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 reporting period work focused primarily in two areas:
(1) Studies aimed at determining the most appropriate mixtures of thiols and LCs for our purposes. As described in the proposal, the LC/thiol ink mixture must possess several properties, including: (a) thiol miscibility in the LC, (b) thiol immiscibility in the SAM, (c) orthogonal anchoring of the LC between the thiols (d) simultaneous co-adsorption of both thiols on the Au substrate. To identify mixtures having all four properties, four carboxylic acid-terminated thiols and three perfluorinated thiols were investigated with two different LCs in over 100 combinations. Miscibility was assessed by differential scanning calorimetry (a bulk measurement) and by atomic force microscopy in SAMs. Monolayer composition as a function of bulk molar ratio, LC and thiol selections was investigated by tensiometry, grazing-angle FTIR, and x-ray photoelectron spectroscopy (XPS). These studies resulted in the identification of a small number of thiol/LC combinations judged to be most suitable for further investigation using stamps.
(2) Investigations of stamped SAMs using LC/thiol inks. Using the aforementioned identified mixtures, samples were prepared by applying ink mixtures to stamps having one-half of the stamp treated to give homeotropic anchoring and the second half treated to give planar anchoring. The stamps were applied to Au substrates and left in contact for varying periods of time, from several hours to several months. Disassembled samples were rinsed and the spatially-dependent composition of the resulting SAMs measured by sessile drop tensiometry, ellipsometry, and XPS. We found that densely-packed SAMs were formed as anticipated, but that the sought-for spatial dependence of composition, correlated to the anchoring direction imposed by the stamp, could not be reproducibly achieved for any of the combinations investigated.
Current work: We hypothesized that the size of perfluorinated and carboxylic acid terminated SAM domains in films prepared using the above-described approach was too small – and hence the entropy of mixing too large – to enable LC energies to exert significant influence. This potential issue was raised as a possibility in the proposal, and seems to have been confirmed. To address it, in current work we begin with Au substrates that have been pre-patterned with micron-sized perfluorinated and carboxylic acid terminated SAM regions formed by microcontact printing, and then apply an inked stamp afterwards. This ensures that substrates start out functionalized with large, phase-separated domains. When placed in contact with an inked stamp, molecular exchange between solution-phase and surface-bound thiols gradually occurs and the film composition, which begins as equimolar in the two thiols, evolves in favor of the thiol whose anchoring matches the stamp. Initial results from this modified approach appear more reproducible, and there are indications from friction-force AFM measurements that the spatially-dependent composition in these samples does evolve over a period of days to weeks in the direction one would predict, correlated to the stamp. We are now working to confirm these results and to study time-dependent morphology and compositional changes in detail.
Student Involvement: Three students have participated in the research so far: Kyle Kheenel and Christopher Grote (both undergraduates at WWU), and Beth Howe, an undergraduate at the University of Cambridge. Ms. Howe, who graduated this year, was in the group of Stuart Clarke at Cambridge, who collaborated with us on many of the measurements. She visited WWU during December 2011.
Related developments: WWU recently acquired its own XPS instrument, which will significantly benefit this project going forward by allowing measurements to be performed locally.
Publications and other products: so far no publications have been submitted from this work. Ms. Howe prepared her undergraduate thesis based on her participation, and Kyle Kheenel presented a poster at WWU’s annual campus research conference.