Adsorption of the liquid crystal molecule 5CB on graphene

We use density functional theory to explore the stable adsorption geometries of the liquid crystal molecule 5CB on a graphene sheet. First, we calculate the dependence of the polarizability of 5CB on its geometry. Our results break down the contributions of the cyano, biphenyl, and alkyl groups to the optical properties of 5CB, confirming the biphenyl group as the most influential. Second, we quantify possible adsorption structures of 5CB on graphene. We find that 5CB can stably adsorb with its biphenyl group oriented intermediate to the armchair and zigzag crystallographic directions, in addition to adsorbing with the biphenyl oriented along those directions.

Figure 1

LC molecule 5CB oriented in the principal axis frame of the polarizability. The angle between the major axis of the polarizability ${\alpha }_1$ and the long axis of 5CB (dashed line) is $4.{05}^{\circ }$. The principal components of the polarizability are shown in the bottom left in units of ${\AA }^3$. The dihedral angle between the alkyl tail and phenyl ring is indicated by ${\varphi }_{\mathrm{t}}$. The dihedral angle between the phenyl rings is indicated by ${\varphi }_{\mathrm{p}}$.

Figure 2

Calculated potential energy (squares, solid line) and isotropic polarizability (circles, dashed line) as a function of the dihedral angle between the alkyl tail and neighboring phenyl ring. The energy is given relative to the optimized ground state energy.

Figure 3

Calculated potential energy (squares, solid line) and isotropic polarizability (circles, dashed line) as a function of the dihedral angle between the phenyl rings. The energy is given relative to the optimized ground state energy.

Figure 4

Highest occupied molecular orbitals when the alkyl tail is oriented ${90}^{\circ }$ and ${180}^{\circ }$ relative to the neighboring phenyl ring. The two colors represent the phases of the orbitals.

Figure 5

Highest occupied molecular orbitals when the biphenyl dihedral is ${90}^{\circ }$ and ${180}^{\circ }$. The two colors represent the phases of the orbitals.

Figure 6

Lowest energy structures for each adsorption orientation. The graphene sheets have been rendered as wire frames for clarity. The thick, black arrow represents the armchair direction of graphene, while the dashed arrow indicates the average orientations. The angles $\theta$ between these lines are given in Table 1.

Figure 7

Out-of-plane displacement for the carbon atoms of graphene. The heavy atoms of the adsorbed 5CB molecule are indicated by the white circles.