Nanoparticle-induced twist-grain boundary phase

By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that surface-functionalized spherical CdSSe nanoparticles induce a twist-grain boundary phase when dispersed in a chiral liquid crystal. These nanoparticles can effectively stabilize the one-dimensional lattice of screw dislocations, thus establishing the twist-grain boundary order between the cholesteric and the smectic-$A$ phases. A Landau–de Gennes–Ginzburg model is used to analyze the impact of nanoparticles on widening the temperature range of molecular organizations possessing a lattice of screw dislocations. We show that in addition to the defect-core-replacement mechanism, the saddle-splay elasticity may also play a significant role.

Figure 1

Atomic force microscopy measurements yield a diameter of $3.4 \pm$ 0.3 nm for the CdSSe NPs.

Figure 2

The $C_p\left(T\right)$ profiles for pure CE8 as well as for the $\chi =0.01$ and 0.05 mixtures, upon cooling with a rate of $0.25 {\mathrm{Kh}}^{-1}$ and heating (only the $\chi =0.05$ mixture is shown) with a rate of $0.15 {\mathrm{Kh}}^{-1}$. In all cases, the low-temperature wings of the peaks display additional features. In the case of the two mixtures, these features correspond to the characteristic thermal pattern of $\mathrm{Sm}A\text{−}{\mathrm{TGB}}_A\text{−}{N}_L^{*}\text{−}{N}^{*}$ phase sequence [7, 27].

Figure 3

Polarizing optical microscopy textures upon heating the $\chi =0.05$ sample from the $\mathrm{Sm}A$ phase; (a) $\mathrm{Sm}A$, (b) ${\mathrm{TGB}}_A$, (c) $N_L^{*}$, and (d) $N^{*}$ phase. The images have been obtained under crossed polarizers.

Figure 4

Polarizing optical microscopy textures upon cooling the $\chi =0.05$ sample from the $N^{*}$ phase; (a) $N^{*}$, (b) $N_L^{*}$, (c) ${\mathrm{TGB}}_A$, and (d) $\mathrm{Sm}A$ phase. The images have been obtained under crossed polarizers.

Figure 5

A simple schematic illustration of the CdSSe NPs trapped in the screw dislocations is shown here.