Graphene and liquid crystal mediated interactions

Abstract

The two-dimensional graphene-honeycomb structure can interact with the liquid crystal's (LC) benzene rings through π-π electron stacking. This LC-graphene interaction gives rise to a number of interesting physical and optical phenomena in the LC. In this paper, we present a combination of a review and original research of the exploration of novel themes of LC ordering at the nanoscale graphene surface and its macroscopic effects on the LC's nematic and smectic phases. We show that monolayer graphene films impose planar alignment on the LC, creating pseudo-nematic domains (PNDs) at the surface of graphene. In a graphenenematic suspension, these PNDs enhance the orientational order parameter, exhibiting a giant enhancement in the dielectric anisotropy of the LC. These anisotropic domains interact with the external electric field, resulting in a non-zero dielectric anisotropy in the isotropic phase as well. We also show that graphene flakes in an LC reduce the free ion concentration in the nematic media by an ion-trapping process. The reduction of mobile ions in the LC is found to have subsequent impacts on the LC's rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off. In a ferroelectric LC (smectic-C Ã phase), suspended graphene flakes enhance the spontaneous polarisation by improving the tilted smectic-C Ã ordering resulting from the π-π electron stacking. This effect accelerates the ferroelectric-switching phenomenon. Graphene can possess strain chirality due to a soft shear mode. This surface chirality of graphene can be transmitted into LC molecules exhibiting two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase, and a macroscopic helical twist of the LC director in the nematic phase. Finally, we show that a graphene-based LC cell can be fabricated without using any aligning layers and ITO electrodes. Graphene itself can be used as the electrodes as well as the aligning layers, obtaining an electro-optic effect of the LC inside the cell. P A 50 m P A Planar alignment 0 V 45 V Homeotropic alignment E ARTICLE HISTORY

Two-dimensional hexagonal boron nitride nanosheet as the planar-alignment agent in a liquid crystal-based electro-optic device

“…It has been experimentally demonstrated that due to this nano-architectural symmetry of the hexagons in both the species, the benzene rings of the LC coherently align on the hexagonal lattice of h-BN [18]. The interactions between the nematic LC and the hexagonal lattice of other different nanomaterials (e.g., graphene, carbon nanotube, molybdenum diselenide, tungsten diselenide) [19][20][21][22][23][24][25][26][27][28][29] have also demonstrated that the LC can achieve a planar-aligned state on the hexagonal nanosurface. The density functional calculations showed that this planar anchoring of the LC on the hexagonal nanostructure is further strengthened with a binding energy, ranging from 0.7 to 2.0 eV/molecule [21,27,39], by the electrostatic energy resulting from a considerable amount of charge transfer between the LC molecules and the hexagonal lattice structure.…”

Evidence of nanodiamond-self-assembly in a liquid crystal, and the consequent impacts on the liquid crystal properties

“…On the honeycomb -like hexagonal pattern, the LC molecules can form short-range order due to ππ stacking, by matching the LC's benzene rings on the honeycomb structure. [32][33][34][35][36][37] See Fig. 1(b).…”

“…The diminished presence of free-ions can reduce the internal friction, resulting in a decrease in the rotational viscosity γ 1 , of the LC media. [29][30][31]34,35 A reduction in γ 1 , therefore, causes a faster rotation of the LC molecules. It is possible that NDs, being a carbon allotrope, also trap some ions and reduce the free-ion concentration in LC, decreasing γ 1 -hence the observed accelerated responses for τ on and τ off in the E7+ND sample.…”

“…It has been shown that LC molecules can anchor to the honeycomb pattern of graphene [32][33][34][35] or carbon nanotubes, 36,37 employing the ππ electron stacking with a binding energy of -2 eV. A nanodiamond crystal possesses graphite-like surfaces 1,38 and can promote ππ stacking, as shown in Fig.…”

Dielectric and Electro-Optic Effects in a Nematic Liquid Crystal Doped with h-BN Flakes

“…The nanoscale interactions between nematic liquid crystals (LCs) and various two-dimensional (2D) hexagonal nanomaterials show interesting phenomena at the macroscopic levels. For example, the epitaxial interactions between the nematic LC and the hexagonal lattice of graphene, molybdenum diselenide, and tungsten diselenide deposited [1][2][3][4][5][6][7][8][9][10] demonstrate that the LC can achieve a planar-aligned state on these 2D hexagonal nanostructures. Our group has recently shown that the 2D hexagonal boron nitride (h-BN) film can be successfully employed as a planar-alignment agent as well as an ion-capturing agent in electro-optic LC devices [11][12][13].…”