J. Phys. Chem. 1995, 99, 16511-16515
16511
Anchoring of Nematic Liquid Crystals on Self-Assembled Monolayers Formed from Alkanethiols on Semitransparent Films of Gold Richard A. Drawhorn and Nicholas L. Abbott* Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616 Received: June 23, 1995; In Final Form: September 19, 1995@
Self-assembled monolayers (SAMs) formed by chemisorption of n-alkanethiols (CH3(CH2),,SH) on films of gold permit manipulation of the mesoscale structure of nematic liquid crystals in contact with these surfaces. Mixed SAMs formed from C H ~ ( C H ~ ) I and ~ S Heither CH3(CH2)4SH or CH3(CH2)9SH homeotropically anchor nematic phases of 4-cyano-4'-pentylbiphenyl (5CB) and p-methoxybenzylidene-p-n-butylaniline (MBBA). Single-component SAMs, in contrast, do not uniformly anchor these nematic phases at room temperature: SAMs formed from CH3(CH2),SH (2 < n < 15) cause either planar or tilted anchoring. Mixed SAMs that homeotropically anchor 5CB and MBBA are conformationally disordered, when characterized prior to contact with the liquid crystal, and have a low number density of long aliphatic chains. We conclude, however, that conformational disorder within the aliphatic chains of SAMs is not a sufficient condition to induce homeotropic anchoring because the aliphatic chains of single-component SAMs with n < 10 are fluidlike (conformationally disordered) at room temperature and do not homeotropically anchor nematic phases. We infer the number density of long aliphatic chains to be an important factor in the anchoring of liquid crystals on mixed SAMs. Self-assembled monolayers formed from n-alkanethiols, in combination with techniques for their patteming on surfaces, form the basis of a procedure to prepare micrometer-scale optical structures from liquid crystals.
Introduction We report the use of self-assembled monolayers (SAMs)' prepared by chemisorption of n-alkanethiols on semitransparent films of gold in fundamental studies of how the molecular-level structure of an organic interface can influence the mesoscale structure of an adjacent phase of nematic liquid crystals. The principal result of this work is the demonstration that mixed S A M s 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 MBBA or 5CB. In contrast, single-component SAMs (CHs(CHz),SH, 2 < n < 15) cause nonuniform, planar or tilted anchoring at room temperat~re.~These results form the basis of a strategy for patteming nematic liquid crystals on surfaces. A complete understanding of the physicochemical mechanisms leading to the anchoring of nematic liquid crystals by organic surfaces does not exist.2 Many studies of the anchoring of nematic liquid crystals use organic surfaces that do not have well-defined structures. The structures of methyl-terminated SAMs formed from n-alkanethiols on gold, in contrast, have been extensively ~haracterized.4-'~Self-assembled monolayers formed on 100 A-thick films of gold transmit %SO% of visible light (A 500nm)'5and, therefore, permit observation of optical textures of liquid crystals using transmitted light. The self-assembly of n-alkanethiols on polycrystalline films of gold with a predominate (1 11) texture results from chemisorption4 of sulfur headgroups onto the Au( 111) lattice. The chemisorbed species form a 4 3 x 4 3 R30" lattice that is commensurate with the underlying gold substrate.6-8~'0~'The organization of the hydrocarbon chains within a SAM is dependent on the length of the chains and temperat~re.~.'.~ Selfassembled monolayers of long aliphatic chains (n 2 lo), when formed under typical conditions (chemisorption from an ethanolic solution of e l mM n-alkanethiol) and when characterized
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Abstract published in Aduance ACS Abstracrs, November 1, 1995.
