Electro-optical Investigations Research Articles

Tunable optical, electro-optical and dielectric properties of eco-friendly graphene quantum dots-nematic liquid crystal composites

ABSTRACT The changes in physical properties obtained by dispersing hydrophilic carboxylated graphene quantum dots (GQDs, <10 nm), which are non-toxic, biocompatible, and eco-friendly, into a room-temperature nematic liquid crystal, 5CB (4-pentyl-4ˈ- biphenylcarbonitrile) are described. Nematic–isotropic transition temperatures, polarising microscopy, electro-optic, photoluminescence, and dielectric investigations are reported for two composites having concentrations of GQDs of 0.25 and 1.0 wt.% in 5CB and compared to pristine 5CB. At the lower concentration (0.25%) of GQDs there is an enhancement in the order parameter by 5.49%, the birefringence by 1.28%, and the activation energy for dielectric relaxation by 4.4% compared to pure 5CB. However, at the higher concentration (1%), aggregation of GQDs occurs. This is apparent in optical textures and resulted in the deterioration of optical and electrical properties compared to 5CB. The photoluminescence spectrum of both concentrations of GQDs are centred at the same wavelength; however, the 1% GQD-5CB mixture exhibits significant quenching and may be suitable for some optical devices. Overall, our results support that, in addition to promoting sustainable, heavy-metal-free, and environmentally safe technologies, a low concentration of GQDs in LCs produces composites that have superior electrical and optical properties; thus, making them suitable for a variety of photonics and display applications.

Novel antiferroelectric materials with resorcinol-based symmetrical fluorinated bent-core mesogens

The synthesis of several symmetrical bent-core compounds with two or four fluorine atoms as substituents on the calamitic units is reported. The influence of fluorine substitution concerning its effect upon the mesophase structure and switching behavior was investigated. The synthesized compounds are built from resorcinol as central core and contain seven phenyl rings linked by ester, imine and azo units with terminal decyloxy/dodecyloxy chains. The mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, electro-optical investigations and X-ray diffraction (XRD) studies. The photochemical characterisation of bent-core azobenzene compounds confirmed the structure of E isomer. The temperature range of mesophases increases upon the introduction of fluorine substituents, as compared with the corresponding unsubstituted compound. All the compounds exhibit antiferroelectric properties. Depending on the number of the F substituents, racemic SmCSPA mixtures and SmCAPA homochiral structures were identified. High values of spontaneous polarization up to 900 nC/cm2 were recorded. DFT studies were used to calculate the minimum energy molecular structures, the dipole moments and the electrostatic potential surfaces of the compounds.

Comprehensive Electro-Optical Investigation of a Ga-Doped AlN Nanowire LED for Applications in the UV-C Range

Comprehensive Electro-Optical Investigation of a Ga-Doped AlN Nanowire LED for Applications in the UV-C Range

AZO Nanoparticles-Decorated CNTs for UV Light Sensing: A Structural, Chemical, and Electro-Optical Investigation

Nanocomposites formed by aluminum-doped zinc oxide nanoparticles (AZO-NP) and multiwall carbon nanotubes (CNT) are proposed here as a promising material for UV light sensing applications, with the great advantage of operating in air, at room temperature, and at low voltage. Nanocomposite layers were prepared with different AZO:CNT weight ratios by a simple methodology at room temperature. They were characterized by means of UV-Vis spectroscopy, scanning and transmission electron microscopies (SEM and TEM), and X-ray photoelectron spectroscopy (XPS). The interaction between the two nanomaterials was demonstrated by comparing the properties of the nanocomposite with the ones shown by the AZO-NPs. Dense AZO-CNT nanocomposite layers were deposited between two metal electrodes on a SiO2/Si substrate, and the electrical properties were investigated in dark condition and under UV light irradiation. The electrical response to the UV light was a sudden current increase that reduced when the light was switched off. Several UV on/off cycles were performed, showing good repeatability and stability of the response. The mechanisms involved in the electrical response are discussed and compared to the ones previously reported for ZnO-CNT nanocomposites.

