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Lateral Self-Organization and Ordering at Nanoheterogeneous Surfaces Hamidou Haidara,* Karine Mougin, Gilles Castelein, and Jacques Schultz Institut de Chimie des Surfaces et InterfacessICSI-CNRS, 15 Rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France Received June 30, 2000. In Final Form: September 22, 2000 We investigated the lateral morphological organization of a liquid crystalline material at binary heterogeneous surfaces, composed of the distribution of nanoscale domains of one molecular compound, within the continuum of a second molecular phase. These surfaces were shown to generate well-organized lateral microstructures, whose length scale, as compared to the underlying nanodomains size, is by 3 orders much larger, demonstrating the absence of any discrete contribution from the nanoscale domains. On the basis of these results, a modulation (cooperative-like effect) of the surface forces arising from the surface structures was proposed, which drives the overall wetting properties, structure formation, and related morphological transitions at these nanoheterogeneous surfaces.
Introduction Heterogeneous surface patterns have received over the past decade a great deal of attention from surface scientists. This growing interest was primarily motivated by the potential applications of these surfaces in new technologies, as well as their relevance as model heterogeneous systems in the fundamental understanding of interface phenomena. Yet, this growing field has produced a variety of surface patterns at both nanometer and micrometer scales, either by microprinting1-3 or nanophase separation of block copolymers.4-6 While the former technique is still limited to well-defined micrometer to millimeter scale geometrical patterns (square, circle, or parallel arrays), the latter has produced more complex geometry at the nanometer scale. Though both were shown to selectively template and drive interface phenomena in a certain way, none of them has so far produced at the nanometer scale, the most common heterogeneity that real surfaces usually offer. This is for instance a simple binary surface, made up of the distribution of curvilinear nanoscale domains of one molecular compound (lyophilic ) wetting, for example), in the continuum of a second molecular phase (lyophobic ) nonwetting). Such nanoheterogeneous molecular surfaces would meet the requirements for the model investigation of number of heterogeneous materials, whose surfaces are either composed of the distribution of crystalline or mesophase domains, in the continuum of an amorphous phase. In this paper, we first report on the possibility of an easy to handle and direct way to elaborate such model nanoheterogeneous molecular surfaces. Then, using liquid crystalline (LC) coatings, these nonuniform nanoheterogeneous structures are shown to template a stable and higher lateral length scale ordering, as compared to the intrinsic size of the underlying nanodomains. Some of the funda* Corresponding author. E-mail:
[email protected]. (1) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Science 1997, 273, 1425. (2) Drelich, J.; Miller, J. D.; Kumar, A.; Whitesides, G. M. Colloids Surf., A 1994, 93, 1. (3) Ondarc¸ uhu, T. J. Phys. II 1995, 5, 227. (4) Walheim, S.; Scha¨ffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (5) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H. J.; Sita, L. R. Langmuir 1998, 14, 241. (6) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610.
Figure 1. AFM picture (in phase contrast mode) of the hydrophobic hts nanoislands (clear domains), at the virginhydroxylated SiO2 substrate. The average domain size corresponding to these 1 min immersions, in 1 mM solutions of hts in carbon tetrachloride, at 21 °C is ∼40 ( 10 nm.
mental issues at the nanometer length scale and potential applications related to these heterogeneous surfaces are herein discussed. Results and Discussion To elaborate these nanoheterogeneous structures, we started from the well-established experimental fact that solvent-coated organosilane films self-assembled onto silicon wafers initially grow up as isolated nanoislands, before they touch to form a continuous and well-packed monolayer.7,8 Our method has then consisted in the unique control of the adsorption time for a given concentration (7) Davidovits, J.; Pho, V.; Silberzan, P.; Goldmann, M. Surf. Sci. 1996, 352-354, 369.
