Spectroscopic insight into molecular fluctuations and phase stability of nematic composites containing gold nanoparticles or carbon nanotubes.

A general model for interactions between nanoparticle dopants and nematic liquid crystals suffers from a lack of experimental data on nanoparticle-host interactions. This dielectric spectroscopy study intends to fill this gap by addressing the impact of gold nanoparticles, carbon nanotubes and toluene molecules on the molecular fluctuation dynamics in the nematic liquid crystal mixture E7. By correlating phase transition temperatures and rotational fluctuation frequencies, we show that the presence of nanoparticles or organic solvent molecules in the nematic host generally destabilizes the nematic state. We also report a clearly different magnitude of destabilization of the nematic state for toluene compared to nanoparticle dopants: while the presence of toluene increases the rate of molecular fluctuations by effectively diluting the host phase, nanoparticle dopants barely affect the molecular fluctuation dynamics. A corresponding trend for the decrease of phase transition temperatures confirms that small organic molecules reduce the strength of intermolecular interactions between host molecules to a significantly larger extent than nanoparticle dopants. We identify the diverse distribution of toluene or nanoparticles in the liquid crystal host phase to play a key role for the resulting effects of doping. The results of our experimental study will help to validate recent theoretical approaches on molecular dynamics in nematic composites and offer a substantial contribution towards stable liquid crystal nanodispersions with tailored properties for plasmonic or electronic applications.

Liquid crystals in micron-scale droplets, shells and fibers.

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this application route strongly influenced academic research, which frequently focused on the performance of liquid crystals in display-like geometries, typically between flat, rigid substrates of glass or similar solids. Today a new trend is clearly visible, where liquid crystals confined within curved, often soft and flexible, interfaces are in focus. Innovation in microfluidic technology has opened for high-throughput production of liquid crystal droplets or shells with exquisite monodispersity, and modern characterization methods allow detailed analysis of complex director arrangements. The introduction of electrospinning in liquid crystal research has enabled encapsulation in optically transparent polymeric cylinders with very small radius, allowing studies of confinement effects that were not easily accessible before. It also opened the prospect of functionalizing textile fibers with liquid crystals in the core, triggering activities that target wearable devices with true textile form factor for seamless integration in clothing. Together, these developments have brought issues center stage that might previously have been considered esoteric, like the interaction of topological defects on spherical surfaces, saddle-splay curvature-induced spontaneous chiral symmetry breaking, or the non-trivial shape changes of curved liquid crystal elastomers with non-uniform director fields that undergo a phase transition to an isotropic state. The new research thrusts are motivated equally by the intriguing soft matter physics showcased by liquid crystals in these unconventional geometries, and by the many novel application opportunities that arise when we can reproducibly manufacture these systems on a commercial scale. This review attempts to summarize the current understanding of liquid crystals in spherical and cylindrical geometry, the state of the art of producing such samples, as well as the perspectives for innovative applications that have been put forward.

Synthesis of liquid crystal silane-functionalized gold nanoparticles and their effects on the optical and electro-optic properties of a structurally related nematic liquid crystal.

Chemically and thermally robust liquid crystal silane-functionalized gold nanoparticles (i.e. AuNP1-AuNP3) were synthesized through silane conjugation. Colloidal dispersions of these particles with mesogenic ligands that are structurally identical (as in AuNP1, AuNP2) or compatible (as in AuNP3) with molecules of the nematic liquid crystal (N-LC) host showed superior colloidal stability and dispersibility. The thermal, optical, and electro-optic behaviors of the N-LC composites at different concentrations of each gold nanoparticle were investigated. All dispersions showed lower values for the rotational viscosity and elastic constant, but only AuNP3 with a dissimilar structure between the nanoparticle ligand and the host displayed the most drastic thermal effects and overall strongest impact on the electro-optic properties of the host. The observed results were explained considering both the structure and the density of the surface ligands of each gold nanoparticle.

Nanoparticle doping in nematic liquid crystals: distinction between surface and bulk effects by numerical simulations.

Doping nematic liquid crystals with small amounts of nanoparticles can significantly alter the electro-optic response of the nematic host. Some of these effects result from nanoparticles influencing the liquid crystal/substrate interface, while other effects are caused by nanoparticles in the bulk. So far, little attention has been paid to the influence of surface interactions on the determination of bulk properties. In the present study, these effects are investigated experimentally and confirmed by numerical simulations. The splay-type Fréedericksz-transition of the nematic liquid crystal 5CB doped with CdSe quantum dots is investigated, as these dispersions are known from earlier studies to affect the initial alignment layers. In comparison, dispersions of chemically and thermally stable silanized gold nanoparticles in the apolar nematic host FELIX-2900-03 are analyzed, which are expected to be bulk-active only. A data fitting routine is presented which allows a distinction between bulk and surface effects of nanoparticle doping. For the quantum dots, an increase of pretilt angle proportional to the doping concentration is found, as well as a slight decrease of the anchoring energy of molecules at the confining substrates. The silanized gold particles show no influence on the boundary conditions up to doping concentrations of 2.5 % (w). For higher concentrations an increase of pretilt angle is reported.

Hydrophobic gold nanoparticles via silane conjugation: chemically and thermally robust nanoparticles as dopants for nematic liquid crystals.

We examine for the first time how chemically and thermally stable gold nanoparticles (NPs), prepared by a silane conjugation approach, affect both the thermal and the electro-optical properties of a nematic liquid crystal (LC), when doped at concentrations ranging from 0.25 to 7.5 wt%. We find that the octadecylsilane-conjugated gold NPs stabilize both the enantiotropic nematic and the monotropic smectic-A phases of the LC host with a maximum stabilization of 2(°)C for the nematic and 3.5(°)C for the smectic-A phases for the mixture containing 1 wt% of the silanized particles. The same mixture shows the lowest values for the Fréedericksz transition threshold voltage and the highest value for the dielectric anisotropy. Generally, all NP-containing mixtures, except mixtures with NP concentrations exceeding 5 wt%, reduce the threshold voltage, increase the dielectric anisotropy and reduce both rise and decay time; the latter particularly at temperatures at least 10(°)C below the isotropic-nematic phase transition on cooling.

Tuning quantum-dot based photonic devices with liquid crystals.

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.

Electroconvection in nematic liquid crystals via nanoparticle doping.

It is known that a small fraction of nanoparticles dispersed in a liquid crystal can alter the electrooptic response, completely. The present study on gold nanoparticles dispersed in 5-n-heptyl-2-(4-n-octyloxy-phenyl)-pyrimidine shows that the contrast inversion observed earlier is initiated by a change from parallel to homeotropic anchoring, thereby causing an instability, which in turn leads to the appearance of convection rolls. After rapid cooling from the isotropic phase, the nanoparticle dispersion shows a regular field-induced Fréedericksz transition, like the pure liquid crystal. The electrohydrodynamic instability is presumably an example for the behavior of (+, -) systems that was predicted by de Gennes, and only recently observed experimentally for the first time.