Graphene as an alignment agent, an electrode, and a source of surface chirality in a smectic-A liquid crystal

A liquid crystal (LC) cell was fabricated by putting together a monolayer graphene-coated glass substrate on one side, and a rubbed planar-aligning polyimide layer on an indium tin oxide (ITO) coated glass substrate on the other side. The monolayer graphene film served as the planar-alignment agent as well as the transparent electrode on one side of the cell. The cell was filled with an achiral LC alkoxyphenylbenzoate (9OO4). The presence of the graphene film on one substrate resulted in an induced chiral signature in the otherwise achiral LC 9OO4. The induced chirality was probed utilizing the electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase. The electroclinic effect showed significant pretransitional behavior on approaching the smectic-A to smectic-C transition temperature from above. The electroclinic effect revealed a low-frequency relaxation process indicating that the chirality was induced on the LC molecules at the graphene interface and did not propagate into the bulk. A soft shear mode can break the symmetry of the hexagonal lattice of graphene on a substrate and, consequently, graphene possesses strain chirality. The noncovalent π-π interaction between the LC and the strained graphene induces molecular conformational deracemization in the LC at the graphene interface, and the LC exhibits surface-induced chirality.

Figure 1

(a) Chemical structure and cooling phase sequence of LC 9OO4. (b) A schematic representation of graphene-PI cell. (c) A picture of the graphene-PI cell. (d) A micrograph of the Sm-A phase of 9OO4 in the graphene-PI cell where the LC director is at 45 ° with respect to the crossed polarizers. (e) A micrograph of the Sm-A phase of 9OO4 in the graphene-PI cell where the LC director is parallel to the polarizer and perpendicular to the analyzer. (f) A schematic representation of a strained graphene film. The Ph-O bond of the LC 9OO4 is twisted on the strained graphene film to maintain the π-π electron stacking interaction.

Figure 2

Electroclinic effect in the Sm-A phase: (a) the effective tilt angle θ vs E ($f=100\mathrm{Hz}$) for 9OO4 in the standard commercial uniform planar cell at two different values of T, listed in the legend; (b) the effective tilt angle θ vs E ($f=100\mathrm{Hz}$) for 9OO4 in the graphene-PI cell at three different values of T listed in the legend. Dotted lines represent linear fits.

Figure 3

(a) The electroclinic coefficient $e_c$ vs T showing pretransitional behavior. (b) The inverse of the electroclinic coefficient $e_c^{-1}$ vs T. A linear fit is shown near $T_{\mathrm{AC}*}(=62{}^{\circ }\mathrm{C})$.

Figure 4

The electroclinic coefficient $e_c$ vs $f$ showing the relaxation dynamics of the ECE at $T=63{}^{\circ }\mathrm{C}$.