0022-3654/95/2099-16511$09.00/0
in air at room temperature, have densely packed (21.4 A2/ chain),I0 pseudocrystalline domains (=90 A)6 of uniform tilt (20-30" from the surface n0rmal).~3"-'~The hydrocarbon chains are extended in their trans conformations12and can form a c(4 x 2) superlattice with respect to the fundamental J3 x 2/3 R30" l a t t i ~ e . ~In. ~ contrast to SAMs formed from nalkanethiols with long chains (n > lo), the aliphatic chains of SAMs formed from short chains (n < 10) are conformationally disordered and fluidlike, and become less densely packed as n decreases.8,' Mixed SAMs formed from n-alkanethiols with long and short aliphatic chains consist of a fluidlike overlayer on an ordered fo~ndation.'~ If both components of the mixed SAM have more than 10 carbon atoms in their aliphatic chains, the structure of the foundation is pseudocrystalline.' The number of gauche conformers (fluidity) within the overlayer of a mixed SAM can be controlled by manipulating the composition of the S A M .We have used both single-component S A M s and mixed SAMs with long and short aliphatic chains to prepare surfaces with welldefined structures in order to establish the role of the packing density and fluidity of an aliphatic monolayer on the anchoring of nematic liquid crystals.
Experimental Section Self-assembled monolayers were prepared by immersing thin films of goldI5 evaporated (100 A at 0.2 ks, P = 5 x lov6 Torr) on clean16 glass microscope slides (Fisher) into ethanolic solutions containing a total concentration of 1 mM of nalkanethiol(s) for 2 h (unless stated otherwise). The compositions of mixed SAMs formed under these conditions were determined by the kinetics of chemi~orption.'~ Both advancing and receding contact angles of hexadecane were measured for each SAM (in air at room temperature) using a goniometer (RamB-Hart). The contact angles reported are the average of 12 measurements taken on four separate samples. The SAMs were subsequently paired and spaced apart by 2;um0 1995 American Chemical Society
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16512 J. Phys. Chem., Vol. 99, No. 45, 1995
A
n
liquid crystal
B NEC
SCB
MBBA
Figure 1. (A) Schematic illustration of optical cell (not to scale). Au, evaporated film of gold; Ti, evaporated film of titanium used to promote adhesion of gold to glass substrates. (B) Chemical structures of MBBA and 5CB. 1
0.9
t40
Om'
0
0.1
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Figure 2. Advancing contact angles, e,, of hexadecane on SAMs formed from ethanolic solutions containing mixtures of CH3(CH2)&H and CHj(CH2)lsSH. xC16. soln is the mole fraction of n-alkanethiol in solution that was CHj(CH2)15SH; (0)homeotropic anchoring of 5CB on the SAM; (0)nonuniform anchoring of 5CB; (0)macroscopic regions of 5CB that differed in anchoring were observed on the same sample (see text for details). The inset shows the hysteresis measured in contact angles of hexadecane advancing and receding across mixed SAMs.
thick films of Mylar to form optical cells (Figure 1A). Nematogens 5CB (BDH Chemical) and MBBA (TCI America), Figure lB, were heated into their isotropic phases, drawn into optical cells by capillary action, and cooled slowly to their nematic phases (-1 "Chin) on a thermal stage (Leica). A polarized light microscope (Olympus) was used to obtain optical textures'' of the liquid crystals at room temperature. Conoscopic interferencefigures1*confirmed the orientation of the direct09 of liquid crystals relative to the substrates in uniformly anchored samples. Clearing-point temperatureswere verified before each experiment (-35.3 "C for 5CB; -46 "C for MBBA) to confirm the purity of the liquid crystals. Patterned SAMs were prepared by "stamping" surfaces of gold film using CH3(CH2),5SH as the "ink"I9 and a block of poly(dimethylsi1oxane) (PDMS) with 1- 100pm-wide grooves in its surface as the stamp. Grooves were seared in the surface of the PDMS with a hot blade of a razor.
