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Nanoheterogeneous Surfaces in the Control of Interface Phenomena H. Haidara,* K. Mougin, and J. Schultz Institut de Chimie des Surfaces et Interfaces, CNRS, 15 Rue Jean Starcky, B.P. 2488, F-68057 Mulhouse Cedex, France Received April 19, 2000. In Final Form: June 26, 2000 We have elaborated model heterogeneous molecular surfaces, which are characterized by the distribution of nanoscale domains of one molecular compound in a continuum of the second. Interestingly, it was found that the macroscopic equilibrium wetting parameters completely fail to account for the interface phenomena at these nanoheterogeneous surfaces, especially the dewetting behavior of confined thin films, as investigated in this work. Though strongly affected by the surface fraction and topological features at the nanoheterogeneous surfaces, the dewetting parameters (kinetics and morphology) do not show any direct correlation to the underlying surface patterns. Instead, a more subtle interplay between the bulk subphase property and the nanoscale top-layer chemistry is observed. On the basis of this interplay, the existence of two dynamic regimes, determined by the time-dependent film thickness ht, was demonstrated during the course of the dewetting process: the early-stage film destabilization dominated by the bulk-subphase property (long-ranged forces) and the late-stage film thinning, nucleation, and hole growth, controlled by the top-layer chemistry (short-range forces and local effects at the wall).
Introduction Surface heterogeneities of either compositional or morphological origin are known to drastically affect interface phenomena. Depending on their chemical and topological pattern, these surface heterogeneities often generate a wide variety of singularities when they are involved in interfacial phenomena (wetting, 2-D diffusion, and aggregation). The recent efforts to elaborate and characterize such heterogeneous surfaces1-8 are motivated by their tremendous importance in both current technologies (painting, printing, lubrication, and templating) and biomedicine (selective adhesion and immobilization of cells and biomolecules). We here report some interesting findings related to model heterogeneous molecular surfaces, either composed of the discrete distribution of nanoscale domains (∼10 nm-∼1 µm) of one molecular compound in the continuous phase of the second or characterized by the bicontinuous 2-D pattern of both molecular phases. An interesting feature of these structures is that they lie at the boundary of two characteristic morphology and length scales: (i) micrometer-millimetersize alternating arrays or patches of two molecular compounds1,4,6-8 and (ii) the “uniform” mixture of different molecules3,5 in which the largest domain size is determined by segregation effects and remains on the molecular scale. The fundamental issue related to these nanoheteroge* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (33) 03 89 60 87 67. Fax: (33) 03 89 60 87 99. (1) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Science 1997, 273, 1425; Biotechnol. Prog. 1998, 14, 356. (2) Walheim, S.; Scha¨ffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (3) Wirth, M. J.; Fairbank, R. W. P.; Fatunmbi, H. O. Science 1997, 275, 44. (4) Ro¨der, H.; Hahn, E.; Brune, H.; Bucher J.-P.; Kern, K. Nature 1993, 366, 141. (5) Folkers, J. P.; Laibinis P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (6) Drelich, J.; Miller, J. D.; Kumar, A.; Whitesides, G. M. Colloids Surf., A 1994, 93, 1. (7) Ondarc¸ uhu, T. J. Phys. II Fr. 1995, 5, 227. (8) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610.
