Thermotropic liquid crystal (5CB) on two-dimensional materials

We present ground-state electronic properties of the liquid crystal 4-cyano-$4^{\prime }$-pentylbiphenyl (5CB) on the two-dimensional materials monolayer graphene, hexagonal boron nitride, and phosphorene. Our density functional theory results show that the physisorption is robust on all surfaces with the strongest binding of 5CB on phosphorene. All surfaces exhibit flexural distortion, especially monolayer graphene and hexagonal boron nitride. While we find type-I alignment for all three substrates, meaning the Fermi level of the system is in the HOMO-LUMO gap of 5CB, the band structures are qualitatively different. Unlike for graphene and phosphorene, the HOMO-LUMO of 5CB appear as localized states within the band gap of boron nitride. In addition, we find that the valence band for boron nitride is sensitive to the orientation of 5CB relative to the surface. The qualitatively different band structures demonstrate the importance of substrate selection for tailoring the electronic and optoelectronic properties of nematic liquid crystals on two-dimensional materials.

Figure 1

Optimized geometry of 5CB on (a)–(d) G, (e)–(h) BN, and (i)–(l) BP. The structures are in top-to-bottom order ECL, AC, INT, and ZZ.

Figure 2

Planar-averaged charge density differences for (a) INT-5CB/G, (b) AC-5CB/BN, and (c) INT-5CB/BP. Positive and negative regions of charge fluctuation are shown in blue and red respectively. Dotted lines trace the relative positions of 5CB and 2D material to aid the eye on the locations of each substance and van der Waals spacing.

Figure 3

Brillouin zones for tetragonal and hexagonal crystals. (a) Tetragonal Brillouin zone for the structures presented in the figures and (b) hexagonal Brillouin zone for G and BN comparison, see Fig. 5. [47].

Figure 4

Band structures for pristine nanosheets (a) G, (b) BN, and (c) BP. The Fermi level has been chosen to be zero.

Figure 5

Electronic structures for lowest energy systems (a) INT-5CB/G, (b) AC-5CB/BN, and (c) INT-5CB/BP. (d) The insets below highlight the band dispersions in the interval $[-0.1,0.0]\mathrm{eV}$ for structures of BN in the ECL, AC, INT, and ZZ structures (left to right). The vertical green line marks the position of the high symmetry point $\mathrm{K}$ in the hexagonal Brillouin zone for G and BN, see Fig. 3.

Figure 6

The projected DOS of (a) ECL-5CB/BN (b) AC-5CB/BN, (c) INT-5CB/BN, and (d) ZZ-5CB/BN. The projections for 5CB are depicted in red.