Carlo Vena

Physical investigation of electrophoretically deposited graphene oxide and reduced graphene oxide thin films

Graphene oxide and reduced graphene oxide thin films are very promising materials because they can be used in optoelectronic devices and in a growing range of applications such as touch screens and flexible displays. In this work, graphene oxide (GO) and thermally reduced graphene oxide (rGO) thin films, deposited on Ti/glass substrates, have been obtained by electrophoretic deposition. The morphological and the structural properties of the samples have been investigated by micro-Raman technique, X-ray reflectometry, and SEM analysis. In order to study the optical and electrical properties, variable angle spectroscopic ellipsometry and impedance analysis have been performed. The thermal annealing changes strongly the structural, electrical, and optical properties, because during the thermal processes some amount of sp3 bonds originally present in GO were removed. In particular, the annealing enhances the Ohmic behavior of the rGO film increasing its conductivity and the estimated optical density. Moreover...

Light Depolarization Effect by Electrohydrodynamic Turbulence in Nematic Liquid Crystals

ABSTRACT A study of the depolarisation effect which occurs in a light beam passing through a thin homogenously oriented layer of nematic liquid crystal (N-(methoxybenzylidene)-4-butylaniline) driven to the turbulence by an external ac voltage is reported. The time behaviour of the transmitted light polarization degree (P) has been recorded for different values of both applied voltage and polarization of the incident light beam. The features of the DSMs regimes strongly depend on the state of polarization of the probe light beam. In particular, the transition can be evidenced by looking at P when the incident states of polarization are oriented at different angles with respect to the liquid crystal anchoring direction. Although the DSMs transition was unambiguously characterized as a transition from a 2-D structured turbulence to a 3-dimensional turbulence (DSM2), our measurements emphasize that oriented turbulent structures coexist in the latter regime.

Thermally induced modifications of the optic properties of lead zirconate titanate thin films obtained on different substrates by sol-gel synthesis

Lead zirconium titanate PbZr0.53Ti0.47O3 (PZT) thin films have been obtained by sol-gel synthesis, deposited on different substrates [float glass, indium tin oxide (ITO)-coated float glass, and intrinsic silicon wafer], and later subjected to different thermal treatments. The morphologic and the structural properties of both PZT thin films and substrates have been investigated by scanning electron microscope and their composition was determined by energy dispersive x-ray (EDX) analysis. Moreover, variable angle spectroscopic ellipsometry provides relevant information on the electronic and optical properties of the samples. In particular, the optical constant dispersion of PZT deposited on ITO-coated float glasses shows a small absorption resonance in the near IR region, not observed in PZT films deposited on the other substrates, so that such absorption resonance can be explained by interfacial effects between ITO and PZT layers. This hypothesis is also supported by EDX measurements, showing an interdiffu...

An approach to a model disordered birefringent medium for light depolarization applied to a liquid crystal device

A general model consisting of micro-wave plates is presented as a straightforward extension of Jones–Mueller calculus in order to describe the effect that undergoes a perfectly polarized quasi-monochromatic light beam transmitted through a disordered birefringent medium. The model treats the birefringent medium as an array of micro-wave plates lying side by side. The transmitted light beam results spatially divided in micro-beams which inherit from micro-wave plates different polarization states and hence, spatial depolarization is achieved. The model is applied to a liquid crystal sandwich-like cell subject to a splay-bend perturbation in the bulk. The model output is cast against the experimental data with a set of parameters throughout simulations. A good agreement is obtained between experimental data and model results.

Stokes parameter analysis of light that is temporally and spatially depolarized

We present a unified treatment of temporal and spatial depolarization in a quasi-monochromatic electromagnetic wave using Stokes parameters analysis. In our approach the Stokes parameters depend on the physical properties of light in a certain space volume. In this volume light can be divided into different light components that have the property of being homogenously polarized and that occupy different volume portions. In particular the obtained Stokes parameters are the weighted average of the Stokes parameters of these light components.

Electrohydrodynamic instabilities in doped M5 nematic liquid crystals

We report the study of the electrodynamic behavior of the nematic mixture M5 doped at several percentages by its reaction precursors 4-(n-Octyloxy)phenol and 4-Heptylbenzoic acid [1]. The influence of the doping impurities on the M5 clearing temperature has been studied by micro differential scanning calorimetry and their conductivities have been determined by impedance analysis as well as their dielectric constants. Finally we studied by polarizing microscopy the influence of the dopants on the voltage threshold for the onset of the orientational instabilities and we determined the cut-off frequency of the low frequency conductive regime.