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Nucleation Processes for Dewetting Initiation of Thin Polymer Films Ioannis Karapanagiotis, D. Fennell Evans, and William W. Gerberich* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received August 24, 2000. In Final Form: January 8, 2001 We study four processes responsible for the dewetting onset of thin (thickness < 100 nm) polystyrene (PS) films spin cast on nonwettable silicon (Si) surfaces. (i) Spontaneous dewetting is initiated by indentlike disturbances formed at the free PS surface, upon heating above the glass transition temperature (Tg) of PS. Indents become steeper as they grow toward the Si substrate, resulting in an increase of the Laplace pressure which opposes dewetting. (ii) Nanoindentation-induced surface defects either grow toward the substrate, similar to the spontaneously formed indents, or heal, resulting in a flat film surface, depending on their curvatures. Artificial indents with curvatures comparable to the ones measured for the spontaneous indents grow. Healing indents correspond to higher curvatures. (iii) Particles initiate dewetting as they sink inside the film and toward the substrate. Performing our experiments in a controlled clean room environment, we found that ∼23% of the developed dry patches were formed because of particle presence. (iv) Indents into the substrate prior to spin coating affect the uniformity of deposited PS films. The developed surface irregularities can also act as nucleation sites for dewetting initiation, upon heating above Tg.
Introduction Polymer thin films are used in many applications such as photoresists, insulating layers in the microelectronics industry, lubricant layers, finish coats in printing, and protective anticorrosive coatings. All these applications require continuous and defect free films. However, whenever these films are placed on nonwettable surfaces, the equilibrium contact angle, θe, between the film and the substrate becomes θe * 0, resulting in a negative value for the spreading coefficient, S, of the system. In this case the polymer dewets from the substrate, resulting in the devastation of the initial continuous film. A prerequisite for dewetting is for the film to be liquidlike. In the context of polymer films, that means that the film must be heated above its glass transition temperature (Tg). In this study we focus on thin (thickness ) h < 100 nm) polystyrene (PS) films spin cast on silicon (Si) surfaces. The stability of such thin films has been extensively studied on a theoretical basis.1-4 For the system of our interest, experimental studies have shown that dewetting is initiated by a spinodal decomposition process, whenever the PS film is extremely thin and comparable to the silicon oxide layer.5 Thicker (several tenths of nm) PS films rupture by indentlike surface disturbances that grow toward the substrate, resulting in the formation of isolated holes (dry patches) that expose substrate to the air.5-7 The lateral growth of a dry patch has been extensively studied and is well understood.8-10 The surface indentations have also been interpreted according to the spinodal (1) Ruckenstein, E.; Jain, R. K. Chem. Soc. Faraday Trans. 1974, 70, 132. (2) Sharma, A.; Ruckenstein, E. J. Colloid Interface Sci. 1986, 113, 456. (3) Sharma, A.; Ruckenstein, E. Langmuir 1986, 2, 480. (4) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (5) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251. (6) Reiter, G. Phys. Rev. Lett. 1992, 68, 75. (7) Reiter, G. Langmuir 1993, 9, 1344. (8) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715. (9) Redon, C.; Brzoska, J. B.; Brochard-Wyart, F. Macromolecules 1994, 27, 468.
