J. Phys. Chem. B 1997, 101, 2143-2148
2143
Surface Plasmon Resonance Imaging of Liquid Crystal Anchoring on Patterned Self-Assembled Monolayers S. D. Evans,* H. Allinson, N. Boden, T. M. Flynn, and J. R. Henderson Centre for Self-Organising Molecular Systems, UniVersity of Leeds, Leeds LS2 9JT, U.K. ReceiVed: October 28, 1996; In Final Form: January 28, 1997X
The anchoring of the nematic liquid crystal (LC) 4′-n-octyl-4-cyanobiphenyl (8CB) at derivatized self-assembled monolayer (SAM) surfaces has been investigated using surface plasmon resonance (SPR) microscopy. Surfaces having distinct areas (patterns) covered by different ω-functionalized groups, one (-CF3) promoting homeotropic anchoring of the nematic director and the other (-OH) planar anchoring, were fabricated from photopatterned SAMs. Our results show that SPR microscopy, which is sensitive to the alignment on the mesoscale (i.e., within 300 nm of the interface) can be used to spatially resolve the anchoring of adsorbed LC films, with a lateral resolution on the order of a few micrometers, as a function of temperature. In addition, we observe surface melting of the crystalline phase at the SAM boundary by both homeotropic and planar aligned melts.
Introduction Self-assembled monolayer (SAM) systems, especially those of alkanethiols on gold, have been intensively investigated in recent years. Their usefulness derives from their ease of formation and the subsequent high quality of the films produced.1 Early research in this field showed that the strong affinity of the thiol (sulfur) group toward the gold meant that the ω-functional group could take a wide variety of forms, X, for alkanethiols of the form (HS-R-X) and where R ) (CH2)n.2 The ultrathin nature of SAMs, typically ∼20 Å thick, has tended to restrict the type and number of techniques that may be applied to their characterization.3 Perhaps one of the oldest and yet still extremely surface sensitive probes is that of contact angle measurements.4 The macroscopic contact angle of an adsorbed drop of liquid is determined by the interfacial tensions of the three interfaces present (and is particularly sensitive to the substrate-liquid surface free energy) and hence is related to the chemical and physical properties of the SAM surface.5 Indeed, in some instances wetting studies can be more revealing than sophisticated surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS). It is not surprising therefore that this simple, but effective, method is widely used as an indicator of the quality of SAMs.6 From the early studies of alkanethiol-derivatized SAMs it became apparent that the inverse is also true, that is, that SAM surfaces may, because of their high quality and reproducibility, be used as model systems for investigating wetting and adsorption phenomena.7 Until recently, such wetting studies had not exploited the novel effects due to the anisotropy of liquid crystalline (LC) films. Recent work by Abbott et al. and Allinson et al. on the interaction of anisotropic fluids with SAM surfaces has demonstrated that control over the surface properties can allow the study of interfacial orientational phase transitions and control over the bulk alignment of LC films.8,9 In the latter study it was shown that the use of SAMs permits a close investigation of orientational wetting and anchoring transitions. Our particular interest is in the behavior of the fluid near the substrate-fluid interface. To probe this region, we have chosen to use techniques that utilize evanescent fields; in particular, * To whom correspondence should be addressed (e-mail s.d.evans@ leeds.ac.uk). X Abstract published in AdVance ACS Abstracts, March 1, 1997.
