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Growth Mechanisms of Octadecylphosphonic Acid Self-Assembled Monolayers on Sapphire (Corundum): Evidence for a Quasi-equilibrium Triple Point Christian Messerschmidt and Daniel K. Schwartz*,† Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received September 5, 2000. In Final Form: October 26, 2000 Self-assembled monolayer growth of octadecylphosphonic acid on single-crystal C-face (1000) and R-face (11 h 02) sapphire (corundum) has been investigated by ex situ tapping-mode atomic force microscopy, contact angle measurements, and Fourier transform infrared spectroscopy. The process of film formation is highly dependent on the temperature, a phenomenon that is explained by a three-phase model for film deposition. At low temperature, close-packed molecular islands nucleate and grow, surrounded by virtually bare substrate, analogous to a two-dimensional (2D) vapor-to-solid transition. At high temperature, on the other hand, the growth involves a 2D liquid-to-solid transition. Different growth kinetics are observed for the two crystal orientations, which can be related to the surface energetics.
Introduction Self-assembled monolayers (SAMs) have been extensively studied in recent years1 due to their applications as protection against corrosion, modifying surface adsorption, coupling of photosensitizers to semiconductors, and immobilization of receptors for molecular recognition in biology and chemistry. To realize these applications, a thorough understanding of SAM growth is needed. It would be desirable, for example, to predict and control the number of defects created under various growth conditions. In earlier publications2-6 we established the growth of an octadecylphosphonic acid (OPA) monolayer on mica as a model system. In this system, atomic force microscopy (AFM) observations showed that growth proceeded via nucleation, growth, coalescence, etc., of densely packed molecular aggregates (islands). Infrared spectroscopy and contact angle measurements suggested that the regions surrounding these islands were essentially bare substrate or contained only a very small density of adsorbed OPA molecules.3,7,8 In the present study we extended our OPA SAM studies to a new substrate. It is hoped that the comparison with the OPA/mica system will lead to a better understanding of how adsorbate-substrate interactions influence the growth mechanisms and kinetics. Sapphire (corundum), the undoped single crystal of R-Al2O3, is widely used as a substrate for thin-film deposition of metals, semiconductors, or insulators.9 Several crystallographic faces of * To whom correspondence should be addressed. Tel.: 504-8623562; fax: 504-865-5596; e-mail:
[email protected]. † Present address: Department of Chemical Engineering, University of Colorado, Boulder, CO 80309; e-mail daniel.schwartz@ colorado.edu. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (3) Woodward, J. T.; Doudevski, I.; Sikes, H. D.; Schwartz, D. K. J. Phys. Chem. B 1997, 101, 7535. (4) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861. (5) Doudevski, I.; Hayes, W. A.; Schwartz, D. K. Phys. Rev. Lett. 1998, 81, 4927. (6) Doudevski, I.; Schwartz, D. K. Phys. Rev. B 1999, 60, 14. (7) Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913. (8) Doudevski, I.; Hayes, W. A.; Woodward, J. T.; Schwartz, D. K. Colloids Surf. 2000, 174, 233.