decomposition theory,11 which predicted successfully that the density, F, of the dry patches should scale with the film thickness, h, as F ∼ h-4.6-7 Other studies, however, suggested that surface indents are nucleated and grow because of intrinsic film defects.12 In addition to the above, which refer to the initiation mechanism of the spontaneous dewetting (meaning that dewetting is initiated without the influence of extrinsic defects), extrinsic defects such as airborne particles have been recognized as another source for dewetting initiation.11 Particles sink into the polymer film, and when they reach the substrate, they initiate dewetting. Apparently any kind of artificial defect that exposes initial substrate area to the air, like an airjet blown in a liquidlike film,9 can initiate dewetting provided that the film is thin enough so that the system is unstable. If the film is in the metastable regime, the imposed artificial defect has to be above a critical size13-15 for lateral growth. Finally, thermal stresses rising from a considerable difference between the thermal expansion coefficients of the film and the substrate have been speculated as another possible mechanism for dewetting initiation.11 Our study aims to elucidate potential sources for dewetting initiation in thin PS films (thickness ) h < 100 nm) spin cast on Si wafers. Indents with depths of penetration lower than the film thickness were imposed on PS coatings using nanoindentation. We found that these artificially induced disturbances either can grow toward the Si substrate and form dry patches, similar to the spontaneous process, or can heal, resulting in a flat polymer surface. Our goal is to test under what conditions pre-existing surface disturbances can create nucleation (10) Jacobs, K.; Seemann, R.; Schatz, G.; Herminghaus, S. Langmuir 1998, 14, 4961. (11) Stange, T. G.; Hendrickson, W. A.; Evans, D. F. Langmuir 1997, 13, 4459. (12) Jacobs, K.; Mecke, K. R.; Herminghaus, S. Langmuir 1998, 14, 965. (13) Sharma, A.; Ruckenstein, E. J. Colloid Interface Sci. 1989, 133, 358. (14) Sharma, A. J. Colloid Interface Sci. 1993, 156, 96. (15) Sykes, C.; Andrieu, C.; De´tappe, V.; Deniau, S. J. Phys. III 1994, 4, 775.
10.1021/la0012337 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001
Dewetting Initiation of Thin Polymer Films
sites for dewetting initiation. For this reason we initially turn our attention to the spontaneous indents, which apparently always grow toward the substrate. We characterize them on a geometrical basis using atomic force microscopy (AFM) and then we compare them with the nanoindentation-induced indents. We note here that as “spontaneous” we describe the indents that formed without the influence of extrinsic defects. Afterward, we compare quantitatively the dry patches formed spontaneously with the ones that formed because of the presence of airborne particles. Last, we show that indentlike defects imposed on uncoated Si wafers, prior to film deposition, can contribute to dewetting initiation. We categorize the substrate defects into “deep” indents which are surrounded by features (rims or cracked portions of the substrate) that protrude above the polymer film and “shallow” indents that are covered by the polymer upon spinning. In both cases dewetting initiation resulting in the formation of circular dry patches was observed at the sites of the substrate defects. Experimental Section Low molecular weight, unentangled, PS (Mw ) 10 900 g/mol, Mw/Mn ) 1.02, Polymer Labs, U.K.) was dissolved in spectroscopic grade toluene. Solutions of 1.0 wt % were spin coated onto 50 mm diameter Si wafers. The wafers were used as they were received from the supplier (Virginia Semiconductor, Fredericksburg, VA). To minimize particle contamination of the resulting PS films, spin coating was performed in a clean room (class 10) and the solutions were filtered using 0.2 µm Teflon filters prior to their deposition on the wafers. The coatings were annealed at 60 °C overnight in a vacuum to remove residual solvent. This annealing temperature is well below the bulk glass transition temperature, Tg, of PS (for Mw ) 10 900 g/mol, Tg ) 91 °C16). Atomic force microscopy (Digital Instruments, Santa Barbara, CA), operated in the contact mode, was then used to image the surface of the films, which appeared to be smooth and defect free. The instrument was mounted with a standard silicon nitride tip with a nominal radius of curvature of 20-60 nm. The thickness (h ) 17 nm) of the PS films was measured by ellipsometry. The same technique revealed the presence of a thin (thickness ) 1.8 nm) native oxide layer on the surface of the uncoated 〈100〉 oriented Si wafers. Several PS coatings were heated at 140 °C to initiate dewetting. Annealing times ranged from 7 min to 2 h. Optical microscopy showed the formation of dry patches which exposed substrate area to the air, while the rest of the PS film appeared to be unperturbed. However, AFM scans on the optically unperturbed surface revealed the presence of several small indents, which pointed toward the Si substrate. Repeated scans on the same polymer area showed no alteration of the surface topography due to the interaction force between the polymer and the cantilever. AFM was also used to scan several dry patches. In some of them the presence of airborne particles, placed at the centers of the circular exposed Si areas, was revealed. In another set of PS coatings (h ) 17 nm), artificial indents were induced using a nanoindentation Hysitron Triboscope (Hysitron Inc., Minneapolis, MN) apparatus mounted with a standard three-sided pyramid (Berkovich) diamond tip of nominal radius 300 nm. The nanoindentation device adapts to an atomic force microscope which utilizes a Hysitron microsensor system instead of the standard AFM head component, for applying loads electrostatically. All indents were shallower than the film thickness, and they were imposed using a force range of 2-5.5 µN. Indentations were arranged in an array as shown in Figure 1. Big marks imposed under high (11 mN) forces helped us to locate the indentation area with an optical microscope and then to scan the small nanoindents with AFM before and after heating at 140 °C. Finally, uncoated Si wafers were indented using an IBM continuous micromechanical tester (MMTsnot commercially available). The MMT utilizes a probe, which measures the depth (16) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley: New York, 1989.
