Ordered interfaces for dual easy axes in liquid crystals

Abstract

Using nCB films adsorbed on MoS 2 substrates studied by x-ray diffraction, optical microscopy and Scanning Tunneling Microscopy, we demonstrate that ordered interfaces with well-defined orientations of adsorbed dipoles induce planar anchoring locked along the adsorbed dipoles or the alkyl chains, which play the role of easy axes. For two alternating orientations of the adsorbed dipoles or dipoles and alkyl chains, bi-stability of anchoring can be obtained. The results are explained using the introduction of fourth order terms in the phenomenological anchoring potential, leading to the demonstration of first order anchoring transition in these systems. Using this phenomenological anchoring potential, we finally show how the nature of anchoring in presence of dual easy axes (inducing bi-stability or average orientation between the two easy axes) can be related to the microscopical nature of the interface. IntroductionUnderstanding the interactions between liquid crystal (LC) and a solid substrate is of clear applied interest, the vast majority of LC displays relying on control of interfaces. However this concerns also fundamental problems like wetting phenomena and all phenomena of orientation of soft matter bulk induced by the presence of an interface. In LCs at interfaces, the so-called easy axes correspond to the favoured orientations of the LC director close to the interface. If one easy axis only is defined for one given interface, the bulk director orients along or close to this axis [1]. It is well known that, in anchoring phenomena, two major effects compete to impose the anchoring directions of a liquid crystal, first, the interactions between molecules and the interface, second, the substrate roughness whose role has been analyzed by Berreman [2]. The influence of adsorbed molecular functional groups at the interface is most often dominant with, for example in carbon substrates, a main influence of unsaturated carbon bonds orientation at the interface [3]. In common LC displays, there is one unique easy axis, but modifications of surfaces have allowed for the discovery of promising new anchoring-related properties. For instance, the first anchoring bistability has been established on rough surfaces, associated with electric ordo-polarization [4] and the competition between a stabilizing short-range term and a destabilizing long-range term induced by an external field, can induce a continuous variation of anchoring orientation [5]. More recently, surfaces with several easy axes have been studied extensively. It has been shown that control of a continuous variation of director pretilt, obtained in several systems [6,7], is associated with the presence of two different easy axes, one perpendicular to the substrate (homeotropic) and one planar [7,8]. Similar models can explain the continuous evolution of anchoring between two planar orientations observed on some crystalline substrates [9]. However, in the same time, two easy axes can also lead to anchoring bi-stability [10,11] or discontinuous transi...

Kinked row-induced chirality driven by molecule–substrate interactions

“…2×T MoS2 with T MoS2 = 3.16 Å, the MoS 2 lattice constant. This confirms an orientation of the biphenyl groups close to being parallel to the [120] direction, as already evidenced for 8CB on MoS 2[48][49][50] .ARTICLEJournal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx…”

“…On MoS 2 , the distance between the alkyl chains is similar to the one on HOPG and the same fan-shape is observed, leading to a distance between the cyanobiphenyl groups of the order of 6 Å in average, close to 2×T MoS2 =6.3 Å. The epitaxy is now driven by the cyanobiphenyl groups oriented in average along the [120] direction of MoS 2 , known to be the favorable average orientation of the cyanobiphenyl groups on MoS 2 [48][49][50] . However, the distance between cyanobiphenyl groups remaining small, a fan-shape of the molecular structure is again induced within the unit-cell in relation with steric interactions.…”

Nanocomposites of 2D-MoS2 Exfoliated in Thermotropic Liquid Crystals

“…RM23 (4′-cyano-[1,1′-biphenyl]-4-yl 4-((6-(acryloyloxy)­hexyl)­oxy)­benzoate), 5CB, 6OCB, and 8CB (4′-pentyl-, 4′-hexyloxy-, and 4′-octyl-4-biphenylcarbonitrile, respectively, see Figure S9), was unsuccessful. This was unexpected, as prior studies have highlighted the formation of kinked lamellar structures of nCB on MoS 2 crystals, driven by nominally favorable interactions and structural commensurabilities. − , …”

Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon

“…The problem is even more acute and of great current interest since recently important progresses have been made in characterizing experimentally the surface-induced order and structuring close to a surface. 8,[19][20][21][22][23][24][25][26] The results of these experiments on thin films are however rather puzzling, particularly about the way the order decays moving away from the surface into the film, and the role of the second confining surface. For instance the groups of Rosenblatt 22,24 and Mu sevi c 23 examined nano-thin films of 5CB close to a surface inducing planar ordering along one direction (rubbed polyimide and mica, respectively).…”