S1089-5647(96)03341-X CCC: $14.00
we have chosen to use surface plasmon resonance and evanescent wave ellipsometry. These techniques have distinct advantages, for the systems being studied here, over conventional ellipsometric and optical techniques. In particular, the probe beam is not required to traverse the birefringent bulk phase, and thus, we are not restricted to studying the isotropic-nematic phase transition. Surface plasmon resonance (SPR) is now becoming a widely used technique for the investigation of adsorbed thin films. However, the imaging mode of SPR remains less widely used. SPR in its spectroscopic mode has, however, been previously applied to the investigation of surfaceinduced alignment and electric field-induced anchoring transitions.10,11 The studies described in refs 10 and 11 are of particular relevance, since they present methodologies for calculating the optical behavior of uniaxial anisotropic systems using scattering matrix methods. The experiments described in this work are focused on 8CB, a rod-like calamitic LC that displays the following phase behavior C-24 °C-SA-34 °C-N-42.6 °C-I. In the liquid state the long molecular axes are isotropically distributed. The transition to the nematic state is characterized by the onset of long-range orientational order of the long molecular axes, and the transition to the smectic A layering is accompanied by the introduction of one-dimensional positional order. Experimental Section Materials. Monolayer films were formed using HS(CH2)n-X, where X ) OH and n ) 15, and HS-(CH2)4-OC6H4-S-CH2-(CF2)9-CF3. The latter material was available to us through previous work and has been shown to produce close packed monolayers with a high degree of conformational order.12 The liquid crystal 8CB (CH3-(CH2)7-C6H4-C6H4-CN) was obtained from Merck Ltd. and had a stated purity level of 99.7% (as determined by HPLC analysis). SAM Formation. Self-assembled monolayers were formed via the spontaneous adsorption of molecules containing an ω-functional group. In all cases 10-3 M solutions in dichloromethane (DCM) were used and the subsequent film quality was monitored using ellipsometry, wetting, and XPS.8,9 For the surface plasmon studies monolayers were formed on 500 Å thick gold films that had been evaporated directly onto © 1997 American Chemical Society
2144 J. Phys. Chem. B, Vol. 101, No. 12, 1997
Evans et al. allowed the temperature to be varied between -40 and 80 °C with a resolution of 0.01 °C. Surface Plasmon Resonance Imaging. The surface plasmon resonance images were obtained using an apparatus, designed and built at Leeds, based on that described by Rothenhausler and Knoll.14 A TIH53 glass prism was used to couple p-polarized radiation from a HeNe laser, 632.8 nm, into the gold sample under investigation. The gold films were evaporated directly onto high refractive index prisms to circumvent the use of index-matching fluid. The subsequent SPR images were collected via the use of a planoconvex lens and a square pixel Sony CCD camera (512 × 484 pixels). High-magnification images were obtained using a zoom lens attached directly to the camera. The images were captured and analyzed using ACCUWARE software. Results and Discussion
Figure 1. Experimental arrangement for SPR experiments. The lower figure shows schematically the exponential decay of the electric field into the LC phase and the interfacial and surface regions of interest.
Figure 2. (Left) Schematic representation (not to scale) of an isotropic liquid phase above a patterned SAM. (Right) Refractive index ellipsoid for the isotropic medium. This is spherical with niso taking an average value closely approximated by niso2 ) (1/3)(n|2 + 2n⊥2) where n| and n⊥ refer to the refractive indices parallel and perpendicular to the director.
the base of a high refractive index prism (n ) 1.85). No chromium, or titanium, adhesion layers were used. The gold films were formed using an Edward’s TMP Auto306 system at a base pressure of 3 × 10-6 mbar. Photopatterning. The patterned SAMs were formed by a two-stage process. Initially, a uniform CF3-terminated monolayer was adsorbed onto the gold surface from solution. The SAM was subsequently rinsed in solvent and Millipore water before being dried under a flow of filtered N2. A metal mask was positioned to be very nearly in contact with the SAM, and the sample was irradiated for 40 min with a UV pen lamp, nominal power of 50 W at 254 nm, then rinsed thoroughly in Millipore water. The patterned sample was then placed in a second alkylthiol solution containing an -OH-terminated thiol derivative. The patterns used here are simple macroscopic stripes. However, much smaller features, certainly down to the 5 µm range, can be produced in a similar way.13 Cell Arrangement. The cells used in the SPR experiments had a geometry as shown schematically in Figure 1 and consisted of the SAM-modified prism as one surface and a clean glass window as the opposite surface. The surfaces were separated using stripped optical fiber with a diameter of 125 µm, and the unsealed cells were filled, by capillary flow, with the 8CB in its nematic or isotropic phase. The glass window was mounted directly on a Peltier effect heater/cooler that