neous surfaces is the way the chemistry, topology, and surface fraction, φs, of such a binary structure can affect the force profile and the emerging interface phenomena at these surfaces. We here bring some striking evidence to this issue based on the investigation of the dewetting behavior of confined-liquid thin films at these model heterogeneous surfaces. These nanopatterned surfaces may also provide a model system for the interface properties of real surfaces that often present mesoscale heterogeneity of one material in the continuum of either an identical or a different material (semicrystalline polymers, block copolymers, and nanomaterials). Results and Discussion For these investigations, we used three model heterogeneous surfaces based on the combination of two molecular compounds: the methyl-terminated n-hexadecyltrichlorosilane Cl3Si(CH2)15CH3, referred to as hts, and the NH2-terminated n-(6-aminohexyl)aminopropyltrimethoxysilane (OCH3)3-Si(CH2)3-NH-(CH2)6-NH2, referred to as nh2. The two first surfaces are composed of a distinct size distribution of the hydrophilic nh2 nanodomains, in the hydrophobic continuum of the hts molecular film (respectively referred to as nh2l/hts and nh2s/ hts, l and s being for large and small domains). The third surface corresponds to a distribution of the hydrophobic hts nanodomains within the continuum phase of the nh2 molecular film (referred to as hts/nh2). These nanoheterogeneous surfaces were complemented with two reference homogeneous surfaces, the uniform hts and nh2 molecular films. The molecular films were formed on silicon wafers (SiO2) according to a standard path of selfassembling of organosilanes from dilute solution onto hydroxylated substrates.9 The heterogeneous surfaces were obtained through a two-step process based on the time control of the early-stage nucleation and domaingrowth mechanisms and well-established for these self(9) Ulman, A. An introduction to Ultrathin Organic Films, from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991; pp 245-253.
10.1021/la0005917 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000
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Figure 1. AFM pictures of the hydrophobic hts nanodomains on the hydrophilic SiO2 substrate: (a) Height mode (nanoislands ) clear domains) and (b) phase mode (nanoislands ) clear nodes).
assembling processes.10,11 The full sample process (adsorption) was performed in a regulated thermal bath at a constant temperature of 20 ( 0.5 °C. After the cleaning and piranha activation step of the wafers,9 the isolated nanodomains were first realized by immersing the wafers into millimolar (1 mM) solutions of hts in carbon tetrachloride (1 min for hts nanodomains) and nh2 in ethanol (3 and 6 min, respectively, for nh2s and nh2l nanodomains). This first step led to well-distributed nanoislands of both molecular films on the silicon wafer, as revealed by different surface techniques. The advancing and receding contact angles and the related hysteresis of water on these surfaces, respectively, were θA ≈ 78° and θR ≈ 54° for hts nanoislands and θA ≈ 60° and θR < 5° for nh2s nanoislands, as compared to θA ≈ θR < 5° for the clean wafers. The average size and surface coverage, respectively, of these nanoislands, as assessed by atomic force microscopy (AFM), were 20 nm and 3% for hts (Figure 1a, height mode, and b, phase mode) as compared to 12 nm and 2% for nh2s. As shown in these AFM pictures and confirmed both by X-ray photoelectron and infrared spectroscopy, the above wetting properties definitely arise from the new surface chemistry and topological organization of these initial discrete distributions of organic nanoislands on the virgin silicon substrate. The final and complete nanoheterogeneous surfaces were obtained by assembling a continuous monolayer of the second molecular phase around the nanodomains in the remaining space of the virgin SiO2 substrate (nh2 around hts nanoislands and vice versa). This final step, involving an adsorption time of ∼10 h is preceded by the solventcleaning and N2-drying of the samples at the end of the first step. It is worth mentioning that notwithstanding the similar hydrophilic character of the hydroxylated virgin SiO2 substrate and the nh2 surface, the substitution of the former for the nh2 monolayer presents a 2-fold advantage. Both nanoislands and continuum phases are of the same nature (organic films), and the surface roughness is reduced to the sub-nanometer range, the ellipsometric thickness of the organic films being, respectively, 2 nm for hts and 1 nm for nh2. As in the first step, X-ray photoelectron and infrared spectroscopy were used to confirm the compositional heterogeneity (hts and (10) Davidovits, J.; Pho, V.; Silberzan, P.; Goldmann, M. Surf. Sci. 1996, 352-354, 369. (11) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354.