Results Figure 2 shows advancing contact angles of hexadecane measured on S A M s formed from CH3(CH2)15SH, CH3(CH2)4-
SH and their mixtures. The advancing contact angle on a S A M formed from CH3(CH2)15SH was 46", while the contact angle for a S A M formed from CH3(CH2)4SH was 14". Contact angles on SAMs formed from mixtures of CH3(CH2)lsSH and CH3(CH2)4SH were between 14" and 46". The inset in Figure 2 shows that the hysteresis in the contact angle of hexadecane is slightly greater on a mixed S A M (-10") than on a singlecomponent S A M ( ~ 5 " ) . The greater hysteresis and lower contact angles measured on mixed S A M s as compared to S A M s formed from C H ~ ( C H ~ ) I ~are S Hconsistent with the presence of conformational disorder within the mixed SAMs.13 The lateral distribution of species within mixed S A M s is unknown: "Islanding" (formation of domains) within mixed S A M s formed from CH3(CH2)15SH and CH302C(CH2)15SH (immersed for 4 duys in solutions of the thiols) has been re~0rted.l~ Figure 3A is the optical texture of 5CB confined by surfaces supporting S A M s formed from CH~(CH~)I~SH. The nematic phase is not uniformly anchored by the pseudocrystalline (ordered) S A M , and no interference figures were observed under conoscopic illumination. The preferred direction visible in the texture runs in the direction of loading of the liquid crystal into the optical cell. Similar results were obtained for MBBA. Textures were consistent with either planar or tilted anchoring.3.20*21 Textures similar to Figure 3A were observed on SAMs formed over periods of time ranging from 2 to 24 h. Nematic phases of either 5CB or MBBA were homeotropically anchored on mixed S A M s with advancing contact angles of hexadecane between 16" and 35". Optical textures of these samples were uniform over 10 mm x 10 mm areas (Figure 3B), and the corresponding interferencefigures were consistent with homeotropic anchoring (Figure 3C).I* Similar results were observed using optical cells with thicknesses ranging from 2 to 150 pm. The hysteresis in the contact angles of hexadecane (inset in Figure 2) does not conelate closely with homeotropic anchoring of 5CB. Mixed S A M s having contact angles of either e38O or -15" produced domains of liquid crystal (-1 mm2) that varied in anchoring. Some domains were homeotropically anchored whereas others produced conoscopic figures consistent with uniform tilted anchoring.I8 Mixed S A M s rich in either CH3(CH&SH (14" < 8, < 16") or C H ~ ( C H ~ ) I ~(38" SH qa < 46") produced nonuniform anchoring. Self-assembled monolayers formed from CH3(CH2)4SH did not uniformly anchor nematic phases of MBBA or 5CB. The schlieren texture shown in Figure 3D is typical for nematic phases having degenerate planar or tilted anchoring.21-22 Finally, we note that the anchoring of MBBA or 5CB on mixed S A M s formed from C H ~ ( C H ~ ) I ~and S HCH3(CH2)9SH was similar to anchoring on mixed S A M s formed from CH~(CH~)ISSH and CH3(CH2)4SH: Homeotropic anchoring was observed in both systems. Because anchoring of nematic liquid crystals on S A M s formed form n-alkanethiols on gold can now be controlled by the composition and structure of the S A M , we have used surfaces patterned with micrometer-scale areas of SAMs that cause either homeotropic (mixed S A M s as selected from Figure 2) or tiltedplanar (SAMs formed from CH3(CH2)lsSH) anchoring to create optical structures from liquid crystals.23 Optical cells were prepared with patterned S A M s (10- 100-pm-wide stripes) on the top surfaces (Figure 4A). The lower surfaces of optical cells supported mixed S A M s with the same composition as the stripes on the top surfaces. Figure 4B is the optical texture of 5CB in a patterned optical cell with a 93-pm-wide stripe on the top surface. Liquid crystal in contact with the stripe was homeotropically anchored, while
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J. Phys. Chem., Vol. 99, No. 45, 1995 16513 I % .
IC
1
A
10 mm
4
.