A simple striped monolayer pattern was used to investigate the melting and spatial anchoring of the liquid crystal 8CB. Figure 2 shows a schematic representation of a striped monolayer sample in contact with the isotropic liquid phase, i.e., well above the nematic-isotropic phase transition temperature TIN (not drawn to scale). The nCB family of liquid crystals have positive optical anisotropy, n| > n⊥, where n| and n⊥ are the refractive indices parallel and perpendicular to the nematic director (direction of average molecular orientation as defined by the long molecular axis; see Figure 2). The values for 5CB, for which the model calculations presented here are based on, are 1.693 and 1.533 for n| and n⊥, respectively.15 We note that for the qualitative discussion given here the absolute values are not needed, and hence, the calculations have not been repeated for 8CB. The refractive index of the isotropic phase is close to the average of these, i.e., Niso ≈ Nav ) 1.586, where Nav2 ) (1/3)(N|2 + 2N⊥2).16 In SPR microscopy, contrast occurs because of changes in optical thickness, ∆n.l , where l is the thickness of a film and ∆n is the difference in refractive index between the film and the ambient.17 Typically, taking ∆n to be constant, the buildup of film thickness can be followed with a high degree of accuracy.18 In contrast, for the problem considered here, it is changes in refractive index that are important. We may assume that l is constant; i.e., the film thickness extends beyond 3λd, where λd is the characteristic decay length of the evanescent field into the bulk. Figure 3 shows the calculated SPR spectra for four idealized cases of alignment. Curves a and b with their resonance positions (minima) at lowest angles of incidence represent two extremes of planar alignment. In both cases the long molecular axis (director) lies within the plane of the surface but in case a it is parallel to the plane of incidence while in case b it is perpendicular to the plane of incidence. Curve c represents the expected curve for the isotropic phase (N ) Nav), and finally, curve d is that calculated for homeotropic alignment. These calculations make several assumptions. First, the anisotropy is assumed to be uniaxial and no account is taken for tilt or twist at some arbitrary angle to the plane of incidence. Second, the anisotropic regions of the film are assumed to be 4000 Å thick and to be sandwiched between isotropic media (gold/SAM and bulk isotropic phase). Finally, all interfaces are assumed to be abrupt and smooth. As far as the surface plasmon mode is concerned, the second assumption is effectively saying that the anisotropic film may be treated as a semi-infinite medium, since the electric field associated with the surface plasmon decays exponentially and is almost totally insensitive to any changes in the dielectric properties beyond ∼3000 Å
Imaging of Liquid Crystal Anchoring
Figure 3. Reflectance curves calculated for p-polarized light as a function of the angle of incidence for a four-layer system comprised of glass, gold, interfacial region, and bulk phase. The following parameters were used in the calculations: nglass ) 1.85; Ngold ) 0.25 3.2j; dgold ) 500 Å; n| ) 1.693; n⊥ ) 1.533; dfilm ) 4000 Å; niso ) 1.586. Curves a and b were obtained for planar oriented situations (see text), curve c for the isotropic phase, and curve d for homeotropic alignment.
Figure 4. Isotropic-to-nematic phase transition. Images a-c represent SPR images for the following situations. For image a, T > TIN, i.e., in the isotropic phase. For image b, T ) TIN, i.e., at the phase transition. Image c represents the situation for T < TIN, i.e., in the nematic phase. In the nematic phase the bright regions correspond to homeotropically aligned films while the dark regions correspond to planar alignment.
away from the surface. The lack of the resonance minimum in the case of homeotropic alignment, curve d, is indicative of the sensitivity of this alignment to film thickness. During the growth of the homeotropic phase one would observe the resonance position moving to higher angles but also dramatically reducing in intensity. Physically, this represents a loss of coupling efficiency to the resonance mode (because of the high refractive index seen for the homeotropically aligned phase). This could be recovered, however, by the use of a higher refractive index prism. When microscopy measurements are made at an angle close to the minimum in Figure 3, curve b, it is evident that the darkest regions in the images correspond to regions of planar anchoring, the brightest regions correspond to homeotropic anchoring, and the isotropic phase will be between these extremes. Isotropic-to-Nematic Phase Transition. Figure 4 shows a selection of low-magnification images taken from a sequence captured during a slow, continuous cooling through the isotropicnematic phase transition (ca. 42 °C). In Figure 4a, T > TIN, the intensity is uniform across the sample and represents the isotropic phase. On decreasing the temperature to 37.9 °C, one obtains a dramatic change in the image (Figure 4b). The stripes
J. Phys. Chem. B, Vol. 101, No. 12, 1997 2145
Figure 5. Schematic representation of the molecular alignment within the nematic phase (T < TIN). Above the CF3-functionalized SAM (left) we find homeotropic alignment, while above the OH-functionalized regions (right) we find planar alignment. These show an idealized picture in the interfacial region and do not necessarily portray the alignment within the first adsrobed LC monolayer.
