in applications like corrosion resistance and adhesion promotion. However, some of the more ambitious and exciting applications of these films, chemical and bio-sensing, affinity chromatography, etc., rely on lateral patterning of the surface. One strategy for patterning involves using established lithographic techniques to pattern the films directly2,3 or to create “stamps” that can then be employed to deposit molecules in an ordered pattern.4 We are especially interested in utilizing the self-organizing properties of the amphiphilic molecules in these films to create patterned surfaces. Similar concepts have been employed successfully in threedimensional applications involving micelles and microemulsions, where the subtle competition between intermolecular interactions results in structures with well-defined length scales that can be tailored from a few nanometers to several micrometers.5 We report here an example of two-dimensional pattern formation in which small changes in molecular length have dramatic effects on the length scale of the pattern. The LB technique involves transferring a monolayer of amphiphilic molecules from the water surface by passing a solid substrate through that surface.6 A multilayer film can be created layer-by-layer; we are concerned here with the first monolayer only. Generally, the molecules on the water surface are compressed into a relatively close-packed liquid condensed (LC), or solid, state before the transfer is accomplished, and although details of molecular arrangement often differ before and after transfer,7,8 the diffeence in packing density is generally negligible. Riegler and co-workers have performed a series of experiments on two-component monolayers in which phase separation is induced by the deposition process.9-12 This suggests that a similar process might be possible even in a onecomponent system. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
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A number of authors have noted that the structure of a deposited monolayer can depend strongly on the choice of substrate. Nearly 30 years ago Spink reported that stearic acid monolayers were smooth and stable when deposited on mica and various metals but rearranged when deposited on silica or glass.13 The dendritic morphology of rearrangement has been observed by AFM in fatty acid salt LB films left underwater for prolonged periods.14,15 AFM has also demonstrated that the molecular arrangement16-18 and surface morphology19 in fatty acid salt LB monolayers and multilayers can be distinctly different on silicon oxide and mica substrates. Fatty acid molecules can even be induced to forsake their usual nearly vertical orientation and lie down when deposited on graphite.20 There are a few reports of monolayers deposited from a dilute state on the water surface, such as the so-called liquid expanded (LE) phase,21-24 or coexistence between LE and LC phases. Molecular aggregates have been reported; however, the aggregates have been alternately suggested to be pre-existing on the water surface23 or created during transfer due to either a dewetting process24 or a transfer-induced phase transition.21 The situation is complicated by the fact that the monolayers used in some of these studies contained polymeric counterions23 or consisted of molecules undergoing polymerization.24 We have, therefore, investigated this phenomenon using fatty acid monolayers, in which the two-dimensional phase diagram is relatively well understood and the dynamics are not complicated by existence of a polymeric constituent. Experimental Details Tetradecanoic (TDA), pentadecanoic (PDA), or hexadecanoic (HDA) acid was spread from chloroform solution to an area of 55 Å2/mol onto a pure water surface (Millipore Milli-Q UV+) contained in a NIMA LB trough held at constant temperature (0.5 °C. For some samples the pH was adjusted by addition of NaOH. The films were transferred to solid substrates by vertical dipping, while the monolayer was held at constant surface pressure (π) in the LE phase. The dipping speed was varied in the range 1-25 mm/min and π from 2 to 8 mN/m. Representative π-A isotherms of the three substances are shown in Figure 1. Polished silicon oxide (SiO2) substrates were cleaned by immersion in a mixture of H2O2/H2SO4 (3:7) held at 90 °C for 1 h. Mica substrates were freshly cleaved immediately before use. Due to the nonzero solubility of these © 1996 American Chemical Society
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Figure 1. Isotherms of fatty acids on water. The LE phase corresponds to the gradually sloping region to the right of the kink. The “plateau” region to the left of the kink represents the coexistence region between the LE phase (two-dimensional liquid) and the more condensed LC phase at smaller area/molecule. Typically, LB films are deposited in the LC phase at high surface pressures; the films in this study were deposited in the LE phase or in the LE/LC coexistence region.
