Confinement-Induced Film Thickness Transitions in Liquid Crystals

Liquid Crystals between Two Alkanethiol Monolayers on. Gold. Marina Ruths,* ... 10 nm of gold on mica without an adhesion layer results. † Max-Planc...
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Langmuir 2001, 17, 6213-6219

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Confinement-Induced Film Thickness Transitions in Liquid Crystals between Two Alkanethiol Monolayers on Gold Marina Ruths,*,†,§ Manfred Heuberger,*,‡ Volker Scheumann,† Jianjiang Hu,† and Wolfgang Knoll† Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany, and Laboratory for Surface Science and Technology, ETH Zentrum, CH-8092 Zu¨ rich, Switzerland Received February 21, 2001. In Final Form: June 20, 2001 We describe the use of two semitransparent gold layers of equal thickness as substrates in the interferometric surface forces apparatus. The continuous gold layers were evaporated onto an adhesion layer of chromium on muscovite mica that had been exposed to water vapor plasma to introduce reactive groups on its surface. Multiple beam interferometry was used to measure the thickness of mixed alkanethiol monolayers covalently bound to the gold and the film thickness of homeotropically oriented liquid crystals confined between these monolayers. In 4′-n-pentyl-4-cyanobiphenyl, quasi-periodic structural forces with transitions comparable to the length of a liquid crystal dimer were observed at film thicknesses below 13 nm, indicating that confinement-induced smecticlike ordering can occur also between two surfaces with a root-mean-square roughness of e1 nm. Similar transitions could also be observed in 4′-n-octyl-4cyanobiphenyl.

Introduction Physisorbed self-assembled or Langmuir-Blodgett deposited monolayers with different packing densities are frequently used model systems in friction and adhesion studies with the surface forces apparatus (SFA)1,2 and are also useful as aligning layers for liquid crystal films.3-6 The substrate supporting the monolayers is typically muscovite mica, an aluminosilicate that can be cleaved to expose macroscopically large, atomically smooth surfaces that are not easily modified chemically. However, selfassembled structures covalently bound to a substrate would be of particular interest for certain applications because of their good stability in solution, their potential as substrates for synthesis,7 and their better wear properties.8 Substrates exposing silica,9-11 alumina,8,12 or †

Max-Planck-Institut fu¨r Polymerforschung. Laboratory for Surface Science and Technology, ETH Zentrum. § Present address: Department of Physical Chemistry, A ° bo Akademi University, Porthansgatan 3-5, FIN-20500 A° bo, Finland. ‡

(1) Yoshizawa, H.; McGuiggan, P.; Israelachvili, J. Science 1993, 259, 1305. Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. (2) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736. Ruths, M.; Granick, S. Langmuir 1998, 14, 1804. (3) Proust, J. E.; Ter-Minassian-Saraga, L. J. Phys., Colloq. 1975, 36, C1-77. Hiltrop, K.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 884; 1981, 85, 582. (4) Horn, R. G.; Israelachvili, J. N.; Perez, E. J. Phys. (Paris) 1981, 42, 39. (5) Cognard, J. In Tribology and the Liquid-Crystalline State; Biresaw, G., Ed.; ACS Symposium Series 441; American Chemical Society: Washington, DC, 1990; pp 1-47 and references therein. (6) Ruths, M.; Steinberg, S.; Israelachvili, J. N. Langmuir 1996, 12, 6637. (7) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592, 602. (8) Berman, A.; Steinberg, S.; Campbell, S.; Ulman, A.; Israelachvili, J. Tribol. Lett. 1998, 4, 43. (9) Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162, 404. Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375. (10) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367. (11) Ruths, M.; Johannsmann, D.; Ru¨he, J.; Knoll, W. Macromolecules 2000, 33, 3860. (12) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3, 413.

silicon nitride13 surfaces have been developed for use in the SFA to meet this need. Despite their widespread use as substrates for covalently bound self-assembled monolayers of thiol and disulfide compounds,14-16 noble metal surfaces are seldom used in the SFA. The main difficulty lies in forming metal layers with sufficient smoothness over large areas to enable measurements at the molecular level. Techniques exist to evaporate metal onto mica or silica, attach the metal layer to another support, and then remove the mica to expose a very smooth metal surface.17 However, in the standard SFA, the separation distance between the front sides of two substrates is deduced from multiple beam interferometry between reflecting (silver) coatings on their backsides.18-20 In contrast to a system of two reflecting surfaces facing each other (frontside mirrors), this setup has the advantage that the separation distance and shape of the surfaces can be determined from interference fringes also when the front surfaces are in or close to contact. The analysis of the optical interference pattern arising from multiple layers is fairly involved and has thus far limited the use of reflective or absorbing materials on the front surfaces in the SFA. Moreover, since such coatings need to be thin for transparency and smoothness, the wellknown dewetting of noble metals from mica and silica surfaces21-26 presents a serious problem. Evaporation of ca. 10 nm of gold on mica without an adhesion layer results (13) Golan, Y.; Alcantar, N. A.; Kuhl, T. L.; Israelachvili, J. Langmuir 2000, 16, 6955. (14) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (15) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (16) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (17) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39. Wagner, P.; Hegner, M.; Gu¨ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867. Stamou, D.; Gourdon, D.; Liley, M.; Burnham, N. A.; Kulik, A.; Vogel, H.; Duschl, C. Langmuir 1997, 13, 2425. Samorı´, P.; Diebel, J.; Lo¨we, H.; Rabe, J. P. Langmuir 1999, 15, 2592. (18) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (19) Mangipudi, V. S. J. Colloid Interface Sci. 1995, 175, 484. (20) Heuberger, M.; Luengo, G.; Israelachvili, J. Langmuir 1997, 13, 3839.

