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Mechanisms of Self-Assembled Monolayer Desorption Determined Using in Situ Atomic Force Microscopy Ivo Doudevski and Daniel K. Schwartz* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received June 6, 2000. In Final Form: August 19, 2000 We have observed the dissolution of a self-assembled monolayer of octadecylphosphonic acid from a mica surface into organic solvent in real time using atomic force microscopy. Holes in the monolayer are observed to nucleate, grow, and percolate across the sample, leaving isolated monolayer islands that gradually decrease in size. The relative rates of hole growth and hole nucleation suggest that removing a molecule from the monolayer/hole boundary is about 5 × 104 times more likely than removing a molecule from within a continuous region of monolayer. The rate of dissolution is increased by flowing solvent through the cell compared to stagnant solvent and increased even further by rapid stirring using the AFM tip. The coverage kinetics can be quantitatively described by a model that incorporates desorption from “hole” regions and diffusive solution-phase transport through a stagnant layer of finite thickness. The coverage kinetics and hole nucleation and growth rates are consistent with a picture of monolayer desorption where molecules detach from the hole/monolayer boundaries and remain adsorbed (at a very dilute density) in the “hole” regions, from which they eventually desorb into the solvent. This is essentially the reverse process as was previously observed for monolayer growth.
Introduction The structure and formation process of self-assembled monolayers (SAMs) have been of increasing interest in recent years due to the utility of these films for surface modification and patterning.1,2 SAMs form spontaneously at the solid/solution interface by sequential processes of adsorption and molecular organization. In many cases, the latter process can be described as two-dimensional cluster nucleation, growth, coalescence, and so forth. A variety of processes are at work simultaneously in this “islanding” mechanism, including transport to and from the surface, adsorption at and desorption from the surface, 2D diffusion on the surface, attachment to and detachment from islands, etc. One way to pick apart the influences of each of these processes is to systematically study SAM growth as a function of solution concentration and temperature. In this manuscript we take a slightly different approach by observing the inverse process, SAM desorption into pure solvent or very dilute solution. Under these conditions, the processes of detachment and desorption are emphasized. A SAM is defined by the fact that one end (the “headgroup”) of the adsorbate molecule has a specific attractive interaction with the solid substrate. A common theme in the growth process of all known SAM systems is the clustering of adsorbate molecules into densely packed two-dimensional (2D) islands which grow, coalesce, and eventually cover the surface.3-9 Except for trichlo* To whom correspondence should be addressed. Telephone: 504/ 862-3562. Fax: 504/865-5596. E-mail:
[email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (3) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (4) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; et al. Langmuir 1995, 11, 2143. (5) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (6) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (7) Eberhardt, A.; Fenter, P.; Eisenberger, P. Surf. Sci. 1998, 397, L285. (8) Doudevski, I.; Hayes, W. A.; Schwartz, D. K. Phys. Rev. Lett. 1998, 81, 4927. (9) Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913.