\t
Figure 4. Anchoring of 5CB on patterned SAMs. (A) Schematic illustration of the patterned SAM on the top surface of the optical cell; (B) optical texture of 5CB in optical cell with top surface supporting a 93-pm-wide stripe of mixed S A M . The 5CB in contact with the stripe was anchored homeotropically.
choring within the stripe over its full length (10 mm). We have used this technique to form structures as small as 10-pm in width.
Discussion
Figure 3. Optical textures of 5CB anchored on S A M s formed from (A) C H ~ ( C H ~ ) I ~ S(B) H ; a binary mixture of CH3(CH2)SH and CH~(CHZ)I~SH with Xcla, = 0.06 and 8, = 28'. The bright spot in the top left comer of the image is light transmitted through a hole in the gold film supporting the S A M ; (C) conoscopic interference figure of sample in (B); (D) optical texture of 5CB in contact with SAM formed from CH3(CH2)4SH. The horizontal dimensions of A, B, and D are 650 pm.
the anchoring of the surroundingliquid crystal was nonuniform. A conoscopic interference figure confirmed homeotropic an-
The anchoring of a nematic phase by a SAM is determined by the excess free energy of the interfacial region between the SAM and nematic phase. Contributions to the excess surface free energy can arise from two classes of interaction^?^ (i) longranged effects associated with an interfacial electric field that acts on the electric dipole and electric quadrupole moments of nematogens and (ii) short-ranged steric and van der Waals interactions between nematogens and the surface. An electric field in the vicinity of a S A M can plausibly arise from sources such as dipolar Au-thiolate bonds, selective adsorption of impurity ions by the or perturbation of nematic order by the S A M . The nematic order can be perturbed in three ways?6*28.29First, distortions (bend, splay) of the director? when coupled with the unsymmetrical shape of molecules having permanent electric dipole moments, can induce an electric polarization called flexoelectric polarization.26 Second, the presence of a surface breaks the translational symmetry of the nematic phase. The resulting gradient of the average thermal fluctuation about the director (the scalar order parameter) can create a gradient in the electric quadrupolar density of the liquid crystal.27 Because a gradient in the quadrupolar density gives rise to an effective dipolar density, an electric polarization results called ordoelectric polarization.2* Third, the two ends of a nematogen can differ in their affinity for the surface. The resulting loss of inversion symmetry at the surface can induce a polari~ation.~~ Because mixed SAMs have a similar density of Au-thiolate bonds for a11 compositions, the associated dipolar contribution
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16514 J. Phys. Chem., Vol. 99, No. 45, 1995
to the interfacial electric field will not be a strong function of the composition of the SAM. However, flexoelectric, ordoelectric, selective ion adsorption, and polar ordering contributions may vary with composition of mixed SAMs and should, therefore, be considered in a description of the anchoring of liquid crystals by these surfaces. Because nematic phases of MBBA and 5CB have dielectric anisotropies that are opposite in sign, an electric field will cause the directors of these nematic phases to assume orthogonal orientations. The same is true for the flexoelectric torques. MBBA and 5CB would, therefore, anchor differently30if either dielectric or flexoelectric effects were the dominant contributions to the excess surface free energy. We note that MBBA and 5CB do anchor differently at their free surface^.^' Because MBBA and 5CB anchor similarly on SAMs of n-alkanethiols on gold, we conclude that dielectric or flexoelectric effects alone are unlikely to cause the observed anchoring of MBBA or 5CB on these surfaces. Ordoelectric or surface polar effects, however, can plausibly cause MBBA and 5CB to anchor in a similar manner on mixed SAMs. Work is in progress to establish (i) the extent of polar ordering of the liquid crystals on SAMs and (ii) the change in the scalar order parameter at the surface as a function of the composition of mixed SAMs. Finally, we consider contributions to the excess surface free energy that arise from short range steric and van der Waals interactions between nematogens and We hypothesized