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in a dewetting layer of flattened islands with a diameter of a few hundred nanometers, separated by gaps right down to the mica surface. Such discontinuous films have been observed at thicknesses of up to 50-100 nm.21,26 Previous work on metal surfaces in the interferometric SFA is generally based on having a thick (and therefore continuous) metal layer on only one of the surfaces, which then acts like a frontside mirror. In early work, Coakley and Tabor studied the van der Waals forces in air between one bare, back-silvered mica sheet and one 50 nm thick silver surface.27 A similar system was later used by Parker and Christenson28 to measure structural forces in octamethylcyclotetrasiloxane and electrostatic forces in water. Around the same time, Smith et al. measured electrostatic forces in water between two back-silvered mica surfaces covered with 4 nm thick layers of platinum.29 Recently, the analysis of multilayer interferometers has been advanced by Levins and Vanderlick.30-32 Vanderlick and co-workers have studied the contact mechanics in air of 50 nm thick, bare or thiol-covered silver30,33 or gold33,34 layers (roughness amplitude of around 5 nm30,33 for silver and root-mean-square (rms) roughness of 1.9 nm34 for gold) in contact with a bare mica surface (silvered on its backside). They have also studied the contact between one atomically smooth gold surface (bare or thiol-covered) and one bare, back-silvered mica surface.35 The smooth gold (rms roughness of 0.4 nm) was formed by utilizing the cold-welding between two compressed, clean gold surfaces to break the contact between evaporated gold and one of the supporting mica surfaces in a 100 µm2 area,35 a development of a technique used by Parker and Christenson for evaporated silver.28 In a recent study, Sheth et al.36 measured the interaction forces in water between one polymer brush (attached to a physisorbed bilayer on a mica surface) and a silver layer (thickness of 50 nm and rms roughness of 1.3 nm) modified by adsorption of alkanethiol. We are interested in preparing gold-covered surfaces with sufficiently low roughness to allow measurements of interaction forces between two identical surfaces at distances well below 10 nm. Gold absorbs visible light strongly, and for this reason, its thickness must not exceed approximately 10 nm for the interference fringes to be visible to the eye. It is known that continuous polycrys(21) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (22) George, M. A.; Bao, Q. C.; Sorensen, I. W.; Glaunsinger, W. S.; Thundat, T. J. Vac. Sci. Technol., A 1990, 8, 1491. (23) Winau, D.; Koch, R.; Fu¨hrmann, A.; Rieder, K. H. J. Appl. Phys. 1991, 70, 3081. (24) Vogt, K. W.; Kohl, P. A.; Carter, W. B.; Bell, R. A.; Bottomley, L. A. Surf. Sci. 1994, 301, 203 and references therein. (25) Russell, S. W.; Rafalski, S. A.; Spreitzer, R. L.; Li, J.; Moinpour, M.; Moghadam, F.; Alford, T. L. Thin Solid Films 1995, 262, 154 and references therein. (26) Liu, Z. H.; Brown, N. M. D.; McKinley, A. J. Phys.: Condens. Matter 1997, 9, 59. (27) Coakley, C. J.; Tabor, D. J. Phys. D 1978, 11, L77. (28) Parker, J. L.; Christenson, H. K. J. Chem. Phys. 1988, 88, 8013. (29) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S.; McClure, D. J. J. Phys. Chem. 1988, 92, 199. Maeda, M.; White, H. S.; McClure, D. J. J. Electroanal. Chem. 1986, 200, 383. (30) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1992, 96, 10405. (31) Levins, J. M.; Vanderlick, T. K. J. Colloid Interface Sci. 1993, 158, 223. (32) Levins, J. M.; Vanderlick, T. K. Langmuir 1994, 10, 2389. (33) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1995, 99, 5067; J. Colloid Interface Sci. 1997, 185, 449. (34) Quon, R. A.; Knarr, R. F.; Vanderlick, T. K. J. Phys. Chem. B 1999, 103, 5320. Quon, R. A.; Ulman, A.; Vanderlick, T. K. Langmuir 2000, 16, 3797. (35) Knarr, R. F.; Quon, R. A.; Vanderlick, T. K. Langmuir 1998, 14, 6414. (36) Sheth, S. R.; Efremova, N.; Leckband, D. E. J. Phys. Chem. B 2000, 104, 7652.

