The present article shows an original transmission-reflection-based method to characterise the microwave dielectric properties of liquid crystals. This new method offering broadband capabilities of transmission-reflection method without the need for vector network analyser calibration. It is based on two measurements in the frequency range: one measurement is performed with the empty cell, the other with the cell filled with the material. The analysis of the two recorded scattering matrices allows to eliminate all error terms and to extract real and imaginary parts of the complex dielectric constant. In this paper, we first present this method, its validation in comparison to other characterisation methods. Secondly, we present an appropriate measuring cell which allows measurements in the frequency range 26 GHz-40 GHz which is also well-suited for temperature-dependent measurements. This method is used to perform dielectric microwave measurements on 5CB liquid crystal. Finally, we discuss the temperature-dependent microwave dielectric properties (permittivity, losses and dielectric anisotropy).

In this paper, we present the development and refinement of a tunable microwave phase shifter featuring a liquid crystal active layer. These phase shifters leverage the unique properties of liquid crystals, such as their microwave dielectric anisotropy and their ability to be controlled by electric fields. We introduce two variations of phase shifters: coplanar phase shifters and coplanar phase shifters employing the half- curve method. Through thorough characterization, we demonstrate that applying a driving electric field of approximately 0.5 V/μm results in a phase change of around l°/cm/GHz. This study highlights the potential of liquid crystal- based phase shifters as promising solutions for microwave applications that require precise and adjustable phase control. Overall, the study explores the dynamic relationship between the applied electric field, the liquid crystal's permittivity, and the resulting phase shift. This understanding of the interaction between the radio-frequency electric field and the liquid crystal's properties paves the way for the development of tunable microwave phase shifters and opens up possibilities for various applications in the field of microwave technology.

Anthracene is considered to be a popular choice as a building block for organic semiconductors. The present work is dedicated to the synthesis and characterization of a novel semiconductor (10-OPIA) possessing mesogenic properties, which allows better control over charge transport in the bulk of a material. A novel anthracene-based molecule is characterized for its potential applications: frontier molecular energy levels are studied by optical spectroscopy and cyclic voltammetry and compared to values obtained via ab initio calculations. Thermophysical and mesogenic properties are investigated by optical microscopy and differential scanning calorimetry. Charge transport properties are characterized by means of an OFET device. It is found that this material can be easily aligned and exhibits a field effect hole mobility of 5.22 × 10-5 cm2 V-1 s-1 and an ON/OFF ratio of 104 in the device prepared by drop casting. Finally, the photoconductive properties of this novel material are addressed in order to investigate its potential applications for organic phototransistors: it exhibits a large photoconductive gain of >100 and a photo-responsivity of >1 A W-1.

We present the effect of polymer network on the ferroelectric phase for a polymer stabilised ferroelectric liquid crystal (PSFLC) system. The ferroelectric liquid crystal (FLC) exhibits a very short helical pitch, large tilt angle and relatively high spontaneous polarisation. Electro-optic and dielectric investigations are performed on films with various polymer concentrations. Measurements in the chiral smectic C* (SmC*) phase of the tilt angle and spontaneous polarisation as a function of electric field strength and polymer concentrations were performed. For low electric fields, a deformed helix ferroelectric (DHF) process was observed for pure FLC as well as for the PSFLC films, showing a linear response of the helical structure to electric field. The influence of the polymer network on the FLC properties is also investigated by linear dielectric measurements. Particularly the Goldstone relaxation mechanism. The results reveal a gradual decrease of the dielectric strength with an increase in the network density. However, the relaxation frequency of this mechanism seems to be insensitive to the polymer network density. These behaviours were interpreted by the increase of both the effective elasticity and the rotational viscosity of the SmC* when stabilised by the polymer network.

We report the electrical characterizations of La2Ti2O7 thin films synthetized by pulsed laser deposition. We observe a significant change in I-V curve in the presence of UV light. The conduction mechanism can be described by two regimes: ohmic at low voltage and space charge limited conduction for higher voltage. Finally, for the first time to our knowledge, the charge carrier mobility of this material is studied; we use the time-of-flight method to investigate this property. We explain the evolution of the carrier mobility with the applied voltage by the presence of a depletion layer between the film and the gold contact, which affects the measurements.