relatively short-chain amphiphiles, we measured the dissolution rate for 10-15 min prior to dipping in order to adjust the transfer ratios. Transfer ratios were typically 70 ( 10% on SiO2 at pH ) 5.8 and 100 ( 10% on mica. Imaging was performed using a Nanoscope III atomic force microscope (AFM) under ambient conditions using a 15 µm × 15 µm or a 150 µm × 150 µm scan head and a silicon nitride tip on an integral cantilever with spring constant 0.12 N/m in contact mode or a silicon tip on a “diving board” cantilever in “tapping mode”. Images were obtained from at least five macroscopically separated areas on each of 32 individual samples; at least two independent samples of each type (i.e., combination of chain length, substrate, dipping speed, surface pressure, and temperature) were prepared and imaged. Representative images are presented below. Results The surface of monolayers of any of the three substances deposited on SiO2 at pH ) 5.8 appeared flat and featureless with the AFM (Figure 2a); the surface roughness is typical of the polished substrate. We attempted to increase the force on the AFM tip to dig through the monolayer in order to measure an approximate film thickness. We were successful in digging such holes; however, we were not able to extract meaningful quantitative information on film thickness. Quite often, the digging procedure resulted in contaminating the tip, which could not be corrected without replacing the tip. Other attempts resulted in clear holes; however, a lot of material piled up around the hole and the uncertainty in the measured depth of the hole was very large. This behavior does, however, suggest the existence of a delicate, loosely bound organic layer on the surface, consistent with the transfer ratio measured during deposition. It is not consistent with control experiments on bare silicon wafers. Monolayers deposited on SiO2 at pH ) 9.3 (Figure 2b), or on mica at pH ) 5.8 (Figure 3), on the other hand, were composed of high islands surrounded by a lower background. The islands were about 2 nm higher than the surrounding area. This height is consistent with approximately fully extended molecules, suggesting a local in-plane molecular area of about 20 Å2/molecule, which is about half of the average molecular area of the monolayer on the water surface (see Figure 1). However, occasionally (15% of the time) films of PDA on mica were composed of high islands immediately surrounded by low “moats” which were themselves surrounded by a region of intermediate height (Figure 4). In these cases, the height
Figure 2. AFM images of hexadecanoic acid deposited on SiO2 at 2 mm/min. (a) At pH ) 5.8 the film is essentially flat and featureless. Small variations in height are consistent with features typically present on the bare polished silicon wafers. We were unable to obtain reliable images on monolayers deposited on SiO2 at higher dipping speeds (i.e. 6 mm/min) since they were easily damaged during imaging. (b) At pH ) 9.3, islands 1.5-2 nm high are observed.
difference from the moat bottom to island top was typically about 2 nm and the height from the moat bottom to the intermediate region top was e1 nm. Detailed studies of island morphology were carried out on mica substrates. The morphology of the islands was divided into two regimes based on island size. Small islands were essentially compact, while larger islands were dendritic and/or porous (Figure 5a-c). No significant differences were observed with a change in dipping speed. Monolayers deposited at different surface pressures had overall differences in surface coverage consistent with the variation in surface density at the water surface; the length scale of the fingers did not change significantly with surface pressure (over the range 2-8 mN/m) or temperature (over the range 22-28 °C for PDA). At sufficiently high temperature (>30 °C for PDA) only small compact aggregates were observed. We attempted to determine a length scale associated with the branching by measuring the width of dendritic fingers. The average widths, based on measurements of approximately 10 fingers/island on 9 islands for PDA and HDA and 4 islands for TDA, were 0.060 ( 0.015 µm (TDA), 2.1 ( 0.8 µm (PDA,), and 5.7 ( 1.2 µm (HDA). Monolayers deposited in the LE/LC coexistence region had large areas consistent with the LC domains observed with fluorescence or Brewster angle microscopy:25 mostly round with a cusplike singularity (Figure 6). However, dendritic aggregates appeared
Pattern Formation in Substrate-Induced Phase Transition
Figure 3. (a) AFM image of pentadecanoic (PDA) acid monolayer deposited on mica. The islands are approximately 2 nm high, consistent with the length of a fully extended molecule. The islands cover about 50% of the surface. (b) A height map across the line marked on part a showing islands and background separated by about 2.0 nm in height.