Figure 2. AFM pictures (phase mode) of the complete nanoheterogeneous molecular surfaces: (a) hts/nh2 surface (hts nanodomains in clear), (b) nh2s/hts (nh2 nanodomains in clear), (c) nh2l/hts (nh2 nanodomains in clear), and (d) cross-sectional analysis corresponding to the height-mode AFM picture of nh2l/ hts surface shown in panelc.
nh2 groups) at the nanoheterogeneous surfaces, the representative AFM patterns of which (phase-contrast mode) are shown in Figure 2a-c. Figure 2d represents a cross-sectional analysis drawn from the height-mode AFM picture of the heterogeneous nh2l/hts surface shown in Figure 2c. This cross-sectional profile leads to an average surface roughness of ∼1.14 nm, in complete agreement with that estimated by ellipsometry from the thickness difference (∆h ∼ 1 nm) between the two uniform molecular films composing the nanohetrogeneous surface. The wetting properties of these final nanopatterned molecular surfaces were, respectively, θA ) 87° and θR ) 56° for hts/nh2, θA ) 110° and θR ) 90° for nh2s/hts, and θA ) 108° and θR ) 86° for nh2l/hts, as compared to the reference uniform monolayers of nh2 (θA ≈ 59°, θR ≈ 21°) and hts (θA ≈ 111°, θR ≈ 101°). As one would expect, the advancing angles of the macroscopic water drop on these surfaces are mainly determined by the hydrophobic hts-domain fraction, whereas the receding ones are representative of the hydrophilic nh2 domains. Interestingly, this secondstep adsorption, especially in ethanol (for the hts/nh2 surface), results in a reorganization of the initial nanoislands and diffuse adsorbed molecules into larger connected clusters, as observed in Figures 1 and 2. Such reorganization can take place through the diffusionaggregation of the initial nanoislands, driven by (i) the solvophobic interactions with the surrounding solvent of the molecules composing the continuous phase and (ii) the growing surface pressure related to the adsorption of these continuous-phase molecules around the nanoislands. In an attempt to check for the influence of these nanoheterogeneous surfaces on the emerging surface forces and related interface phenomena, we chose to study the dewetting behavior of thin silicon-oil films confined between the model surfaces and a surrounding water
Nanoheterogeneous Surfaces
Figure 3. Dewetting kinetics of the confined 100-mPa‚s silicon oil on the 2-D nanoheterogeneous surfaces in hole radius (Rt - R0) versus time: (b) uniform hts molecular film, R0 ) 46 µm, V ∼ 0.03 µm/s; (9) nh2s/hts surface, R0 ) 88 µm, V ∼ 0.44 µm/s; and (2) nh2l/hts surface, R0 ) 56 µm, V ∼ 0.71 µm/s. For both uniform nh2 and heterogeneous hts/nh2 surfaces, a quasiinstantaneous dewetting process (nucleation and hole opening) is obesrved within a time scale t , 1s.
medium. Because dewetting (film destabilization, holegrowth rate) is driven by both long- and short-range molecular forces, one expects the whole dewetting dynamics and final morphology to strongly depend on the specific feature of the nanoheterogeneous surfaces. For the dewetting experiments, a 300-nm silicon-oil film (viscosity, 100 mPa‚s) was spin-coated onto the model surface, and the sample was immediately immersed in water (width of the surrounding water, ∼5 mm). The full experiment was followed and recorded using a video camera. The most significant results of these studies are given in Figures 3 and 4a-f for the dewetting velocities and the final dewetting morphologies, respectively. These results revealed few remarkable differences between these nanoheterogeneous surfaces, some of which were unexpected, regarding the basic physics involved in these dewetting processes. For instance, it has been shown12,13 for these confined films that when the equilibrium contact angle, θE, and relative viscosity, ηfilm/ηsm, between the dewetting film and the surrounding medium, sm, satisfy θE < 1 < ηfilm/ηsm, the hole opening velocity then scales as V ∼ (γ/η)θE3. Because the water-oil interface tension γOW and the viscosity, η, of the dewetting film are identical for both surfaces, one expects the hole opening velocity, V, to differ only by θE3 for nanoheterogeneous surfaces of θE < 1 rad (∼58°). We measured these equilibrium θE’s of siliconoil droplets in water medium to be, respectively, 11° for the hts surface, 14° for nh2s/hts, ∼23° for nh2l/hts and hts/nh2, and 92° for nh2. On the basis of the unique θE3 dependence of V, one would expect the lowest dewetting velocity to be similar for hts and nh2s/hts, while for nh2l/ (12) Brochard-Wyart, F. J. Phys. II Fr. 1994, 4, 1727. In this paper (p 1728), which considers the dewetting of a confined water film of thickness, e, between a solid substrate and a rubber sheet, a criterion for a hole of radius, R, to expand is given as R > e2/h0, where h0 ) |S|/µ and µ ) the elastic modulus of the rubber. Taking the adimensional ratio (R/e) from these relationships then results in the criterion R/e > µe/|S| ∼ γ/|S|. (13) Joanny, J. F.; Andelman, D. J. Colloid Interface Sci. 1987, 119, 451.
