Uniform Anchoring of Nematic Liquid Crystals on Self-Assembled

Self-assembled monolayers supported on obliquely deposited gold permit uniform anchoring of nematic .... The Journal of Physical Chemistry B 0 (proofi...
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Langmuir 1996, 12, 2587-2593

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Uniform Anchoring of Nematic Liquid Crystals on Self-Assembled Monolayers Formed from Alkanethiols on Obliquely Deposited Films of Gold Vinay K. Gupta and Nicholas L. Abbott* Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616 Received January 2, 1996X This paper demonstrates that self-assembled monolayers (SAMs) formed by chemisorption of alkanethiols on the surface of obliquely deposited, 10 nm-thick, films of gold are useful in studies of the anchoring of liquid crystals (LCs) on surfaces. Self-assembled monolayers supported on obliquely deposited gold permit uniform anchoring of nematic LCs over centimeter-scales. In contrast, SAMs prepared on films of gold deposited with no preferred direction cause nematic phases to form 10-100 µm scale domains with optical axes having random azimuthal orientations. Self-assembled monolayers prepared from either CH3(CH2)15SH or CH3(CH2)9SH on films of gold deposited obliquely induce near-planar and azimuthally uniform anchoring of nematic phases of 4-cyano-4′-pentylbiphenyl (5CB). Mixed SAMs formed by coadsorption of CH3(CH2)15SH and CH3(CH2)9SH cause homeotropic anchoring of nematic phases of 5CB on films of gold deposited without a preferred direction and produce a small tilt (3-5°) from the normal on gold deposited obliquely onto glass substrates. The contact angles (advancing and receding) of hexadecane are indistinguishable when measured on SAMs supported on gold with and without a preferred direction. Liquid crystals can be used to detect anisotropy in these surfaces that cannot be measured by the wetting of simple fluids such as hexadecane.

Introduction We aim to manipulate the molecular-scale and mesoscale structure of organic surfaces so as to control the properties of complex fluidsssuch as liquid crystalline phasessplaced in contact with these surfaces. Recent advances in the synthesis of organic surfaces by molecular self-assembly form the basis of the work reported herein.1 We report control of the orientation of nematic phases of 4-cyano-4′-pentylbiphenyl (5CB) by manipulation of the molecular-level order within self-assembled monolayers (SAMs) formed from alkanethiols and by manipulation of the mesoscale properties of gold substrates that support the SAMs. The “anchoring” of uniaxial nematic phases by surfaces is specified, in general, by the orientation of the director of the bulk nematic phase.2 The orientation of the director, relative to the surface, is described by a polar angle (measured from the normal of the surface) and an azimuthal angle (measured in the plane of the surface). We report control of the polar angle of nematic phases on SAMs by the choice of the alkanethiol(s) used to form the SAMs; control of the azimuthal angle of the nematic phase is achieved by manipulation of the manner of deposition of the film of gold supporting the SAM. Studies in the past have established that chemisorption of organosulfur compounds onto the surface of gold can result in the formation of densely-packed and ordered monolayers of molecules supported on the surface of the * Author to whom correspondence should be addressed. E-mail: [email protected]. Telephone: (916) 752-6527. Fax: (916) 7521031. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (c) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87 and references cited therein. (2) The term “anchoring” refers to the alignment of liquid crystals by surfaces in contact with them. The “director” of a nematic phase is the axis of symmetry of the orientational distribution function for a chosen molecular axis. Homeotropic anchoring refers to the director lying perpendicular to the plane of the substrate. Planar anchoring refers to the director lying parallel to the substrate with either uniform or nonuniform azimuthal alignment.

gold and that the properties of surfaces so-formed can be controlled by manipulation of the structure of the sulfurcontaining molecules.3 The self-assembly of alkanethiols on gold provides, therefore, the capability to construct well-defined surfaces to test hypotheses regarding the interactions of fluids, such as those exhibiting liquid crystalline phases, with organic surfaces. Our work aims to use liquid crystals (LCs) supported on SAMs to develop new principles for (i) amplification of molecular-level events on surfaces into macroscopic signals, (ii) selfassembly of micrometer-scale optical structures in three dimensions, and (iii) fabrication of “aligning” surfaces for tunable filters, electro-optic modulators, and flat panel displays. The anchoring of liquid crystals on the surfaces of solids has been the subject of extensive theoretical and experimental investigation and forms the basis of applications of liquid crystals in optical devices such as flat panel displays.4 Numerous techniques and materials exist to affect the alignment of liquid crystals. A complete picture of the mechanisms controlling anchoring of liquid crystals on either inorganic or organic surfaces is, however, yet to be elucidated. It is not yet possible to design a surface to give a desired anchoring of a liquid crystal. (3) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (c) Bain, C. D.; Troughton, B. E.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (e) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (f) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (g) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (h) Camillone, N. III; Chidsey, C. E. D.; Liu, G. -Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (i) Butt, H.-J.; Seifert, K.; Bamberg, E. J. Phys. Chem. 1993, 97, 7316. (j) Stranick, S. J.; Weiss, P. S.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., A 1993, 11, 739. (k) Camillone, N. III; Chidsey, C. E. D.; Liu, G. -Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (l) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (m) Rolandi, R.; Cavalleri, O.; Toneatto, C. Thin Solid Films 1994, 243, 431. (n) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (o) Scho¨nenberger, C.; Sondag-Hu¨thorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (p) Sondag-Hu¨thorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (q) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (r) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Hu¨thorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259.

