Andrei Th. Ionescu

Optical and electrical characterization of a gold nanoparticle dispersion in a chiral liquid crystal matrix

We report on the effect of gold nanoparticle (Au NP) dispersion in a chiral nematic liquid crystal (LC). Polarized optical microscopy and X-ray diffraction measurements evidence the insurgence of an order change in the LC host. Moreover, a comparative analysis based on dielectric and voltammetric spectroscopies performed on pure LC and on Au NP-doped LC shows that Au NP’s presence besides affecting LC order influences its electric properties: ion conductivity results importantly reduced, and beyond a threshold value of the applied field electrophoresis phenomena are induced.

Effects of Gold Nanoparticle Dispersion in a Chiral Liquid Crystal Matrix

In this article, we report experimental studies on the effect of gold nanoparticle dispersion in a cholesteric liquid crystal matrix. Besides deeply affecting liquid crystal structural order upon inducing phase transition toward unexpected smectic-like phases, the presence of nanoparticles causes important changes in the liquid crystal electric properties, increasing liquid crystal conductivity and influencing ions diffusion.

Dielectric investigations on a bent-core liquid crystal

Dielectric measurements on a bent-core liquid crystal were carried out in the frequency range from 10 mHz to 100 kHz in planar aligned cells. Four relaxation ranges were detected during heating condition: two in a low frequency range of a few hertz probably due to conductivity and interface relaxation phenomena, another between 10 and 20 Hz, and another one in a range between 10 kHz and 100 kHz in smectic as in nematic and isotropic phases. The third relaxation response is no more visible during cooling conditions. Dielectric increments, distribution parameters, and relaxation frequencies have been evaluated at different temperatures by fitting data with Havriliak-Negami (H-N) relaxation function, which is an empirical modification of the Debye relaxation model. The presence of a relaxation response between 10 and 20 Hz and the relatively great values of the permittivity could suggest the presence of a ferroelectric response due to the presence of cybotactic clusters.

Fast electro-optic switching in nematic liquid crystals

The switching between on and off states of nematic liquid crystal pixels can be controlled with the well-known electro-optic effect. This effect however presents a fast response at switching on the electric field but a slow response at switching it off. Here we show a suitable choice of materials, cell geometries, surface preparations, and time dependence of the applied voltage that leads to a switch off response as fast as that at switch on. This is due to the particular conductivity mechanism in polymers with aromatic rings leading to rectifying properties when deposited on top of indium tin oxide surface. A transient negative charge at polymer–liquid crystal interface favors a faster and stronger planar orientation of the nematic molecules diminishing drastically the switch off time and increasing the contrast ratio. These facts, interesting enough from a fundamental point of view, could also lead to important technological consequences.

Mechanisms leading to fast relaxation of liquid crystal cells aligned with conductive polymers

Using nematic liquid crystal cells aligned by conductive polymers, like polyaniline or doped polypyrrole, a very fast electro-optic response is observed. We show that when a switch that interrupts the voltage across the cell is placed between the cell and the ground, the largest voltage drop is on the switch in its open position. The voltage distribution among the nematic cell and the switch is evaluated. The role played in this very fast electro-optic response of the ionic charges built on the interfaces together with the redox processes among the free charges in the polymer-liquid crystal interface is also described. For an asymmetric cell (only one side covered with a conductive polymer) a rectifying effect appears. In this case a circuital model is used to mimic the steplike behavior of the transmitted light during the relaxation of the system.

Use of dielectric spectroscopy for evidencing subphases in an antiferroelectric liquid crystal

The analysis of an orthoconic antiferroelectric liquid-crystalline mixture, using dielectric spectroscopy, has revealed a plurality of ferroelectric smectic C* subphases. A great variety of relaxation responses that could indicate the presence of different SmC* subphases has been observed with the differential scanning calorimetry (DSC) technique in partial agreement with other techniques as DSC and optical microscopy. They represent smectic intermediate variants between the ferroelectric phase and the antiferroelectric one, whose study is still open and is collecting a lot of interest in academic field. The results obtained by dielectric spectroscopy were compared with other techniques as differential scanning calorimetry and polarized optical microscopy. The results demonstrate the possibility of using dielectric spectroscopy to highlight subphases that cannot be observed with conventional experimental techniques.

Influence of an elastic deformation on the scalar nematic order parameter

Abstract The influence of an elastic deformation on the scalar nematic order parameter is considered. The analysis is performed starting directly from the definition of the scalar order parameter, without any use of phenomenological model. Our calculations show that the deformation imposed on the nematic director reduces the scalar order parameter. The reduction of the order is proportional to the square of the deformation, in agreement with the result of Landau–de Gennes-like models.

Role of dopants on the electro-optic effect in nematic liquid crystals aligned with doped polypyrrole

In order to estimate electro-optical response times, measurements of the transmitted light intensity through positive uniaxial nematic thin films in planar alignment have been performed. Very fast relaxation times were experimentally found for nematic samples aligned with conductive polymers, namely, polypyrrole doped with 5 types of anions having different molecular sizes and geometries. Using undoped polypyrrole alignment layers, slower electro-optical responses were obtained. Given that the improvement in the relaxation times is induced by the presence of the anions in the polypyrrole films, the increasing of the total restoring torque of the nematic molecules to the confining surfaces—responsible for the short relaxation times—is related to the charges accumulated at the nematic-conductive polymer interface.

Statistical analyses of repolarisation current of a PZT film deposited on ITO electrode with different thermal treatments

In the vast application fields of lead zirconate titanate (PZT) thin films, of particular interest are the interaction effects occurring at the ferroelectric–substrate interface [E. Bruno, M.P. De Santo, M. Castriota, S. Marino, G. Strangi, E. Cazzanelli and N. Scaramuzza, J. Appl. Phys. 103 (2008) p.064103; S. Dunn and R.W. Whatmore, J. Eur. Ceram. Soc. 22 (2002) p.825]. Relevant for this purpose are polarity-sensitive liquid crystals (LC) cells and micro- and nanoelectronic applications [S. Marino, M. Castriota, G. Strangi, E. Cazzanelli and N. Scaramuzza, J. Appl. Phys. 102, selected for Virtual Journal of Nanoscale Science & Technology, 30 July 2007 (2007) p.013002]. The polarisation current was investigated of a PZT film (PbZr0.47Ti0.53O3) obtained by sol–gel synthesis and deposited by spin coating on an indium tin oxide (ITO) electrode. The different behaviour exhibited by such a system when the support electrode was previously submitted to a thermal treatment was attributed to the change of the electrical properties of the ITO layer. In particular, a higher negative charge in the conductive band of the ITO electrode seems to be responsible for a higher order in the ferroelectric film.

Anomalous conductivity in PZT thin film deposited on copper substrate electrode

Electrical properties of ferroelectric films are influenced by factors that include methods of synthesis and characteristics of the substrate electrode. Conductivity measurements were performed on PZT (lead zirconate titanate) thin films deposited by sol–gel synthesis on a copper electrode to investigate electric properties and isolate the principal charge carriers. A semiconducting PZT/Cu interface appears during thermal treatment, significantly influencing electric conduction. A power law, describing the transport mechanism across the PZT film, was found empirically.