P. K. Karahaliou

Biaxial nematics: symmetries, order domains and field-induced phase transitions †

We studied the symmetry and spatial uniformity of the orientational order of the biaxial nematic phase in the light of recent experimental observations of phase biaxiality in thermotropic bent-core and calamitic-tetramer nematics. Evidence is presented supporting monoclinic symmetry, instead of the usually assumed orthorhombic symmetry. The use of deuterium nuclear magnetic resonance to differentiate between the possible symmetries is described. The spatial aspects of biaxial order are presented in the context of the cluster model, wherein macroscopic biaxiality can result from the field-induced alignment of biaxial and possibly polar domains. The implications of different symmetries on the alignment of biaxial nematics and on the measurements of biaxial order are discussed in conjunction with the microdomain structure of the biaxial phase.

Symmetries and alignment of biaxial nematic liquid crystals

The possible symmetries of the biaxial nematic phase are examined against the implications of the presently available experimental results. Contrary to the widespread notion that biaxial nematics have orthorhombic symmetry, our study shows that a monoclinic (C(2h)) symmetry is more likely to be the case for the recently observed phase biaxiality in thermotropic bent-core and calamitic-tetrapode nematic systems. The methodology for differentiating between the possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy measurements is presented in detail. The manifestations of the different symmetries on the alignment of the biaxial phase are identified and their implications on the measurement and quantification of biaxiality as well as on the potential use of biaxial nematic liquid crystals in electro-optic applications are discussed.

Dielectric studies of a laterally-linked siloxane ester dimer

This paper reports measurements of the dielectric response over the frequency range 102 to 109 Hz of a liquid crystal dimer consisting of two ester mesogens laterally linked by an alkoxy chain containing a siloxane group. The synthesis and phase behaviour of the siloxane dimer are also reported. Results show that there are two relaxations in the isotropic phase and four in the nematic phase of the material. The possible molecular origins for these modes are given. It is found that there is a coupling between internal and external modes which gives rise to a cooperative mode as the temperature in the nematic phase is lowered towards a glass transition.

Columnar phase structures of an organic-inorganic hybrid functionalized with eight calamitic mesogens

A liquid-crystalline octapode, formed by laterally connecting calamitic mesogens to an inorganic silsesquioxane cube through flexible siloxane spacers, is studied using polarized light microscopy, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The studies are extended to mixtures of the octapode with the respective monomer mesogens. The monomer and the octapode show a nematic phase. At lower temperatures, the octapode exhibits additionally a columnar hexagonal phase (6 lattice), which, on further cooling, undergoes a transition to a columnar rectangular phase (2 lattice). A similar phase-transition sequence is observed for mixtures of the octapode with moderate concentrations of the monomer. The columnar-columnar transition is discussed combining XRD and DSC results, and a possible model of the molecular self-organization is presented.

Transient and ac electrical transport under forward and reverse bias conditions in aluminum/porous silicon∕p-cSi structures

Dielectric impedance measurements as well as transient current–voltage (I–V) characteristics under conditions of forward and reverse bias are reported in aluminum/porous silicon (PS)∕p-cSi structures at different temperatures. Under reverse bias conditions, the electrical conduction of the structures can be modeled by a simple equivalent circuit of two parallel RC networks in series combination, representing a bulk and a junction region. The bulk conduction is ohmic. From the detailed analysis of the PS/cSi junction, the carrier concentration NA and the depletion layer width W are evaluated at each temperature. The elapsed time t0 from the onset of the square pulse, at which the transition from the dominant bulk resistance to junction conduction takes place, is a function of temperature. As the temperature decreases, conduction due to bulk resistance dominates over junction conduction. On the other, hand forward conduction is dominated from the bulk. Two conduction mechanisms are present. As the time proc...