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Control of the anchoring of nematic liquid crystals has, in the past, been largely based on the use of organic surfaces prepared by coating surface-active molecules or polymer films on inorganic (e.g. silicon oxide) substrates followed by surface treatments such as rubbing. These systems are prone to contamination (especially during rubbing) and lack structural reproducibility and stability: systematic manipulation of the physical and chemical properties of such surfaces is not possible. Past efforts aimed at addressing this issue have included the preparation of surfaces through the reaction of organosilanes with various substrates.5 The literature describing the properties of these surfaces suggests that this system too has been plagued by irreproducibility and limited control of the extent of organization of molecules within the adsorbed layer.6 The use of SAMs formed from alkanethiols on thin, semitransparent films of gold7 in studies of the anchoring of liquid crystals on surfaces was reported by Drawhorn and Abbott.8 The principal result of that work was the demonstration that mixed SAMs formed from n-alkanethiols with long (CH3(CH2)15SH) and short (CH3(CH2)4SH or CH3(CH2)9SH) aliphatic chains can homeotropically anchor2 nematic phases of either N-(p-methoxybenzylidene)-p-n-butylaniline (MBBA) or 5CB. In contrast, single-component SAMs (CH3(CH2)nSH, 2 < n < 15) caused non-uniform, planar, or tilted anchoring at room temperature. These results were reported for SAMs supported on films of gold deposited onto a substrate with no preferred direction. Here we report that SAMs supported on obliquely deposited films of gold cause uniform, planar or tilted anchoring of 5CB at room temperature. These results emphasize the role of the gold supporting the SAM in determining the interaction of liquid crystalline phases with self-assembled surfaces. Past studies of the anchoring of liquid crystals on obliquely deposited inorganic surfaces have established their usefulness in fundamental studies and devices.9 We report the anchoring of liquid crystals on SAMs on gold to be the combined result of anisotropy within the polycrystalline film of gold and molecular-level order within the SAM. We compare and contrast the influence of anisotropy within the gold and molecular-level order within the SAM on the wetting of simple fluids and the anchoring of liquid crystals on these surfaces. Experimental Section Thin films of gold (99.999%, International Advanced Materials, New York) were deposited onto clean10 glass microscope slides (Fisher Finest, Premium Grade) using an electron-beam evaporator (model SEC600, CHA industries, Fremont). Titanium was used to promote adhesion between the gold and the glass (4) (a) Castellano, J. A. Mol. Cryst. Liq. Cryst. 1983, 94, 33. (b) Hiltrop, K.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 582. (c) Proust, J. E.; Ter-Minassian-Saraga, L. Solid State Communications 1972, 11, 1227. (d) Je´roˆme, B. Rep. Prog. Phys. 1991, 54, 391. (e) Cognard, J. Mol. Cryst. Liq. Cryst. Suppl Ser. 1982, 1, 1. (5) (a) Yang, J. Y.; Mathauer, K.; Frank, C. W. In Microchemistry: Spectroscopy and Chemistry in Small Domains; Masuhara, H., DeSchryver, F. C., Kitamura, N., Tamai, N., Eds.; North-Holland: Amsterdam, 1994; p 441. (b) Peek, B.; Ratna, B.; Pfeiffer, S.; Calvert, J.; Shashidhar, R. Proc. SPIE 1994, 2175, 42. (c) Papanek, J. Mol. Cryst. Liq. Cryst. 1990, 179, 139. (d) Mullin, C. S.; Guyot-Sionnest, P.; Shen, Y. R. Phys. Rev. A 1989, 39, 3745. (e) Kahn, F. J. Appl. Phys. Lett. 1973, 22, 386. (6) (a) Ulman, A. Adv. Mater. 1990, 2, 573. (b) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (c) Ulman, A. MRS Bull. 1995, 20, 46. (7) DiMilla, P. A.; Folkers, J. P.; Biebuyck, H. A.; Harter, R.; Lopez, G. P.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 2225. (8) Drawhorn, R. A.; Abbott, N. L. J. Phys. Chem. 1995, 45, 99, 16511. (9) (a) Janning, J. L. Appl. Phys. Lett. 1972, 21, 173. (b) Hiroshima, K.; Mochizuki, M. Jpn. J. Appl. Phys. 1980, 19, 567. (c) Crossland, W. A. Appl. Phys. Lett. 1975, 26, 598. (d) Urbach, W.; Boix, M.; Guyon, E. Appl. Phys. Lett. 1974, 25, 479.