that both the degree of fluidity of aliphatic chains within a SAM and the density of packing of chains within a SAM would influence the penetrability of a SAM to nematogens (interdigitation) and thereby affect the anchoring of nematic phases on the SAM. Because neither single-component, fluidlike SAMs (n < 10) nor pseudocrystalline SAMs ( n > 10) were found to homeotropically anchor nematic phases, we conclude that the conformational order within a S A M does not solely determine whether or not a SAM can induce homeotropic anchoring. Because mixed SAMs that homeotropically anchor 5CB or MBBA are conformationally disordered and have a low density of long aliphatic chains, we infer that the density of long aliphatic chains within a mixed SAM is an important factor in the homeotropic anchoring of nematic phases. A recent report by Shen and co-workers that describes anchoring of 8CB on a loosely packed silane monolayer (CH~(CHZ)(M~)~N+(CH& Si(OMe)3Cl-, DMOAP) on glass demonstrates that liquid crystals can penetrate into a monolayer and reduce conformational defects in the aliphatic chains.34 Hiltrop and S t e g e m e ~ eand r ~ ~Fang and Wei36have observed changes in the anchoring of MBBA as a function of the packing density of monolayers of lecithin (L-p,y-dilauroyl-a-lecithin and dipalmitorylphosphatidylcholine) on glass. Hiltrop and Stegemeyer proposed a model in which the liquid crystal molecules interact sterically with a preorganized monolayer: the nematogen was assumed to penetrate into defects in the monolayer structure. In contrast, the outer regions of mixed SAMs formed from long and short chains are fluidlike. Reorganization of the SAM when placed in contact with a liquid crystal is, we believe, an important part of the mechanism of anchoring of liquid crystals on SAMs. Finally, we note that the planar and tilted anchoring that we have observed for nematic liquid crystals on SAMs formed from long-chain n-alkanethiols on gold contrasts with the homeotropic anchoring reported by others for SAMs formed from alkyltrichlorosilanes on glass.35 We do not yet understand the origin of this difference.
Conclusion In summary, we report that SAMs formed from alkanethiols on gold form a useful experimental system for studies of the anchoring of liquid crystals on organic surfaces. By controlling the composition and structure of mixed SAMs formed from long and short aliphatic chains, we have achieved homeotropic, tilted, and planar anchoring. Our observations of uniform tilted domains on mixed SAMs suggest that control over the structure of mixed SAMs may permit manipulation of the tilt (polar angle) of liquid crystals on surfaces. SAMs formed from w-functionalized n-alkanethiols, in combination with techniques for their patterning on surfaces, can, we believe, provide additional routes for manipulation of mesoscale structure within nematic phases (e.g., bistable anchoring).
Acknowledgment. This work was supported in part by the MRSEC Program of the NSF (DMR-9400354) and a NSF CAREER award to NLA (CTS-9502263). We thank Hans Biebuyck, Vinay Gupta, William Miller, Debra Rolison and George Whitesides for useful comments. References and Notes (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Elodgett 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 therein. (2) The term “anchoring” refers to the alignment of liquid crystals by surfaces in contact with them. For a recent review of this topic see: JtrBme, B. Rep. Prog. Phys. 1991, 54, 391. 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 anchoring of a nematic phase with the director perpendicular to the plane of the substrate. ,,. (3) Nonuniform anchoring refers to the formation of many individual domains with optical axes that have differing azimuthal orientations. For planar anchoring the optical axes are parallel to the substrate, while for tilted anchoring they are tilted from the normal to the surface by an angle 0. (4) (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.; Chidsey, C. E. D.: Liu, G.-Y.: Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (0Widrig, 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) Butt, H.