rosilane-based SAMs that are capable of covalent crosslinking,3,4 one expects that the molecules in islands are in dynamic exchange with a dilute, disordered phase of molecules adsorbed on the regions of surface between islands and possibly with adsorbate molecules in the adjacent solution phase. For example, the rounded (compact) shapes of islands commonly observed suggest that rearrangements take place on the island edges in order to lower the boundary free energy. Experiments under growth conditions explore this issue only indirectly, however, since the growth kinetics are dominated by the net adsorption and attachment rates. In this paper, we report in situ atomic force microscopy (AFM) observations of the dissolution/desorption of SAMs into pure solvent and very dilute solution. In the early stages of dissolution we directly measure the nucleation rate of holes in the monolayer and the growth rate of individual holes. These rates can be related to the probability of removing a molecule from the island interior and from an island edge, respectively. Not surprisingly, we find that removal from island edges is much more likely. This observation combined with the distinctive form of the coverage kinetics during dissolution leads to a picture of desorption that involves molecules detaching from the island edge and moving into adjacent “open” areas of the surface followed by desorption from the open areas into solution. Experimental Details AFM images were obtained with a Nanoscope III MMAFM (Digital Instruments, Santa Barbara, CA) in contact mode. To avoid surface contamination during in situ imaging, the deposition solution came into contact with only glass, PTFE Teflon, and a fluoropolymer Kalrez O-ring (Dupont). Initially the liquid cell was filled with clean tetrahydrofuran (THF) and images were obtained of the clean mica substrate. Then solution containing approximately 0.5 mM octadecylphosphonic acid (OPA), CH3(CH2)17PO(OH)2sdissolved in THFswas allowed to flow into the liquid cell. After the monolayer was nearly complete, the deposition solution was replaced by pure THF and the monolayer proceeded to dissolve into the solvent. The dissolution was performed using either a fixed volume of stagnant THF or by
10.1021/la0008028 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000
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Figure 1. In situ AFM images (500 nm × 500 nm) showing the surface topology of an OPA monolayer during the early stages of dissolution into about 30 µL of stagnant THF. The annotation on each image reflects the exposure time of the sample when the image was captured. Numerous examples of hole nucleation and growth can be observed on the images at 1620, 2340, and 3120 s which show the same area of the film. The arrows indicate one example of a hole that grows considerably in size. flowing clean THF continuously through the cell. The dissolution was monitored in situ by AFM. Typically images were obtained over a 2 µm × 2 µm area. At several stages during monolayer growth and dissolution, the scanned area was increased to 5 µm × 5 µm to check that the smaller initial scanning area contained no evidence of damage due to scanning. Images of dissolution were obtained at regular time intervals. Typically, between image acquisitions the scanning was stopped and the tip moved about 50 µm away from the surface in order to minimize the effect of convection (stirring) due to tip scanning. In some experiments, however, the AFM tip was used to deliberately stir the solvent (between image acquisitions) by moving the tip a distance of approximately 10 µm from the surface and scanning rapidly. Image analysis was performed using NIH Image software on images that were 2 µm × 2 µm in area. Methods for determining surface coverage were discussed in detail previously.8,10 Hole sizes were determined using an elliptical approximation by measuring the width at half-maximum of cross-sectional plots through the holes in horizontal and vertical directions. Simulations of monolayer coverage during dissolution were performed to determine the coupled effects of desorption from the surface and diffusive transport away from the surface through a finite stagnant boundary layer. In such a model, convection caused by stirring or flow is assumed to reduce the thickness of the stagnant layer. Various models for the desorption kinetics were considered as discussed below. The diffusive transport was calculated by dividing a fluid layer of finite thickness l into either N ) 20 or 40 slabs (as a consistency check) and writing the following discrete version of the diffusion equation for cn (the concentration in slab n):
∂cn ∂2cn (cn-1 - 2cn + cn+1) )D 2 )D ∂t ∂z (∆z)2 where D is the molecular diffusivity and ∆z ) l/N is the slab thickness. These coupled differential equations were integrated numerically by the Euler method using Microsoft Excel with the following boundary conditions: (1) all material desorbed from the substrate was placed into the fluid slab adjacent to the surface (affecting the concentration c0) and (2) the concentration after the stagnant layer was zero (cn+1 ) 0). The time steps used were varied from 1 to 5 s to check for consistency of the results.
Results Figure 1 shows a sequence of AFM images typical of the early stages of SAM dissolution into stagnant solvent. The first image shows the surface topology 2 min after exposing the nearly complete monolayer to pure solvent. Holes about 2 nm deep develop in the monolayer and gradually grow and coalesce. The magnitude of the height difference suggests that the high areas are regions of densely packed adsorbate molecules standing on end and the low areas are either bare substrate or a low-density phase of adsorbed molecules. Infrared spectroscopy and contact angle experiments on quenched partial monolayers were consistent with this picture.9,11 These images are (10) Doudevski, I.; Schwartz, D. K. Phys. Rev. B 1999, 60, 14. (11) Woodward, J. T.; Doudevski, I.; Sikes, H. D.; Schwartz, D. K. J. Phys. Chem. B 1997, 101, 7535.
