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Tapping Mode AFM Studies of Nano-Phases on Fluorine-Containing Polyester Coatings and Octadecyltrichlorosilane Monolayers Bryan B. Sauer,* R. Scott McLean,† and Richard R. Thomas‡ Central Research and Development, DuPont, Wilmington, Delaware 19880-0356, and Jackson Laboratory, DuPont Specialty Chemicals, Deepwater, New Jersey 08023 Received December 3, 1997. In Final Form: February 19, 1998 Lightly cross-linked films were made by solvent casting hydrocarbon polyester solutions containing trace fluorocarbon in the bulk and then curing or cross-linking the film to anchor the fluorine groups. The surface nano-morphology was studied by AFM techniques based on tapping mode AFM which allows very low contact forces and gives indirect surface chemical mapping of different domain morphologies with high lateral resolution. The contrast is usually from different local stiffness variations of domains at or near the surface. More conventional friction force microscopy techniques were also applied to verify the results. The percent coverage of fluorine-rich domains was quantitatively related to that determined from hexadecane contact angles. A low-coverage self-assembled octadecyltrichlorosilane (OTS) monolayer system, exhibiting nano-patches on Si substrate, was used as a control for a newly applied ultra-light-tapping technique which derives its chemical resolution from hydrophilicity differences of the phases at the surface. The hydrophilicity differences possibly modify the water meniscus forces on the scanning tip and allow indirect assignment of domains based on different hydrophilicities in cases where there are no a priori stiffness assignments of the different domains.
Introduction Advances in scanning probe microscopies continue at a rapid pace.1-3 While scanning tunneling microscopy is very effective at characterizing conductor and semiconductor surfaces, it is less useful for characterizing polymer and organic layer surfaces, especially for many situations where one wishes to learn about the bulk polymer nanomorphology from studies of the outermost free surface. AFM techniques include contact AFM, contact AFM in the light repulsive mode, lateral force AFM or friction force AFM3, and force modulated AFM. Chemical resolution has been demonstrated in studies of mixed hydrocarbon and fluorocarbon monolayers,3-6 and we will extend these studies to thicker fluorocarbon containing films using tapping and friction force techniques. Octadecyltrichlorosilane and related self-assembled monolayers have also been excessively studied by AFM techniques,7,8 including the morphology during deposition starting with lower coverages.7 We use this system here as a control where nano-domains can be studied under different tapping AFM instrument set points. * To whom correspondence should be addrressed at DuPont. † Central Research and Development, DuPont. ‡ Jackson Laboratory, DuPont Specialty Chemicals. (1) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, Germany, 1996. (2) Chernoff, D. A. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1996, 37 (2), 599. (3) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (4) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (5) Kajiyama, T.; Tanaka, K.; Ohki, I.; Ge, S.-R.; Yoon, J.-S.; Takahara A. Macromolecules 1994, 27, 7932. (6) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898. (7) Banga, R.; Yarwood, J.; Morgan, A. M.; Evans, B.; Kells, J. Langmuir 1995, 11, 4393. (8) Garcia-Parajo, M.; Longo, C.; Servat, J.; Gorostiza, P.; Sanz, F. Langmuir 1997, 13, 2333.