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talline Au(111) layers with a thickness of 10 nm and up are readily evaporated onto glass or silica if an “adhesion layer” (typically chromium or titanium) is used between the gold and the substrate.24,25 This approach has been used for force measurements in aqueous solution between alkanethiol monolayers on gold in a MASIF instrument by Ederth et al.37 (The instrument, which is based on a piezoelectric bimorph force sensor,38,39 has also been used to measure the van der Waals forces between two 53 nm thick silver layers in air39). The solid glass spheres used as substrates were covered with 1.0-1.6 nm of titanium and 10 nm of gold (rms roughness of 0.15-0.2 nm).37 We found that on thin quartz-glass sheets that can be used as substrates in the interferometric surface forces apparatus,9 a continuous metal layer with an rms roughness of 0.4-0.7 nm could be formed by thermal evaporation of 1.5 nm of chromium and 8 nm of gold, without any pretreatment of the surface. However, two such quartzglass sheets are always of different thicknesses, and the thickness may vary across each sheet. It would greatly simplify the calibration procedures and the calculations of distance (film thickness) from the interference pattern if a symmetrical interferometer could be assembled. The adhesion layers used on glass or silica substrates consist of a transition metal with a solid solubility in gold.24,25,40 Metal atoms from the adhesion layer that are in contact with the substrate are able to react with oxygen chemically bound to the substrate surface, which prevents dewetting.24,25,40 By introducing reactive groups (mainly hydroxyl groups) on the mica surface by means of an established procedure, water vapor plasma treatment,41,42 we found that a chromium adhesion layer could be used to promote the formation of continuous, thin gold layers on mica. Because of the complex multilayer structure of our interferometer, accurate distance measurements were not feasible with the conventional linear approximation.18,43 In particular, the gold layers inside the interferometer exhibit substantial absorption and phase change. As a consequence, the interference fringes are shifted and only every second fringe is visible (cf. Figure 2b). An alternative data evaluation procedure was selected, which takes all these effects into account. The wavelengths of the fringes were determined manually as in conventional SFA experiments. By using the theory of wave propagation in stratified media,44 we calculated the thickness of all optical layers and finally created precise conversion tables to directly transform measured wavelengths into distances in each experiment. To evaluate the feasibility of force measurements between rough surfaces, we have investigated confined films of molecules that in the bulk form liquid crystals,45 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-n-octyl-4-cyano(37) Ederth, T.; Claesson, P.; Liedberg, B. Langmuir 1998, 14, 4782. Ederth, T.; Liedberg, B. Langmuir 2000, 16, 2177. Ederth, T.; Claesson, P. M. J. Colloid Interface Sci. 2000, 229, 123. (38) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (39) Parker, J. L. Langmuir 1992, 8, 551. (40) Onyiriuka, E. C.; Kinney, L. D.; Binkowski, N. J. J. Adhes. Sci. Technol. 1997, 11, 929 and references therein. (41) Parker, J. L.; Cho, D. L.; Claesson, P. M. J. Phys. Chem. 1989, 93, 6121. (42) Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.; Tidblad, J.; Blomberg, E. J. Colloid Interface Sci. 1990, 134, 449. (43) Hunter, S. C.; Nabarro, F. R. N. Philos. Mag. 1952, 43, 538. (44) Born, M.; Wolf, E. Principles of Optics, 6th ed.; Pergamon Press: Oxford, 1980. (45) Karat, P. P.; Madhusudana, N. V. Mol. Cryst. Liq. Cryst. 1976, 36, 51.