Figure 2. (a) Hole density calculated as the number of holes extracted from images such as those shown in Figure 1 divided by the approximate molecular cross-sectional area (0.25 nm2). This density was then divided by the fractional coverage, so the plotted quantity is the number of holes per filled surface site. The solid line (slope ) 3.5((0.3) × 10-8 s-1) represents the best linear fit to the early hole nucleation regime. (b) Size evolution of individual holes during the hole nucleation regime. The hole area, in units of molecules, was divided by the hole perimeter, in units of molecules. To reduce statistical noise, the areas of four holes that were initially identical in size (within experimental uncertainty) were averaged. The solid line (slope ) 1.9((0.4) × 10-3 s-1) represents the best linear fit to the data.
analogous to images observed during monolayer formation, where “islands” of molecules were observed to nucleate and grow.8 Although it is usually not possible to obtain images of the exact same field of view over the entire dissolution process (due to scanner drift, etc.), it can generally be achieved over periods of 30-50 min at a time. For example, the images at 1620, 2340, and 3120 s in Figure 1 show the evolution of the same region of monolayer over a 25 min period. Nucleation and growth of holes can be directly observed on these images, and the arrows indicate one example of a hole that grows significantly in size. Figure 2a shows the hole number density divided by fractional surface coverage as a function of time for dissolution into stagnant solvent. This quantity is defined as the number of holes per covered “site” (taken to be the approximate molecular cross-sectional area, 0.25 nm2). Over the first 3000 s, the number of holes gradually increases with time (hole nucleation regime). To create a
Mechanisms of Self-Assembled Monolayer Desorption
Figure 3. Time dependence of the surface coverage during dissolution experiments in stagnant solvent/solution (filled circles), continuously flowing solvent (open squares), and flowing solvent combined with stirring using the AFM tip (open circles). The solid lines represent the results of simulations that incorporate desorption from open areas on the surface (regions between islands) combined with diffusive molecular transport through a finite stagnant layer that has a different thickness in each of the three experiments.
new hole, a molecule must be removed from a region of continuous SAM, that is, the interior of an island. Therefore, the slope of the hole density versus time curve in the nucleation regime is related to the rate at which such molecules are removed. The best fit (indicated by the solid line in Figure 2a) is consistent with a rate of 3.5 (( 0.3) × 10-8 s-1. At later times the number of holes is relatively constant and then slowly decreases due to hole coalescence. Ultimately, continuous paths of bare substrate traverse the image from side to side. In stagnant solution, the concentration of adsorbate molecules builds up in solution so that a steady state is eventually reached between desorption and readsorption and the coverage stabilizes at a nonzero value. In flowing solvent, ultimately only isolated islands of molecules remain and these are gradually etched away, leaving a bare surface. Figure 2b shows the growth kinetics of individual holes in the SAM over part of the time covered in Figure 2a. To reduce statistical noise, the sizes of four individual holes that were the same initial size (within experimental uncertainty) were averaged. The hole size has been expressed as the hole area (in units of number of molecules) divided by the hole perimeter (in units of number of molecules). This calculation results in a quantity that is essentially proportional to the perimeter (for compact island shapes). It is useful to consider the area/perimeter, however, since the change in this quantity per unit time is precisely the rate at which molecules are removed from the edge of an island. The best fit (indicated by the solid line in Figure 2b) is consistent with a rate of 1.9((0.4) × 10-3 s-1, nearly 5 orders of magnitude faster than removal of molecules from the island interior. Figure 3 shows the coverage kinetics for dissolution experiments involving stagnant solution, flowing solvent, and flowing solvent with stirring using the AFM tip. Clearly, solvent flow results in faster dissolution than stagnant solution and stirring removes the monolayer even faster. Therefore, the removal rate appears to be affected by bulk transport, at least in some cases. The coverage versus time curves have distinctive downward-curving shapes in all cases. When the solvent is not replaced by flow, the solution concentration gradually increases and the coverage reaches a minimum of about 0.5 after about 11 000 s. The concentration of OPA molecules in solution at this point is about 5 µM. After this time, the coverage remains approximately constant with occasional fluctua-
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Figure 4. Rate of monolayer removal (-dθ/dt) and the total length of the island edge plotted versus time for the experiment using flowing solvent whose coverage kinetics are displayed as open squares in Figure 3. The rate of removal was calculated directly from the coverage data after slight data smoothing. The edge length is expressed as the total number of molecules found along island edges per molecular cross-sectional area (estimated at 0.25 nm2).