Dielectric response and ac conductivity analysis of hafnium oxide nanopowder

The dielectric response of hafnium oxide nanopowder was studied in the frequency range of 10−2–106 MHz and in the temperature range of 20–180 °C. Broadband dielectric spectroscopy was applied and the experimental results were analyzed and discussed using the electric modulus (M*) and alternating current (ac) conductivity formalisms. The analyses of the dc conductivity and electric modulus data revealed the presence of mechanisms which are thermally activated, both with almost the same activation energy of 1.01 eV. A fitting procedure involving the superposition of the thermally activated dc conductivity, the universal dielectric responce and the near constant loss terms has been used to describe the frequency evolution of the real part of the specific electrical conductivity. The conductivity master curve was obtained, suggesting that the time–temperature superposition principle applies for the studied system, thus implying that the conductivity mechanisms are temperature independent.

CuO/Ta2O5 core/shell nanoparticles synthesized in immersed arc-discharge: production conditions and dielectric response

We reported recently on a novel nanostructured material produced by the arc-discharge in water method, and extended studies were realized to identify the nature of this material, i.e., CuO/Ta2O5 core/shell crystalline nanoparticles (NPs). As a continuation of this investigation on the possibility of complex NP synthesis using immersed arc-discharge, the production conditions of the CuO/Ta2O5 NPs are herein presented in detail and the electrical properties of the nanopowder are examined comprehensively. The discharge is thus probed in situ by electrical measurements, optical emission spectroscopy and high speed imaging, and the electrical behavior of the NPs is considered by means of broadband dielectric spectroscopy. This combined study provides an integrated characterization of this new material, unveils its potential applications, and makes available suggestions on the process control.

Tilt order parameters, polarity, and inversion phenomena in smectic liquid crystals

The order parameters for the phenomenological description of the smectic-A to smectic-C phase transition are formulated on the basis of molecular symmetry and structure. It is shown that, unless the long molecular axis is an axis of twofold or higher rotational symmetry, the ordering of the molecules in the smectic-C phase gives rise to more than one tilt order parameter and to one or more polar order parameters. The latter describe the indigenous polarity of the smectic-C phase, which is not related to molecular chirality but underlies the appearance of spontaneous polarization in chiral smectics. A phenomenological theory of the phase transition is formulated by means of a Landau expansion in two tilt order parameters (primary and secondary) and an indigenous polarity order parameter. The coupling among these order parameters determines the possibility of sign inversions in the temperature dependence of the spontaneous polarization and of the helical pitch observed experimentally for some chiral smectic-C* materials. The molecular interpretation of the inversion phenomena is examined in the light of this formulation.

On the molecular theory of dimeric liquid crystals

We present a statistical mechanics approximation scheme for the explicit treatment of spacer‐mediated configurational correlations among the mesogenic units that form a dimer molecule. The approximation is applied to the description of the nematic phase of linear uniaxial dimers interacting via a standard molecular pair‐potential. Transition temperatures, order parameters and pair correlation averages are calculated for different spacer lengths. The results readily reproduce the experimentally observed trends of phase transition thermodynamics and of dipolar correlations deduced from dielectric studies.

Interface traps density of anodic porous alumina films of different thicknesses on Si

Impedance Spectroscopy was employed in order to investigate the electrical properties of thin porous anodic alumina films on Si, of thicknesses in the range of 50-200 nm, fabricated by anodization in sulfuric acid. C–V and G–V measurements were performed in the voltage range +5.0 V to −10.0 V and the frequency range 1 MHz to 100 Hz. The typical form of C–V and G–V curves of a metal-insulator-semiconductor (MIS) structure was obtained. The effective dielectric constant e was calculated with a typical value of 6.5, in good agreement with previous published results. C-ω and G-ω measurements were performed as a function of the applied gate voltage in the depletion region in order to calculate the interface trap density Dit and interface trap time constant τit. Dit and τit were evaluated following the conductance method. The evaluated Dit values are of the order of 1012 eV−1 cm−2 and their observed thickness dependence is rather attributed to differences of the porous alumina/p-Si interface, introduced during the formation process, than to sample thickness.