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Figure 1. Illustration of two schemes used to deposit films of gold. (A) Uniform depositionsrotation of the substrate holder about axes X and Y changed both the angle and direction of incidence of the gold during deposition (see text for details). (B) Oblique depositionsthe substrate was not rotated and the angle of incidence of the gold was fixed at 50°. substrate. The rate of deposition of gold was 0.2 Å/s (P e 5 × 10-6 Torr). The gold was deposited using two schemes. First, glass microscope slides were mounted on the substrate holder (planetary) of the electron-beam evaporator (Figure 1A). The holder rotated the slides in an epicyclic manner with respect to the gold source. The epicyclic rotation caused changes in both the angle of incidence (polar angle between the incoming stream of gold atoms and the normal of the surface of the slide) and the direction of incidence (direction of the incoming stream of gold atoms projected onto the plane of the surface of the slide) of gold during deposition (Figure 1A). Because this scheme does not introduce a preferred direction into the films of gold, we refer to films of gold prepared by this scheme as “uniformly” deposited films or “films without a preferred direction”. Second, glass microscope slides were mounted in a customized holder that was held stationary (Figure 1B). The gold was deposited onto the surface of the microscope slide at a constant angle of incidence of β ) 50° and from a single direction. We refer to films of gold prepared by this scheme as “obliquely” deposited films or “films with a preferred direction”. In both deposition schemes, a quartz crystal microbalance (QCM) placed directly above the source of gold was used to measure the thickness of gold deposited onto a substrate oriented normal to the incident flux of gold. “Uniformly” deposited films were prepared by deposition of 10 Å of titanium and 100 Å of gold onto the QCM. Approximately 14 Å of titanium and 140 Å of gold were deposited onto the QCM in order to produce obliquely deposited films with the same thickness of gold and titanium as deposited onto the “uniform” (10) Glass slides were cleaned by immersion in piranha solution (70% H2SO4, 30% H2O2) for 30 min followed by sequential rinses of deionized water and methanol, followed by drying under a stream of nitrogen. Caution: piranha solution is a strongly oxidizing mixture that can undergo explosion upon contact with organic materials.

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Figure 2. Images of films of gold obtained by scanning tunneling microscopy. (A) Uniformly deposited gold. (B) Obliquely deposited gold. The insets show each image in Fourier space (1/500 nm < q/2π < 1/4 nm). films of gold. Scanning tunneling microscopy of the substrates of gold was performed using a Nanoscope III (Digital Instruments, Santa Barbara, CA) STM operated under ambient conditions with a platinum-iridium tip. Self-assembled monolayers were prepared by immersion of films of gold into ethanolic solutions containing a total concentration of 1 mM of n-alkanethiol(s) for 2 h. Single component SAMs were formed from either CH3(CH2)15SH or CH3(CH2)9SH. Mixed SAMs were formed from solutions containing 0.2 mM CH3(CH2)15SH and 0.8 mM CH3(CH2)9SH. The composition of the mixed SAMs formed under these conditions was determined by the kinetics of the processes leading to the formation of the SAMs. These SAMs were reported previously to induce homeotropic anchoring of 5CB on uniformly deposited films of gold.8 Both advancing and receding contact angles of hexadecane were measured on each SAM by using a Rame´-Hart (Mountain Lakes, NJ) goniometer. Hexadecane was purified by passage through a column of alumina before measurement of contact angles. The measurements were performed in two orthogonal directions on each SAM. These directions coincided with the direction of incidence of the stream of gold vapor (AA′) and the direction perpendicular to it (AB) (Figure 1B) on the obliquely deposited films. The contact angles reported here are the average of 36-54 measurements. Optical cells were prepared by pairing two gold surfaces, either untreated or supporting SAMs, and by spacing them apart using 2 µm thick films of mylar. The liquid crystal 5CB (EM Sciences, New York) was heated into its isotropic phase, drawn into optical cells by capillary action, and cooled in air to room temperature (25 °C). A polarized light microscope (BX60, Olympus) was used to observe the optical textures formed by light transmitted through the optical cell. Orthoscopic and conoscopic images were recorded.11 All images were obtained using a 20× objective lens with a 550 µm field of view, unless otherwise stated. Because the director of a nematic phase of 5CB is parallel to the optical axis with the high refractive index (i.e. slow axis), orientation of the director relative to the direction of incidence of gold during oblique deposition (AA′) was inferred from measurement of the orientation of the slow axis of the nematic phase. The slow axis was determined by observing the direction of shift of interference colors12 produced by the sample when an accessory (either a quarter-wave plate or a quartz wedge) was inserted in the optical path. The interference colors shifted toward higher retardation (11) Nematic phases are birefringent, i.e. possess anisotropic refractive indices. Variations in the orientation of the director within a nematic phase appear as variations in the intensity of light transmitted through the sample using crossed polars.