tions although the island size distribution continues to evolve due to renucleation of new islands.12 Analysis and Discussion It is not surprising that molecules are removed more easily from the island edge than from the island interior. For one thing, molecules at the edge have lateral interactions with fewer other molecules. Also, a molecule at an edge of an island may detach from the island but remain adsorbed to the surface (it can move into the “bare” region adjacent to the island) while a molecule within an island interior must desorb from the surface completely. Therefore, it is significantly more costly, in terms of free energy, to remove a molecule from within an island and consequently less likely to occur. What consequences does this have for the kinetics of desorption? The simplest approach for describing the timedependence of the coverage θ during a desorption experiment is to postulate first-order kinetics, that is, dθ/dt ) -kθ, where k is a rate constant. This leads to the familiar exponential rate law, θ ) θ0e-kt, where θ0 is the initial coverage. Given the fact that removal from the island interior is unlikely, we should not expect this expression to describe the kinetics well and, as Figure 3 demonstrates, it is qualitatively inconsistent with the shape of the coverage versus time curves (e.g. it is concave-upward). If we make the simplifying assumption that the loss of molecules from the island interior is negligible, then two limiting possibilities come to mind: (1) molecules desorb directly from the island edge into solution or (2) molecules detach from the island edge and move into an open region adjacent to the island from which they eventually desorb. In case 1 above, one might expect a rate law of the form dθ/dt ) -k(total edge length), while in case 2 the form dθ/dt ) -k(1 - θ) might be appropriate. As shown in Figure 4, there is a poor correlation between the rate of removal and the total edge length during the experiment. The amount of edge is typically small at first when few holes are present, goes through a maximum for coverage in the range 0.3-0.5, and then decreases for lower coverage, since the remaining individual islands are gradually shrinking. The removal rate does not have this form. After an initial short period of rapid removal, the removal rate becomes quite small and gradually increases as the monolayer is desorbed. This suggests that desorption directly from island edges into solution is not the preferred mechanism. (12) Doudevski, I.; Schwartz, D. K. J. Phys. Chem. B 2000, 104, 9044.
