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Chapter 7
Orientations of Liquid Crystals on Self-Assembled Monolayers Formed from Alkanethiols on Gold Nicholas L. Abbott, Vinay K. Gupta, William J. Miller, and Rahul R. Shah Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616 Recent investigations of the anchoring of liquid crystals on self-assembledmonolayers (SAMs) formed from alkanethiols on gold are discussed. The structures of alkanethiols used to form SAMs (e.g., odd and even alkanethiols or long and short alkanethiols), the manner of deposition of films of gold used to support SAMs (uniform or oblique deposition), as well as the procedures used to deposit alkanethiols (spontaneous adsorption or microcontact printing) can all be exploited to control the orientations of liquid crystals on these surfaces. A liquid crystal (LC), when placed into contact with a surface, will generally assume a restricted set of orientations defined with respect to the surface (1-6). This phenomenon, referred to as the "anchoring" of a L C by a surface, is the result of orientation-dependent interactions between the surface and LC. Because control of the orientations of LCs near surfaces is central to the principles of operation of almost all devices based on these optically anisotropic fluids, a substantial effort in the past has been directed towards both elucidation of fundamental forces that control the orientations of LCs near surfaces as well as the development of procedures for the fabrication of surfaces that permit manipulation of these forces in a predictable and systematic manner (4,7-21). In this chapter, we describe recent advances in the anchoring of LCs on surfaces which have been enabled by an experimental system that permits the design and synthesis of surfaces with a remarkable level of control over structure and chemical functionality: this system is based on the chemisorption and self-assembly of monolayers of organomercaptans on the surface of evaporated films of gold. Although some questions regarding the details of the structure of these surfaces remain to be answered (see below), in comparison to surfaces used in the past for studies of the anchoring of LCs, self-assembled monolayers (SAMs) formed from organomercaptans and organodisulfides on the surface of gold offer a level of control, stability and reproducibility that is not possible when using procedures such as the rubbing of films of polymers, deposition by the method of Langmuir and Blodgett or the self-assembly of organosilanes on metal oxide surfaces (22,23). Surfaces prepared by the selfassembly of sulfur-containing molecules on thin films of gold are, in our opinion, ideally-suited for use in studies of the anchoring of LCs by surfaces. ©1998 American Chemical Society
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82 Our investigations (24-30) of the anchoring of LCs by surfaces have been motivated by two principal questions. The first of these questions deals with the use of liquid crystalline fluids for the study and characterization of surfaces. Whereas recent investigations of the anchoring of LCs by surfaces have focused on the issue of the design and fabrication of surfaces for manipulation of the orientations of LCs, we believe the "inverse" pursuit - that of using LCs to probe the structure of organic surfaces - holds substantial promise as a surface-analytical tool. Measurement of the contact angle of a droplet of an isotropic fluid placed on a surface (Figure la) forms the basis of one of the oldest and most widely used methods for the characterization of surfaces (31,32). This method is an ideal one for the characterization of organic surfaces because it is generally non-destructive, simple to perform and can yield quantitative information about the thermodynamic state of a surface (32,33). We believe, for reasons discussed below, that the incorporation of orientational or positional order within a fluid (Figure lb) - such as found in LCs - can extend the use of fluids for characterization of surfaces to types of surfaces that cannot be readily distinguished by measurement of contact angles of an isotropic fluid. Three facts lead us to believe that LCs can form the basis of a useful system for the characterization of organic surfaces. First, the orientations of mesogens within LCs are correlated over distances of micrometers (4). The influence of a surface on the orientation of a L C can, therefore, propagate from the near-surface region (first few nanometers from the surface) into the bulk of the L C (as far as 100 micrometers from the surface). These micrometer-scale orientational correlation lengths of LCs can form the basis of a mechanism by which information about the structure of a surface can be amplified into the properties of a bulk medium. Because LCs possess anisotropic optical properties (birefringence), measurement of the bulk orientation of the L C can be achieved by using any one of a variety of