sapphire are available, two of which have been studied here, namely, C-face (1000) and R-face (11h 02). After annealing, the substrate’s atomically flat terraces become accessible, separated by parallel step edges. This substrate, therefore, provides the opportunity of observing SAM growth under the influence of two parameters: the different crystal orientations and the step edges vs the terraces. In the most simplified view, the different surface sites that are available can be assigned varying values of interfacial free energy. For example, R-face is generally thought to have lower interfacial free energy than C-face;10-13 one expects step-edge sites to be associated with even higher energy. Experimental Section R-face sapphire was obtained from Union Carbide (Danbury, CT) and C-face sapphire from Bicron (Washougal, WA) as polished wafers. The wafers were divided into approximately square pieces (about 12 mm on a side) by a diamond scribe and cleaned by piranha solution [3:1 sulfuric acid/hydrogen peroxide (30%)] for 30 min at 80 °C. After thorough rinsing with water from a Millipore Milli-Q UV+ (Bedford, MA) system, the samples were blown dry with nitrogen and heated to 1300 °C in a furnace (Thermolyne 46100) for various periods of time. Prior to use they were UV-oxygen-cleaned (Boekel Industries) for 30 min to minimize the effect of adsorbed contaminants. The synthesis of OPA was described previously.2 Tetrahydrofuran (99.9%; Fisher Scientific, Pittsburgh, PA) was used as a solvent for SAM deposition. SAMs were prepared from 0.050.2 mM solutions. Solutions for deposition at temperatures lower than room temperature were immersed in a Fisher Scientific 9101 (Pittsburgh, PA) cooling bath and allowed to equilibrate at the desired temperature for 30 min. The substrates themselves were cooled for the same amount of time in an empty glass vial immersed in the same water bath. Millipore water and hexadecane (99%, Aldrich) were used for contact angle measurements. The samples were imaged with a Nanoscope III MMAFM (Digital Instruments, Santa Barbara, CA) in tapping mode under ambient conditions. Silicon tips (Nanosensors, Wetzlar, Germany) with a spring constant of 45-60 N/m and a resonance frequency (9) Dorre, E. Alumina: Processing Properties and Applications; Springer-Verlag: New York, 1984. (10) Guo, J.; Ellis, E.; Lam, D. J. Phys. Rev. B 1992, 45, 13647. (11) Gignac, W. J.; Williams, R. S.; Kowalczyk, S. P. Phys. Rev. B 1985, 32, 1237. (12) Ciraci, S.; Batra, I. P. Phys. Rev. B 1983, 28, 982. (13) Wiederhorn, S. M. J. Am. Ceram. Soc. 1969, 37, 42.
10.1021/la001266m CCC: $20.00 © 2001 American Chemical Society Published on Web 12/22/2000
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Figure 1. AFM images (1 µm × 1 µm) showing the evolution of the structure of OPA monolayers formed on C-sapphire substrates at room temperature. The substrates were removed from 0.05 mM solution (with THF as a solvent) after immersion times indicated by the annotation. The cross-section height plot (lower right) shows typical feature heights along the line indicated in the image at lower left. in the range of 270-350 kHz were used. The scanning rate was 1.5 Hz. Height and phase images were recorded simultaneously under light tapping conditions (rsp > 0.9).14 Contact angles were measured using a custom-built contact angle goniometer following a procedure described by Bain et al.15 in which a drop was formed at the end of the needle and brought into contact with the surface. The needle was removed and the contact angle was measured. Results were reproducible within (2°. Fourier transform infrared spectroscopy (FTIR) was performed with a Mattson Galaxy spectrophotometer in transmission mode. The sample spectra were normalized against a background of bare sapphire. Periodic interference fringes in the transmitted spectra were removed via Fourier filtering with the Igor software package.
Results The sapphire surface is known to rearrange in the temperature range 1000-1400 °C.10,16 To obtain parallel terraces of relatively uniform width, we found that annealing at 1300 °C for 5 h gave the best results. For lower annealing temperatures, terraces were less uniform, and at 1350 °C some terraces narrowed and eventually merged. We measured step heights consistent with reported values16 for C-sapphire (0.22 ( 0.02 nm) and for (14) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, 385. (15) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (16) Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Ishiyama, O.; Shinohara, M.; Kubo, M.; Miura, R.; Miyamoto, A. Appl. Phys. Lett. 1995, 67, 2615.
R-sapphire (0.36 ( 0.03 nm). Heating to temperatures higher than 1350 °C resulted in greater step heights, corresponding to previous reports.16 In addition, at these higher annealing temperatures, the terrace width was less reproducible. SAMs of OPA were prepared at room temperature on both C- and R-sapphire substrates. The water contact angle was 90° after short immersion times at low concentrations (e.g., 1 min for 0.05 mM solutions). This is in contrast to the OPA monolayers on mica,2,3 where similar contact angles at this concentration were reached only after several hours of immersion. Although the contact angle implied the presence of a surface film, no distinct topographical features were initially observed on the samples (Figures 1 and 2) by AFM. After longer immersion times, higher structures with poorly defined boundaries began to appear (see Figures 1 and 2). With increasing immersion time, these features covered a larger fraction of the surface. In addition, the characteristic heights of the features gradually increased with immersion time but never reached a height consistent with a fully extended OPA molecule (about 2 nm). The tallest features observed were only 0.8 ( 0.06 nm (C-sapphire) or 0.55 ( 0.09 nm (R-sapphire) higher than the surrounding regions (Figure 3a). The water contact angle stayed at 92° within experimental uncertainty regardless of immersion time (after 1 min). The hexadecane contact angle, however, rose from 31° to 36° as the higher structures evolved
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Figure 2. AFM images (1 µm × 1 µm) showing the evolution of the structure of OPA monolayers formed on R-sapphire substrates at room temperature. The substrates were removed from 0.05 mM solution (with THF as a solvent) after immersion times indicated by the annotation.