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On the other hand, the second rate law leads to an expression of the form θ ) 1 - (1 - θ0)ekt, which (for θ0 < 1) has a downward-curving shape. In fact, the solid line drawn through the open circles in Figure 3 is exactly this expression with k ) 3.9 × 10-4 s-1. Thus, the qualitative appearance of the coverage kinetics data, in particular the downward-curving shape, is consistent with a mechanism where molecular desorption into solution occurs predominantly from the regions between islands. Since Figure 3 shows that the desorption kinetics are affected significantly by flow and stirring of the solvent, it is clear that bulk transport of adsorbate molecules can influence the desorption kinetics. However, even in stagnant solution, where the desorption is much slower, the downward-curving shape of the coverage versus time data is still apparent, particularly at short times. This suggests that we should incorporate both desorption from the surface (using the mechanism discussed above) and bulk transport in a realistic model. The solid lines drawn through the data in Figure 3 are the result of such a model where we have attempted to minimize the number of free parameters. The effect of flow or stirring was incorporated as a decrease in the thickness of the stagnant fluid layer. It was assumed that the net removal rate from open areas on the surface will be reduced by the accumulation of adsorbate molecules in the subsurface layer. The expression used was
(
)
c0 dθ ) -k 1 - (1 - θ) dt cj where c0 is the adsorbate molecule concentration in the subsurface layer and cj is the concentration at which there is no net desorption (i.e. the desorption rate equals the adsorption rate in the regions between islands). The subsurface concentration, c0, was taken to be the concentration of the first “slab” of the fluid layer within which diffusive transport was calculated as described above in the experimental section. Thus, four parameters were necessary to describe the model, k, cj, D, and the fluid layer thickness l. Only l should change from one experiment to another. Since our goal was to show that this type of model could realistically explain the results (and not to determine the parameters with great accuracy), we judged it most important to reduce the number of free parameters to a minimum. Therefore, we estimated the molecular diffusivity in solution to be D ≈ 2 × 10-6 cm2/s11 and did not allow it to vary. Since the experiment in stagnant solvent indicated that desorption halted after about 0.5 of the monolayer had dissolved, cj was calculated as the concentration obtained from dissolving half of the number of molecules in a close-packed monolayer into the volume of solvent contained in the liquid cell (about 5 µM). We determined k by assuming that stirring using the AFM tip very close to the surface effectively removed the stagnant fluid layer. Thus, the data represented by open circles in Figure 3 was fit to a simplified model ignoring bulk transport, and the best value of the rate constant was determined to be k ) 3.9 × 10-4 s-1. Using these values of D, cj, and k and taking the fluid layer
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thickness to be that of the entire liquid cell (l ) 0.13 cm) resulted in the solid line drawn through the filled circles, in reasonable agreement with the early part of the dissolution into stagnant solvent. The line drawn through the open squares corresponded to the same values of D, cj, and k and a reduced value of the fluid layer thickness or l ) 0.07 cm, apparently created by solvent flow. Although the model is unsophisticated, we suggest that the minimal number of free parameters involved makes the qualitative results quite believable. The dissolution kinetics data are consistent with desorption from regions of the surface not covered by monolayer islands combined with diffusive transport through a fluid layer whose thickness may be reduced by convection. It is worth noting that the increasing influence of diffusive transport for the thicker fluid layers has a lessening effect on the downwardcurving form of the kinetics of surface desorption, particularly at later times when the removal becomes essentially bulk diffusion-limited. This is consistent with experimental data. Given this behavior, it is clear that the addition of diffusive transport to one of the other surface desorption models (first-order or island-edge desorption) could not reproduce the downward-curving form observed. Conclusions The desorption/dissolution of a self-assembled monolayer of octadecylphosphonic acid from a mica surface was observed in situ using atomic force microscopy. The qualitative mechanism involved nucleation, growth, coalescence, percolation, and so forth of holes in the monolayer. The depth of the holes was consistent with the approximate molecular length, suggesting that, as during SAM growth, the molecular density in the regions between monolayer islands is negligible for this system. In the early time regime of hole nucleation, the relative rates of hole growth and hole nucleation suggest that removing a molecule from the monolayer/hole boundary is about 50 000 times more likely than removing a molecule from within a continuous region of monolayer. The rate of dissolution is slowest in stagnant solvent and is increased by convection caused by solvent flow or stirring using the AFM tip. A numerical model incorporating desorption from “bare” surface regions and diffusive solution-phase transport through a stagnant layer of finite thickness was developed. This model successfully described the form of the dissolution kinetics in stagnant, flowing, or stirred solution using a limited number of free parameters. The results of this study are consistent with a picture of monolayer desorption where molecules detach from the monolayer island boundaries and remain adsorbed on the “bare” regions of the surface, from which they eventually desorb into the solvent. Acknowledgment. This work was supported by the National Science Foundation (Grant Number CHE9980250) and the Camille Dreyfus Teacher-Scholar Awards Program. LA0008028