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biphenyl (8CB). Certain self-assembled mixed monolayers of alkanethiols (SAMs) can orient 5CB and 8CB molecules homeotropically (perpendicularly) to a single surface.46,47 We have studied the interactions of two alkanethiolcovered gold surfaces with an rms roughness of 0.7-1.0 nm across thin films of 5CB and 8CB. Even though our surfaces were significantly rougher than the previously studied surfactant monolayers physisorbed on mica substrates,4,6 we could still observe an induced smecticlike ordering of these cyanobiphenyls at small film thicknesses as a result of confinement. Experimental Section The interaction forces were measured with a Mark II surface forces apparatus.48 In this instrument, the force acting between two crossed, cylindrically curved, semi-transparent surfaces is calculated from the deflection of a double-cantilever leaf spring supporting one of the surfaces. The deflection is deduced from the change in distance between the surfaces, measured by multiple beam interferometry (cf. separate section below) as the base of the spring is moved a known distance. A calibration of this movement is done at large separation where no forces act between the surfaces. The spring constant used in our experiments was k ) 5.3 × 105 mN/m. The preparation of substrates started with the standard procedure for preparing mica surfaces for the SFA: Step-free sheets of muscovite mica with uniform thickness (chosen in the range of 4-5 µm) were cleaved from thicker blocks (grade no. 2 ASTM V-2, S & J Trading, Glen Oaks, NY). The thin sheets were cut in smaller pieces with a hot platinum wire, and these were placed on a thick support sheet of mica for thermal evaporation of 53 nm of silver (Unaxis, purity 99.99%). The silver layer forms the reflecting mirror (backside of the substrates) in our experiments. Later, separate measurements of the interferometer thickness were done in mica-mica contact, with Cr/Au layers, and after deposition of SAMs on the gold. For this reason, each cleaved thin sheet needed to be large enough so that six smaller pieces with identical thickness could be obtained from it. Since only every second interference fringe can be observed after absorbing metal layers are deposited on the front surfaces (cf. Figure 2b), it was not necessarily an advantage to use thinner mica as this gives fewer fringes to observe. For the water vapor plasma treatment, four silvered mica pieces of equal thickness (each with an area of 0.7-1 cm2) were placed with their silvered side down onto small droplets of water on a new, freshly cleaved mica support sheet. The water droplet under each small piece spread and caused it to adhere temporarily to the support sheet while its edges were covered carefully with narrow strips of freshly cleaved mica (thickness of 1-2 µm). This procedure is necessary to prevent damage to the silver during the plasma treatment and causes the piece to stick securely to the support sheet in all subsequent deposition steps. The water droplets under the pieces evaporated as they were kept in the vacuum chamber before the plasma treatment. Each support sheet with pieces was exposed to water/Ar plasma under continuous wave mode at an RF power of 37 W for 3 min. The Ar flow was controlled at a flow rate of 5 sccm by an MKS gas flow meter (type 1259C), while water vapor was led to the reactor through a floating flowmeter. The process pressure of the plasma treatment was maintained at 0.20 mbar (MKS type 647B). These conditions, which are similar to the ones used by Parker et al.,41,42 are expected to render the mica surface reactive (mainly by the formation of silanol groups) without affecting its thickness or smoothness.41,42,49 We used atomic force microscopy to verify that plasma treatment at these conditions did not damage the mica surface, since only occasional particles could (46) Gupta, V. K.; Miller, W. J.; Pike, C. L.; Abbott, N. L. Chem. Mater. 1996, 8, 1366. (47) Miller, W. J.; Gupta, V. K.; Abbott, N. L.; Tsao, M.-W.; Rabolt, J. F. Liq. Cryst. 1997, 23, 175. (48) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc, Faraday Trans. 1 1978, 74, 975. (49) Senden, T. J.; Ducker, W. A. Langmuir 1992, 8, 733.

Figure 1. (a) AFM image (area 1 µm × 1 µm) obtained in tapping mode in air of 8.2 nm thick gold layer on 1.4 nm chromium, evaporated on water vapor plasma treated mica. The rms roughness of this surface was 0.7 nm. (b) Height profile along the horizontal line in (a). be observed, as for untreated mica.50 Plasma treatment for 10 min caused damage (holes with a depth of 0.5-1 nm), and exposure to reactive plasma for long times at high power is indeed known51 to damage mica. After the plasma treatment, the support sheets with mica pieces were placed in an evaporator equipped with several resistive heaters so that materials could be deposited in sequence without opening the chamber (Balzers BAE 250, used also for the silver evaporation mentioned earlier). At a pressure of e5 × 10-6 mbar and room temperature, 1.5-2.0 nm of chromium (Unaxis, purity 99.9%) and 8.0-9.0 nm of gold (Unaxis, 99.99%) were evaporated at rates below 0.1 nm/s. The thicknesses and deposition rates were measured with a calibrated quartz crystal monitor. Chromium was evaporated from a covered molybdenum boat (Unaxis, model BD 482 062-T), and gold was evaporated from a conventional open tungsten boat. No difference in the quality of the metal film was observed when the samples were inserted into the chamber immediately (a few minutes) after the plasma treatment or up to 1 h later. Other times of exposure to the ambient air were not investigated. The roughness of the gold layers was investigated with an atomic force microscope (Nanoscope IIIa, Digital Instruments) used in contact or in tapping mode. Each gold layer was continuous and of uniform roughness as measured from varying scan sizes (1 µm × 1 µm or 6 µm × 6 µm areas) chosen at random locations on the surfaces. Occasional features on the surfaces of some samples were possibly the result of contamination in the plasma treatment. Such samples were not used for the following measurements, which are based on uniform surfaces with an rms roughness of 0.7-1.0 nm (Figure 1). After the evaporation, some matched pairs of mica pieces (still attached to the support sheet) were immediately immersed in a solution of 1-decanethiol (Aldrich, 96%) and 1-hexadecanethiol (Aldrich, 92%) in ethanol (Riedel-de Hae¨n, >99.8%) for 24 h. The (50) Ohnishi, S.; Hato, M.; Tamada, K.; Christenson, H. K. Langmuir 1999, 15, 3312. (51) Liu, Z.-H.; Brown, N. M. D.; McKinley, A. Appl. Surf. Sci. 1997, 108, 319. Anderson, C. A.; Brown, N. M. D.; Cui, N.; Liu, Z.; McKinley, A.; Walker, C. G. H. Surf. Coat. Technol. 1997, 97, 151.

