Direct Observation of Smectic Layers in Thermotropic Liquid Crystals

We demonstrate subnanometer resolution cryo-TEM imaging of smectic layers in the smectic and nematic phases of two bent-core liquid crystals. Our results show perfect periodicity over several hundred layers in the smectic phase and also provide the first direct evidence of smectic clusters on length scales of 30–50 nm in a nematic liquid crystal. The results are corroborated with small angle x-ray scattering measurements. The observation of smectic clusters in the nematic phase is of special interest in bent-core liquid crystals, where the smectic clusters are stable over wide temperature ranges, in contrast to the well-known pretransitional “cybotactic” clusters that appear only in the vicinity of a bulk smectic phase. The means to characterize and manipulate this nanoscale molecular order could open up completely new liquid crystal-based technologies.

Figure 1

Summary of sample preparation and TEM experimental conditions. (a),(b) Molecular structure, code, and phase sequence on heating and cooling of the compounds 3RBC-S and 3RBC-N, respectively. (Cr, N, SmA, and Iso denote crystal, nematic, smectic-$A$, and isotropic liquid phases, respectively; numbers are transition temperatures in °C). (c) Illustration of the LC films deposited on TEM grids coated with a carbon film. (d) Polarizing optical microscopy texture of a 100 nm thick LC film. The gray image indicates planar alignment (average molecular long axis parallel to the substrates). (e) Illustration of the TEM imaging when the layers are perpendicular to the substrates (left) and when they are tilted by more than a few degrees (right).

Figure 2

A typical cryo-TEM image of 3RBC-S quenched from 87 °C. (a) A $350\mathrm{nm}\times 500\mathrm{nm}$ area of the sample. Dark domains correspond to thick areas, and the periodic stripe texture corresponds to a less than 100 nm thick area with smectic layers perpendicular to the substrate. (b) Enlarged image of the $70\mathrm{nm}\times 70\mathrm{nm}$ area enclosed by the rectangle in (a), illustrating the details of the layer structure. Dark stripes correspond to the aromatic cores and lighter areas to the hydrocarbon tails.

Figure 3

Summary of the cryo-TEM observations on 3RBC-N. TEM images after quenching from 90 (a), 80 (b), 70 (c), and $65°\mathrm{C}$ (d). Striped domains are smectic clusters of molecules, which are oriented in different directions even in overlapping spots at different depths in the sample. (e) Histogram of the width of the layered clusters at $70°\mathrm{C}$. (f) Histogram of the length of the clusters at $70°\mathrm{C}$. The cluster size increases to lower quench temperatures.

Figure 4

Summary of the experimental results from SAXS measurements (a) for 3RBC-S and (b) for 3RBC-N. The wave number ($q$) corresponding to the peak scattered intensity (left vertical axis) and the full width at half maximum ($\Delta q$) (right vertical axis) are plotted as a function of temperature for the two compounds. Also shown are 2D diffraction patterns for various temperatures.