in the Michel-Le´vy chart13 when the slow axis of the sample and the accessory coincided: the interference colors shifted to lower retardations when the slow axis of the sample and the accessory were orthogonal to each other. The intensity of light transmitted through each optical cell was recorded during rotation of the sample between crossed polars. This measurement was used to characterize the uniformity of anchoring of liquid crystals on SAMs. The sample was illuminated with light of wavelength 600 nm, obtained by placing an interference filter on the iris diaphragm of the polarizing microscope. The background intensity of light transmitted through crossed polars and the maximum intensity of light transmitted through parallel polars were recorded for an empty optical cell. The values of intensity reported in Figure 4 are corrected for the background intensity of light passed through crossed polars and are normalized by the intensity of light measured to pass between parallel polars (both empty cells). For comparison, a uniform, linearly birefringent standard was assembled from a quarter-wave plate (147.3 nm) placed on top of an empty gold cell, and its optical properties were measured using the methods described above. All intensities of transmitted light are averaged across the 550 µm wide field of view.

Results Images of films of gold obtained by scanning tunneling microscopy (STM) are shown in Figure 2. Gold deposited both uniformly and obliquely possessed a pebbly texture.14 By using STM, we were not able to find differences between (i) the real-space images of uniformly and obliquely deposited gold or (ii) the orthogonal directions of obliquely deposited gold: values of root-mean-square roughness for the obliquely deposited gold measured in the orthogonal (12) Under illumination with white light and crossed polars, a material possessing birefringence (∆n ) ne - no, where ne is the extraordinary refractive index and no is the ordinary refractive index) causes, in general, the formation of interference colors. These colors result from unequal transmission by the analyzer of the different wavelengths within white light. The extent to which the analyzer transmits (or absorbs) each wavelength depends on the retardation (∆nd) of light during its passage through the birefringent material of thickness d. The resulting interference colors are divided into “orders” according to whether they result from retardations of 0-550 nm (firstorder colors), 550-1100 nm (second-order), 1100-1650 nm (third-order), and so on. (13) Hartshorne, N. H.; Stuart, A. Crystals and the Polarising Microscope; Edward Arnold & Co.: New York, 1970.

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Table 1. Advancing and Receding Contact Angles of Hexadecane Measured on SAMs Supported on Obliquely and Uniformly Deposited Golda along AA′

a

along AB

type of surface

advancing

receding

advancing

receding

CH3(CH2)15SH on uniformly deposited gold CH3(CH2)15SH on obliquely deposited gold CH3(CH2)9SH on uniformly deposited gold CH3(CH2)9SH on obliquely deposited gold mixed SAM on uniformly deposited gold mixed SAM on obliquely deposited gold

46 ( 1 46 ( 1 44 ( 1 45 ( 1 37 ( 1 37 ( 1

42 ( 1 42 ( 1 39 ( 1 40 ( 1 33 ( 1 34 ( 1

46 ( 1 46 ( 1 45 ( 1 45 ( 1 36 ( 1 37 ( 1

41 ( 1 42 ( 1 40 ( 1 40 ( 2 32 ( 1 33 ( 1

Contact angles were measured by advancing and receding drops of hexadecane along the directions AA′ and AB in Figure 1B.

