Thickness Dependent Phase Behavior of Antiferroelectric Liquid Crystal Films

Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-$C_{\alpha }^{*}$ phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-$C_{FI2}^{*}$ phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-$C_{\alpha }^{*}$ phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems.

Figure 1

$\Delta$ as a function of temperature upon cooling from films with thickness $N=6$, 10, and 34 layers with $\alpha =90°$ (black squares) and 270° (red dots). $T0$, $T1$, $T2$, $T3$, and $T4$ mark the transitions into $\mathrm{Sm}{C}^{*}$, $\mathrm{Sm}{C}_{\alpha }^{*}$, $\mathrm{Sm}{C}_{FI2}^{*}$, $\mathrm{Sm}{C}_{FI1}^{*}$, and $\mathrm{Sm}{C}_A^{*}$ phase. On the top is the chemical structure of MHPBC.

Figure 2

$\Delta$ data (symbols) and fitting (lines) as a function of $\alpha$ from the 6-layer film at (a) $T=80.3°\mathrm{C}$ ($\mathrm{Sm}C*$ phase), (b) $T=67.6°\mathrm{C}$ ($\mathrm{Sm}{C}_{FI2}^{*}$ phase), (c) $T=65.7°\mathrm{C}$ ($\mathrm{Sm}{C}_{FI1}^{*}$ phase). (d) Top views of structures for $\mathrm{Sm}{C}_{FI2}^{*}$ and $\mathrm{Sm}{C}_{FI1}^{*}$ phase, arrows represent the tilt direction of each layer, numbers represent the layer index within the unit cell.

Figure 3

Thickness dependent phase diagram of MHPBC free standing films obtained from cooling runs.

Figure 4

(a) $\Delta T(\mathrm{Sm}{C}_{\alpha }^{*})$ as a function of $N$ (b) free energy per layer of the $\mathrm{Sm}{C}_{\alpha }^{*}$ structure (black squares) and $\mathrm{Sm}{C}^{*}$ structure (red dots) as a function of film thickness calculated from Eq. (1).

Figure 5

(a) $\Delta T(\mathrm{Sm}{C}_{FI2}^{*})$ as a function of $N$ (black square) and free energy per layer calculated with Eq. (2) (red line). (b) Cartoon of a film with even number of layers in the $\mathrm{Sm}{C}_{FI2}^{*}$ phase.