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Figure 1. Indentation pattern. Big indents at the corners served as markers, necessary to locate the area on which the smaller nanoindents were induced. of penetration while a piezostack drives the tip inside the sample. Unlike the Hysitron Triboscope, which requires small, AFM size, samples (diameter < 10 mm) for testing, the MMT can accept the 50 mm Si wafers that were used in this study. Using whole circular wafers, instead of small cleaved pieces, was important since we wanted to avoid edge effects which can occur upon coating small rectangular pieces of substrate. Indents were induced using a conical tip of nominal radius 1 µm in a similar array as shown in Figure 1. The indented wafers were then spin coated with 1.0 and 2.5 wt % PS solutions in toluene, and the residual solvent was removed upon annealing in a vacuum. The resulting film thicknesses were measured to be 17 and 50 nm using ellipsometry. Samples were then cleaved into small sizes to fit in the atomic force microscope, which was used to scan the areas of the films in which the substrate indents were induced, before and after heating the samples at 140 °C for 5-7 min. Then, samples were extensively (2-3 h) heated at 140 °C to reach the final dewetting stage, characterized by the formation of PS droplets. During the thermal treatment, samples were occasionally observed with an optical microscope to record any dry patch formed in the area of the substrate indents. Upon reaching the final dewetting stage, samples were rinsed with acetone to remove PS. AFM was utilized again to record the shape of the revealed (initial) substrate indents.
Results and Discussion Figure 2 presents an AFM image of a PS coating, which has been heated above Tg. A dry patch and several indentlike disturbances at the free PS surface are observed. Depending on the formation mechanism, we separate these indents into two categories:11 (i) spontaneous, isolated, indents formed without the presence of extrinsic defects; (ii) particulate indents, formed as airborne particles sink into the polymer film. Figure 3a shows a spontaneous indent, and Figure 3b an indent formed due to particle presence. Operating the atomic force microscope in the height mode, the particle is identified as a bump at the bottom of the indent. This enables us to distinguish the two mechanisms for indent formation. In the following we first focus on the spontaneous indents and leave the particulate indents for later discussion. Several samples were heated at 140 °C for different annealing times. Spontaneous indents were captured with AFM, over a broad range of indent depths, at an intermediate stage while they grew toward the substrate. Figure 4 shows the radii of several indents as a function of their corresponding depths. Deeper indents correspond to higher radii. The radius of an indent is defined at the
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Figure 2. AFM image of a PS coating after heating at 140 °C for 40 min. A dry patch and at least six surface disturbances are detected in this view. Film thickness is 17 nm.