well-established experimental techniques. These techniques are based on the use of polarized light and are typically no more complicated than a standard goniometer used for measurements of contact angles. Unlike contact angles of fluids supported on surfaces, which reflect the relative properties of three interfaces (vapor-solid, vaporliquid and liquid-solid), LCs can, in principal, be used to yield information about the state of a single interface (LC-substrate). Second, it is well known that LCs can support elastic deformations (bend, splay and twist) over distances ranging from a few nanometers to micrometers (3,4). Isotropic fluids, in contrast, can not support such distortions. The capability of LCs to store energy in these deformations can provide a sensitivity to the structure of surfaces on the mesoscale (nanometers to micrometers) that is not possible when using isotropic fluids. The effects of roughness of surfaces are typically reflected in the hysteresis of advancing and receding contact angles on surfaces: the contribution of roughness on scales of tens of nanometers to hysteresis of contact angles is, however, vanishingly small. Third, the molecular-level nature of interactions between a L C and an organic surface can differ substantially from that of an isotropic fluid. The orientational order within a liquid crystal can, for example, impose itself on disordered organic surfaces and thereby permit differentiation of disordered surfaces that are indistinguishable when characterized by using isotropic fluids (26). Alternatively, an ordered surface can have a pre-existing structure that is complementary or not to the structure within a L C : the matching or mismatching of the molecular-level structure of molecules supported on surfaces with LCs can, we believe, lead to a level of discrimination between surfaces that is not possible when using isotropic fluids. The second question that motivates our studies of the orientations of LCs on SAMs formed from organomercaptans and organodisulfides is related to the broadly explored issue of how to design surfaces to anchor LCs in prespecified orientations. Even though a great deal of effort in the past has been directed towards this issue, a
Frank; Organic Thin Films ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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83 variety of types of anchoring of LCs have eluded easy preparation (e.g., high angles of pretilt from a surface). Furthermore, the development of new classes of optical devices demand types of anchoring that have not been required in the past: for example, in recent years, optical displays with wide viewing angles have created the need for procedures leading to patterned alignment of LCs on surfaces with micrometer-scale resolution (11-14^34-37). Because a variety of methods for patterning alkanethiols on the surface of gold have been reported (38-46) over the past five years - including microcontact printing, micromachining and photoassisted oxidation - this system is, we believe, an ideal one for use in studies of patterned alignment of LCs on surfaces. The second objective of the studies reported herein is, therefore, to assess the utility of patterned SAMs as templates for the fabrication of complex optical structures from LCs. Planar Anchoring on SAMs Formed from C H 3 ( C H ) S H on Gold 2
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The first experiments described in this chapter revolve around our investigation of the anchoring of LCs on SAMs formed from C H ( C H ) S H on the surface of gold. These surfaces have been widely studied by a variety of surface analytical techniques and thus they form a logical starting point for an investigation of the anchoring of LCs (22,47). In the following section, we summarize what is known about the structures of SAMs formed from alkanethiols on gold and then we compare the structure of these surfaces to SAMs formed from alkyltrichlorosilanes supported on substrates of metal oxides. This comparison is a useful one because past studies have reported measurements of both contact angles of isotropic fluids and orientations of LCs on SAMs formed from organosilanes on metal oxides. Whereas the contact angles of isotropic fluids on these two types of SAMs are not measurably different, the orientations of LCs are found to be strikingly different. Knowledge of differences in the structures of these two types of SAMs (alkanethiols on gold and organosilanes on oxide surfaces) is used to explore the question of how the structure of a S A M influences the anchoring of a L C . Long-chain alkanethiols (for example, C H 3 ( C H ) i 7 S H ; Figure 2) and alkylsilanes (for example, octadecyltrichlorosilane or OTS) can react with the surfaces of gold and hydroxylated S1O2, respectively, and thereby form monolayer-thick hydrocarbon films (SAMs) tethered to the surfaces of their respective substrates (23). The structures of these SAMs, however, differ from one another in three important ways at least. First, surfaces of evaporated films of gold used to support SAMs formed from alkanethiols are predominantly Au(l 11) (48). In contrast, the surface of S1O2 is amorphous. Second, the sulfur head groups of alkanethiols chemisorb onto the gold surface to form a densely-packed (V3x\3)R30° lattice that is commensurate with the underlying Au(l 11) (49). Periodicity present in the surface of the gold is thereby communicated to the monolayer of organic molecules. The silane headgroups of SAMs formed from OTS are, in contrast, believed to polymerize in the plane of the S1O2 surface to form a network of - S i - O - S i - bonds which likely attaches to the surface at randomly spaced surface sites (hydroxy 1 groups) (50). Whereas the lateral structure of the surface of gold is imposed on SAMs formed from alkanethiols, there appears to be relatively little lateral coupling between metal oxide surfaces and SAMs formed from organosilanes. Third, alkyl chains within SAMs formed from long-chain alkanethiols on gold spontaneously organize into nearly all-trans conformations that tilt by «30° away from the normal of the surface (51-53). The twist angles of the chains within these SAMs are not all the same, and variation in the twist from one chain to the next gives rise to a c(4x2) superlattice that is commensurate with the underlying lattice of sulfur atoms (54,55). The alkyl chains within carefully prepared SAMs formed from OTS on S1O2, in contrast, assume nearly all-trans conformations that typically tilt by 3
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Frank; Organic Thin Films ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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Figure 1. (a) Cartoon of a drop of a partially wetting, isotropic fluid on a surface. The contact angle (Θ) is governed by a balance of interfacial tensions at the threephase contact line, (b) Droplet of a liquid crystalline fluid: nematic, smectic and columnar phases are depicted within the droplet.
Figure 2. Cartoon of the structure of a S A M formed from CH3(CH2)nSH on gold. Details of the bonding between the sulfur and gold (and the superlattice) are not shown.
Frank; Organic Thin Films ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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«10° away from the surface normal (50). No superlattice (or diffraction patterns) have been reported for SAMs formed from OTS. A number of past reports describe measurements of contact angles of isotropic fluids on SAMs formed from long-chain alkanethiols on gold and OTS on S1O2 (5659). Contact angles measured on these two types of SAMs are similar and consistent with the exposure of methyl groups at their surfaces. Contact angles of hexadecane and water, for example, are 45-47° and 110-113°, respectively, on both types of SAMs, and the critical surface energies of both surfaces are approximately 20±1 mJ/m (56,59). Although differences in the structure of SAMs formed from OTS on S1O2 and longchain alkanethiols on gold are known to exist (see above), these structural differences do not appear in contact angles of isotropic fluids measured on these surfaces. The behavior of liquid crystals, in contrast to isotropic liquids, is remarkably different on SAMs formed from OTS on S i 0 and SAMs formed from long-chain alkanethiols on gold. Whereas past studies (1,60,61) have reported the axis of symmetry (the so called "director") of nematic phases of 5 C B anchored on SAMs formed from OTS on S1O2 to be normal to these surface (so called "homeotropic anchoring"), the anchoring of nematic phases of 5 C B on SAMs formed from C H ( C H ) i 7 S H on gold is planar (parallel to surface) (28). The optical texture of a nematic phase of 5 C B confined between two SAMs formed from CH (CH2)i7SH on semi-transparent films of gold is grainy when viewed between crossed polarizers (Figure 3a). The grainy appearance of the optical texture reflects a non-uniform azimuthal orientation (orientation of the director in the plane of the surface of the sample) of the L C director. The azimuthal domains within the L C extend over distances of approximately 5 μηι or less. Because standard methods for measurement of the tilt angle of the director away from the surface (e.g., crystal rotation method) generally require samples with a uniform azimuthal orientation over a macroscopic area, the tilt angle of the director cannot be determined in samples of the type shown in Figure 3a by using these methods (2). In order to measure the tilt angles of liquid crystals anchored with textures of the type shown in Figure 3a, in collaboration with Prentiss and coworkers, we used a reflection interferometric technique based on circularly polarized light (29). Estimates of the birefringence obtained by using this method allowed us to conclude that the director of a nematic phase of 5 C B anchored on a S A M formed from C H ( C H ) S H (7