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mole fraction of 1-hexadecanethiol of the total alkanethiol in solution (total concentration of 1 mM) was 0.2, which has been shown to result in a mole fraction in the mixed SAM of ca. 0.6.46 After the adsorption, the support sheet with the mica pieces was rinsed with ethanol and blown dry with N2 gas. The advancing contact angle of water on the mixed SAM was ca. 105°. These SAM-covered mica pieces were later glued onto half-cylindrical fused silica support disks with a thermosetting epoxy glue (EPON 1004F, Shell) for experiments with the SFA. Corresponding matched pairs of untreated gold-covered mica pieces were glued onto support disks with a mixture of dextrose and galactose and used for measurements of the interference spectra arising with the bare metal layers in contact. Typically, this was done within a few hours of removing the gold-covered mica from the evaporator. After this calibration, we immersed also these bare gold surfaces in alkanethiol solution (in a separate container). The sugar mixture is not completely insoluble in ethanol, but no appreciable change in the measured contact angle or monolayer thickness was found when the adsorption was done in this manner. We did not find any changes in the roughness of the gold after gluing the pieces on support disks with EPON 1004F (melting temperature52 of ca. 96 °C) or the sugar mixture (melting temperature of ca. 170 °C), likely because the time of heating the gold layer was short.22 Rapid cooling was ensured by moving the support disks with molten glue from the hot plate to the stainless steel bottom plate of the laminar flow cabinet immediately before the mica pieces were put on. As judged from contact angle and thickness measurements, the SAM was not damaged in the gluing with EPON 1004F. The liquid crystals, 5CB and 8CB, were obtained from Merck. A small amount of each liquid crystal was centrifuged at 5000 rpm to remove possible dust particles before a drop was injected between the surfaces. Our experiments were done at 25 ( 2 °C (the room temperature was not controlled), where bulk 5CB is in the nematic phase and 8CB is in the smectic phase.45 Both phases consist of dimers with overlapping cyanobiphenyl moieties.53 The mixed SAM was expected to induce homeotropic orientation (i.e., the long axis of the molecules oriented perpendicular to the substrates) in both 5CB and 8CB,46 which we also observed experimentally from the optical properties of films confined between two surfaces (cf. refs 4 and 6). The atmosphere in the instrument chamber was kept dry with P2O5 during the experiments.

Multilayer Multiple Beam Interferometry. To determine the separation distance between the front surfaces (film thickness), we used multiple beam interferometry.54 In contrast to the customary situation of a 3or 5-layer interferometer,18,19,55 our setup consisted of up to 11 optical layers (Figure 2), some of which were absorbing (metal) films. This rather complex interferometer calls for a nonstandard approach to transform measured wavelengths into real-space surface separations. On the basis of previous work by Clarkson56 and by Levins and Vanderlick,32 we used a numerical inversion of the multilayer matrix method to calculate distances from measured interference patterns. Our experimental data consist of the wavelengths of one or more interference fringes (fringes of equal chromatic order (FECO)). Instead of matching the measured spectrum of an entire spectral range, Imeas(λ), with theory,32 we used a variant of fast spectral correlation.57 The measured spectrum is represented as a sum of delta functions peaking at the wavelengths of the manually measured fringes. The (52) Manufacturer’s data. (53) Leadbetter, A. J.; Frost, J. C.; Gaughan, J. P.; Gray, G. W.; Mosley, A. J. Phys. (France) 1979, 40, 375. (54) Tolansky, S. Multiple Beam Interferometry of Surfaces and Films; Oxford University Press: London, 1949. (55) Horn, R. G.; Smith, D. T. Appl. Opt. 1991, 30, 59. (56) Clarkson, M. T. J. Phys. D.: Appl. Phys. 1989, 22, 475. (57) Heuberger, M. Rev. Sci. Instrum. 2001, 72, 1700.