between these domains, and branched protrusions were observed growing from the domains themselves. Discussion We believe that these results firmly establish the cause of the aggregation or islanding to be a two-dimensional condensation phase transition that occurs during LB transfer, as suggested by Mikrut et al.21 Such substrate-mediated condensation has been observed previously in two-component systems9-12 and is consistent with differences in the IR spectrum of monolayers before and after LB transfer.22 Certainly, it is clear that the large dendritic islands in HDA or PDA are not present in the monolayer before transfer, since they are not observed with fluorescence or Brewster angle microscopy.25,26 The systematic trend of length scale with chain length suggests that the aggregates in TDA are formed via the same mechanism as the other substances. Furthermore, if aggregates already existed on the water surface before transfer, why would they be transferred to some substrates and not to others? We, therefore, conclude that these aggregates are not present in the pretransfer Langmuir monolayer. An argument has also been made that aggregates could form as a result of the natural dewetting of the substrate due to drainage or evaporation. However, this does not explain the dramatic change in length scale due to chain length nor the difference between SiO2 and mica substrate. In fact, similar aggregation has been observed21 on silicon oxide and glass at pH ) 5.5 using a phosphatidic acid, whereas we did not observe aggregation of carboxylic acids on silicon oxide at a similar pH. This suggests that the aggregation phenomenon
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Figure 4. AFM image of pentadecanoic acid monolayer deposited on mica. This type of surface profile occurred approximately 15% of the time. The islands are approximately 2 nm higher than the bottom of the low “moats” surrounding the islands. The intermediate height areas surrounding the moats are e1 nm higher than the moat bottom. (b) A height map across the line marked on part a showing the three height levels in the image.
is sensitive to the particular head-group-substrate interaction and is not simply due to a difference in the dewetting process between silicon oxide and mica. Furthermore, dendritic dewetting morphology has not been observed27,28 nor is it theoretically expected.29 Although the equilibrium monolayer phase on an infinitely thick water subphase is the LE (two-dimensional liquid) under deposition conditions, as the water layer thins during drainage, long-range interactions between the substrate and the surfactant molecules (e.g. electrostatic, van der Waals) perturb the free energy of the monolayer. Since the monolayer is at a density near the onset of a condensation transition (Figure 1), a small perturbation could effect a similar transition on the substrate. We, therefore, postulate that the equilibrium arrangement of the fatty acid molecules on a substrate with a significant negative surface charge (such as mica or SiO2 at pH g 930) is relatively close-packed with the chains fully extended and perpendicular to the surface. It is reasonable that the preferred molecular arrangement on the approximately uncharged SiO2 surface at pH ) 5.8 is similar to that on the water surface (loosely packed, two-dimensional liquid). Although the most dramatic change caused by the change in pH from 5.8 to 9.3 is the increase in surface charge on the silicon oxide substrate,30 there is also a significant change in the head-group charge. At pH ) 5.8 the fatty acids are approximately 75% ionized, and they become completely ionized at pH g 6.2.31,32 Therefore, the increased negative charge on the substrate and the monolayer may both be factors in causing the aggregation to occur at higher pH. Because the pKa’s of silica and fatty acids are fairly close in
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Figure 6. AFM image of a hexadecanoic acid monolayer deposited in the LE/LC coexistence region, 32 °C, 4 mN/m. Large rounded domains with a cusplike singularity are consistent with domain morphology observed in the coexistence region by fluorescence and Brewster angle microscopy. Dendritic domains and protrusions of approximately the same height as the rounded domains are also visible.
