59th Annual Report on Research 2014

The goal of this research was 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. The central idea we investigated concerned 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. 

Over the course of the grant research was undertaken to identify (1) the optimum mixtures of thiols and LCs; (2) the optimum timescale and temperature 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; and (4) for the most promising materials, determine the extent of compositional influence on deposited monolayers. Based on the results of these studies, the most promising combination of materials was found to involve the smectic-A LC 4’-cyano-4-octyl-biphenyl (8CB) and two thiols: 11-mercaptoundecanoic acid and tridecafluoro-1-octane thiol. 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 – appeared to increase with LC layer thickness and so we used the thinnest practical cell geometry, having a LC thickness of approximately 500 nm. The optimum incubation time, i.e. the time at which systems seemed to have reached a limiting state of monolayer composition, was found to be about 1 week at room temperature. 

The major experimental effort was aimed at attempting to convincingly demonstrate the LC could exert a measurable effect on thiol monolayer composition. Our conclusion is that this has not yet been accomplished. While many samples – even a majority of samples – showed systematic differences in the water contact angles measured between planar and homeotropic regions after the LC was been removed (consistent with selective adsorption of perfluorinated vs. carboxylic acid-terminated thiols), the variability in the contact angle results was too large to serve as a definitive indicator. Extensive XPS, grazing-angle FTIR, and polarizing optical microscopy likewise failed to find statistically convincing evidence for the hypothesized effect. Thus we were unable to accomplish our goal of using LC solvents to influence monolayer composition in these Au/SAM films. 

In our original proposal we discussed several potential reasons that the sought for effect might turn out not to be observable. One potential reason had to do with the entropy of mixing, which tends to drive a randomized monolayer composition antagonistic to the influence of the LC. We attempted to address this early on by using differential scanning calorimetry to identify immiscible mercaptan pairs. Additionally, we investigated the use of pre-pattered monolayers using soft lithography to establish phase-separated micropatterns right from the beginning of the experiment. Neither approach appeared to make much difference in the outcome. A second potential reason concerns the equilibration time for monolayer formation, which as discussed in the original proposal was hypothesized to proceed in two steps. The first step involves initial thiol adsorption to give a complete monolayer. This step is known to happen relatively rapidly, being substantially complete within 24 hours. The second, presumably slower step involves the evolution of the composition of the monolayer from one that is initially quasi-random, to one enriched in the thiol providing a favorable anchoring match to the stamp. An extensive set of experiments was performed to identify the appropriate timescale, ranging from about 10 minutes to over 1 month of incubation time. Further experiments investigated the effect of incubation temperature, though this was practically limited to a narrow range around room temperature bounded by the phase transition temperatures of the LCs. The results from these studies suggested that after about 1 week of incubation at room temperature the influence of LC alignment reached a limiting value. It is possible however that the kinetics are simply so slow to observe at laboratory timescales and this remains a possible explanation or at least a contributing factor. A third potential reason concerned the weak anchoring forces LCs exert on surfaces, which is limited by the lesser of the anchoring energy and bulk elasticity. We therefore explored a range of different LCs, eventually identifying the smectic-A LC 8CB, which though not a nematic LC as originally proposed, was found to display the best combination of elasticity and anchoring. In the end however, after an extensive series of experiments investigating different liquid crystals and thiols, molar ratios and concentrations of thiols, cell design, incubation time and temperature, and surface patterning, we were unable to definitively and reproducibly demonstrate the sought for effect.

Student Involvement: Five students participated in this research: 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 three 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, and all students made regular presentations at group meetings.