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spectral correlation function then becomes a finite sum of theoretical transmission intensities, evaluated at the measured FECO wavelength. For the determination of all parameters describing our 11-layer interferometer, successive deposition of the optical layers was chosen, which is similar to a method described by Mangipudi19 for the case of a 5-layer interferometer. First, we determined the uniform mica thickness from typically 7-10 measured fringe wavelengths in Ag/mica-mica/Ag contact, using known refractive index data for mica.58 Second, a measurement of five fringe wavelengths in the Ag/mica/Cr/ Au-Au/Cr/mica/Ag contact, together with known refractive index data,59 was used to obtain an independent thickness measurement of the metal layers. The interferometrically measured thickness was always found to be within 5% of the values measured with the quartz crystal microbalance during the deposition. In a third step, the mixed SAM was deposited onto the gold and five fringe positions were measured in Ag/mica/Cr/Au/SAM-SAM/ Au/Cr/mica/Ag contact. The SAM thickness was determined assuming a dispersion-free refractive index similar to that of alkanes, nSAM ) 1.45. Once this was established, we measured five fringe wavelengths at several different, stable film thicknesses of 5CB and 8CB and calculated these distances using the refractive indexes n5CB ) 1.54 and n8CB ) 1.52.45 The force versus distance measurements in 5CB were based on the wavelength shift of one fringe and evaluated using a conversion chart. The numerical method to calculate such charts is essentially similar to the illustrations given by Clarkson.56 We decided to measure the wavelength of only one lead fringe to increase the speed of measurement and thus reduce effects of possible drift.60 Figure 3 illustrates the conversion charts used for force measurements in 5CB between mixed SAMs in two different experiments (different mica thicknesses). While such a conversion chart is accurate in the sense of taking into account all aspects of the complex multilayer interferometer, it is specific to each experimental situation and must be recalculated for each interferometer. The effects of surface roughness on the width and position of interference fringes has been discussed by Levins and Vanderlick.31 Generally, roughness induces a shift of the fringes to longer wavelength. This shift has been shown to be small for a roughness amplitude below 5 nm, so that the change in fringe position caused by the introduction of an additional material in the interferometer corresponds well to the one observed for perfectly smooth substrates.31 The width of the fringe at an rms roughness of 1 nm is not significantly larger than that for bare mica surfaces and has therefore only a small influence31 on the accuracy of the manual measurement of wavelengths. The nominal distance resolution in our interferometric measurements can be estimated from the error in the fringe wavelength, that is, the standard deviation σλ. The wavelength of a primary interference fringe can be determined manually to σλ < (30 pm (using a graticule in the eyepiece and a grating with 1180 grooves per mm in a 0.5 m spectrometer). In our experiments, we determined M ) 5 wavelengths of odd-order fringes, that is, λi)1,...,M. These wavelengths are dependent variables,18 and the error of fast spectral correlation57 is thus reduced by a factor of M0.5 ∼ 2.2. For a mica thickness of 4-5 µm, one observes fringes of chromatic order N ) 29-35. Using (58) Bailey, A. I.; Kay, S. M. Br. J. Appl. Phys. 1965, 16, 39. (59) Weaver, J. H.; Frederikse, H. P. R. In CRC Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Frederikse, H. P. R., Eds.; CRC Press: Boca Raton, FL, 1996; pp 12/126-149. (60) Za¨ch, M.; Heuberger, M. Langmuir 2000, 16, 7309.

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Figure 2. (a) Schematic drawing of the multilayer interferometer containing metal layers on mica confined by two 53 nm thick Ag mirrors. The Au layers are used for the thiol linking of the self-assembled monolayer (SAM), and the Cr layer is an adhesion promoter for Au on mica. These inner metallic layers introduce phase change in reflection and transmission as well as absorption, which must be taken into account for the exact FECO evaluation. (b) Typical interference fringes (FECO) arising from the symmetrical 11-layer interferometer in (a) with a mica thickness of 4.7153 µm, SAM thickness of 1.58 nm, and 5CB thickness of 5 nm (actual experimental data). The illustration, which accurately reproduces the interference pattern observed in the experiment, was calculated based on an analytical algorithm similar to the one described in ref 20. Because of the optical absorption of the gold layers, the spectrum exhibits dark regions at wavelengths coinciding with the secondary fringe pattern (ref 20). Primary fringes of all chromatic orders vanish in these dark bands. In particular, when this symmetrical interferometer confines a very thin film (in our case 5CB), every second fringe vanishes. This is an excellent demonstration of the fact that the characteristic fringe shape (e.g., concave or convex) and, with it, the “sensitivity to refractive index” of a primary fringe depend primarily not on its chromatic order but rather on its wavelength relative to the underlying secondary fringe pattern. The three long vertical lines in the figure indicate spectral lines from a mercury lamp, which are used to calibrate the wavelength measurement.