Tapping mode AFM1,2,9 is a contact technique, but the ca. 300 kHz tapping in the direction normal to the surface allows one to lessen the capillary and adhesion forces which typically reduce resolution of static scanning AFM methods. Tapping mode AFM with simultaneous topographical and phase detection has been used to demonstrate subnanometer surface chemical resolution via mechanical differences of the different domains.1,2,9-11 “Phase” information is obtained by detecting the phase shift between the driving and actual tip response oscilation signals at ca. 300 kHz, and its advantage over the topographical data is that sample quality, mainly long wavelength roughness, does not significantly influence resolution. At 300 kHz, the phase information and sometimes the topographical data, give information on domains at or just below the surface, in a way that can be controlled by various AFM set points.9-13 In many cases of copolymers or blends, one component will be lower in surface energy and will dominate the top few angstroms of the surface.10,12 Tapping mode reduces the duration of tip-sample contact, leading to less deformation.1,9 Although the forces are low for tapping mode AFM, they are still finite, and for soft polymers one must check for damage, in addition to considering artifacts in topographical data caused by different tip interactions as one scans across regions of varying stiffness. The phase data are quite useful for interpretation of the data as will be shown below. The forces for contact AFM1 are ca. 5-500 nN (nN ) nanonewton) vs maximum forces per oscillation of 0.1-1 nN for tapping mode AFM.1 These low forces combined with intermittent contact lead to very low lateral drag forces, (9) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. Lett. 1997, 375, L385. (10) McLean, R. S.; Sauer, B. B. Macromolecules 1997, 30, 8314. (11) Van Noort, S.J. T.; Van der Werf, K.O.; De Groth, B.G.; Van Hulst, N. F.; Greve, J. Ultramicroscopy 1997, 69, 117. (12) Magonov, S. N.; Cleveland, J.; Elings, V.; Denley, D.; Whangbo, M.-H. Surf. Sci. 1997, 389, 201. (13) Brandsch, R.; Bar, G.; Whangbo, M. H. Langmuir 1997, 13, 6349.
S0743-7463(97)01334-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/23/1998
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and for reducing damage of soft polymers, these low lateral forces may be even more important than the force normal to the surface. For contact AFM under liquid immersion the forces are ca. 0.01-1 nN.14 Experimental Section Materials. “Concentrated” OTS solutions were prepared after Sagiv15 where OTS was dissolved in CHCl3 and then added to a CHCl4/hexadecane mixture. This solution was diluted 50 times with hexadecane, so low-coverage patchy monolayers could be formed. We chose a 120 s adsorption time for one representative system studied here. The Si wafer substrates were cleaned in water and then rinsed with methanol before gently flaming in a CH4/O2 torch, and then treating with OTS within a few minutes. A typical alkyd resin is the condensation product of unsaturated fatty acids with various polyhydric alcohols and diacids.16 The unsaturated fatty acids are capable of oxidative cross-linking by a free radical reaction through the double bonds.17 This crosslinking establishes a three-dimensional network to the coating system and helps to impart mechanical properties. In a modification to our alkyd system, a fluorinated ester of an unsaturated fatty acid was added. The ester was prepared by condensation of perfluorooctylethyl alcohol (F(CF2)8(CH2)2OH) with oleic acid. The relatively low surface tension of the ester, along with unsaturation in the hydrocarbon tail, should allow it to segregate to the coating surface and participate in oxidative cross-linking. Solution mixtures of the alkyd resin and the fluorinated acid ester were prepared and films were produced by spin coating at 1 krpm onto Si wafers and cured slowly at room temperature for 2 weeks. The material will be called F-PE and contains about 1% bulk weight fraction fluorine. Methods. Height and phase data were recorded simultaneously on a Nanoscope IIIa AFM from Digital Instruments, Santa Barbara, CA. Microfabricated cantilevers or silicon probes (Nanoprobes, Digital Instruments) with 125 µm long cantilevers were used at their fundamental resonance frequencies which typically varied from 270 to 350 kHz depending on the cantilever. Cantilevers had a small tip radius of 5-10 nm. The so-called “phase” is the phase lag between the driving and actual tip response oscilation signals at ca. 300 kHz. In our results a high phase is a high phase lag presumably caused by higher interactions with the surface. In tapping mode several instrumental factors can affect the results.1,9,10-13,18 The level of force applied to the surface can dramatically change the data, especially the phase data.9 The forces are roughly adjusted by the magnitude of the free air amplitude (Ao) and the ratio (rsp) of the engaged (A) or set point amplitude to Ao (rsp ) A/Ao).9 It is not possible to exactly define the tip forces because of many unknowns including the tip contact area and the energy dissipated in the sample. This set point amplitude, which is used in feedback control, was adjusted to 40-70% of the free air amplitude for “moderate-force” imaging, and to 75-90% of the amplitude for low-force imaging.9 One must be very careful with the position of the oscillation frequency relative to the ca. 300 kHz cantilever resonance peak, and should make sure that it does not shift when adjusting rsp. To our knowledge, little or no consideration is given to this in the literature. The full width at half-height of the resonance peak is very small and is about 0.001 kHz, and typically the frequency is positioned to give an amplitude about 80% of the resonance peak height amplitude, which is on the lower frequency side of the resonance maximum. For a frequency position on the high-frequency side of the resonance peak, the tip-sample interactions can be uncontrollably high because of slow feedback response with the Digital instrument, and this can dominate the results and lead to artifacts. If one wishes to vary rsp or the free air amplitude, one should not ignore the position relative to the resonance peak, which may also vary unintentionally. We have (14) Weisenhorn, A. L.; Maivald, P.; Butt, H.-J.; Hansma, P. K. Phys. Rev. B 1992, 45, 11226. (15) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (16) Paul, S. Surface Coatings Science and Technology; Wiley: New York, 1985; Chapter 2. (17) Surface Coatings; Tafe Educational Books: Randwick, Australia, 1974: Vol. 1, Chapter 3.