level of the flat free polymer surface (Figure 3a). With a simple linear extrapolation of the data, we calculate that when an indent reaches the substrate (i.e. indent depth ) film thickness ) 17 nm), the radius is, on average, 1.65 µm. For unentangled polymer films, dewetting kinetic measurements have shown that the radius of a dry patch, Rdp, that grows laterally scales linearly with time,8,9 Rdp ∼ t. However, a linear extrapolation of the data at t ) 0 does not correspond to Rdp ) 0 but rather to a particular positive value. By using such data17 for low Mw PS (Mw ) 22 000 g/mol) thin (h ) 23 nm) films on Si, we calculate that, at t ) 0, Rdp ) 1.2 µm. Figure 5 shows schematically the cross section of a dry patch and the adjacent polymer that forms a rim. By θr we denote the receding contact angle between the rim and the substrate. From Figure 5 we have
R ) Rdp + (h/tan θr)
(1)
Using θr ) 12° 17 and values for h and Rdp as they were mentioned above, we calculate (eq 1) that, at t ) 0, R ) 1.3 µm. This is close to the radius size of 1.65 µm that corresponds to a spontaneous indent which reaches the Si substrate, considering (i) the difference in the molecular weights between the data of Figure 4 (Mw ) 10 900 g/mol) and the data used to calculate Rdp at t ) 0 (Mw ) 22 000 g/mol) and that (ii) the two sets of data (Figure 4 and data for the growing dry patches17) are independent measurements and they refer to different growing indents and advancing dry patches. We now turn our attention to nanoindentation indents, induced at the free surface of PS films. Figure 6 shows an AFM image of such a nanoindent and the corresponding cross section. A comparison of this indent with the spontaneously formed indent of Figure 3a shows an important difference in the heights of the rims. In a spontaneous indent the rim is more “diffused” in the lateral direction and therefore hardly noticeable. We observed this in other experimental studies as well.11 However, the steeper rims of the nanoindentation-induced defects did not seem to affect their final evolution upon heating above Tg. Nanoindents with similar rims either grew toward the substrate or healed (leveled), resulting in a flat polymer surface, upon heating above Tg. We have studied the (17) Stange, T. G. Ph.D. Thesis, University of Minnesota, 1997.
Figure 3. AFM image and the corresponding cross section of a spontaneously formed indent (a) and an indent that formed because of a particle (b). The radius of the spontaneous indent is denoted by R, and the maximum depth by zA. Film thickness is 17 nm.
dynamics of the healing process elsewhere.18,19 Here, we attempt to establish a criterion that can predict the final evolution (growing or healing) of the indents, upon heating above Tg. To achieve that we first turn attention to the spontaneous indents. The formation and growth of these indents toward the substrate induces a Laplace pressure (18) Karapanagiotis, I.; Schmidt, R. H.; Haugstad, G.; Gladfelter, W. L.; Evans, D. F.; Gerberich, W. W (submitted to Langmuir). (19) Karapanagiotis, I.; Evans, D. F.; Gerberich, W. W. Macromolecules, accepted.
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Figure 4. Radii, R, of spontaneous indents versus their corresponding depths, zA (Figure 3a), for a 17 nm thick PS film.
Figure 5. Schematic cross section of a dry patch of radius Rdp. The dewetting material is accumulated into an advancing rim. R is the radius of the dry patch at the level of the flat free film surface, and θr is the receding contact angle.
which tends to level the film. For the driving force of indent growth, contradictory suggestions can be found in the literature. A nonfavorable van der Waals interaction between the film and the substrate, rising from the positive sign of the Hamaker constant (A > 0) of the system,20 has been suggested as responsible for indent growth.7,11,20 Other studies, however, suggested that for films thicker than the oxide layer (which is the case in our study) the Hamaker constant becomes negative 21 and therefore film rupture cannot be attributed to the van der Waals interactions between the film and the substrate. No matter what the driving force is, indent formation and growth clearly induces a Laplace penalty which is given by22
∆P ) 2γH
(2)
where ∆P is the extra pressure due to the presence of the curvature H and γ is the surface tension of PS. The mean curvature of a surface at a point A0 is defined as
H|A0 )
(|
1 1 2 R1
A0
+
|)
1 R2
(3)
A0
where R1 and R2 are the minimal and the maximal values of the curvature radius R, of a section normal to the surface at the point A0. In 2D the curvature of a function y ) f(x) at a point A0(x0,y0) is defined as
|
f ′′(x0)| 1 ) R A0 (1 + f ′(x )2)3/2
(4)
0
(20) Sharma, A.; Reiter, G. J. Colloid Interface Sci. 1996, 178, 383. (21) Mu¨ller-Buschbaum, P.; Stamm, M. Physica B 1998, 248, 229. (22) Evans, D. F.; Wennerstro¨m, A. The Colloidal Domain; VCH Publishers: New York, 1994.
Figure 6. AFM image and the corresponding cross section of a nanoindentation-induced indent. Film thickness is 17 nm.