directions (AA′, AB) were indistinguishable and were estimated to be approximately 5.5 ( 0.5 Å. A small degree of anisotropy can be seen in the Fourier-transformed image of the texture shown in the inset of Figure 2B. This anisotropy was, however, not observed consistently in images obtained from different samples and was not observed in different areas within the same sample. All gold films were determined to be continuous by STM. The advancing and receding contact angles of hexadecane measured on SAMs formed from CH3(CH2)15SH or CH3(CH2)9SH were consistent with values reported in the past for densely-packed, pseudocrystalline SAMs (Table 1).1,3 Contact angles measured on mixed SAMs formed from CH3(CH2)15SH and CH3(CH2)9SH were lower than contact angles measured on single-component SAMs because the mixed SAMs were conformationally disordered. As shown in Table 1, the advancing and receding contact angles of hexadecane were (i) indistinguishable when measured on SAMs on gold deposited uniformly or obliquely and (ii) indistinguishable when measured in the orthogonal directions (AA′, AB) on obliquely deposited gold. The optical textures of 5CB in contact with SAMs supported on films of gold deposited uniformly or obliquely revealed that the manner of deposition of the gold supporting the SAM does impact the anchoring observed on the SAM. Figure 3 shows the optical textures of 5CB on uniformly deposited films of gold that were (A) “bare” or (B) covered by a SAM formed from CH3(CH2)15SH or (C) CH3(CH2)9SH. All textures were nonuniform, although the presence and type of SAM influenced the type and density of defects present within the liquid crystalline phase. These differences are easily seen in Figure 4. Figure 4 shows the intensity of light transmitted through the nematic phase and cross-polars as a function of the angle of rotation of the optical cell with respect to the polarizer. Nematic phases of 5CB anchored on SAMs formed from CH3(CH2)15SH on uniformly deposited gold (no preferred direction) yielded an intensity of transmitted light that was independent of the angle of rotation of the sample. Because the azimuthal anchoring of the nematic phase was correlated over spatial scales of only 1-10 µm (see Figure 3B), when averaged over the 550 µm scale field of view used in the measurement of transmitted light, there was no preferred direction of azimuthal anchoring. In contrast, the azimuthal anchoring of 5CB on SAMs formed from CH3(CH2)9SH on uniformly deposited gold was correlated over spatial scales of ≈100 µm. The longrange correlation of the azimuthal anchoring of 5CB on SAMs formed from CH3(CH2)9SH on uniformly deposited gold caused a small degree of modulation of the intensity of transmitted light during rotation of the sample between cross-polars (Figure 4). (14) The morphology of gold within thin films depends on the substrate supporting the gold and the temperature during deposition of the gold. Past studies have shown that gold deposited onto glass at room temperature has a pebbly texture. See: ( a) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahar, S. Surf. Sci. 1988, 200, 45. (b) Hwang, J.; Dubson, M. A. J. Appl. Phys. 1992, 72, 1852. (c) DeRose, J. A.; Lampner, D. B.; Lindsay, S. M.; Tao, N. J. J. Vac. Sci. Technol., A 1993, 11, 776.

Figure 3. Optical textures of 5CB in contact with surfaces formed from either uniformly deposited gold or obliquely deposited gold (the cell is viewed between cross-polars). Uniformly deposited films of gold: (A) “bare” or (B) supporting a SAM formed from CH3(CH2)15SH or (C) CH3(CH2)9SH. Obliquely deposited films of gold (antiparallel alignment): (D) “bare” gold or (E) supporting a SAM formed from CH3(CH2)15SH or (F) CH3(CH2)9SH. The inset in (E) shows the texture magnified ×2.5: the horizontal dimension of the region shown in the inset is 110 µm. The horizontal dimension of each sample shown is 550 µm.

The optical textures of 5CB were uniform over centimeter scales when 5CB was anchored on SAMs supported on gold deposited obliquely onto glass substrates (Figure 3D-F). The retardation colors created by white light transmitted through the optical cells and crossed polarizers were compared to those in a Michel-Le´vy chart.13 From the comparison of colors we estimated a birefringence of 0.16 ( 0.04 within optical cells with SAMs formed from either CH3(CH2)15SH or CH3(CH2)9SH. This birefringence is consistent with an optical axis of symmetry of the liquid crystal (director) that lies in the plane of the substrate (so-called planar or near-planar alignment of 5CB).15 The in-plane azimuthal alignment of the director of the liquid crystal on “bare” gold was perpendicular to the direction of incidence of the gold during deposition. The directors of liquid crystals in contact with SAMs (15) The extraordinary refractive index (ne) for a nematic phase of 5CB has been reported as 1.706 (for light with a 633 nm wavelength) or 1.736 (at 514.5 nm), and the ordinary refractive index (no) as 1.531 (at 633 nm) or 1.544 (at 514.5 nm). (See Blinov, L. M.; Chigrinov, V. G. Electrooptic Effects in Liquid Crystal Materials; Springer-Verlag: New York, 1994). The maximum birefringence (ne - no) of a nematic phase of 5CB does, therefore, lie between 0.17 and 0.19. Planar anchoring of 5CB will give rise to a birefringence of 0.17-0.19.