Sauer et al. found that this can entirely dominate the topography and phase signals and can even lead to a reversal in sign of one or both. A new technique explored below is ultra-light-tapping, where rsp is set to ca. 95% and one is careful to control the frequency relative to the resonance peak as is discussed above. In this mode the feedback loop is barely operational because the tracking forces are so low, and the results are strongly influenced by capillary condensation or related forces, which translates into indirect chemical sensitivity with high lateral resolution because of hydrophobicity differences of the different domains. Related AFM studies of the effect of contamination layers have been presented recently.11,13 AFM was also performed in the standard contact mode for simultaneous topographical and lateral friction data. A triangular cantilever (from Digital) with a larger tip radius (Si3N4) of about 20-60 nm was used. The scan frequency was about 1.5 Hz. The measurements were made in the weakest repulsive mode possible, without causing lifting off of the tip. Unless otherwise indicated, data presented here were obtained in moderate-force mode. In this mode, high phase corresponds to high modulus, and low phase to low modulus,1,9 i.e., the relatively commonly used stiffness contrast. Many controls in our laboratory were performed on macroscopic patchy layers of polymers to confirm this.10
Results Tapping mode AFM data for a sample with relatively low OTS coverage are shown in Figure 1. The high spots are the nano-patches of OTS on a Si background. A similar trend was seen for material deposited out of the dilute OTS solution with a lower immersion time, leading to about 3 times lower percent coverage of patches (not shown here). For all of these samples made from dilute solutions in hexadecane, the OTS possibly adsorbs from micellelike solution aggregates, giving rise to these small patches about 1.6 nm high and about 20 nm wide. At higher magnification such as in a 100 nm × 100 nm scan box, the height of the patches decreases to about 1.2 nm, because of higher elastic deformation of the soft monolayer by the tip. At high magnifications, the tip resides longer at each given point on the surface, contributing to larger effective tip forces. The phase data on the right image in Figure 1 are dark because the patches are slightly softer, and under these moderate-tapping force conditions this is the expected trend. The “mechanical” contrast between the patches and the bare substrate is actually very low because the layer is so thin. Apparently, the effective stiffness of such a thin layer is elevated because of the near proximity of the silicon. At higher tapping forces and the same magnification, the contrast in phase can be flipped (Figure 2). Under these “ high” tapping force conditions, the patches of OTS become light and the Si substrate becomes dark. This reversal has been demonstrated previously,9 with some theoretical justification, and is useful as an interpretation tool. A new method which can be used to assign various phases is coined “ultra-light-tapping”. The forces in tapping AFM are lowered to the level where the tip barely remains in contact with the surface, and the position relative to the resonance peak is carefully monitored (see Experimental Section). This gives apparent topographical data on “smooth” surfaces where the hydrophobic patches (OTS patches) appear as low spots, and the hydrophilic areas appear as high spots (Figure 3). The phase data for the patches are white and show a reversal of phase expected for low- to moderate-tapping forces. Such reversals have been measured recently for monolayers on gold13 and other systems.11 Although the contrast is high, the resolution is slightly worse than it was for moderate tapping because of the very low forces. A signature of
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Figure 1. AFM moderate-tapping mode data for OTS deposited on Si at low coverage. On the left is topographical (gray scale 0-7.5 nm) and on the right is phase (scale 0-30°). Scan box is 500 × 500 nm.