Using eqs 3 and 4, we calculated the curvatures of the spontaneous indents at point A, which corresponds at the maximum depths of the indents (Figure 3a) as follows: Using the topographical AFM images of the spontaneous indents, we took cross sections at several directions. We then curve fitted the portions of the cross sections at the vicinity of point A with fourth order polynomial functions and calculated 1/R|A at the chosen directions, from eq 4. By taking the minimal and maximal values of 1/R|A, we calculated the mean curvature H|A, using eq 3. We note here that the difference between the minimal and maximal values of 1/R|A was small and sometimes negligible, indicating that the bottom of a spontaneous indent approaches a spherical shape. Similarly we calculated the curvatures, H|A, that corresponded to the nanoindentation-induced indents. The results are summarized in Figure 7, which shows the calculated H|A values versus the corresponding depths for three types of indents: (i) spontaneous indents, (ii) nanoindentation-induced growing indents, and (iii) nanoindentation-induced healing indents. Growing indents induced by nanoindentation have H|A values in the vicinity of the H|A of the spontaneous indents. Healing indents correspond to higher H|A values, giving rise to the Laplace penalty that the system has to overcome for indent growth (eq 2). For the growing artificial indents the Laplace penalty is comparable to the corresponding values for the spontaneously formed and growing indents. Consequently, Figure 7 provides a clear distinction between the growing and the healing process of artificially imposed surface defects. In the following, we address two interesting points associated with the techniques that have been used to induce and image the indents. First, the results presented in Figure 7 might be affected by tip size effects. The measurements in Figure 7 were performed using a standard AFM tip of nominal radius of curvature 20-60 nm, which corresponds to a curvature of 0.05-0.016 1/nm. Consequently, the tip curvature is about an order of
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Karapanagiotis et al. Table 1. Air Particle Concentration Measurements, Inside the Clean Room at Several Locationsa particle concentration (particles/ft3) at the following particle sizes location
0.3 µm
0.5 µm
1.00 µm
5.00 µm
1 2 3 4 5 6 7
125 165 745 30 145 145 90
105 105 335 25 85 120 35
35 40 115 15 25 25 0
0 0 0 0 0 0 0
a Locations 1, 2, 3, and 4 are close to the spinner, used for sample preparation. Locations 5, 6, and 7 are close to the thermal chamber, used to heat the samples above Tg.
Table 2. Comparison of Dry Patches Formed Spontaneously with Ones Formed by particles
Figure 7. Mean curvatures, H|A, of spontaneous, nanoindentation-induced growing and nanoindentation-induced healing indents versus their corresponding depths, zA. Growing indents have lower H|A values than the healing ones. Film thickness is 17 nm.
magnitude higher than the indent curvatures, shown in Figure 7. It appears that this difference is sufficient for the instrument to resolve the growing from the healing indents (Figure 7). Second, because nanoindentation experiments were performed using the same Berkovich tip in every case, one might expect all artificially induced indents to have the same curvature. However, we have noticed that the Hysitron Triboscope, which utilizes a piezoelectric scanner to move the sample, drifts laterally during indentation. The lateral drift is negligible when hard materials are indented and/or high forces are used. In our case a soft polymer coating is indented under very low forces ( 50 µm in Figure 1) are accompanied by an extensive lateral drift of the tip, resulting in residual indents which resemble small scratches. These indents have small curvatures, and they grow toward the substrate upon heating above Tg (growing nanoindentation indents in Figure 7). On the contrary, whenever small lateral offset values (x or y < 50 µm in Figure 1) are used, the extent of the lateral drift is diminished and steep indents are induced which heal upon heating above Tg (healing nanoindentation indents in Figure 7). In Figure 1 the indentation pattern is shown. Growing indents correspond to x (or y) ) 80 µm, and healing indents to x (or y) e 40 nm. Choosing discrete positions for the indentations, we were able to exploit the drift effect as a tool to induce indents with considerably different shapes and curvatures (Figure 7). Other minor sources for the curvature range observed in the imposed indents are the contamination of the tip surface, expected after the imposition of a series of indents, and the tilt of the piezoelectric scanner that occurred when large lateral offset values were used. Finally, we note that we found it very difficult to induce blunt, growing, indents with penetration depths < 12 nm (Figure 7). At large lateral offset values (>50 µm) low
sample 1 sample 2 sample 3
spontaneous dry patches
particulate dry patches
54 57 50
16 13 20
