Graphene as transmissive electrodes and aligning layers for liquid-crystal-based electro-optic devices

In a conventional liquid crystal (LC) cell, polyimide layers are used to align the LC homogeneously in the cell, and transmissive indium tin oxide (ITO) electrodes are used to apply the electric field to reorient the LC along the field. It is experimentally presented here that monolayer graphene films on the two glass substrates can function concurrently as the LC aligning layers and the transparent electrodes to fabricate an LC cell, without using the conventional polyimide and ITO substrates. This replacement can effectively decrease the thickness of all the alignment layers and electrodes from about 100 nm to less than 1 nm. The interaction between LC and graphene through $\pi -\pi$ electron stacking imposes a planar alignment on the LC in the graphene-based cell—which is verified using a crossed polarized microscope. The graphene-based LC cell exhibits an excellent nematic director reorientation process from planar to homeotropic configuration through the application of an electric field—which is probed by dielectric and electro-optic measurements. Finally, it is shown that the electro-optic switching is significantly faster in the graphene-based LC cell than in a conventional ITO-polyimide LC cell.

Figure 1

(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 glass substrate under a crossed polarized microscope, showing a bright state, an intermediate state, and a dark state, respectively. (f) Normalized intensity as a function of the relative angle of rotation of the LC-coated graphene film on the glass substrate. The average director ($\widehat{n}$) orientation for the microphotograph (c) is ${45}^{\circ }$ with the crossed P and A—which produces a bright state. The average director ($\widehat{n}$) orientation for the microphotograph (e) is $0^{\circ }$ with P (or A)—which produces a dark state. The white bar in micrograph (e) presents $50\mu \mathrm{m}$.

Figure 2

(a) A schematic representation of a conventional LC cell containing a layer of ITO and a layer of polyimide coating with unidirectional rubbing on each glass slide, (b) the picture of a conventional LC cell, (c) a schematic representation of a graphene-based cell which contains a single layer of graphene on each glass slide, (d) the picture of a graphene-based LC cell, and (e) the Raman spectrum confirming the presence of the monolayer graphene on the glass substrate.

Figure 3

(a–c) Micrographs of the bright, intermediate, and dark states, respectively, under the crossed polarized microscope of the conventional LC cell filled with E7 liquid crystal; (d) normalized transmitted intensity as a function of the relative angle of rotation for the conventional LC cell; (e–g) micrographs of the bright, intermediate, and dark states, respectively, under the crossed polarized microscope of the graphene-based LC cell filled with E7 liquid crystal; (h) normalized transmitted intensity as a function of the relative angle of rotation for the graphene-based LC. The white bar in micrograph (g) presents $50\mu \mathrm{m}$.

Figure 4

Dielectric constant $\varepsilon$ as a function of applied rms field $\vec{E} (f=1000\mathrm{Hz})$ in the nematic phase $(T=22{}^{\circ }\mathrm{C})$ of E7 liquid crystal in two different LC cells listed in the legend. This shows that the LC can exhibit a typical Fréedericksz transition in the graphene-based LC cell. The inset shows that the threshold field in the graphene-based cell is more than that of the conventional cell.

Figure 5

Electrically controlled birefringence effect of E7 liquid crystal in the graphene-based cell. (a–d) Micrographs of the graphene-based LC cell filled with E7 liquid crystal under the crossed polarized microscope at 0, 5, 12, and 23 V, respectively. (e) The transmittance $\frac{I}{I_o}$ of E7 liquid crystal $(T=22{}^{\circ }\mathrm{C})$ in the graphene-based cell as a function of applied ac voltage $(f=1000\mathrm{Hz})$. The inset shows the transmittance of E7 liquid crystal in the conventional ITO-polyimide cell as a function of applied ac voltage $(f=1000\mathrm{Hz})$. The white bar in micrograph (d) presents $50\mu \mathrm{m}$.

Figure 6

Electro-optical switching of E7 liquid crystal in the graphene-based cell. (a) A schematic presentation of the experimental setup. (b) Normalized transmitted intensity as a function of time when a peak-to-peak voltage $(V_{\mathrm{pp}}=27\mathrm{V})$ is turned off at $t=0$, and then turned on at $t=25\mathrm{ms}$, for the two cells listed in the legend $(T=22{}^{\circ }\mathrm{C})$. The dashed line represents the applied voltage profile. (c) Top panel: ${\tau }_{\mathrm{on}}$ as a function of $V_{\mathrm{pp}}$ for E7 liquid crystal for the two cells listed in the legend. Bottom panel: ${\tau }_{\mathrm{off}}$ as a function of $V_{\mathrm{pp}}$ for E7 liquid crystal for the two cells listed in the legend.