on the right of the image are due to the different anchoring behaviors of the LC on the -CF3 and -OH functionalized regions of the SAM, respectively. With reference to Figure 3 it is apparent that the bright regions correspond to regions of homeotropic anchoring (CF3 substrate) while the dark regions to those of planar anchoring (OH substrate). The uniform region on the left side of the image corresponds to the isotropic phase. The presence of both the isotropic and nematic phases in Figure 4b is due to a slight temperature gradient across the sample (left side is greater than the right side by less than 0.2 °C). On the length scale imaged here the boundary between the isotropic phase and the nematic phase appears sharp, as do the boundaries between the regions with different anchoring. Further cooling to T - TIN ≈ -6 °C, Figure 4c, results in a reduction in the intensity in the planar anchored regions, associated with increasing planar order, and careful analysis enables one to distinguish between domains with different in-plane director distributions (see below). Unfortunately, the high-intensity homeotropically anchored regions contain little retrievable information, since coupling to the surface plasmon mode is not possible in this configuration (without the use of a higher refractive index prism). The transition between isotropic and nematic anchoring appears abrupt with no evidence of any pretransitional behavior (orientational wetting) for the planar aligned films. This result is in agreement with results from our recent work, using evanescent wave ellipsometry, on single component surfaces that demonstrated that planar anchoring occurred for 8CB on -OH- and -COOH-functionalized surfaces and, in contrast to the growth of homeotropic films on CF3 surfaces, showed no evidence of pretransitional effects.8 Figure 5 summarizes schematically the orientation of the nematogens on CF3, OH-patterned regions of monolayer. Nematic Phase. The increased contrast seen in the planar anchored regions, with decreasing temperature, is associated with the corresponding increase in the nematic order parameter.16,19 The images in Figure 6 show the growth of domains as the temperature is reduced. These images represent the interfacial region only, with most information coming from the first 1000 Å, or less, of material, and do not necessarily indicate the growth of larger domains in the bulk phase. The regions of different contrast within the planar anchored phase appear to represent regions with different in-plane orientations with respect to the plane of incidence. The brighter areas correspond to regions in which the average orientation of the long molecular axis lies closer to, if not in, the plane of incidence, while the darker regions correspond to regions displaying larger angles with respect to the plane of incidence. The index ellipsoids
2146 J. Phys. Chem. B, Vol. 101, No. 12, 1997
Figure 6. SPR images of the nematic phase. Image a was obtained at T - TIN ) 6.0 °C, and image b was obtained at T -TIN ) 6.4 °C. The bright central region in each image corresponds to the homeotropic phase. The contrast observed with the planar region is discussed within the text.
Evans et al.
Figure 8. Sequence of images taken during a slow heating of the substrate from the crystalline phase (image a) through the smectic to the nematic phase (image e).
Figure 7. Half-plane index ellipsoids for the extreme cases of planar alignment. In case a, the director is in the plane of incidence and parallel to the surface. In case b, the director is anchored perpendicular to the plane of incidence and parallel to the surface. k represents the direction of the wave vector associated with the evanescent wave, θ represents the angle of refraction within the LC medium, and np(θ), ns, and np are the relevant refractive indices.
representing the extreme cases of the planar alignment in-plane and normal to the plane of incidence, respectively, are illustrated in Figure 7. Such an interpretation assumes the systems are uniaxial and does not take account of any possible tilt with respect to the surface, which could also lead to increases in intensity. With more detailed experiments it should be possible, by varying the angle of incidence and by making suitable gray scale analyses, to obtain further information regarding the orientation and growth of these domains within the planar regions. Indeed, from histograms of the intensity obtained by sampling areas purely within the planar phase, it is apparent that there is preferred growth of certain distributions over others as the temperature is reduced. This leads to an almost bimodal distribution of orientations.20 Nematic-to-Smectic A-to-Crystal Transition. On further cooling below the transition to the crystalline phase ca. 18 °C, we obtained an image with almost uniform contrast and with an average intensity similar to that obtained for the isotropic phase, Figure 8a, indicating that there is no net anisotropy within this phase (no complete wetting of oriented crystal domains). A closer examination of this phase, however, shows the presence of some structure in the form of elongated splayed features running from the top to the bottom of the images. These features bear no relation with the patterned SAM but are in the flow direction from which the cell was filled.
Figure 9. Histograms obtained from a gray scale analysis of images obtained during heating from the crystalline to the nematic phase.