Figure 5. AFM images of fatty acid LB monolayers on mica. (a) Tetradecanoic acid deposited at 22 °C, 4 mN/m. The length scale of the dendritic structure is about 60 nm. (b) Pentadecanoic acid deposited at 22 °C, 2 mN/m. The length scale of the dendritic structure is about 2 µm. (c) Hexadecanoic acid deposited at 32 °C, 3 mN/m. The length scale of the dendritic structure is about 6 µm. Smaller compact domains are also visible.
value, experiments would have to be performed with a different oxide substrate or a different head-group in order to separate these two effects. The details of LB monolayer structure have been previously shown to be substrate dependent. For example, a fatty acid monolayer deposited from the condensed L2 phase on glass has an untilted hexagonal structure,7 while a monolayer deposited from the same phase on Formvar has a centered rectangular structure.8 However, this is the first observation of such a
dramatic structural dependence (a factor of 2 in in-plane density) on substrate. Also, the pronounced dependence on pH provides an important clue as to which properties of the substrate are important in determining film structure. The observed morphology can be explained as a consequence of the kinetic processes of nucleation and growth. When the water layer thins sufficiently, a domain of the two-dimensional condensed phase nucleates and attaches to the substrate. The nucleus grows as LE phase molecules are transported toward the domain until no molecules remain in a two-dimensional liquid state. The resulting monolayer should appear as in Figure 3: islands with a height corresponding to a fully extended molecule. Since the molecular area in such an island should be about half that of the average molecular area in the monolayer, the coverage should be about 50%, as we observe. We interpret the type of monolayer shown in Figure 4 as one in which the growth process was interrupted. In other words, the mobility of the molecules in the LE phase was lost before they were all transported to the growing solid domains, perhaps because of complete drying of the thin water film. The region of intermediate height surrounding the moats is consistent with the density of the LE phase. The existence of the moats suggests that during the growth process there is a depletion region surrounding each growing domain. The dendritic morphology of the islands has a strong resemblance to structures that result from instabilities during solidification,33 e.g. the Mullins and Sekerka instability. Generally, interfacial tension inhibits the instability for short perturbation wavelengths, and there is a critical wavelength for unstable growth that involves a competition between interfacial free energy and the driving force for growth. Thus, in a sample of growing pieces of solid phase it is typical to see small compact shapes and large dendritic shapes, exactly as we see in the monolayers on mica. The change in length scale with increasing chain length may be due to an increase in the line tension of the condensed phase domain with chain length. We suggest that the driving force for the instability is due to a surface tension gradient. When the initial nucleus is formed, there is a depletion of surfactant molecules in the liquid phase surrounding the nucleus (because the molecules in the solid phase take up only half the area). Molecules are transported toward the nucleus because of a surface tension gradient (Marangoni flow). However, as molecules reach the domain they are converted to
Pattern Formation in Substrate-Induced Phase Transition a solid phase, and a steady-state surface tension gradient is maintained. This gradient is enhanced near a protrusion that occurs as a fluctuation in the shape of a growing domain, and therefore, the compact shape is inherently unstable. Although this explanation is attractively simple, other possible mechanisms could be at work. For example, the hydrodynamics of the exclusion of the water film from beneath the growing domain should be considered. Although the fatty acid monolayers on water are all in the region of their respective phase diagrams, generically labeled the LE phase (and thought to be an isotropic two-dimensional liquid phase), we cannot exclude the possibility that the structural or transport properties might vary from one chain length to another. Such a variation could cause the driving force for the growth instability to change in strength with chain length and, hence, affect the dendritic length scale. However, if we make the assumption that the driving force for the instability is not particularly sensitive to the chain length, the critical length scale for instability should scale as λ1/2, where λ is a line tension. This would imply, however, that the line tension increases by a factor of 10 000 from TDA to HDA (such a dramatic increase would be feasible only near a critical point). Intuitively, we would expect that there should only be a minimal change in line tension that would scale with the chain length. Conclusion We have observed that fatty acid LB monolayers deposited from the LE phase undergo a substrate-induced phase transition on mica and on SiO2 at pH ) 9.3 (where the SiO2 has a negative surface charge) but not on SiO2 at pH ) 5.8 (where the SiO2 is approximately uncharged), in which the area/molecule is reduced by about half. A novel growth instability results in domains of this new phase being dendritic if they are larger than a critical length scale. The length scale of the instability increases dramatically from 60 nm to 6 µm simply by changing the molecular chain length from 14 to 16. This pattern formation illustrates the promise of utilizing self-organizing interactions for lateral patterning. Significant questions remain as to the nature of the growth instability that can result in such a large variation in length scale. Future experiments will systematically explore the temperature and surface pressure dependence of the aggregation. Acknowledgment. The authors acknowledge helpful conversions with Howard Stone and Don Gaver. This work was supported by the Camille and Henry Dreyfus New Faculty Award Program, the Exxon Education Foundation, the Petroleum Research Fund, and the Center for Photoinduced Processes funded by the National Science Foundation and the Louisiana Board of Regents. H.D.S. was supported by a Coca-Cola Foundation fellowship.