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Figure 4. Fractional transmittance of light between crossed polars and 5CB anchored on SAMs on gold as a function of the angle between the sample and polarizer. Fractional transmittance is the ratio of the intensity of light transmitted through the optical cell (containing liquid crystal) and cross-polars to the intensity of light transmitted through an empty cell under parallel-polars (see Experimental Section for details). The surfaces of the optical cells supported SAMs formed from (]) CH3(CH2)15SH on uniformly deposited gold, (O) CH3(CH2)9SH on uniformly deposited gold, ([) CH3(CH2)15SH SAM on obliquely deposited gold, and (b) CH3(CH2)9SH SAM on obliquely deposited gold. For comparison, the intensity of light transmitted through a quarter-wave plate is also shown (4). The wavelength of light was 600 nm.

formed from either CH3(CH2)15SH or CH3(CH2)9SH were, in contrast, aligned parallel to the direction of incidence of gold during deposition. Because the anchoring of 5CB on SAMs formed on gold deposited obliquely was uniform over centimeter scales, we observed strong modulation of the intensity of light transmitted through cross-polars during rotation of the sample with respect to the polarizer (Figure 4). Unlike the uniformly birefringent quarter-wave plate, however, nematic phases supported on SAMs prepared on obliquely deposited gold did not completely extinguish light transmitted through the cell at any angle of the sample relative to the polarizer. The ratio of the maximum to minimum intensity of light was approximately 20. Inspection of the texture of 5CB between cross-polars revealed the presence of a fine structure on spatial scales of micrometers (inset Figure 3E). This fine structure within the nematic phase prevented complete extinction of light transmitted through the optical cell. We prepared optical cells with SAMs supported on obliquely deposited gold such that the preferred directions in the two surfaces of the cell were orthogonal. Surfaces paired in this manner induced a twist in the nematic phase. Figure 5 shows the optical characteristics of such a twisted nematic cell. The sample appeared bright through crosspolars and dark under parallel polars, indicating that linearly polarized light was rotated by 90° upon progression from one face of the cell to the other. Finally, we report the influence of the manner of deposition of the gold on the anchoring of 5CB on mixed SAMs formed from CH3(CH2)15SH and CH3(CH2)9SH. These SAMs caused homeotropic anchoring of 5CB on uniformly deposited gold. Figure 6 shows conoscopic interference figures obtained for 5CB anchored on SAMs prepared on uniformly and obliquely deposited gold: the director of 5CB was uniformly tilted approximately 3° from the normal (“tilted homeotropic”) when supported on the mixed SAM prepared on the obliquely deposited gold. Discussion The principal result of the work reported herein is the demonstration of uniform anchoring of nematic phases

Figure 5. Optical textures of 5CB in a twisted nematic cell. The cell was assembled from obliquely deposited films of gold that supported SAMs formed from CH3(CH2)15SH. The preferred directions within the top and bottom gold surfaces were mutually orthogonal. (A) Cross-polars. (B) Polarizer and analyzer at 45°. (C) Parallel polars. The horizontal dimension of each image is 550 µm.

Figure 6. Conoscopic images of 5CB anchored on mixed SAMs prepared from ethanolic solutions of 0.2 mM CH3(CH2)15SH and 0.8 mM CH3(CH2)9SH. (A) Homeotropic anchoring of 5CB on the mixed SAM supported on a uniformly deposited gold surface. (B) Tilted homeotropic anchoring of 5CB on the mixed SAM supported on obliquely deposited gold. By using the radial location of the melatope, the numerical aperture of the lens (≈0.4), and the mean refractive index of the nematic sample, the tilt in B was estimated to be approximately 3° from the normal.

on SAMs formed from alkanethiols on obliquely deposited, semitransparent films of gold.16 Past work on the anchoring of liquid crystals on SAMs formed from alkanethiols used uniformly deposited gold that caused

