Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements

Indium tin oxide (ITO) layers are widely used to make electrodes in measuring cells, because these layers are transparent and electrooptical investigations can be performed using such prepared cells. It was found during the dielectric spectroscopy measurements, performed for smectic liquid crystalline mixture, that it is not possible to detect some important relaxation modes in paraelectric SmA*, ferroelectric SmC*, and antiferroelectric SmC_A* phases for the frequencies higher than 300 kHz. The measuring cell does not allow to measure relaxations, because its own dielectric behaviour covers the dielectric response of a liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to the finite resistance of ITO layers. The theoretical model was introduced, which shows how to calculate relaxations related to liquid crystals from dielectric response of the empty and filled measuring cell. The proof of strong influence of cell properties on effective (measuring) values of dielectric permittivities was shown.     

Technical aspects of dielectric spectroscopy measurements of liquid crystals

The dielectric spectroscopy measurements were performed for antiferroelectric liquid crystalline mixture. For this purpose, the cells with ITO electrodes were prepared. It was found that it is not possible to detect some important relaxation modes in Sm A^*, Sm C^*, and Sm CA^* phases. The own cell mode (related to cell properties, i.e., capacity and resistivity) covers the dielectric response of liquid crystalline medium. Dielectric measurements in cells with gold electrodes were done to show all possible relaxations in antiferroelectric liquid crystals (LCs).     

Dielectric properties of LC mixture with induced antiferroelectric phase

Mixture of two liquid crystalline components exhibits the antiferroelectric phase in a broad temperature range at room temperatures, though the two components separately do not show an antiferroelectric phase in a temperature range applied. The dielectric spectroscopy technique combined with measurements of the selective light reflection was used for identification and characterization of the phases and subphases existing in the mixture. In the SmC*_A phase, the low frequency mode characteristic of antiferroelectric phase has been detected. In the broad temperature range between SmC*_A and SmA* phases, no relaxation mode has been detected. The soft mode registered near the phase transition to SmA* phase follows the Curie-Weiss law.     

Characteristic dielectric parameters of para-, ferro- and antiferro-electric phases of (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate

Frequency and temperature dependence of dielectric parameters of a liquid crystalline compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate under planar orientation of the molecules have been investigated in the frequency range 1 Hz-10 MHz. This compound possesses smectic paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC_A*) phases. Dielectric spectroscopy suggests the existence of a relaxation mechanism in the SmA* phase, which behaves as a soft mode. In the SmC* phase two relaxation modes are observed. One mode continues from the SmA* phase with decreasing dielectric strength and the other has characteristics of the Goldstone mode. Two dielectric relaxation modes have been observed for the SmC_A* phase. These two modes are related to the antiferroelectric ordering and the helical structure of the SmC_A* phase.     

Dielectric spectroscopy of the SmQ* phase

Liquid crystal possessing two biphenyl moieties in the molecular core and lateral chlorine substitution far from the chiral chain has been studied by dielectric spectroscopy. On cooling from the isotropic phase, the material possesses the frustrated smectic Q* (SmQ*) and SmC_A* phases. It has been confirmed by dielectric spectroscopy that the SmQ* phase can be related to the SmC_A* anti-ferroelectric phase. However, only one relaxation process has been observed in the SmQ* phase, while in the SmC_A*, two relaxations are clearly detectable. It seems that the mode found in the SmQ* can be connected with high-frequency anti-phase mode observed in the SmC_A* phase. Its relaxation frequency is similar to P_H relaxation frequency, but is weaker. The same relaxation has been observed even a few degrees above the SmQ*–Iso phase transition. Another explanation for the mode detected in SmQ* and isotropic phases can be molecular motions around short molecular axis.     

Small-signal ac response of Ba1−xLaxF2+x crystals

The small-signal ac response of single crystalline Ba_1?xLa_xF_2+x solid solutions has been studied in the temperature region 300–820 K, and in the frequency range 4×10^?2–5×10⁴ Hz. At low temperatures the frequency dispersion is dominated by processes in the bulk. Apart from the bulk conductance these processes include a dielectric response determined by broad distributions of dipole-like relaxations. The increase of the static dielectric constant with increasing solute content is explained. At higher temperatures the frequency dispersion reflects interfacial phenomena. In addition to an almost ideal interface capacitance, phenomena are observed which indicate the growth of a surface layer due to a reaction between the electrolyte, and residual oxygen and/or water vapour from the ambient.     

Dc-bias-field-induced dielectric relaxation and ac conduction in CaCu3Ti4O12 ceramics

The dielectric relaxation and ac conduction of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated at different temperatures under a dc bias. The dc bias gives rise to space charge accumulation, i.e. an electrode response, resulting in the significant increase of dielectric permittivity and dielectric loss tangent. Two Debye-like relaxations, arising from electrode and grain boundary responses, are present at low frequency with an increase of the dc bias. The electrode and grain boundary relaxations are distinguished according to the impedance spectroscopy and the frequency-dependent ac conductivity. The relaxation times of electrode and grain boundary relaxation are 0.955 ms and 0.026 ms, respectively, with a dc bias of 10 V at 328 K.     

