Reports: UR551329-UR5: Stamping Ordered Molecular Monolayers Using Liquid Crystal Inks

David L. Patrick, Western Washington University

The goal of this research is to investigate nematic liquid crystal (LC) inks combined with patterned anchoring alignment stamps to create new methods for preparing multi-component molecular thin films with controlled organization and composition.  In particular, the central idea we are investigating concerns use LC solvents to influence and guide the formation of mixed alkylthiolate self-assembled monolayers (SAMs) by coupling molecular order and composition in the SAM to a pattern on a stamp through an elastic strain field produced by competitive LC anchoring at the stamp- and SAM surfaces.  The concept resembles the well understood situation of heteroepitaxial growth in the presence of elastic surface strain, which can lead to grain refinement, compositional changes, and pattern formation such as stripes, periodic droplets, etc. when a significant mismatch exists between adsorbate and substrate lattice constants, however in the present case, the strain field is provided by a nematic LC, rather than the substrate, and results from anchoring and elastic forces, rather than an epitaxial mismatch.

 During the previous reporting periods work was undertaken to identify (1) the optimum mixtures of thiols and LCs for our purposes; (2) the optimum timescale for the experiment; (3) whether patterning of the thiol monolayer could be used to amplify the anticipated effect of the LC solvent / stamp influence by reducing the entropy of mixing.  Based on the results of these studies, during the most recent period the focus narrowed to one particular type of system consisting of the smectic-A LC 4’-cyano-4-octyl-biphenyl (8CB) and two thiols: one perfluorinated and the other hydrogenated and terminated in a carboxylic acid.  The decision to focus on 8CB (as opposed to the other main liquid crystal investigated so far: the structurally similar nematic LC 5CB) entails making a trade-off between longer equilibration times (due to 8CB’s much higher viscosity) and increased elastic forces (due to 8CB’s greater stiffness toward twist and splay distortions).  The equilibration time – by which is meant the period of time required for the LC director field to reach its minimum energy configuration in the cell – depends strongly on the thickness of the LC layer.  We hypothesize that it is this timescale, rather than the timescale for thiol monolayer formation, that is limiting.  Consequently we began working with the thinnest possible cells we could construct, ultimately limited by particles in the laboratory environment, which act as spacers.  A new HEPA-filtered hood system was constructed for this purpose and is now used to prepare all samples.

 The major experimental effort has been aimed at trying to convincingly demonstrate that the LC is exerting a measurable effect on thiol monolayer composition.   A conservative assessment is that this has not yet been accomplished.  While many samples – even a majority of samples – show systematic differences in the water contact angles measured between differentially stamped Au regions after the LC has been removed (consistent with selective adsorption of perfluorinated vs. carboxylic acid-terminated thiols), the variability in the contact angle results remains too large to serve as a definitive indicator.  Accordingly, we are pursuing a focused effort combining contact angle measurements, XPS, grazing-angle FTIR, and polarizing optical microscopy applied systematically to each sample, aiming to correlate the results in order to provide a firmer picture of the surface composition.  These measurements are ongoing and are being conducted by two WWU undergraduates: Andrew Lindsey and Evan Yuhas.

 Student Involvement: Five students have participated in the research so far: Kyle Kheenel, Christopher Grote, Andrew Lindsey, and Evan Yuhas (all undergraduates at WWU), and Beth Howe, an undergraduate at the University of Cambridge.  Ms. Howe, who graduated two years ago, was in the group of Stuart Clarke at Cambridge, who collaborated with us on many of the measurements.  She visited WWU during December 2011.

 Publications and other products: so far no publications have been submitted from this work.  Ms. Howe prepared her undergraduate thesis based on her participation, Kyle Kheenel presented a poster at WWU’s annual campus research conference.