Surface Plasmon Raman Scattering Studies of Liquid Crystal Anchoring on Liquid-Crystal-Based Self-Assembled Monolayers

Abstract

We studied the anchoring of 6CB on a series of self-assembled monolayers (SAMs) with a functional group that mimics that of the nematic liquid crystal (LC). The SAMs were first characterized by wetting, Fourier-transform infrared spectroscopy, and surface potential measurements. We found that, in two of these SAMs, the end group dipoles were oriented close to the normal of the surface and that these promoted homeotropic anchoring. In the case of the other SAM, the dipole was oriented parallel to the surface, and planar anchoring was obtained. Raman scattering by adsorbates on thin metal films is enhanced by the electromagnetic fields of surface plasmon polaritons (SPPs). Despite the inherent polarization of SPPs, there have been few reports in which SPP Raman scattering has been used to study molecular orientation. We have developed optical instrumentation to provide efficient excitation and collection of SPP Raman scattered light using attenuated total reflection geometry. The Kretschmann prism coupling configuration was used to excite SPPs on thin (500 A) gold films with adsorbed SAMs of alkanethiols in contact with thin films (50 microm) of the nematic liquid crystal 4'-hexylbiphenyl-4-carbonitrile (6CB, Merck). The anchoring and orientational wetting of the LC 6CB at the interface with omega-functionalized SAMs was studied using this arrangement. In agreement with the results of previous studies, a high-energy surface (-COOH) was found to promote planar anchoring, whereas a low-energy surface (-CF(3)) was found to induce homeotropic anchoring.

Alignment of a Columnar Hexagonal Discotic Liquid Crystal on Self-Assembled Monolayers

“…This strengthens the view that “alignment of Col h phases of DLCs is intrinsically homeotropic regardless of the nature of the DLC or the nature of the surface”: that the surface acts purely as a mechanical barrier, and although it is known that a specific interaction between the surface and the discogen can be used to enhance the ease or quality of the alignment, this factor seems to be of secondary importance. This is quite different to the situation which pertains in the case of calamitic liquid crystals where control of the surface chemistry, for example, control over the nature of a SAM surface or over the orientation within the “seed layer” provides a very simple way in which the alignment of the liquid crystal can be controlled. ,,,, …”

Surface Dipole Control of Liquid Crystal Alignment

“…43,45,53 In the case of alignment layers treated with SAMs, different LC orientations have been observed using polar and nonpolar adsorbate molecules. 44,38,51 Additionally, chiral and "odd−even" 54 effects have been observed, showing that LC alignment is sensitive to variations in the symmetry 55−57 and orientation 46,47 of the exposed moieties of the terminal functionality of the SAM. Self-assembled adsorbates used in previous studies typically varied in two or more of these factors simultaneously (e.g., comparing structural analogues with different exposed moieties: −CH 3 , −OH, and −COOH).…”

“…Abbott and others have shown that SAMs also influence the alignment of LCs, − with the ability to control both azimuthal and polar orientations, which have found use in sensors . However, a convolution of steric effects, surface topography, and intermolecular forces complicates our understanding of the mechanisms responsible for alignment. ,,− Molecular adsorbates, in the form of either well-organized SAMs or adventitious surface contamination, can alter LC arrangement by changing the preferred in-plane alignment axis or inducing homeotropic alignment, normal to the surface. ,, In the case of alignment layers treated with SAMs, different LC orientations have been observed using polar and nonpolar adsorbate molecules. ,, Additionally, chiral and “odd–even” effects have been observed, showing that LC alignment is sensitive to variations in the symmetry − and orientation , of the exposed moieties of the terminal functionality of the SAM. Self-assembled adsorbates used in previous studies typically varied in two or more of these factors simultaneously (e.g., comparing structural analogues with different exposed moieties: −CH 3 , −OH, and −COOH).…”

Revisiting Surface Enhanced Raman Scattering on Flat Metallic Surfaces

“…Over the last decades, reports of SERS in the KC have not focused on the understanding of the method, but mostly on its practical use in research as a characterization tool, for example to determine molecular orientations on the surface [113,114]. Such a move toward applications of the techniques further emphasize the need to complete our fundamental understanding of the underlying mechanisms.…”

“…• Since all the cited studies but two [112,114] were carried out on Ag films, can this method be convincingly extended to Au films, which are a better alternative for applications, and are already extensively used in SPRS instruments.…”