Quantum-dots-dispersed bent-core nematic liquid crystal and cybotactic clusters: Experimental and theoretical insights

We study a quantum-dots (QDs) dispersed bent-core nematic liquid crystalline system in planar geometry and present experimental measurements of the birefringence ($\mathrm{\Delta }n$), order parameter ($S$), dielectric dispersion, absorption spectra, and optical textures with attention to variations with temperature. A bent-core liquid crystal (LC) $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ is used as the host material and CdSe/ZnS core-shell type QDs are used as the dopant. The nematic ($N$) phase of the pristine (undoped) LC $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ contains cybotactic clusters, which are retained by its QDs incorporated LC nanocomposite. Our experimental findings support: (i) reduced orientational order parameter of the QDs dispersed LC system compared to its pristine counterpart at fixed temperatures, (ii) reduced cybotactic cluster sizes due to the incorporation of QDs, and (iii) increased activation energies related to reduced cluster sizes. We complement the experiments with a novel Landau–de Gennes-type free energy for a doped bent-core LC system that qualitatively captures the doping-induced reduced order parameter and its dependence on the properties of the QDs and its variation with temperature.

Figure 1

Visibly homogeneous solutions of (a) $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$, (b) CdSe/ZnS QDs, and (c) nanocomposite ($14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3+0.5\mathrm{wt}\%\mathrm{CdSe}/\mathrm{ZnS}\mathrm{QD}$) in chloroform (${\mathrm{CHCl}}_3$).

Figure 2

Birefringent textural color variation with temperature of (a)–(e) the bent-core LC $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ and (f)–(j) the 0.5 wt % CdSe/ZnS QDs dispersed $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$, respectively, during cooling. In each image, $\mathit{r}$ indicates the rubbing direction, and the scale bar indicates $100\mu \mathrm{m}$. The periodic white spots in the background of the images are features of the LC cell (Instec Inc., USA) caused by the PI printing fabric in the production line.

Figure 3

Experimental values of birefringence ($\mathrm{\Delta }n$) for the LC $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ (half-filled squares) and its nanocomposite (half-filled circles); the solid lines (pure LC: red and LC nanocomposite: blue) represent the four-parameter fit to the experimental data using Eq. (3). The related fitting parameter values are shown in the figure. The fitting parameters ${\chi }^2$ and $R^2$ are generated by the fitting algorithm so that ${\chi }^2\sim 0$ and $R^2\sim 1$ describe good fits.

Figure 4

Orientational order parameter ($S$) as a function of temperature for the bent-core LC $14\text{-2}\mathrm{M}\text{−}{\mathrm{CH}}_3$ and its 0.5 wt % CdSe/ZnS QDs incorporated nanocomposite.

Figure 5

Frequency-dependent real (${\epsilon }^{\prime }$) and imaginary (${\epsilon }^{\prime \prime }$) parts of dielectric permittivity of (a) and (b) pristine LC ($14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$) and (c) and (d) QDs dispersed LC nanocomposite ($14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3+0.5\mathrm{wt}\%\mathrm{CdSe}/\mathrm{ZnS}\mathrm{QDs}$) at different temperatures during cooling ($f$ in hertz).

Figure 6

DC bias suppression of the low-frequency relaxation mode ($M_1$) in (a) pure $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ at $129{}^{\circ }\mathrm{C}$ and (b) 0.5 wt % QDs incorporated $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ at $121{}^{\circ }\mathrm{C}$ ($f$ in hertz).

Figure 7

Dielectric permittivity of $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ and its nanocomposite at 10 kHz as a function of temperature.

Figure 8

Simultaneous fitting of the real (${\epsilon }^{\prime }$) and the imaginary (${\epsilon }^{\prime \prime }$) parts of complex dielectric permittivity (on the logarithmic scale) using the H-N equations in (a) pure $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ and (b) 0.5 wt % QDs incorporated $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$ ($f$ in hertz). Experimental data—The green squares represent ${\epsilon }^{\prime }$, and the hollow circles represent ${\epsilon }^{\prime \prime }$. Fit data—The red solid line represents fit to ${\epsilon }^{\prime }$, and the blue solid line represents fit to ${\epsilon }^{\prime \prime }$. The fitting parameters ${\chi }^2$ and $R^2$ are generated by the fitting algorithm so that ${\chi }^2\sim 0$ and $R^2\sim 1$ describe good fits.

Figure 9

Temperature-dependent variation of the relaxation frequency ($f_R$) and the dielectric strength ($\delta \epsilon$) corresponding to $M_1$ and $M_2$ of (a) and (b) the pristine LC and (c) and (d) 0.5 wt % QDs incorporated LC.

Figure 10

Arrhenius plot of the $M_1$ relaxation frequency ($f_{R1}$) in the $N$ phase of (a) pristine and (b) 0.5-wt % CdSe/ZnS QDs incorporated $14\text{−}2\mathrm{M}\text{−}{\mathrm{CH}}_3$. The activation energy ($E_a$) is calculated from the slope of the linear fit, represented by the solid red line.

Figure 11

Bulk mean order parameter as a function of temperature for the undoped (green circle) and the doped (red square, blue triangle, and purple diamond) cases (temperature $T$ is in kelvins).