We report the synthesis of oleylamine capped CdS nanowires and we have dispersed a small optimized amount of these NWs in the Colh phase of a recently synthesized phenazine-fused-triphenylene discotic liquid crystal to understand the temperature-dependent charge transport.

We explore the molecular nature of doping in organic semiconductors (OSCs) by employing a liquid crystalline organic semiconductor based on phenyl naphthalene as a model. The mesophase nature of composites that include a charge transfer complex (CTC) between the OSC (8-PNP-O12) and an electron acceptor (F4TCNQ) has been investigated by means of differential scanning calorimetry, polarized optical microscopy and X-ray scattering. Optical and vibrational spectroscopies allow us to explore the characteristics and the amount of charge transfer in the CTC and expose some properties that appear only in the complexed state. We have found this system to exhibit partial charge transfer, which manifests itself in all the phase states of the host 8-PNP-O12, as well as in solution. Due to the lowering of molecular symmetry as a result of the charge transfer, one of the previously IR-only vibrational bands of the nitrile group is found to be now active in the Raman spectrum. We have also made an attempt to further investigate the influence of dopant introduction on the bulk hole mobility of 8-PNP-O12. It is found that the presence of the CTC promotes the hole transport in the Smectic B mesophase, however it seems to have a somewhat negative influence in the less ordered smectic A mesophase. This work aims to establish the link between the inevitable change of molecular geometry that occurs on charge transfer with the results obtained by spectroscopic techniques and electronic charge carrier mobility measurements.

Molecular arrangement of 8CB on the GO surface and its I–V curve.

The influence of polymer network on hole transport in discotic liquid crystal (LC) is investigated by the time of flight charge carrier mobility measurements. Composites of discotic LC and nematic reactive mesogen undergo UV-activated polymerization in columnar phase which yields the formation of chemical gels. Polarized optical microscopy allows us to observe alignment effect, whether scanning electron microscopy helps us to obtain characteristic imprint of polydomain columnar phase in created polymeric network. Hole mobility is improved, presumably due to the stabilization of the alignment of discotic LC. This type of composite is relevant for potential multilayer and flexible device applications.

The effect of Sn2P2S6 ferroelectric nanoparticles dispersed in 8CB liquid crystal on the dielectric properties of the nanocolloids is studied. The experimental results are discussed by taking into account different effects: the dielectric permittivity contribution of nanoparticles, the effect of permanent polarization of nanoparticles on the liquid crystal order and the anchoring interactions between nanoparticles and liquid crystal molecules. For a homogeneous nanoparticles dispersion, the doped liquid crystal shows an increase of the dielectric anisotropy which is more improved by increasing nanoparticles concentration. For high concentrations, the presence of aggregates decreases the dielectric anisotropy due to a decrease of polarization effect. Near the nematic-isotropic phase transition, the study evidences similar behavior between the shift of the dielectric anisotropy and that of the phase-transition temperature. Near the smectic-A-nematic phase transition, the results show that the shifts of the dielectric anisotropy and of the smectic-A-nematic phase-transition temperature present different behaviors. This difference is interpreted by the coupling between smectic and nematic ordering.

Photo-carrier transport process and mobility in the different phases including crystalline and isotropic phases of a liquid crystalline photoconductor material from the 2-phenylnaphthalene derivative, 2-(4’-octylphenyl)-6-dodecyloxynaphthalene (8-PNP-O12) was re-investigated as function of electric field and temperature by using time of flight (TOF) transient photocurrent technique. Charge carrier transport is discussed in different phases from the measurement of the transient photocurrent. In the mesophases, experimental data, in agreement with previous results, highlight electronic and ionic conductions. Moreover, were revealed electronic transport in isotropic phase of a liquid crystalline material and crystalline phase of this compound.

Broadband dielectric spectroscopy is used to determine the dielectric properties and phase transitions of the 4-n-octyl-4′-cyanobiphenyl liquid crystal (8CB) doped with harvested and non-harvested ferroelectric nanoparticles.

