Ion trapping, reduced rotational viscosity, and accelerated electro-optic response characteristics in gold nano-urchin–nematic suspensions

The free-ion concentration in a nematic liquid crystal (LC) is found to be significantly reduced when gold nano-urchins (AuNUs) of 50-nm diameter are dispersed in the LC in dilute concentrations. The nano-urchins on AuNUs trap a significant amount of mobile ions, reducing the free-ion concentration in the LC media. The reduction of free ions results in a decreased rotational viscosity and accelerated electro-optic response of the LC. The study is carried out with several AuNUs concentrations in the LC, and the experimental results consistently suggest that there exists an optimal concentration of AuNUs, above which they tend to aggregate. At the optimal concentration, the ion trapping is maximum, rotational viscosity is at its lowest, and the electro-optic response is the fastest. Above this optimal AuNUs concentration, the rotational viscosity is found to increase, and consequently, the LC no longer exhibits an accelerated electro-optic response.

Figure 1

Schematic illustration of the presence of ions and AuNUs in the nematic phase. Small spheres represent the ions, the ellipsoids represent LC molecules, and the big spheres with spikes represent the AuNUs. (a) Random distribution of free ions in a nematic phase. (b) AuNUs' ion-trapping process in a nematic phase. (c) SEM image of a AuNU particle. Optical microphotographs under a cross-polarized microscope for (d) E7+AuNU1, (e) E7+AuNU2, (f) E7+AuNU3, and (g) E7+AuNU4 samples. The 20-µm spacer particles from the LC cells are visible, and no graphene aggregates are observed for E7+AuNU1 and E7+AuNU2. In addition to the spacer particles, several small aggregates are observed as small dark spots for E7+AuNU3 and E7+AuNU4. (h) A $10\times$ magnified image of the dashed rectangular region of (g). Two such AuNU aggregates are shown in two dashed circles. Scale bar is shown in each microphotograph.

Figure 2

(a) Ion current, $I_{\mathrm{ion}}$, as a function of time for E7, E7+AuNU1, and E7+AuNU2 at $35{}^{\circ }\mathrm{C}$ after the voltage is inverted across the cells. The peak represents the ion bump when positive and negative ions meet in the middle of the cell. (b) Free-ion concentration, $n_{\mathrm{i}}$, as a function of temperature for E7 and four different E7+AuNU samples listed in the legend. Typical error bars are shown.

Figure 3

(a) Rotational viscosity, ${\gamma }_1$, as a function of temperature for E7 and four E7+AuNU samples, listed in the legend. Inset: Transient current, $I$($t$), as a function of time for E7, E7+AuNU1, and E7+AuNU2 at $T=30{}^{\circ }\mathrm{C}$. Typical error bars are shown. (b) ${\gamma }_1$ (right-$Y$ axis) and $n_{\mathrm{i}}$ (left-$Y$ axis) as a function of AuNU concentration at $T=35{}^{\circ }\mathrm{C}$. Dotted lines are a guide to the eye.

Figure 4

Dielectric anisotropy, $\mathrm{\Delta }\varepsilon$, as a function of temperature for E7 and E7+AuNU samples, listed in the legend. Inset: Dielectric anisotropy, $\mathrm{\Delta }\varepsilon$, as a function of AuNU concentration at $T=35{}^{\circ }\mathrm{C}$. Typical error bars are shown.

Figure 5

Dynamic electro-optic switching of E7 and E7+AuNU samples. (a) The left-$Y$ axis shows the normalized transmitted intensity as a function of time when a 40-V applied voltage is turned off, for the test cells listed in the legend $\left(T=20{}^{\circ }\mathrm{C}\right)$. The right-$Y$ axis shows the applied voltage profile. (b) Left-$Y$ axis shows the normalized transmitted intensity as a function of time when a 40-V applied voltage is turned on, for the test cells listed in the legend $\left(T=20{}^{\circ }\mathrm{C}\right)$. The right-$Y$ axis shows the applied voltage profile. (c) Optical switching off, ${\tau }_{\mathrm{off}}$, and (d) optical switching on, ${\tau }_{\mathrm{on}}$, as a function of applied voltage for E7 and E7+AuNU samples listed in the legend. Typical error bars are shown.

Figure 6

(a) Free-ion concentration, $n_{\mathrm{i}}$, as a function of temperature for E7 and two E7+AuNU samples of different sizes (100 and 50 nm), as listed in the legend. (b) Rotational viscosity, ${\gamma }_1$, as a function of temperature for E7 and two E7+AuNU samples of different sizes (100 and 50 nm), as listed in the legend. Typical error bars are shown.