the common linearized approximation for small distances,18 one can show that the error estimation60 is given by σD ≈ ((σλN)/(2nmicaM0.5) ≈ (0.2 nm. Since the thickness of the mica is determined in a similar, separate measurement, the estimated overall error is σ ) (σD2 + σD2)0.5 ≈ 0.3 nm. Results When two gold-covered surfaces without alkanethiol monolayers were brought toward each other in dry air, no force (neither attractive nor repulsive) was observed until the surfaces came to undeformed (rounded), nonadhesive contact. Unless the surfaces were pressed together, they could be separated from rounded contact without any observable adhesion. The contact area would increase slightly after the surfaces had been left in contact for some minutes or after slight compression. A change in distance, indicating flattening of protrusions on the gold, could only be seen during strong compression, that is, where the softer glue layer under the mica also deformed (F/R > 15 mN/m). The surfaces then came gradually closer together. The total change in separation distance at strong com-

pression was about 1 nm, suggesting a flattening of 0.5 nm per surface. After even a slight compression, the two bare gold surfaces usually adhered strongly and separated with damage (cohesive failure) in the gold. In contrast to observations on gold layers directly evaporated on mica,35 the damage of our layers did generally not happen at the metal-mica interface, possibly because of the adhesion layer. Also, the interactions between two SAMs on gold were very small on approach in air. Generally, the surfaces could be compressed and then separated back to rounded contact and apart without a jump out. Occasionally, a small jump apart could be observed, suggesting an adhesion of |(F/R)adh| < 0.5 mN/m. These observations are in qualitative agreement with theoretical predictions for the adhesion between rough surfaces by Maugis61 and others,62 where van der Waals attraction and repulsion (61) Maugis, D. J. Adhes. Sci. Technol. 1996, 10, 161. (62) Fuller, K. N. G.; Tabor, D. Proc. R. Soc. London, Ser. A 1975, 345, 327. Johnson, K. L. In The Mechanics of Contact between Deformable Solids; de Pater, A. D., Kalker, J. J., Eds.; Delft University Press: Delft, The Netherlands, 1976; pp 26-40.

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Figure 3. Conversion chart used to transform manually measured lead fringe wavelengths from two separate experiments (setup as in Figure 2a) into surface separations (5CB film thickness), D. The thickness of each of the two mica sheets (cf. Figure 2a) was 4.7153 µm in one experiment (corresponding to the fringe pattern in Figure 2b) and 4.5602 µm in the other. The conversion correctly accounts for all properties of the optical layers in a specific experiment, including phase change and absorption at the inner metallic layers.

between compressed protrusions result in a near-cancellation of observable forces, so that the surface energy cannot be obtained from the pull-off force as for smooth surfaces. The change in separation with strong compression was smaller than or comparable to the one for bare gold layers. The SAM-covered surfaces were generally not damaged even after several approaches and separations, and they did not appear to flatten permanently on subsequent compressions, nor did the adhesion develop to become stronger. In all our experiments, the thickness of one mixed monolayer of 1-decanethiol and 1-hexadecanethiol was between 1.6 and 2.0 nm, which corresponds well with the expected average of 1.9 nm based on the thicknesses of single-component layers15 and assuming a mole fraction of 0.6 of 1-hexadecanethiol.46 When the liquid crystals were present between the SAMs, we observed several stable distances upon compression of the films to small separations. The surfaces moved suddenly from one such distance to the next both on compression and on separation. Similar structural forces have been observed for both 5CB and 8CB between close-packed, smooth monolayer surfaces.4,6 The stable layer thicknesses were 2.5, 5.4, 8.1, and 11.1 nm for 5CB (indicated in Figure 4) and 3.0, 6.6, and 10.5 nm (accuracy of (0.3 nm) for 8CB (more distances were not studied for 8CB). The values obtained for the thinnest layers are in good agreement with multiples of the dimer lengths of these molecules determined by small-angle X-ray diffraction, 2.5 nm63 and 3.17 nm,64 respectively, and with the known film thickness transitions for these compounds confined between smoother surfaces.4,6 In 8CB, determinations of the magnitude of the force are very difficult because of a strong background force arising from the layering in the smectic phase, resisting both compression and separation at all separations. In contrast, in 5CB we did not observe a force until at a separation below ca. 15 nm, where a structural force (63) Leadbetter, A. J.; Mehta, A. I. Mol. Cryst. Liq. Cryst. Lett. 1981, 72, 51. (64) Safinya, C. R.; Sirota, E. B.; Bruinsma, R. F.; Jeppesen, C.; Plano, R. J.; Wenzel, L. J. Science 1993, 261, 588.

Ruths et al.