Figure 3. Time evolution of OPA monolayers grown on C- and R-sapphire from 0.05 mM THF solution and removed after specified immersion times. (a) Typical feature heights. (b) Hexadecane contact angles.
(Figure 3b). Growth of the high structures was slower on R-sapphire, where the hexadecane contact angle reached 36° after 1 h in 0.05 mM solution as opposed to about 30 min for C-sapphire (Figure 3b).
The growth process at lower temperature was qualitatively different. Figure 4 shows the growth of an OPA monolayer on C-sapphire at 2 °C. At this temperature the initial stages of monolayer growth were marked by nucleation of distinct molecular islands with well-defined boundaries. Water contact angles for these samples were much lower than the samples covered by the continuous phase that formed initially at higher temperature (e.g., only 45° after 30 min; see Figure 4b). Gradual filling-in after longer immersion times increased the contact angle, similar to the results obtained on mica2,3 (Figure 4b). The heights of these islands (approximately 1.1 ( 0.3 nm) were somewhat greater than those of the features on the films grown at room temperature but still significantly less than the length of a fully extended OPA molecule (about 2.5 nm). This suggests substantial molecular tilt (probably due to the large phosphonate headgroup size) and is consistent with our previous observations of OPA SAM thickness on mica.2,3 The surface coverage continued to increase, but was not yet complete, up to the longest immersions we performed, 15.5 h. FTIR spectra, in the C-H stretch region, supported the AFM and contact angle results. The positions of the methylene stretch peaks, on films grown at room temperature, gradually shifted to lower wavenumbers for longer immersion times (Figure 5a). For example, the antisymmetric methylene stretch peak was centered at 2922 cm-1 (indicating significant alkyl chain disorder) for a film immersed for 1 min but gradually moved to 2916 cm-1 (consistent with an all-trans alkyl chain) after about 1 h. This trend suggested a conversion of disordered alkyl chains to ordered chains during film growth. Samples grown at 2 °C, however, did not show such a shift. The antisymmetric methylene stretch peak was centered at 2917 cm-1 after 10 min of immersion and remained at this value for longer immersion times (Figure 5b). This implied that, during all stages of growth at low temperatures, all alkyl chains were well-ordered, consistent with discrete islands of upright standing molecules surrounded by essentially bare sapphire.
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Figure 5. Transmission FTIR spectra (C-H stretch region) of OPA monolayers grown on C-sapphire by immersion in 0.05 mM THF solution for the times indicated. (a) Films prepared at room temperature. The methylene stretch peak gradually shifts to lower wavenumbers, indicating an increase in alkyl chain order during film growth. (b) The peaks remain in the same position, consistent with well-ordered alkyl chains, throughout film growth.
Figure 4. AFM images (1 µm × 1 µm) showing the evolution of the structure of OPA monolayers formed on C-sapphire substrates at 2 °C. The substrates were removed from 0.05 mM solution (with THF as a solvent) after immersion times indicated by the annotation. The graph at lower right shows the evolution of the water contact angle with time on samples prepared under the same conditions.
