Two-Dimensional Assemblies of Banana-Shaped Liquid Crystal Molecules with Short Alkyl Chains at Solid/Liquid Interface

Abstract

Two-dimensional assemblies of a series of banana-shaped liquid crystal molecules with short alkyl chains, (LCn, n = 5, 6), are investigated at the solid/liquid interface on graphite surface by using scanning tunneling microscopy (STM) under ambient conditions. The high-resolution STM images of the LCn assemblies demonstrated a lamellae structure with bright bands and dark stripes, in which the alkyl chains of neighboring rows were either interdigitated or tail-to-tail due to the van der Waals interactions. The results in this paper are different with other reported banana-shaped liquid crystals, in which the core may play a dominant role in the self-assembly and the solvent molecules may also play a role in the assemblies. It may be helpful in understanding the assembly mechanism of banana-shaped liquid crystal molecules.

Structure formation in monolayers composed of hard bent-core molecules

“…Structures that are left are polar nematics, N SB and commensurate or incommensurate smectics of A or C type, among which the most relevant are shown in Fig. (4). Thus, for the case of perfectly aligned boomerangs only two variables are needed to parametrize one particle distribution function.…”

“…For N SB we will assume the director to be a periodic function in x-direction, which means that P (X) ≡ P (s, n(x)), where s = ±1 represents two opposite orientations of the steric dipole with respect to the local director, Fig. (4). For lamellar structures we will use vertical coordinate y and s to parametrize P : P (X) ≡ P (s, y).…”

“…Since the steric polarization is always perpendicular to the local director we assume direction of the former to rotate uniformly between −θ max /2 and θ max /2 for 0 ≤ x < d/2, Fig. (4), where θ max and d should be determined from the free energy minimum. Assuming the structure to be globally nonpolar we take…”

“…In general, the most probable two-dimensional structures that can be expected in boomerang systems are given in Fig. (4). The low density phase, the nematic phase, can be here of two types: the standard nematic phase (N ) in which the same (on average) number of boomerangs is pointing to the right as to the left.…”

“…Two-dimensional structures made by complicated macromolecules are recently of great interest due to their potential applications, mainly in photoelectronic and biosensor area [1][2][3][4][5][6][7][8]. In contrast to assemblies of spherical objects like, for instance, colloidal or nanosilica spheres, in case of anisotropic or irregularly shaped particles, there is a possibility to realize monolayers exhibiting very regular patterns which, next, can be utilized as a matrix capable to orient liquid crystal or to fabricate elements of electronic devices [5].…”