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azimuthally nonuniform anchoring of liquid crystals.8 Because methods for quantitative measurements of the anchoring of nematic liquid crystals generally require azimuthally uniform samples, the capability to uniformly anchor nematic phases will, we believe, facilitate the use of SAMs formed from alkanethiols in fundamental studies of the anchoring of liquid crystals on surfaces. The capability to uniformly anchor liquid crystals on SAMs can, we believe, also form the basis of principles for amplification of phenomena on surfaces into optical signals (sensors), and the synthesis of mesoscale structures from liquid crystals using patterned SAMs as templates.8 Past studies of anchoring of liquid crystals on uniformly deposited silicon oxide (SiOx) surfaces have reported azimuthally random anchoring.4,9 The directors of nematic phases align parallel to the surface on films of SiOx deposited either at normal incidence or “omniazimuthally” (constant angle of incidence but azimuthal rotation).9,17 Films of SiOx, when coated by a surface-active agent (e.g. organosilanes, lecithin), induce homeotropic anchoring of liquid crystals.9 We have also observed azimuthally nonuniform anchoring of liquid crystals on films of gold deposited at changing angles of incidence and with azimuthal rotation (uniformly deposited gold).8 The anchoring remains azimuthally nonuniform and near planar when the uniformly deposited gold is coated by a single-component SAMs formed from either CH3(CH2)15SH or CH3(CH2)9SH, whereas a transition to homeotropic anchoring can be induced by formation of a mixed SAM by coadsorption of CH3(CH2)15SH and CH3(CH2)9SH.8 The anchoring of liquid crystals on surfaces of obliquely deposited SiOx (thicknesses of 50-1000 Å) has also been reported.9,18-21 The anchoring on these surfaces depends on the angle of incidence of the SiOx during deposition: (a) angles of incidence less than 40° (from the normal) cause the liquid crystal to align randomly in the plane of the surface, (b) angles between 40° and 75°-80° induce alignment of the liquid crystal in the plane of the surface with the optic axis of symmetry perpendicular to the direction of incidence of the SiOx during deposition, (c) angles greater than 80° cause uniform alignment of liquid crystals with a tilt of 20°-40° from the surface. Obliquely deposited films of SiOx (thicknesses of 3003200 Å, deposited at 15-20 Å/s) contain columnar deposits arranged with periods of hundreds of angstroms.19b Studies in the past have correlated the anchoring of liquid crystals on obliquely deposited SiOx surfaces with the sizes and shapes of these “columnar” structures. An “elastic model”sin which the energy of distortion of the director of a nematic phase about these columnar structures dictates the orientation of the liquid crystalshas been proposed to describe the influence of these microstructures (with dimensions of hundreds of angstroms) on the anchoring of liquid crystals.18,19b,23 Our (16) Transparent, thin films of gold have formed the basis of devices such as infrared reflectors, electrode coatings for deicing and defogging of automobile and aircraft windows, RFI/EMI shielding, and antistatic coatings. See: (a) Haacke, G. Annu. Rev. Mater. Sci. 1977, 71, 73. (b) Smith, G. B.; Niklasson, G. A.; Svensson, J. S. E. M.; Granqvist, C. G. J. Appl. Phys. 1986, 59, 571. (17) Hiroshima, K.; Obi, H. Proc. SID 1984, 25, 287. (18) Guyon, E.; Pieranski, P.; Boix, M. Lett. Appl. Eng. Sci. 1973, 1, 19. (19) (a) Cheng, J.; Boyd, G. D.; Storz, F. G. Appl. Phys. Lett. 1980, 37, 716. (b) Goodman, L. E.; McGinn, J. T.; Anderson, C. H.; Digeronimo, F. IEEE Trans. Electron Devices 1977, 24, 795. (20) Guyon, E.; Urbach, W. In Nonemissive Electrooptics Displays; Kmetz, A. R.; von Willisen, F. K., Ed.; Plenum Press: New York, 1976; p 121. (21) (a) Uchida, T.; Ohgawara, M.; Wada, M. Jpn. J. Appl. Phys. 1980, 19, 2127 (b) Heffner, W. R.; Berreman, D. W.; Sammon, M.; Meiboom, S. Appl. Phys. Lett. 1980, 36, 144. (22) Crossland, W. A.; Morrissey, J. H.; Needham, B. J. Phys. D 1976, 9, 2001.