-J.; Seifert, K.; Bamberg, E. J. Phys. Chem. 1993, 97, 7316. (i) Stranick, S. J.: Weiss, P. S.: Parikh, A. N.: Allara, D. L. J. Vac. Sci. Technol. A 1993, 11, 739. (j) Camillone, N.: Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (k) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, IO, 2853. (1) Rolandi, R.: Cavalleri, 0.: Toneatto, C. Thin Solid Films 1994, 243, 431. (m) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, I O , 3383. (n) Schonenberger, C.: Sondag-Huethorst, J. A. M.: Jomtsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 61 1. ( 0 ) Sondag-Huethorst, J. A. M.; Schonenberger, C.: Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (p) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (q) Schonenberger, C.; Jomtsma, J., Sondag-Huethorst, J. A. M.: Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (5) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.: Scoles, G. J. Chem. Phys. 1993, 98, 3503. (6) Camillone, N.: Chidsey, C. E. D.: Eisenberger, P.; Fenter, P.: Li, J.; Liang, K. S.: Liu, G.-Y.: Scoles, G. J. Chem. Phys. 1993, 99, 744. (7) Fenter, P.: Eisenberger, P.: Liang, K. S. Phys. Rev. Lerr. 1993, 70. 2447. (8) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559 (b) Nuzzo, R. G.; Zegarski, B. R.: Dubois, L. H. J. Am. Chem. SOC. 1987, 109, 733 (c) Strong, L.; Whitesides, G. M. Langmuir 1988,4,546 (d) Bain, C. D.; Troughton, E. B.: Tao, Y.-T.: Evall: J.; Whitesides, G. M.: Nuzzo, R. G. J. Am. Chem. SOC. 1989, 111, 321 (e) Dubois, L. H.; Zegarski, B. R.: Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (9) Badia, A.; Back, R.: Lennox, R. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 2332. (10) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6 , 682. (11) Samant, M. G.; Brown, C. A.; Gordon, J. G. Langmuir 1991, 7, 437.
Letters (12) (a) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990,93,767. (b) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. SOC. 1990, 112, 558. (c) H h e r , G.; Kinzler, M.; Thiimmler, C.; Wo11, Ch.; Grunze, M. J. Vac. Sci. Technol. A 1992, IO, 2758. (13) (a) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Adhes. Sci. Technol. 1992, 6, 1397. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC. 1989, 111, 7164. (d) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (e) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (14) Stranick. S . J.: Parikh. A. N.: Tao. Y.-T.; Allara. D. L.: Weiss, P. S . J . Phys. Chem. 1994, 98, 7636. (15) DiMilla. P. A.: Folkers, J. P.; Biebuvck, H. A,: HBrter, R.; Lbpez, G. P.; Whitesides, G. M. J. Am. Chem. Soc:1994, 116, 2225. (16) Glass slides were cleaned by immersion in piranha solution (70% H2S04. 30% H202(30%)) for 1 h followed by a rinse of deionized water. Warning: piranha solution reacts strongly with organic compounds and should be handled with extreme caution: do not store the solution in closed conrainers. (17) Nematic phases are birefringent. Variations in the average molecular orientation of a nematic phase appear as variations in the intensity of light transmitted through crossed polarizers. (18) Conoscopic interference figures were obtained using a Bertrand lens and a wide aperture light source. The interference figure for a homeotropically anchored sample is a Maltese Cross that remains stationary as the sample is rotated between crossed polarizers; for tilted anchorings, the cross rotates as the sample is rotated. Hartshorne, N. H.; Stuart, A. Crystals and the Polarising Microscope; Edward Arnold & Co.: New York, 1970. (19) (a) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (b) Kumar, A.; Biebuyck, H. A,; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. SOC. 1992, 114, 9188. (c) Kumar, A,; Biebuyck, H. A,; Whitesides, G. M. Langmuir 1994, IO, 1498. (d) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600. (e) Xia, Y.; Whitesides, G. M. J. Am. Chem. SOC. 1995,117, 3274. Kumar, A,; Abbott, N. L.; Kim, E.; Biebuyck, H.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219. (20) The response of MBBA to an electric potential applied across the optical cell (25 V using the gold films as electrodes) was consistent with planar anchoring: No change in optical texture was observed upon application of the electrical potential. Application of an electrical potential to cells filled with 5CB induced homeotropic anchoring. (21) Demus, D.; Richter, L. The Textures of Liquid Crystals; Verlag-Chemie: Weinheim, 1978. (22) The textures of MBBA and 5CB on SAMs formed from CHdCH2)4SH were observed to be variable. We also observed textures that were not schlieren textures.