Crystal-to-Smectic A-to-Nematic Transition: Premelting. From a minimum temperature close to 8.5 °C the sample was slowly reheated. The sequence of images in Figure 8 (a-d) shows the gradual appearance of the smectic A-nematic phase beneath overlying bulk crystalline texture. Both the planar and the homeotropic anchorings grow gradually out from the surface as the temperature is increased. Figure 9 shows histogram plots of frequency versus pixel intensity, obtained by analyzing the gray scale distributions within the images for a selection of temperatures. The crystalline phase, curve a, is symmetric and fitted by a single Gaussian distribution. Further heating results in the growth of two new contributions. To higher intensity (higher pixel number) we find a contribution from the emerging homeotropic phase, while at lower intensity, we see the growth of the planar phase. During the growth of the nematic phases, i.e., with increasing temperature, we find that the peaks
Imaging of Liquid Crystal Anchoring
J. Phys. Chem. B, Vol. 101, No. 12, 1997 2147 Conclusions
Figure 10. Increased resolution SPR images obtained at constant temperature showing the isotropic-nematic interface. The circular fringes are due to problems with our optical setup and sample preparation. The fringes that appear within the isotropic phase (as bright lines) and the nematic phase (as darker lines) may possibly be associated with guided wave modes.
associated with the planar and homeotropic anchoring converge toward specific intensities and increase in area at the expense of the crystalline contribution. The lowest curve shows a histogram obtained for T > TSN, i.e., for the pure nematic phase. It appears from Figure 9 that the homeotropic phase emerges slightly earlier than the planar phase, i.e., at lower temperatures. The presence of both the nematic and crystalline phases implies that the crystalline interface melts prior to the bulk melting transition and that the nematic phase grows outward from the SAM surface. The ability of a crystalline phase to melt continuously via surface melting can be viewed as complete wetting of the solid by fluid.21 Here, we appear to be seeing that the crystal phase is nonwetting on both hydrophobic and hydrophilic SAM surfaces so that complete wetting of the SAM crystal interface occurs by both planar and homeotropically aligned LC phases. Higher Magnification Images of the Isotropic-to-Smectic A-to-Nematic Phase Transition. Raising the temperature to TIN and sitting at this temperature led to the observation of the isotropic phase sweeping from left to right and then back again. This behavior is somewhat reminiscent of waves lapping a shoreline and is presumably due to tiny temperature oscillations. Figure 10 shows images captured during the motion of the isotropic-nematic interface. In Figure 10a the isotropic phase was “advancing”, i.e., moving from left to right, while in Figure 10b the isotropic phase was “receding”. In all such experiments we found similar behavior, with the “advancing” interface having a smooth, abrupt edge, while the “receding” interface possessed a jagged appearance. By changing the angle of incidence so that the isotropic phase was on resonance (Figure 3, curve c), we were able to obtain more contrast within the planar region of the film in parts c and d of Figure 10. In these images both the planar and homeotropic anchoring appear brighter, with the brightest region corresponding to the homeotropically anchored film. Previous ellipsometric experiments revealed that pretransitional, orientational wetting behavior occurred only for the homeotropic anchoring.8,9 In parts c and d of Figure 10 this would be manifest by the brighter homeotropic region extending further into the isotropic phase than the planar anchored region. Although careful image analysis appears to suggest that this is the case, these observations are not nearly as unambiguous as the Brewster angle ellipsometry data.22
These SPR imaging experiments have demonstrated how patterned SAMs, with their surface properties tuned by using different ω-functional groups, can be used to control the anchoring of nematic fluids. Conversely, they also illustrate how nematic anchoring can be exploited to probe the chemical nature of SAM surfaces. SPR microscopy is most sensitive (for the systems of interest here) to the orientational order within the first 1000 Å or so from the surface. Within this interfacial layer SPR can be used to image the anchoring of the nematic director and to investigate pretransitional behavior. The latter was especially successful when applied to surface-melting phenomena of crystalline phases. In our investigations we could not detect changes between the nematic and smectic phases. This, we believe, is due to the small differences in the optical properties between these phases. We also would like to make it clear that the alignment close to the surface may be smectic when the bulk phase is nematic, or even isotropic, and thus our studies cannot be used to distinguish between these