J. Phys. Chem., Vol. 100, No. 21, 1996 9097 References and Notes (1) Knobler, C. M.; Desai, R. C. Annu. ReV. Phys. Chem. 1992, 43, 207. (2) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (3) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; et al. Appl. Phys. Lett. 1993, 62, 476. (4) Lopez, G. P.; Biebuyck, H. A.; Frisbie, C. D.; Whitesides, G. M. Science 1993, 260, 647. (5) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (6) Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007. (7) Shih, M. C.; Peng, J. B.; Huang, K. G.; Dutta, P. Langmuir 1993, 9, 776. (8) Steitz, R.; Mitchell, E. E.; Peterson, I. R. Thin Solid Films 1991, 205, 124. (9) Riegler, J. E.; LeGrange, J. D. Phys. ReV. Lett. 1988, 61, 2492. (10) Spratte, K.; Riegler, H. Makromol. Chem., Macromol. Symp. 1991, 46, 113. (11) Riegler, H.; Spratte, K. Thin Solid Films 1992, 210/211, 9. (12) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25, 211. (13) Spink, J. A. J. Colloid Interface Sci. 1967, 23, 9. (14) Schwartz, D. K.; Garnaes, J.; Viswanathan, R.; Zasadzinski, J. A. N. Science 1992, 257, 508. (15) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 10444. (16) Schwartz, D. K.; Viswanathan, R.; Garnaes, J.; Zasadzinski, J. A. N. J. Am. Chem. Soc. 1993, 115, 7374. (17) Viswanathan, R.; Zasadzinski, J. A. N.; Schwartz, D. K. Science 1993, 261, 449. (18) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; et al. Science 1994, 263, 1726. (19) Viswanathan, R.; Schwartz, D. K.; Garnaes, J.; Zasadzinski, J. A. N. Langmuir 1992, 8, 1603. (20) Kuroda, R.; Kishi, E.; Yamano, A.; Hatanake, K.; et al. J. Vac. Sci. Technol. 1991, B9, 1180. (21) Mikrut, J. M.; Dutta, P.; Ketterson, J. B.; MacDonald, R. C. Phys. ReV. B 1993, 48, 14479. (22) Rana, F. R.; Widyati, S.; Gregory, B. W.; Dluhy, R. A. Appl. Spectrosc. 1994, 48, 1196. (23) Chi, L. F.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Thin Solid Films 1994, 242, 151. (24) Fang, J.; Knobler, C. M. J. Phys. Chem. 1995, 99, 10425. (25) Schwartz, D. K.; Tsao, M.-W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 8258. (26) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588. (27) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. ReV. Lett. 1991, 66, 715. (28) Elender, G.; Sackmann, E. J. Phys. II 1994, 4, 455. (29) Brochard-Wyart, F.; deGennes, P. G. AdV. Colloid Interface Sci. 1992, 39, 1. (30) Abendroth, R. P. J. Colloid Interface Sci. 1970, 34, 591. (31) Petrov, J. G.; Kuleff, I.; Platikanov, D. J. Colloid Interface Sci. 1982, 88, 29. (32) Kobayashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1988, 159, 267. (33) Mullins, W. W.; Sekerka, R. F. J. Appl. Phys. 1963, 34, 323.
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