At intermediate temperatures there was a regime where both growth modes were present simultaneously at different surface locations. Panels a and b of Figure 6 show a C-sapphire sample, prepared at 15 °C. The topographic (height) image (Figure 6a) shows regions displaying distinct island growth (e.g., the region marked L) coexisting with flat continuous regions (marked H) at a height intermediate between the islands and the bare areas immediately adjacent to the islands. Since a thin film of disordered molecules should have dramatically different material properties than bare sapphire (in particular, it should be “softer”), one would expect that phase imaging should provide additional information. According to the theory of phase imaging,14 harder parts of the sample have greater phase shifts, under the light tapping conditions employed here, and thus appear lighter in color. This has been verified for polymers,14 mixed Langmuir-Blodgett films,17 and bilayers of amphiphiles.18 In Figure 6b there is a smaller phase shift (consistent with a softer film) for the parts of the sample covered by the continuous phase (meaning they appear dark in the image) than for the low regions surrounding the islands. (17) Messerschmidt, C.; Schulz, A.; Rabe, J. P.; Simon, A.; Marti, O.; Fuhrhop, J.-H. Langmuir 2000, 16, 1299. (18) Messerschmidt, C.; Svenson, S.; Stocker, W.; Fuhrhop, J.-H. Langmuir 2000, 16, 7445-7448.
These samples permit a direct comparison of the film properties during the two types of growth and confirm that the early stages of growth at higher temperature involve a soft, thin layer of adsorbate molecules that completely covers the substrate, while the island growth at lower temperatures leaves large parts of the sapphire between molecular islands essentially bare to the solution. It was difficult to determine a precise range over which the two growth mechanisms occurred simultaneously; it appeared to be somewhat substrate-dependent. For example, a subtle effect of step edges was revealed. In Figure 6c one can detect a preference for island growth on the terraces while the step edges remain covered by the continuous phase. As a consequence, less regular substrates with wider step edges showed the two-phase behavior at higher temperatures than very regular substrates with narrower terraces. Discussion The two types of growth observed here are closely related to recent observations in other SAM systems. The OPA/ mica system was shown to form via island nucleation and growth on an otherwise essentially bare substrate.2-6 The current observations of OPA growth on sapphire at low temperatures are qualitatively consistent with these experiments. The initial stages of octadecyltrimethylammonium bromide SAM growth on mica from aqueous solution, on the other hand, involved a thin continuous layer within which the molecules had substantial disorder.7,8 Only later did densely packed islands of wellordered, vertically oriented molecules nucleate and grow from this disordered phase. This description can be applied to the current observations of OPA growth on sapphire at room temperature. Related behavior has been observed for alkyltrichlorosilane SAMs. Growth modes involving a continuous dis-
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Figure 6. AFM images of OPA monolayer prepared at 15 °C. (a) Topographic image showing the coexistence of both growth modes at this temperature. The region marked H is representative of the high-temperature continuous liquid-phase film, while that marked L shows island growth characteristic of low temperatures. (b) Phase contrast image of the same sample region shown in panel a. (c) Topographic image showing the preference of the continuous liquid phase for step edges.
ordered layer or discrete well-ordered islands have been observed, depending on deposition conditions (temperature, water content, etc.).19-22 The suggestion has been made that self-assembled monolayers can be compared to Langmuir monolayers in terms of their phase behavior at different temperatures.19,20 For octadecyltrichlorosilanes this meant that raising the temperature to 40 °C resulted in a continuous “liquid-expanded” layer20 as opposed to the fractal island growth commonly observed at room temperature.23 For the silane system, the analogy to Langmuir monolayers was intended to be quite literal. The existence of a thin water layer on the substrate was postulated upon which the adsorbate molecules organized into the appropriate phase(s) prior to immobilization via cross-linking (polymerization). We interpret our results in a way similar to the Langmuir monolayer analogy mentioned above.19,20 However, since there is no polymerization involved in our system, we suggest a generalized 2D phase diagram containing solid, liquid, and vapor phases with temperature and adsorbate surface concentration as the axes (Figure 7). If equilibration of the 2D structure occurs on a time scale that is rapid compared to deposition from solution, the partial monolayer structure during growth will be consistent with an isotherm on the phase diagrams a vertical line in Figure 7. We refer to such a process as quasi-equilibrium growth. If the isotherm line is drawn to the right of the triple point, the monolayer passes during growth through the liquid phase (continuous phase), a liquid/solid coexistence region, and finally the solid phase. Lowering the temperature below the triple point, however, avoids the liquid phase and leads to solid-phase growth directly from the 2D vapor. Samples prepared at intermediate temperature that displayed both growth mechanisms simultaneously would be consistent with film growth at (or very near) the triple point. Since it is possible that the ultimate macroscopic properties of the film (e.g., defect density) may depend on the growth mode, this picture implies that growth conditions should be selected in order to optimize these properties. This picture is certainly simplistic, however, and does not satisfactorily explain all aspects of the observations. For example, in quasi-equilibrium 2D liquid-to-solid (19) Parikh, A. N.; Allara, D. L.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (20) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441. (21) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190. (22) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H.; Basnar, B.; Vallant, M.; Friedbacher, G. Langmuir 1999, 15, 1899. (23) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354.
