Georg H. Mehl

Nematic twist-bend phase with nanoscale modulation of molecular orientation.

A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).

Microsecond linear optical response in the unusual nematic phase of achiral bimesogens

Some hydrocarbon linked mesogenic dimers are known to exhibit an additional nematic phase (Nx) below a conventional uniaxial nematic (Nu) phase. Although composed of non-chiral molecules, the Nx phase is found to exhibit linear (polar) switching under applied electric field. This switching has remarkably low response time of the order of a few microseconds. Two chiral domains with opposite handedness and consequently opposite responses are found in planar cells. Uniformly lying helix, electroclinic, and flexoelectric effects are given as possible causes for this intriguing phenomenon.

Structure–property relationships in nematic gold nanoparticles

Design rules for the investigation of nanoparticles covered with an organic monolayer are explored. The combination of small “spherical” particles in conjunction with nematogenic mesogens and hydrocarbon chains allows for the rational design of materials with nematic phase behaviour. The usefulness of the approach is demonstrated for two systems, where the particles have been characterised by NMR and TEM and mesomorphic properties have been identified using DSC and OPM.

Do the short helices exist in the nematic TB phase

Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical textures related to the formation of arrays of focal conic defects (FCDs). Some of the focal conics exhibit submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, that might suggest surface freezing.

Field-induced periodic chiral pattern in the Nx phase of achiral bimesogens

Some hydrocarbon-linked mesogenic dimers are known to exhibit an additional nematic phase (Nx) in the temperature range below the conventional nematic (Nu) phase. One of the features of this phase is the presence of optical response typically found in chiral systems, while the involved molecules are non-chiral. We demonstrate that the two domains of opposite handedness found in planar cells can be controlled/induced by the external electric field and these form periodic striped patterns. The effect of frequency and amplitude of the electric field on the periodicity and formation of the domain pattern is investigated.

Polyhedral liquid crystal silsesquioxanes

Synthetic routes towards monodisperse polyhedral liquid-crystalline silsesquioxanes are described. The liquid-crystalline phase behaviour of these structurally uniform materials, with various silsesquioxane core and mesogenic side-chain structures, were analysed and compared with those of related oligomeric and polymeric materials. Common features of the solid-state behaviour of these organic/inorganic hybrids, which form self-organized structures in their soft-matter states, were identified. Copyright

Elastic properties of bimesogenic liquid crystals

The second nematic phase found in some bimesogenic liquid crystals with an odd flexible spacer has aroused considerable interest due to many unusual properties exhibited by them. However, the reason for such molecules to exhibit transitions to the modulated phase is still unclear. Dozov [Europhys Lett. 2001;56:247] predicted that negative K 33 can lead to a modulated phase where the director has either splay-bend or twist-bend distribution. Though various theoretical studies have suggested that this scenario may be valid, no experimental evidence has so far been given. In order to study the influence of the elastic constants, we measured the splay and bend elastic constants in the ordinary nematic phase of a dimer material, CBC11CB, for temperatures down to 0.6°C above the Nx–Nu transition. Our results show that the bend constant is reduced by a factor of 2 compared with that closer to the I-Nu transition but is positive and the trend does not seem that K 33 will extrapolate to zero or fall below it for the temperature range investigated. Compared to K 11, K 33 is reduced by a factor of 3, close to the Nu–Nx transition temperature.

Liquid crystals with restricted molecular topologies: supermolecules and supramolecular assemblies

The term liquid crystal is often associated with fascinating compounds that exhibit unusual melting or solubilisation properties, however, it also represents a unique collection of mesophases that exist between the solid state and the amorphous liquid. As such, this unique state of matter can be accessed by a wide variety of materials from low molar mass to polymeric systems. In this Feature Article we describe some recent studies concerning the liquid-crystalline behaviour of ‘in-between’ materials that have discrete molecular structures, and which are oligomeric but not low molar mass or polymeric systems. Thus, these materials could be described as having supermolecular architectures. We examine some of their mesophase properties and their abilities to form supramolecular assemblies. The development of liquid crystals that have large molecular structures or consist of large scale assemblies is one step towards creating novel self-organising systems which are of a similar dimension to certain biological materials, such as proteins.

Direct observation of liquid crystals using cryo‐TEM: Specimen preparation and low‐dose imaging

Liquid crystals (LCs) represent a challenging group of materials for direct transmission electron microscopy (TEM) studies due to the complications in specimen preparation and the severe radiation damage. In this paper, we summarize a series of specimen preparation methods, including thin film and cryo‐sectioning approaches, as a comprehensive toolset enabling high‐resolution direct cryo‐TEM observation of a broad range of LCs. We also present comparative analysis using cryo‐TEM and replica freeze‐fracture TEM on both thermotropic and lyotropic LCs. In addition to the revisits of previous practices, some new concepts are introduced, e.g., suspended thermotropic LC thin films, combined high‐pressure freezing and cryo‐sectioning of lyotropic LCs, and the complementary applications of direct TEM and indirect replica TEM techniques. The significance of subnanometer resolution cryo‐TEM observation is demonstrated in a few important issues in LC studies, including providing direct evidences for the existence of nanoscale smectic domains in nematic bent‐core thermotropic LCs, comprehensive understanding of the twist‐bend nematic phase, and probing the packing of columnar aggregates in lyotropic chromonic LCs. Direct TEM observation opens ways to a variety of TEM techniques, suggesting that TEM (replica, cryo, and in situ techniques), in general, may be a promising part of the solution to the lack of effective structural probe at the molecular scale in LC studies. Microsc. Res. Tech. 77:754–772, 2014.

Design, synthesis, and characterization of mesogenic amine-capped nematic gold nanoparticles with surface-enhanced plasmonic resonances.

The use of the liquid-crystalline state to control the assembly of large (>5 nm) gold nanoparticles (NPs) is of considerable interest because of the promise of novel metamaterial properties of such systems. Here we report on a new approach for the preparation of large nematic gold NPs using a bifunctional capping agent that enables control over the particle size and serves as a linkage for subsequent functionalization with mesogenic groups. Properties of the NPs were characterized by HRTEM, NMR, DSC, TGA, UV/vis, OPM, and XRD studies. The results confirmed the formation of a stable nematic mesophase above 37.5 °C for NPs in the 6-11 nm size range.