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Figure 4. Snapshots of the final dewetting morphologies on the 2-D nanoheterogeneous and reference uniform surfaces. The patterns are formed by the droplets of the dewetting siliconoil film: (a) uniform hts surface; (b) uniform nh2 surface, where one can notice the characteristic size and quasi-periodic distribution of the polygonal cells; (c) hts/nh2 surface; (d) nh2s/ hts surface; (e) nh2l (6-min adsorption)/hts surface; and (f) nh2 (10-min adsorption)/hts.
hts, hts/nh2, and nh2, V should be higher, with Vnh2 > Vhts/nh2 ) Vnh2l/hts. Our results do show some discrepancy, as shown in Figure 3, where the magnitude of the dewetting velocity at the nh2s/hts surface is found to be an order higher than that at the hts surface, whereas Vnh2 ≈ Vhts/nh2 > Vnh2l/hts. The above discrepancy cannot be accounted for by the overestimation14 of the dynamic dewetting contact angle, θD ≈ θE/x2, involved in the expression of V and which arises from the assumptions that (i) the velocity of the moving rims at both film and hole sides are identical and (ii) the contact angle of the rim is zero at the film side. Furthermore, such an overestimation would have resulted in a global lowering of V, keeping their relative magnitude identical, for a similar θE. What these results clearly point out is that the equilibrium wetting parameters are irrelevant in accounting for the interface phenomena at these nanoheterogeneous surfaces. In addition to these dynamic aspects, the final dewetting morphology, both patterns and characteristic sizes, though strongly affected by the surface fraction and topological feature of the thin top layer, does not show any direct relationship to the heterogeneous surface pattern (Figures 2, 4). For instance, both of the nh2/hts surfaces exhibit quite a similar dewetting morphology (average droplets size and patterns), even for the much larger nh2-domain fraction, as shown in complementary experiments where nh2/hts surfaces corresponding to 10-min nh2-domain growth have been used (Figure 4f). In fact, there is no gradual evolution of these dewetting morphologies, with the increasing nh2-domain fraction within hts, toward the well-ordered polygonal dewetting morphology of the uniform nh2 surface. Instead, the hts/ nh2 surface (1-min hts-domain growth) in which the domain fraction of hts and nh2 compounds is comparable (14) Andrieu, C.; Sykes, C.; Brochard, F. J. Adhes. 1996, 58, 15.
