Using droplets of nematic liquid crystal to probe the microscopic and mesoscopic structure of organic surfaces

We report a simple and general procedure that uses nematic liquid crystals (NLCs) to probe the structure of organic surfaces prepared by molecular self-assembly. The procedure involves placement of a submillimeter-sized droplet of NLC on the surface of a self-assembled monolayer (SAM)of organic molecules and observation of the droplet under illumination with polarized light. Because NLCs are optically anisotropic, polarized light permits characterization of the distortion of the NLC within the droplet, which we demonstrate here to reflect the structure of surfaces on spatial scales that range from the molecular tg the mesoscopic. We demonstrate the use of droplets of NLCs (i) to probe the molecular-level structure (Angstrom-scale) of surfaces by distinguishing between SAMs formed from odd or even chain-length alkanethiols or SAMs coadsorbed from mixtures of long and short alkanethiols; (ii) to probe the nanometer scale texture of polycrystalline films of gold used to support SAMs, including gold substrates prepared by oblique deposition from a vapor of gold; (iii) to image the micrometer-scale structure of SAMs patterned by using microcontact printing; and (iv) to follow, in situ, reactions on surfaces, using as an example the displacement of a SAM formed from CH3(CH2)(6)SH by CH3(CH2)(15)SH dissolved into the NLC. Because either reflection or transmission polarization microscopy can be used to image the NLC drops, this method permits characterization of monolayers of organic molecules on both transparent and opaque substrates. To clarify the relationship between the optical appearance of the NLC droplets, the distortions of NLCs within the droplets, and the orientations of NLCs near SAMs, we report numerical simulations of droplets of NLCs supported on surfaces and calculations of the optical textures of these droplets.