10.1021/la000915r CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
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Figure 2. AFM pictures (in phase contrast mode) of the complete nanoheterogeneous molecular surfaces: (a) hts nanoislands (clear) as shown in Figure 1, within the hydrophilic nh2 continuum (hts-nh2) and (b) nh2 nanodomains (clear), corresponding to 6 min adsorption, in hts continuum (nh2-hts). One can notice the well-defined topological structure of the hts domains, as compared to the shorter nh2 chains whose domains may grow in a less organized way as already demonstrated for alkyltrichlorosilanes.9
and temperature of the organosilane solution, to produce the desired number density and size of nanoislands. These nanodomain-coated samples were thoroughly rinsed with pure solvent to remove free and loosely adsorbed molecules and dried under nitrogen. The complete heterogenous binary molecular surfaces were obtained from the nanodomain-coated samples, by immediately adsorbing the continuous molecular phase in the remaining free space of the silicon substrate (SiO2), either by solvent or by vapor deposition.9,10 Both methods were successfully used, though only nanoheterogeneous surfaces processed by vapor deposition were considered in the following investigations. Two organosilane molecules,11 the hexadecyltrichlorosilane (referred to as hts) and the (((6-aminohexyl)amino)propyl)trimethoxysilane (referred to as nh2), soluble in carbon tetrachloride and ethanol, respectively, were used, at a concentration of 1 mM. The homogeneous molecular films of these organosilanes, which were made as reference surfaces, are, hydrophobic and hydrophilic, respectively. The SiO2 substrates were cleaned as described in the literature,12 prior to their immersion in the silane solution maintained at a temperature of 21 ( 0.5 °C. The adsorption times used in this study for the hts and nh2 nanoislands were 1 and 6 times, respectively. An atomic force microscopy (AFM) picture corresponding to hts nanoislands at the virgin SiO2 substrate is shown in
Figure 1. The nanodomain-coated wafers were then exposed to the vapor of the continuous molecular phase, in a dynamically evacuated chamber (1 h at ∼10-2 Torr). The AFM pictures of the resulting nanoheterogeneous molecular surfaces, composed of the distribution of hts nanodomains in the amine-terminated nh2 continuum (referred to as hts-nh2), and vice versa (nh2-hts), are shown in Figure 2.13 The topology, surface fraction, and characteristic domain size in these heterogeneous binary molecular surfaces can both be varied over a large scale, by changing any of the fundamental control parameters (C, T°, and t). To illustrate the potential applications related to these nanoheterogeneous molecular surfaces as model systems in the fundamental understanding and technological use of nanoscale effects in interface phenomena (film stability, pattern formation, aggregation morphology), we have chosen to investigate the stability and structure formation by LC coatings at these surfaces. This choice for LC coatings was primarily motivated by the existing literature14 on their temperature-driven morphological transitions (annealing), which may serve as reference results for this work. The LC used in this work was the 4-cyano4-n-hexyl-biphenyl (6-CB) at a concentration of 2.5 mM in cyclohexane. The advancing contact angles θa which are the most representative for this volatile LC solution
(8) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (9) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (10) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (11) Both methyl-terminated hexadecyltrichlorosilane, Cl3Si(CH2)15CH3, and (((6-aminohexyl)amino)propyl)trimethoxysilane, (OCH3)3Si(CH2)3-NH-(CH2)6-NH2, were from ABCR GmbH & Co. KG, Karlsruhe, Germany. (12) Ulmann, A. An introduction to Ultrathin Organic films, from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991; pp 245-253.
(13) The typical roughness at these nanoheterogeneous surfaces, which mainly arises from the difference in the thickness of the two molecular phases 1 and 2 nm for the nh2 and hts domains, respectively), lies in the subnanometer range, as estimated by ellipsometry. The compositional heterogeneity at the molecular scale, revealing the presence of nh2 and hts compounds, has been analyzed, both by X-ray photoelectron and by infrared (IRRAS) spectroscopy. This compositional heterogeneity has also been characterized by the advancing and receding contact angles (θA, θR) of a water droplet, leading to 87° and 56° for hts-nh2 and 108° and 86° for nh2-hts, as compared to the uniform monolayers of nh2 (59°, 21°) and hts (111°, 101°). (14) Sheiko, S.; Lermann, E.; Mo¨ller, M. Langmuir 1996, 12, 4015.
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Figure 3. Selected optical micrographs (in the medium range of the drop radius) of the LC lateral structure at the nanoheterogeneous surfaces (T ) 21 °C): (a) uniform nh2 molecular surface; (b) nanoheterogeneous hts-nh2 surface; (c) nanoheterogeneous nh2-hts surface as appearing in Figure 2b; (d) nanoheterogeneous nh2-hts surface with a lower surface fraction of nh2 domains as compared to (b).
are, respectively, 26° on hts, and about 7° on both nh2, hts-nh2, and nh2-hts, at 20 °C. In comparison, the equilibrium contact angles θe of the pure LC liquid are 60° on hts,