phases. The technique has a spatial resolution down to the micrometer scale, while the high sensitivity enables small changes in the dielectric properties of the interface to be followed quite readily. In our case, the photopatterned SAMs were readily imaged and led to a very clear spatial resolution of planar and homeotropically aligned nematic films. With better alignment and higher contrast resolution this technique should prove useful for probing pretransitional wetting phenomena and for studying the uniformity of anchoring. Owing to the evanescent nature of the probing field, this technique permits a wider exploration of the phase behavior than is normally possible for such highly birefringent materials and, as has been shown here, can equally be used to investigate adsorption isotropic, LC, and crystalline phases. A particularly nice feature of the experimental setup used in this study is that by imaging in the evanescent field, one sees the evolution of structure above the surface. In particular, our results show that when 8CB is cooled from the isotropic to the nematic phase, above a patterned SAM, quite distinct anchoring behavior is displayed depending on the nature of the functional group. Perfluorinated groups yield homeotropic alignment, while hydroxyl and carboxylic acid groups yield planar alignment. Contrast was observed within the planar aligned regions, which may be due to domains with different orientation with respect to the plane of incidence or possibly due to differences in molecular tilt. On further cooling, we observed the onset of crystallization, and on reheating, we found that surface melting occurred preferentially before the bulk transition. Our attempts to observe orientational wetting by homeotropic layers in the isotropic phase, known to occur on the CF3 surfaces, were not particularly successful because of the lack of a resonance with the homeotropic phase. This aspect of the experiments would be better performed with a higher refractive index prism, thereby lowering the angle of the resonance minimum to lie within the observable range. Acknowledgment. We acknowledge the EPSRC and the Royal Society for funding that enabled this work to be carried out. One of us T.M.F. also thanks the EPSRC for the award of a Ph.D. studentship. Finally, we are grateful to Professor A. Ulman for the provision of the CF3-functionalized alkylthiol derivative. References and Notes (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wayne, K. J.; Yu, H. Langmuir 1987, 3, 932.
2148 J. Phys. Chem. B, Vol. 101, No. 12, 1997 (2) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. Allara, D. L. Biosens. Bioelectron. 1995, 10, 771. (3) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press, New York, 1991. Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: Cambridge, 1994. (4) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. (5) Chaudhury, M. K. Mater. Sci. Eng. 1996, R16. (6) Tao, Y. T.; Lee, M. T. Thin Solid Films 1994, 244, 810. Tao, Y. T.; Lee, M. T.; Chang S. C. J. Am. Chem. Soc. 1993, 115, 9547. Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761. Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (7) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L. Langmuir 1988, 4, 365. Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, C. C. J. Am. Chem. Soc. 1991, 113, 1499. (8) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. Drawhorn, R. A.; Abbott, N. J. Phys. Chem. 1995, 99, 16511. Allinson, H.; Boden, N.; Evans, S. D.; Henderson, J. R. Phys. ReV. Lett., submitted. (9) vans, S. D.; Allinson, H.; Boden, N.; Henderson J. R. Faraday Discuss., in press. (10) Sprokel, G. J.; Santo, R.; Swalen, J. D. Mol. Cryst. Liq. Cryst. 1981, 68, 977. Sprokel, G. J. Mol. Cryst. Liq. Cryst. 1981, 68, 987.
Evans et al. (11) Welford, K. R.; Sambles, J. R.; Clark, M. G. Liq. Cryst. 1987, 2, 91. Welford, K. R.; Sambles, J. R. Appl. Phys. Lett. 1987, 50, 871. Ko, D. K. K.; Sambles, J. R. J. Opt. Soc. Am. A. 1988, 5, 1863. (12) Evans, S. D.; Uranker, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (13) Evans, S. D.; Flynn, T. M. Unpublished results. (14) Rothenhausler B.; Knoll W. Nature 1988, 332, 615. (15) Beaglehole, D. Mol. Cryst. Liq. Cryst. 1982, 89, 319. (16) Chandrasekhar, S. Liquid Crystals, 2nd ed.; Cambridge University Press: Cambridge, 1992. (17) Hickel, W.; Rothenhausler, B.; Knoll, W. J. Appl. Phys. 1989, 66, 4832. Evans, S. D.; Flynn, T. M.; Ulman A. Langmuir 1995, 11, 3811. (18) Morgan, H.; Taylor, D. M. App. Phys. Lett. 1994, 64, 1330. Berger, C. E. H.; Kooyman, R. P. H; Greve, J. ReV. Sci. Instrum. 1994, 65, 2829. Schmitt, F. J.; Knoll, W. Biophys. J. 1991, 60, 716. (19) deGennes, P. G.; Prost, J. The Physics of Liquid Crystals; Oxford University Press: Oxford, 1995. (20) A note of caution should be made, since without a full gray scale analysis over a range of angles, it could be the case that the increasing peak in the gray scale analysis could refer to an increase in the random in-plane distribution of molecules. (21) Lipowsky, R. Phys. ReV. Lett. 1986, 57, 2876.