Figure 7. Schematic quasi-equilibrium phase diagram consistent with the two growth modes observed at 2 °C and room temperature (about 22 °C), assuming the existence of a triple point between these temperatures.
coexistence, the two phases should each remain at a constant density throughout the transition. This is inconsistent with the time evolution of the apparent feature heights (presumably the thickness difference between liquid- and solid-phase films) shown in Figure 1. A possible explanation for this lies in the fact that the growth is expected to be quasi-equilibrium only if the deposition rate is much slower than the rates of all surface processes (nucleation and growth of solid-phase domains, for example). If this condition is not met, the 2D liquid phase would become “superconcentrated” (i.e., it would have a density greater than the equilibrium coexistence concentration), and thus its density (and therefore thickness) could vary considerably during the growth of the solid phase. This deviation from quasi-equilibrium growth is also a possible explanation for the range of temperatures over which both growth mechanisms are observed on the same sample. Under quasi-equilibrium conditions, of course, three phases would coexist only at one temperature. As suggested above, a variety of surface sites having a range of energies could also be part of the explanation. Calculations by Guo et al.10 revealed that the surfaces with the lowest cleaving energies for C- and R-sapphire are chemically different. While C-sapphire is terminated with an Al layer, R-sapphire has an oxygen layer on its surface. In addition, the C-face surface is generally found to have a higher interfacial free energy.10-13 These factors may contribute to the faster growth kinetics and taller features on C surfaces. If, in fact, the adsorbate molecules are organized in a particular epitaxial arrangement
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relative to the substrate atoms, the in-plane density of the SAM (and hence the film thickness) would be influenced by the unit cell areas of C-face (19.64 Å2) vs R-face (22.42 Å2). This could also explain the taller features on C-sapphire. If we put our previous results on OPA growth on mica into the context of the present observations, it is clear that the OPA/mica system is below the triple point even at room temperature. This suggests that the triple point may be influenced by substrate surface energy or the binding strength of adsorbate to substrate. This is consistent with the frequent observation of the 2D liquid phase in proximity to the higher energy step-edge region even at temperatures for which the low-temperature growth mode is observed on terraces. This would also predict that the triple point of the lower energy R orientation should be at a somewhat higher temperature than that of the C orientation if we were able to measure it with sufficient accuracy. As a word of caution, we note that the present experiments were performed ex situ on partial SAMs whose growth was quenched by removal from solution. There should always be concern that the process of quenching may introduce changes in the partial monolayer structure. In our previous studies of OPA SAMs on mica, the morphology of quenched films3 was qualitatively similar to that of SAMs grown in situ;5 i.e., isolated islands of close-packed molecules. However, the quantitative details of coverage and island size distribution were altered by
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the quenching process. In future experiments, we will study OPA SAM growth on sapphire in situ to explore this issue. Conclusions OPA monolayer growth on sapphire at different temperatures proceeds via different mechanisms. At room temperature, a continuous 2D phase of disordered molecules is formed initially and later evolves to a thicker, more ordered film via the gradual growth of higher structures. At lower temperatures, the monolayer forms by nucleation and growth of islands in which the molecules are close-packed and vertically oriented. These results were interpreted in terms of a simple phase diagram model where the transition temperature represents a triple point. The growth appears to be biased toward the higher temperature mode in the vicinity of substrate step edges. By choosing the temperature accordingly, one may be able to control the defect density in the final monolayer film. Acknowledgment. This work was supported by the National Science Foundation (award CHE-9980250) and the Camille Dreyfus Teacher-Scholar Awards Program. C.M. acknowledges support from a grant by the Federal and Regional Special Program III for Universities awarded by the DAAD. LA001266M