A proposed new experimental setup for determination of refractive indices of liquids by mode line measurements

A new experimental method is proposed for measuring the refractive indices of liquids based on mode line measurements. The experimental setup consists of a dielectric substrate in which a rectangular hollow is formed containing the concerned liquid. This is covered by a trapezium-shaped prism. The liquid, confined between both basin walls and the prism, forms a wavegude. Thus, the refractive index of the liquid can be determined by measuring the propagating modes in the structure. Using this technique, the refractive indices of liquids can be measured to an accuracy of the order of 1×10^-4.     

How to determine parameters of soft mode from dielectric spectroscopy performed using cells with ITO electrodes?

Indium tin oxide (ITO) electrodes are widely used for liquid crystal applications as well as for measuring cells. Unfortunately, ITO layer possesses its own non zero resistivity R, which produces (with the cell capacity C₀) the cut-off frequency f₀ of RC₀ circuit. From this reason, dielectric spectroscopy for high frequencies cannot be performed directly. The limits of classical method of extracting high frequency losses are shortly discussed. The new method of extracting high frequency losses is used for the first time for the experimental data. The new method can also evaluate the shrinkage of measuring cell gap after filling with liquid crystal.     

Structural and chemical analysis of a model Si–SiO2 interface using spatially resolved electron-energy-loss spectroscopy

This work reports an experimental electron-energy-loss spectroscopy study carried out on a model thermal Si–SiO₂ interface. Valence-loss spectra and core-loss spectra (Si L_2,3 and O K edges) were recorded across the interface in line-spectrum mode with a high spatial resolution in a field emission gun scanning transmission electron microscope. From the analysis of the line spectra and on the basis of high-resolution electron microscopy and high-angle annular dark-field experiments, it is concluded that the interface is not sharp but extends over about three atomic planes consisting of Si and O atoms arranged in a structure evolving between crystalline SiO and SiO₂ before growing as an amorphous SiO₂ layer. In addition, from the analysis of the valence-loss spectra in terms of energy-loss function or dielectric function ε, we show that valence-electron-energy-loss spectroscopy could be a relevant alternative method for determining the electron properties, for example the bandgap, and the dielectric constant of dielectric gates on a nanometre scale.     

Conductivity relaxation in NiTiO3 at high temperatures

NiTiO₃ ceramics were prepared via the traditional solid-state reaction route. The dielectric properties of NiTiO₃ ceramics have been systematically investigated in the temperature range from room temperature to 1073 K NiTiO₃ ceramics exhibit intrinsic dielectric response in the temperature range below 400 K. Two relaxations were observed in the temperature range higher than 400 K. The relaxation activation energy is 0.95 eV and 1.17 eV for the low- and high-temperature relaxations, respectively. Our results strongly indicate that the two relaxations are related to conductivity relaxation associated with the singly and doubly ionized oxygen vacancies.     

Detection of early stage precursor during formation of plastic crystal ethanol from the supercooled liquid state: a simultaneous dielectric spectroscopy with neutron diffraction study

Transformation of deuterated ethanol from supercooled liquid into a plastic crystal or rotator phase is investigated by means of a particular experimental setup combining simultaneously dielectric spectroscopy with neutron diffraction techniques. We demonstrate that, previous to the growth of the bcc lattice of the plastic crystal phase, the formation of a precursor or intermediate phase through a liquid-liquid phase separation takes place. Once this precursor phase is formed, subsequent (plastic) crystalline nucleation and growth is expected to develop.     

Light scattering and dielectric manifestations of secondary relaxations in molecular glassformers

Photon correlation data of the molecular glass-forming materials 2-picoline, dimethylphthalate (DMP) and salol are compared with their dielectric loss spectra in the time-frequency range where the dielectric data reveal secondary relaxations. Slow secondary relaxation processes in molecular liquids are commonly studied by dielectric spectroscopy (DS) and, based on such studies, believed to be characteristics of the deeply super-cooled liquid state. However, there has been no direct experimental evidence that they are similarly detected by other experimental techniques. In the present study, we experimentally address this question for the anisotropic (depolarized) light scattering (LS). In the first approximation, DS and LS probe the same molecular reorientation dynamics, and therefore are expected to provide qualitatively similar spectra. We find however that this is not the case, namely i) the magnitude of the slow secondary relaxations is much less in LS than in DS data, which is the opposite to expectations; ii) the shape of the relaxation spectrum is qualitatively different, concerning both the main and secondary processes. We discuss possible sources of these differences in the context of related data from the literature.Received: 21 July 2003, Published online: 23 December 2003PACS: 64.70.Pf Glass transitions - 77.22.Gm Dielectric loss and relaxation - 78.35. + c Brillouin and Rayleigh scattering other light scatteringJ. Mattsson: Presently at: DEAS and Physics Department, Harvard University, Cambridge, MA 02138, USA     

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T _C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for f_NRM, δε^NRM and distribution parameter α_NRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT _C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.     