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to the nh2(10 min)/hts sample (Figure 4f) does exhibit the characteristic polygonal dewetting feature (Figure 4b) of the uniform-nh2 surface. This result constitutes interesting experimental evidence of the predominant influence of the topological feature on nanopatterned surfaces for comparable surface fractions of the molecular compounds. Finally, the most unexpected feature of these dewetting patterns is their drastic change with the chemistry of the very thin topmost molecular layers (1-2 nm) for an identical bulk substrate (silicon wafer). The silicon film (300 nm, 100 mPa‚s) and boundary phase being identical, one actually would expect, based on the predominant effect of long-range van der Waals forces over the interface, the dewetting patterns at hts, hts/nh2, nh2, and nh2/hts surfaces to be controlled by the bulk silicon substrate, irrespective of the nature and organization of the nanometer-size top layer. Our results do not totally corroborate this prediction. Instead, they clearly seem to reveal some more subtle interplay between the bulk subphase properties and the chemistry and organization of the thin top layer. This result can be understood from the development of the instability in the dewetting film, which should bring its initial thickness, h0, to some critical value, hc (hc/h0 ∼ 0), at hole-nucleation sites. The influence of the thin-top-layer chemistry in the late-stage evolution of the decreasing film thickness (ht/h0 , 1) may become predominant to definitely control the local critical value hc and the overall features of the dewetting dynamics (characteristic time, hole density, pattern). It then seems clear from our results that one should distinguish between two regimes when considering the stability and dewetting characteristics of thin coating in relation to the influence of the chemistry of a nanometer-scale top layer, either adsorbed (hydration layer, for instance) or grafted onto a bulk support. In the first regime, above hc (hc/ht , 1), the interface dynamics and film stability are controlled by long-range forces, which, for a given film and boundary phase, is completely dominated by the bulk substrate contribution. In the second regime, around hc (hc/ht ∼ 1), short-range forces and local effects (boundary conditions and friction at the wall) may become predominant across the whole remaining film thickness, and the late stage evolution of the dewetting (hole opening and growth) is critically determined by the top-layer chemistry. Because the hole density was shown12,15 to be proportional to a characteristic adimensional number related to the interface energies, (∼ γOW/|S|, S ) γSW - γSO - γOW being the spreading parameter), it turns out that what ultimately drives these morphologies is modulation of the surface forces emerging from the nanoheterogeneous structure. The comparison of Figures 2 and 4 actually does support this conjecture, because it precludes any direct epitaxial hole-nucleation process. Furthermore, in these experiments, where the silicon-oil film is bounded by the pure bulk-water phase, we do not expect more (15) Haidara, H.; Vonna, L.; Schultz, J. Langmuir 1998, 14, 3425; see Appendix.
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contaminants (defects) to be present within the system as compared to the air environment, where both systems remain stable. This allows us to reasonably assume the predominant influence of surface forces over any defectinduced nucleation in the dewetting process. Though the subtle mechanisms of this surface force modulation remain unclear, they should involve some lateral fluctuation of the Hamaker constant16 over the nanodomain and continuum phase, to affect the late-stage dewetting process (hc/ht ∼ 1) through the van der Waals excess free energy of the film, g ) -A*/12πh2 (h, film thickness; A*, laterally fluctuating Hamaker constant). Actually, for our confined films of constant thickness, h, interface tension, γOW, and viscosity, η, the wavelength of the surface instabilities (q-1 ∼ h2xγ/A*) which leads to spontaneous dewetting and, thus, determines the final morphology and the characteristic time scale for hole nucleation exclusively depends16,17 on the magnitude of the Hamaker constant. How the Hamaker constant A* emerging from the 2-D nanoheterogeneous patterns scales with the characteristic size and surface density of the phase domains constitutes some of the fundamental questions to elucidate. And yet, the possibility this method offers for the control of both the surface fraction and the topological features of the chemical phases in these nanoheterogeneous systems constitutes a determining result regarding many aspects of interface phenomena (e.g., biointerface, nanotribology, thin films, and patterning). Though these investigations have mainly been concerned with nanoheterogeneous surfaces presenting two molecular phases of completely different chemical natures, the above modulation of A*, through the longrange van der Waals forces, can potentially be observed on less well-defined heterogeneous surfaces. In fact, any surface distribution of mesophase domains (crystalline, for instance) in an amorphous continuum of the same material, such as in polyethylene, may result in similar physics. This only arises from the dependence of A* on the mole-number density, F, of the material18 (A* ) π2F2Cij), which significantly varies with the local organization {F(amorphous) , F(crystalline) for identical molecules and pair potential interaction parameter, Cij}. Regarding both of these issues, the present results may constitute, among others, a path toward a better understanding of the interplay between 2-D nanostructuration and the control of interface phenomena. Acknowledgment. The authors gratefully thank Dr. G. Castelein for his appreciable help with atomic force microscopy (AFM). LA0005917 (16) Safran, S. A.; Klein, J. J. Phys. II Fr. 1993, 3, 749. (17) Sharma, A.; Khanna, R. Phys. Rev. Lett. 1998, 81, 3463. (18) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992; pp 176-177.