J. Phys. Chem., Vol. 99, No. 45, 1995 16515 (23) In contrast to other methods employed to attain lateral control over the anchoring of nematic liquid crystals, patterning of SAMs formed from n-alkanethiols requires only chemical modification of the substrate. Other methods include the following: (i) Patterning of underlying electrode materials and application of external electric fields: Patel, J. S . : Rastani, K. Opt. Lett. 1991, 16, 532. (ii) Patterning of substrate topology which exploits associated elastic effects: Kawata, Y . ; Takatoh, K.1 Hasegawa, M.; Sakamoto, M. Liq. Cryst. 1994, 16, 1027. KLnel, H. V.; Litster, J. D.; Melngailis, J.; Smith, H. I. Phys. Rev. A 1981, 24, 2713. Nakamura, M.; Ura, M. J. Appl. Phys. 1981, 52, 210. (iii) Photopolymerization techniques: Shannon, P. J.; Gibbons, W. M.: Sun, S. T. Nature 1994, 368, 532. (iv) Vapor deposition of metals with conflicting surface anchorings: Ong, H. L.; Hurd, A. J.; Meyer, R. B. J. Appl. Phys. 1985, 57, 186. (24) Barbero, G.; Chuvyrov, A. N.; Krekhov, A. P.: Scaldin, 0. A. J. Appl. Phys. 1991, 69, 6343. (25) Barbero, G.; Durand, G. J. Phys. (France) 1990, 51, 281. (26) Meyer, R. Phys. Rev. Lett. 1969, 22, 918. (27) The quadrupolar density is a macroscopic volume average of molecular quadrupolar moments calculated by integrating the microscoplc charge density of a molecule over the molecular volume; Prost, J.; Marcerou, J. P. J. Phys. (France) 1977, 38, 315. (28) Barbero, G.; Dozov, I.; Palierne, J. F.; Durand, G. Phys. Rev. Lett. 1986, 56, 2056. (29) Petrov, A. G.; Derzhanski, A. Mol. Cryst. Liq. Cryst. 1977,41,41. (30) For an electric field created by a negatively charged surface, the dielectric effect stabilizes homeotropic anchoring of 5CB but destabilizes homeotropic anchoring of MBBA; in contrast, the flexoelectric effect destabilizes homeotropic anchoring of 5CB, but stabilizes homeotropic anchoring of MBBA. Although unlikely, we cannot rule out a cancellation of the two effects. Barbero, G.; Petrov, A. G. J. Phys.: Condens. Matter 1994, 6, 2291. (31) (a) Bouchiat, M. A,; Langevin-Cruchon, D. Phys. Lett. 1971, 34A, 331. (b) Warengham, M.; Isaert, N.; Bernard, P. J. Opt. 1984, 15, 133. (32) Hiltrop, K.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 582. (33) Proust, J. E.; Ter-Minassian-Saraga, L. Solid State Commun. 1972, 11, 1227. (34) Huang, J. Y; Superfine, R.; Shen, Y. R. Phys. Rev. A 1990, 42, 3660. (35) (a) Yang, J. Y.; Mathauer, K.; Frank, C. W. Microchemistry 1994, 441. (b) Peek, B.; Ratna, B.; Pfeiffer, S.; Calvert, J.; Shashidhar, R. Proc. SPZE 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. Lert. 1973, 22, 386. (36) Fang, J.; Wei, Y. Mol. Cyst. Liq. Cryst. 1992, 222, 71. JP95 1748K