The influence of a low ac electric field on phase transitions is discussed in the case of a nematic liquid crystal 4-n-octyl-4^{'}-cyanobiphenyl (8CB) doped with Sn_{2}P_{2}S_{6} ferroelectric nanoparticles. The phase-transition temperatures obtained from temperature-dependent dielectric measurements were higher than those determined by the calorimetric method. This difference is explained by the presence of the measuring electric field which induces two effects. The first one is the amplification of the interactions between the nanoparticle polarization and the liquid-crystal order parameter. The second one is the field-induced disaggregation or aggregation process at high nanoparticle concentrations.

ABSTRACT We present the dielectric properties of two pure ferroelectric liquid crystal materials (C11 and C12) exhibiting cholesteric (N*), paraelectric smectic A (SmA*), and ferroelectric chiral smectic C (SmC*) phases. In the dielectric measurements carried out without a DC bias field, we studied the soft-mode relaxation in the SmA* phase. From experimental data and using the results of a Landau model, we evaluate the electroclinic coefficient in the SmA* phase. A soft-mode-like mechanism was also observed in the N* phase for a compound with a shorter chain (C11). This relaxation process is not detected for the homolog with a longer chain (C12). The observation of this mechanism is linked to smectic order fluctuations within the N* phase whose amplitude is more enhanced close to the SmC*−SmA*−N* multicritical point.

für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. Dielectric Relaxation in Nematic and Isotropic Phases of 4-(f#*a/is-4'-n-Propylcy clohexyl) iso thiocyana to benzene

Polymer/liquid crystal (LC) films were elaborated by photopolymerization using UV radiation of mixtures made up of a LC, a monomer and a photoinitiator. Three acrylic difunctional tripropyleneglycol based monomers possessing the same chemical structures were studied. The monomer/LC films exhibit distinguished morphologies presenting specific LC domain sizes, which were observed by polarized optical microscopy. These results were correlated with the electro-optical responses of the corresponding polymerized/crosslinked systems. A detailed characterization by linear dielectric spectroscopy of monomers and corresponding polymers was carried out as a function of temperature in the frequency range from 20 Hz to 1 MHz.

Thermotropic liquid crystals composed of rod like molecules are known as calamitic liquid crystals. Flatten molecules of some organic compounds form discotic liquid crystals. Both kinds of compounds may exhibit nematic phase. Calamitic mesogens may also form lamellar smectic structures being so important for living systems, whereas discotic molecules make columnar mesophases. Banana-shaped (bow-shaped or bent-core) ferroelectric liquid crystals have been discovered in the last decade of the 20-th century (Noiri et al., 1996). Since then there has been a great interest in their dielectric and electrooptic properties due to potential applications (Sekine et al., 1997; Link et al., 1997; Pelzl et al., 1999). At the beginning some of the bent-core compounds were not chemically stable enough as to study them experimentally during cooling and heating runs in long lasting experiments (Wrobel et al., 2000). It came out that bent-core achiral thioesters are very stable materials showing either B1 or B2 phase (Rouillon et al., 2001; Ossowska-Chruściel, 2007, 2009). In this article we present complementary studies on B1 and B2 phases of 1,3-phenylene bis{4-[(4-alkoxybenzoyl)-sulfanyl]benzoates} (in short: nOSOR) having achiral symmetric bent-core molecules shown in Fig. 1.

Polymer Dispersed Liquid Crystal films were elaborated by photopolymerization using ultraviolet radiation of liquid crystal/monomer mixtures. Three acrylic difunctional propyleneglycol based monomers were used; differing only by their chain lengths in terms of their molecular weight. Infrared spectroscopy investigation made it possible to obtain the monomer conversion rates, in order to determine the effect of the presence of liquid crystal on the kinetics of polymerization and phase separation. The characterization by linear dielectric spectroscopy of the monomers and polymers was carried out as a function of temperature in the frequency range from 20 Hz to 1 MHz.

A ferroelectric liquid crystal showing a de Vries phase is studied by the mean of dielectric and electro-optical measurements. Due to a strong electro-clinic effect close to the smectic A*-smectic C* phase transition, a non linear electrical response is observed. The non linear dielectric characterization of this compound and the confrontation with a theoretical model allow us to extract the Landau's parameters of this material. The results are discussed on the basis of other data obtained by different measurement techniques. As an example, we confirm the second order smectic A*-smectic C* phase transition.