Figure 2. Moderate to hard tapping data on the patchy OTS layer. The phase data on the right have been flipped compared to Figure 1. Scan box is 500 × 500 nm.
this lower resolution is given by the horizontal streakiness in Figure 3. Results for the alkyd resin modified with the fluorinated acid ester (F-PE), with about 1% bulk weight fraction fluorine, are given in Figure 4 and were taken under standard moderate-tapping forces. It is seen that the high spots are also high in phase. From our knowledge of AFM under moderate tapping,9,10 this suggests that the patches are the stiffer material. The patches are typically 1.01.4 nm high and 30 nm wide. Another feature of this
system is shown in Figure 5 where lower magnification data show the presence of a larger aggregate in addition to the small 30 nm wide patches. The aggregates were typically a few hundred nanometers wide and are very flat, characterized by a uniform thickness of about 1.2 nm. They were seen consistently in several of the scans. AFM lateral friction data taken using a different tip are given in Figure 6 for the same fluorinated film. Higher radius tips are used for friction force measurements, while the same 6-10 nm (sharp) tips are used for all other
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Figure 3. Ultra-light-tapping AFM data for same sample as in Figure 1. On the left is topographical (scale 0-5 nm) and on the right is phase (scale 0-30°). These data are taken at very low tip contact forces (see text). Scan box is 250 × 250 nm.
Figure 4. Moderate-tapping data for the alkyd resin modified with the fluorinated acid ester (F-PE, contains about 1% bulk weight fraction fluorine). Light is high on the left and high phase on the right (scan box 1 × 1 mm).
measurements. It is seen that the small patches have low friction, and the results in Figure 5 showed that they were the stiffer material. There is no way of unambiguously assigning domain composition using this information, partly because we are not sure if the fluorine-rich regions are the stiffer phase. Also, previous friction AFM data show that for ordered (crystalline) perfluorinated monolayers that the fluorinated domains had the higher friction, but in our system, the discontinuous phase has the lower friction. This is
probably because of the high cohesive strength of the domains of longer perfluorinated chains,3 while our system has shorter(CF3[CF2]7-) and much more dilute chains which must be less organized. Thus, it is not unreasonable that the patches are fluorine-rich even though they are low friction. The ultra-light-tapping data are shown in Figure 7 for the same F-PE sample. From our knowledge of fluorocarbon wettability, we can be quite sure that any fluorinerich regions must be more hydrophobic. Thus, the dark
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Figure 5. Lower magnification scan of F-PE showing a large flat patch (scan box 2 × 2 mm).
Figure 6. Lateral friction force AFM for the sample in Figure 4. On the left is topographical (scale 0-5 nm) and on the right is friction (scale 0-100 V). Light regions are high friction. Dark regions are fluoropolymer rich in the friction data.
regions in the topographical data in Figure 7 are the more hydrophobic ones for reasons discussed below. Discussion Why is the ultra-light-tapping technique important? In many cases, one does not know which phase is stiffer or lower in friction, but one can easily predict which phase is more hydrophilic or can determine this from contact angle analysis. Recall that with ultra-light-tapping domain stiffness differences become less important, and hydrophobicity differences seem to become important
because of capillary condensation of water between the tip and the surface. The explanation for the reversal of the topographical data (Figures 3 and 7) is related to the capillary condensation of water. At the hydrophilic regions, the condensation of water between the tip and the sample would be stronger, and the tip feedback compensates by increasing the distance from the surface in order to keep the force constant. This explains the perplexing question of how a patch, such as an OTS patch on silicon, can appear as a hole or a depression. Notice that for our two examples, in Figure 3 the “holes” are the
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Figure 7. Ultra-light-tapping data on F-PE film. The patches appear as apparent holes (see text) and the contrast in phase improves relative to moderate-tapping data for the same film (Figure 5).