Figure 4. Interaction force F (normalized by the radius of curvature, R) as a function of film thickness of 5CB (distance, D) confined between two identical mixed monolayers of 1-decanethiol and 1-hexadecanethiol on gold substrates with rms roughness of e1 nm. One fringe was chosen for the measurements, and the distances were determined from a conversion chart as shown in Figure 3. Open symbols denote approach (compression) and filled symbols denote separation of the surfaces. Each of the three shown compression-separation cycles took about 15 min. The shaded vertical lines indicate the stable distances (estimated error σ ) (0.3 nm) determined in consecutive measurements of five fringes at each film thickness, as described in the section on multilayer multiple beam interferometry.

appeared superposed on a repulsive background (Figure 4). The layer compressibility in the direction normal to the substrates can be determined from the shape of the force versus distance curve.65 Values of the layer compressibility modulus, B ) (2.1 ( 0.9) × 107 N/m2 for the thinnest layer (around D ) 2.5 nm) and B ≈ (0.6-1.2) × 107 N/m2 for the other three, are in good agreement with results obtained for 8CB at 35 °C (nematic) between smoother surfaces.6 In smectic liquid crystals, the layer compressibility modulus in the bulk is typically (1-3) × 107 N/m2, depending on the temperature,66 and the induced positional ordering of compressible layers seen in the 5CB film is thus smecticlike. Discussion The measured interaction force in our system can be viewed as the sum of locally similar interactions at many points in a multiasperity contact. The surface profile in Figure 1b suggests that the radius of such local contact points is a few tens of nanometers (similar to the size of an AFM tip). A multiasperity contact is necessarily less ideal than a contact between smooth surfaces, and a smearing out of the measured interaction forces is expected to result from the distribution of separation distances. The effects of surfaces with different roughnesses on solvation forces has been discussed for confined model fluids by Frink and van Swol.67 Roughness is expected to cause a decrease in the height and number of the observed film thickness transitions and a shift of each layer to a larger distance.67 A decrease in the number of film thickness transitions and in the magnitude of the force compared to that between two mica surfaces has been observed for octamethylcyclotetrasiloxane between rough (65) Richetti, P.; Kekicheff, P.; Barois, P. J. Phys. II (France) 1995, 5, 1129. (66) Collin, D.; Gallani, J. L.; Martinoty, P. Phys. Rev. A 1986, 34, 2255. Yamamoto, J.; Okano, K. Jpn. J. Appl. Phys. 1991, 30, 754. (67) Frink, L. J. D.; van Swol, F. J. Chem. Phys. 1998, 108, 5588.

Liquid Crystals between Two Alkanethiol Monolayers

surfactant monolayers68 and between one mica and one silver surface.28 For 5CB between SAMs on gold, we observed only 5 transitions (Figure 4) as compared to 7-15 in nematic 8CB and 5-7 in 5CB between smoother monolayer surfaces,4,6 and the outer ones are indeed shifted outward (i.e., the film thickness at the transition corresponds less well with that expected from a multiple of the dimer length). The calculations for differently structured surfaces by Frink and van Swol indicate that a periodic structural force would not be observable at an rms roughness larger than 0.3 times the molecular diameter.67 In homeotropically oriented cyanobiphenyls, the relevant dimension is the dimer length (2.5 and 3.17 nm for 5CB and 8CB, respectively). We are probably at the lower limit of molecular size to observe structural forces between our surfaces (rms roughness e 1 nm), which is intuitively reasonable considering the surface profile in Figure 1b. A practical difference between our surfaces and the model67 is that the model assumes a lateral size of the corrugations equal to the diameter of the molecule in the lateral direction. On our surfaces, there are clearly some larger areas (diameter of 10-15 nm) of similar height. A repulsive background force (superposed on the periodic thickness transitions) has been observed both experimentally4,6 and in computer simulations69 of homeotropically oriented liquid crystals confined to small film thicknesses. This phenomenon has been ascribed to reorientation of molecules at intermediate film thicknesses that do not correspond to fully developed molecular (68) Christenson, H. K. J. Phys. Chem. 1986, 90, 4. (69) Sonnet, A. M.; Gruhn, T. J. Phys.: Condens. Matter 1999, 11, 8005.

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layers.69 In Figure 4, this background force is somewhat stronger and has a shorter range than that observed for 5CB between smoother substrates,4 but it is weaker than that for 8CB in the nematic phase.6 It is likely caused by the difficulty in accommodating molecules in the confinement-induced structure at intermediate film thicknesses and not by direct contact between asperities on the surfaces, since the compressibility agreed well with data for other liquid crystals on smoother surfaces. Conclusions We describe an experimental protocol for the preparation of continuous, transparent gold layers on mica. The usefulness of these easily modified substrates for measurements of film thicknesses and interaction forces also at small separations with a conventional interferometric surface forces apparatus was demonstrated for mixed alkanethiol monolayers and a thermotropic, homeotropically oriented liquid crystal, 5CB. We also discuss the use of fast spectral correlation interferometry57 for our system, which contains several absorbing and reflecting layers. Limitations were discussed with respect to surface roughness effects on structural forces and the accuracy of the manual optical measurements. Acknowledgment. We thank M. Schallehn and R. Fo¨rch for discussions on the plasma treatment and K. Morigaki and K. Petersen for AFM measurements. Discussions with F.-J. Schmitt and T. Gruhn are gratefully acknowledged. M.R. thanks the Alexander von Humboldt Foundation and the Max-Planck-Gesellschaft, and M.H. thanks the Holderbank Stiftung for financial support. LA010272V