loads (∼2 µN) were adequate to produce relatively deep indents (Figure 7). Our next goal was to evaluate quantitatively the extent at which the particulate nucleation mechanism participates in dewetting initiation. A similar extensive study has been done elsewhere.11 Here we briefly present our results. Table 1 provides airborne particle concentration data, measured with an APC-1000 particle counter (Biotest Diagnostics Corporation, Denville, NJ), at several locations inside the clean room (class 10) in which three PS coatings were prepared and annealed above Tg. In this case Si wafers were spin coated and then directly heated above Tg, without any prior removal of the residual solvent. Heating was performed in a thermal chamber, located inside the clean room. The samples were exposed to the clean room air for 5 min. Then the samples were cleaved (after thermal treatment) to fit into the AFM holder, and 70 dry patches were scanned (Figure 2) from each coating to evaluate the initiation mechanism in each case. We distinguished the particulate-initiated dry patches, by recognizing bumps at the centers of the exposed Si areas. Dry patches formed spontaneously did not appear to have any irregularities at their centers. Table 2 shows that the spontaneous mechanism for dewetting initiation dominates. However, a relative augmented role of the particulate nucleation is observed comparing our results with other studies.11 This is justified by the different airborne particle concentrations, that are expected in different laboratories. Finally, we note that particles with sizes < 1 µm were detected as dewetting initiators. In Figure 4 such a particle ( 0.3 µm was used. Cracking was also recorded in (23) Cook, R. F.; Pharr, G. M. J. Am. Ceram. Soc. 1990, 73, 787.
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the load-displacement curves, received during the indentation tests, as a displacement excursion upon loading. By taking AFM cross sections of the indents after spin coating and comparing them with the corresponding cross sections of the impressions revealed upon heating above Tg and rinsing with acetone (not shown here), we found that PS material was present at the bottom of the indents after spin coating. Cracked portions (or rims) of deep indents protruded above the unperturbed PS surface while the indent bottoms were at a lower vertical level than the smooth PS surface. After spin coating, samples were annealed at 140 °C and dewetting was initiated from the sites of the deep indents (Figure 10b). Conclusion We have studied four different processes that can initiate dewetting of thin PS films (h < 100 nm) on Si surfaces, upon heating above Tg. (i) Spontaneous dry patches are formed from indentlike disturbances developed at the free PS surface upon heating above Tg. The growth rate of these indents has been studied elsewhere.11 Here we characterized the indents on a geometrical basis by measuring their radii, R, and their curvatures, H|A, as a function of their corresponding depths (both R and point A are defined in Figure 3a). We found that deeper indents correspond to higher values for both R and H|A. We can use this result to monitor the growth process of an indent, considering that the growth mechanism for all the spontaneously formed disturbances is the same. As a particular indent grows toward the substrate, it “expands” laterally by increasing its radius, R, at the level of the flat film surface and it becomes steeper at the bottom. The
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increasing curvature results in a proportional increase of the Laplace pressure. (ii) Nanoindentation-induced surface disturbances either can grow toward the substrate and initiate dewetting or can heal, resulting in a flat film surface. By measuring the curvatures of these artificial indents, we were able to distinguish the growth from the healing process. Growing indents have H|A values comparable to the corresponding curvatures of the spontaneous indents. Healing indents appear to have higher H|A values, leading to an augmented Laplace pressure (Figure 7). (iii) Dewetting initiation (i.e. formation of dry patches) is also achieved by airborne particles which sink toward the Si substrate. Performing experiments in a clean room, characterized in Table 1, we found that ∼23% of the dry patches formed on a 50 mm wafer were initiated by particles. Samples were exposed to the clean room air for a total of 5 min. (iv) Substrate indents induced prior to the film deposition can initiate dewetting. Deep indents are accompanied by features (e.g. indent rims) higher than the film thickness with uncovered substrate areas serving as nucleation sites for dewetting initiation. Shallow indents are surrounded by rims lower than the thickness of the deposited film. Upon spin coating and drying, shallow substrate indents result in disturbances at the film surface which resemble the shape of the covered substrate indents. Upon heating above Tg, these surface disturbances can also grow and initiate dewetting. Acknowledgment. Support by the Center for Interfacial Engineering (CIE), a National Science Foundation Engineering Research Center, is gratefully acknowledged. LA0012337