Evidence of antiferroelectric-like ordering from the study of dielectric permittivity–voltage behavior in bent core liquid crystalline material showing B2 phase

Detailed dielectric relaxation behavior of a bent core liquid crystalline material showing B₂ phase has been investigated. Only one relaxation mode is detected in the dielectric relaxation spectrum which was attributed to the long-axis rotation process. Though the field-dependent dielectric study does not reveal any other relaxation mode, the appearance of two humps in the dielectric permittivity–voltage spectrum, similar to the antiferroelectric (AFLC) case, confirms the signature of weak AFLC ordering in the B₂ phase.     

Preliminary investigation into feasibility of dissolved methane measurement using cavity ringdown spectroscopy technique

For the exploration of gas hydrate resources by measuring the dissolved methane concentration in seawater, a continuous-wave cavity ringdown spectroscopy (CW-CRDS) experimental setup was constructed for trace methane detection. A current-modulation method, rather than a cavity-modulation method using an optical switch and a piezoelectric transducer, was employed to realize the cavity excitation and shutoff. Such a current-modulation method enabled the improvement of the experimental setup construction and stability, and the system size and stability are critical for a sensor to be deployed underwater. Ringdown data acquisition and processing were performed, followed by an evaluation of the experimental setup stability and sensitivity. The obtained results demonstrate that great errors are introduced when a large fitting window is selected if the analog-to-digital converter has an insufficient resolution. The ringdown spectrum of methane corresponding to the 2v ₃ band R(4) branch was captured, and the methane concentration in lab air was determined to be 2.06 ppm. Further experiments for evaluating the quantitative ability of this CW-CRDS experimental setup are underway from which a high-sensitivity methane sensor that can be combined with a degassing system is expected.     

Dielectric and molecular dynamics study of the secondary relaxations of poly(styrene-co-methylmethacrylate) copolymers: Influence of the molecular architecture

The effect of the structure of copolymers (random, alternate or diblock) on their dynamics has been studied by dielectric spectroscopy. Six copolymers of styrene and methyl methacrylate (three diblocks, one alternate and two random) have been studied. The results show that the sub- T _g transitions of the diblock samples can be described by one asymmetric Havriliak-Negami (HN) function, while two are necessary for the rest of the copolymers (β and γ relaxations). The characteristic times of the sub- T _g relaxations show an Arrhenius temperature dependence and there is a strong coupling of the α and β relaxations at high temperatures. The deconvolution of the merging relaxations has been made in the framework of the Williams Ansatz set out in terms of Havriliak-Negami distributions. Because the 2D ²H-NMR results excluded any significant contribution from the rotation of the methoxy group of the methacrylate group around the C-OCH₃ bond, the γ relaxation may be assigned to the rotation of the methyl methacrylate group in a styrene-rich environment. The Molecular Dynamics simulations of a poly(methyl methacrylate) homopolymer and of the alternate copolymer are in qualitative agreement with the experimental results, although they predict smaller values for the activation energy of the sub- T _g relaxations.     

Local structure and orientational ordering in liquid bromoform

The neutron diffraction data of liquid bromoform (CHBr₃) at 25°C was analysed using the Empirical Potential Structure Refinement technique in combination with H/D isotopic substitution. Compared to liquid chloroform (CHCl₃), CHBr₃ displays more spatially defined intermolecular contacts. A preference for polar stacking with collinear alignment of dipole moments is observed for the most closely approaching CHBr₃ molecules, although to a lesser extent than in chloroform. Consistent with this, and in line with dielectric spectroscopy, the Kirkwood correlation factor from the structural model of CHBr₃ is smaller than that of CHCl₃. The net antiparallel alignment of dipole moments in CHBr₃, as suggested by dielectric spectroscopy, must be due to weak but persistent long-range orientation correlations in CHBr₃, which counteract the local polar stacking.     

Dielectric spectroscopy investigations of nanostructured silicon

Dielectric spectroscopy measurements were performed on planar silicon nanostructures buried within a crystalline Si that form a nanoscale Si-layered system. An insight into the specific behavior of the free-carrier population confined in the surface potential well was then made possible. It was found that the presence or the absence of the SiO₂ passivation modifies considerably relaxation responses of the studied structures. A clear differentiation of two dielectric responses: from the same sample with and without electronic passivation allowed determination of the conduction behavior in the surface c-Si delimited by the nanoscale Si-layered system. The sample with a 100 nm thick SiO₂ layer (and an excellent quality of the SiO₂/c-Si interface) exhibits a fractional power-law dielectric response, corresponding clearly to the generalized Mittag–Leffler pattern. Simultaneously, the dielectric response of a bare sample (after the total RIE of the previously deposited SiO₂ layer, about 10 nm native SiO₂ layer and poor quality of the SiO₂/c-Si interface) is dominated by the conductivity term.