Enhancement of polar anchoring strength in a graphene-nematic suspension and its effect on nematic electro-optic switching

A small quantity of monolayer graphene flakes is doped in a nematic liquid crystal (LC), and the effective polar anchoring strength coefficient between the LC and the alignment substrate is found to increase by an order of magnitude. The hexagonal pattern of graphene can interact with the LC's benzene rings via $\pi –\pi$ electron stacking, enabling the LC to anchor to the graphene surface homogeneously (i.e., planar anchoring). When the LC cell is filled with the graphene-doped LC, some graphene flakes are preferentially attached to the alignment layer and modify the substrate's anchoring property. These spontaneously deposited graphene flakes promote planar anchoring at the substrate and the polar anchoring energy at alignment layer is enhanced significantly. The enhanced anchoring energy is found to impact favorably on the electro-optic response of the LC. Additional studies reveal that the nematic electro-optic switching is significantly faster in the LC-graphene hybrid than that of the pure LC.

Figure 1

(a) Dielectric constant, $\varepsilon$ for E7 and E7-GP as a function of $V_{\mathrm{rms}}$ at $T=30{}^{\circ }\mathrm{C}$. The inset shows the Fréedericksz threshold voltage, $V_{\mathrm{th}}$ for the samples. (b) Dielectric constant, $\varepsilon$ for E7 and E7-GP as a function of $1/V_{\mathrm{rms}}$ at $T=30{}^{\circ }\mathrm{C}$. The solid lines represent the linear fit in the high-voltage linear regime (as $1/V_{\mathrm{rms}}\to 0, V_{\mathrm{rms}}\to \infty$). The extrapolated $y$ intercept of the linear fit gives the value of ${\varepsilon }_{\parallel }^{\mathrm{extp}}$.

Figure 2

Polar anchoring strength coefficient, $W_{\theta }$ as a function of temperature for E7 and E7-GP on the linear scale. Typical error bars are shown. The dotted lines are guide-to-the-eye. The inset shows $W_{\theta }$ as a function of temperature for E7 and E7-GP on the logarithmic scale.

Figure 3

(a) A schematic representation of the alignment of nematic LC molecules on graphene due to $\pi –\pi$ electron stacking. The ellipsoids are LCs and the black honeycomb structure is the graphene surface. The LC molecular structure is shown in the ellipsoid on the graphene surface. The $\pi –\pi$ electron stacking is illustrated by matching the LC's benzene rings on the graphene-honeycomb structure. The nematic director ($\widehat{n}$) is orientated at ${45}^{\circ }$ with respect to the crossed polarizer (P) and analyzer (A). This orientation, therefore, produces a bright state. (b) The system is rotated through ${45}^{\circ }$ and the nematic director ($\widehat{n}$) is parallel to A—which produces a dark state. (c)–(e) Microphotographs of a thin layer of nematic LC on a monolayer graphene film on a copper substrate under a reflected crossed polarized microscope, showing a bright state, a dark state, and a bright state of the highlighted domain at $0^{\circ }, {45}^{\circ }$, and ${90}^{\circ }$, respectively, with respected to the crossed polarizers. (f) Normalized intensity of the highlighted domain as a function of the relative angle of rotation. The white bar in micrograph (e) presents $50\mu \mathrm{m}$.

Figure 4

(a) Microphotograph of the bare alignment substrate used in the E7 cell. Randomly dispersed spacer particles with uniform size are visible. Two of them are highlighted with dotted circles. (b) Microphotograph of the bare alignment substrate used in the E7-GP cell. The dark spots, much smaller than the spacer particles, are graphene aggregates. These are small aggregated graphene flakes attached to the substrate. A spacer particle is highlighted inside a dotted circle and a few graphene flakes are highlighted inside a solid circle. (c) A $7\times$ magnified image of the dotted square region of panel (b). A spacer particle is highlighted inside a dotted circle at the bottom left corner. All other smaller dark spots are graphene flakes in small aggregates.

Figure 5

(a) Splay elastic constant, $K_{11}$ as a function of temperature for E7 and E7-GP. Typical error bars are shown. (b) $\frac{K_{11}}{W_{\theta }}$ as a function of temperature for E7 and E7-GP.

Figure 6

Electro-optic switching of E7 and E7-GP cells. The left $y$ axis shows the normalized transmitted intensity as a function of time when an applied voltage is turned off at $t=0$, and then turned on at $t=25\mathrm{ms}$, for E7 and E7-GP, listed in the legend ($T=30{}^{\circ }\mathrm{C}$). The right $y$ axis shows the applied voltage profile across the cells.

Figure 7

(a) Optical switching on, ${\tau }_{\mathrm{on}}$ and (b) optical switching off, ${\tau }_{\mathrm{off}}$ as a function of temperature for E7 and E7-GP listed in the legend.