Figure 8. moderate-tapping on a cross-linked coating based on incorporation of a “long chain” fluorinated alkane alcohol into an acrylic copolymer backbone. The fluorocarbon is about 1.5 wt % of total bulk solids, and the surface has a large excess of F compared to bulk.
softer material, and in Figure 7 the “holes” are the harder material. These are evidence that the phenomena are not related to stiffness or other viscoelastic interactions of the tip. We have found that for surfaces with slightly higher roughnesses with “abrupt” steps on the order of several nm or more, it becomes very difficult to obtain ultra-light-tapping data, because the tip loses contact with the surface and cannot regain contact unless the contact forces are readjusted.
Although one loses resolution with ultra-light-tapping AFM, there is some advantage for studying F-PE. With contact angles of 55°/25° (adv/rec) in hexadecane, it is not immediately obvious which phase is continuous, partly because we do not know the composition of the fluorinerich regions, although we can guess that the hydrocarbon rich regions are depleted in fluorine and are characterized by a 0° contact angle. The contact angles are comparable to those of pure poly(tetrafluoroethylene) [46°/42°,
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hexadecane],19but are much lower than the hexadecane angles of 80°/76° expected for a close packed -CF3 surface19 such as that found for oriented perfluorinated monolayers. Ultra-light tapping gives a strong indication that the nanopatches are the fluoropolymer phase. Because they appear as “low spots” in the topographical data (Figure 7 left), this means that they are more hydrophobic as would be expected for fluorocarbon-rich domains. The hexadecane contact angles of 55°/25° can also be explained by the theoretical treatment described in Figure 1 in the work of Johnson and Dettre.20 Computer analysis of the area fraction of patches for the phase data in Figure 3 gives 9 ( 1%. According to Figure 1 of ref 17, 10% coverage of patches with inherent 80°/80° (adv/rec) hexadecane contact angles, and the other 90% bare substrate characterized by 0°/0° (adv/rec), would give 57°/16°. In our case the composition of the patches is unknown so a logical assumption is that the patches may have a high level of -CF3 groups characterized by 80°/80° (adv/rec) contact angles. It is known that pure hydrocarbon film does have 0°/0° (adv/rec) hexadecane contact angles,19 justifying the above analysis. Similar analysis was found to describe the contact angles for patchy OTS films such as those in Figure 1. Quantitative determination of the percent coverage of these nano-phases is novel to AFM and gives important information on the mechanism of formation of domains on all of these surfaces. Finally, a system with a more uniform surface was (18) Magonov, S. N.; Elings, V.; Papkov, V. S. Polymer 1997, 38, 297. (19) Wu S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982. (20) Johnson, R. E., Jr.; Dettre, R. H. J. Adhes. 1970, 2, 3.
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fabricated in order to compare with contact angle data. Figure 8 shows the topographical and phase data for a cross-linked acrylate backbone polymer containing 1.5 wt % fluorine incorporated into the backbone in the form of F(CF2)6(CH2)2CO2C(CH3)dCH2, with the detailed composition given previously.21 The solution in methyl amyl ketone was spin-coated onto a silicon wafer, and cured at 80 °C for 1 h. In this system, a very fine network of fluorine-rich domains is seen (Figure 8). Imaging of such domains requires very high lateral resolution which results from combined effects of sharp tips, low deformation, and high spring-constant cantilevers. In moderate-tapping, the domains are “high” spots in topography, and high in phase, consistent with F-PE. The nano-phase separation occurs at the surface of this system as the fluorinated species are driven to the surface during slow solvent removal, and the inherent incompatibility of fluorocarbon and hydrocarbon in the solvent depleted state causes the observed structures. The hexadecane angles of (39°/31°) indicate a relatively homogeneous surface coverage on a microscopic scale, consistent with the very fine network. The low bulk levels of fluorine make this an economically efficient use of fluorine, and one finds from the AFM data and from previous interface profiling data that there is an ∼20-fold excess of fluorine at the surface relative to the bulk.21 The previous study also showed that the excess was mostly confined to the outer 5 nm of the air/polymer interface.21 LA971334D (21) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Sauer, B. B.; Stika, K. M.; Swartzfager, D. G. Macromolecules 1997, 30, 2883.