Gupta and Abbott

attempts to probe the structure of our films of gold on spatial scales from 1 to 500 nm did not reveal anisotropy within these surfaces. The anisotropy in the shapes of grains (∼250 Å in size) within our obliquely deposited films of gold was below the threshold detectable by scanning tunneling microscopy, plausibly because of the high mobility of adatoms of gold, the low rate of deposition (∼0.2 Å/s), and the thinness of our films of gold. Because the anchoring of 5CB (measured on micrometer scales) on 100 Å thick gold films deposited obliquely or uniformly is differentsthe surfaces of gold deposited obliquely (at 50°) align 5CB with a uniform azimuthal orientation, whereas the azimuthal orientation of 5CB on uniformly deposited gold is randomswe conclude that the properties of our surfaces that control the anchoring of the nematic liquid crystals are not revealed by our imaging of these surfaces by STM. Several investigators have modified the surfaces of obliquely deposited SiOx with either polymerized N,Ndimethyl-N-[3-(trimethoxysilyl)propyl]octadecylammoniumchloride (DMOAP) or physisorbed cetyltrimethylammonium bromide (CTAB).21,22 Surfaces of obliquely deposited SiOx, when coated with CTAB or DMOAP, induce a small tilt (≈0°-30°) of the optical axis of nematic phases from the normal (tilted homeotropic).18,20,21 The tilt was proposed to stem from the locally homeotropic anchoring of the liquid crystal on the tilted columns of SiOx coated by a surface-active agent. The anchoring of liquid crystals on surfaces of SiOx coated by surface-active agents is, therefore, either homeotropic or tilted homeotropic, depending on whether the SiOx is deposited at normal incidence or at an oblique angle. We report uniform, near-planar anchoring of liquid crystals using SAMs formed from either CH3(CH2)15SH or CH3(CH2)9SH on obliquely deposited gold and homeotropic or tilted homeotropic anchoring using mixed SAMs formed by coadsorption of CH3(CH2)15SH and CH3(CH2)9SH on uniformly or obliquely deposited gold, respectively. Because SAMs formed from CH3(CH2)15SH or CH3(CH2)9SH are densely packed and pseudocrystalline, and because mixed SAMs formed by coadsorption of CH3(CH2)15SH and CH3(CH2)9SH are conformationally disordered, we conclude that the different anchoring of LCs on these surfaces can be attributed to the organization of the aliphatic chains within the SAMs. In contrast, the anchoring of liquid crystals on films of SiOx coated or not coated with CTAB or DMOAP can plausibly arise from factors other than organization of the aliphatic chains. Changes in the density of charges at the surfacescaused, for example, by the adsorption of CTAB or DMOAPscan influence the anchoring of liquid crystals on these surfaces. Selfassembled monolayers on gold do, therefore, form the basis of a useful experimental system for fundamental studies of the combined influence of the topology of substrates and the structure of organic monolayers on the anchoring of liquid crystals. Measurements of the wetting of fluids on surfaces are easy to perform, are surface-sensitive, and have, therefore, become a widely accepted method for the characterization of surfaces. Here we report that surfaces measured to be indistinguishable by advancing and receding contact angles of hexadecane can cause large and easily detected changes in the anchoring of liquid crystals. Past attempts to understand the anchoring of liquid crystals on surfac(23) The tilt of a liquid crystal from a surface has been found to depend on the liquid crystal as well as the material deposited (e.g. SiOx, C, CaF2, Au, Ag). Alignment of liquid crystals on obliquely deposited films cannot, therefore, be described purely on the basis of an elastic interaction of the liquid crystal with the topography of the surface. See refs 4 and 22. (24) This work was supported in part by the MRSEC Program of the NSF (Grant DMR-9400354) and an NSF CAREER award to N.L.A.

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Alkanethiols on Obliquely Deposited Films of Gold

tant-coated surfaces as well as SAMs formed from organosilanes have correlated the wetting of fluids on surfaces with the alignment of nematic liquid crystals. Our results illustrate that microscopic interactions responsible for the wetting of simple isotropic fluids such as hexadecane can differ from those interactions that govern alignment of liquid crystals by surfaces. A future report will compare and contrast the interactions of isotropic fluids and anisotropic fluids using surfaces formed by the self-assembly of alkanethiols on gold. Conclusion We report the preparation of SAMs formed from alkanethiols on the surfaces of obliquely deposited films of gold. Self-assembled monolayers supported on obliquely

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deposited gold uniformly anchor nematic phases of liquid crystals, whereas SAMs supported on gold deposited without a preferred direction do not. Control of the packing and conformational order of aliphatic chains within a SAM, in combination with manipulation of the manner of deposition of the gold substrate supporting the SAM, permits control of both the polar and azimuthal anchoring of nematic phases on these surfaces. Nearplanar and azimuthally uniform anchoring can be achieved by using single-component SAMs formed from hexadecanethiol or decanethiol supported on obliquely deposited gold. The tilt of a nematic phase can be controlled by using mixed SAMs prepared by coadsorption of alkanethiols with long and short alkanethiol chains: we report homeotropic and tilted homeotropic (small pretilt from the normal) anchoring. LA960003I