Anisotropic transport investigation through different etching depths in InAs/InAsSb T2SL barrier midwave infrared detector

In this paper, an electro-optical investigation is carried out to study the anisotropic behavior of a mid-wavelength infrared (MWIR) Ga-free InAs/InAsSb type-II superlattice (T2SL) XBn barrier detector. The main goal was to improve our understanding of the transport mechanisms in this device. Quantum efficiency measurements are performed on mesa devices with different etching depth. Absorption and lifetime measurements are also realized in order to extract the transport properties and, in particular, the hole vertical mobility. The low value obtained does not seem compatible with miniband transport. A discussion is then conducted, to shed light on the different mechanisms involved in the hole vertical transport.

Hockey-Stick Polycatenars: Network formation and transition from one dimensional to three-dimensional liquid crystalline phases

Photo switchable liquid crystalline (LC) materials are of great interest for optical and photonic applications. Herein we report the design, synthesis, and molecular self-assembly of the first examples of photosensitive hockey-stick (HS) polycatenars. Therefore, two new series of HSLCs derived from 4-cyanoresorcinol bent-core unit connected to a short azobenzene-based side arm with one variable alkoxy chain and a long ester-based wing terminated with two alkoxy chains at 3 and 5 positions of the terminal benzene ring are reported. They differ from each other in the length of the terminal chains connected to the long arm. The LC self-assembly of these HSLCs was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electro optical investigations. Depending on length of terminal chains a transition from one dimensional (1D) tilted and non-tilted smectic phases to three dimensional (3D) achiral bicontinuous cubic phases with Ia3¯d symmetry (Cubbi/Ia3¯d) upon chain elongation is observed. Moreover, achiral isotropic liquid networks were observed for medium and long chain homologues. Most of mesophases are room temperature LCs phases with wide ranges as observed in the cooling cycles, where once they are formed on heating, no sign of crystallization is detected down to ambient temperature. Finally, UV light irradiation results in fast and reversible photoinduced transformation between different types of LCs phases as well as between LC phase and isotropic liquid.

4H-SiC p-n Junction-Based Near IR Photon Source

We propose a Near Infra Red photon emitting 4H-SiC compact vertical pn diode. The “as fabricated” diode exhibits a strong visible emission at 500 nm associated with the intrinsic and/or processing induced D1 centers in the semiconductor. Thanks to opportune He <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion irradiation, we are able to obtain color centers in the junction region responsible for photon emission around 900 nm while simultaneously quenching unwanted visible emission. Electro-optical investigation was conducted by Electro-Luminescence measurements in the 300 nm -1100 nm range and by Emission Microscopy, both with pumping a current between of 0.1 and 2 mA. The proportionality of the emission intensity to the pumping current opens the interesting perspective of engineering the concentration of defects with the aim of obtaining single defects and thus single photon source devices. This is of considerable interest in low-power proximity sensors for consumer, industrial, automotive and telecommunications applications.

Electro-optic and dielectric properties of polymer networks stabilised short pitch chiral smectic C* liquid crystal

ABSTRACT 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.

Effect of intermolecular charge transfer between Ni(II)Pc and CdSeS/ZnS QD on dielectric relaxation mechanism of 5CB nematic liquid crystals in the presence of UV illumination

This paper is a report on electro-optical and dielectric parameters of Ni(II)Pc (nickel(II) phthalocyanine) and CdSeS/ZnS (cadmium selenide sulfide/zinc sulfide) quantum dot (QD) doped 5CB (4-pentyl-4′-cyanobiphenyl) nematic liquid crystal (LC) in the dark and under UV illumination condition. Electro-optical and dielectric measurements of the samples were carried out using electro-optical switching and dielectric spectroscopy methods, respectively. Dielectric anisotropy, relaxation frequency, dielectric strength, and threshold voltage values were determined using the experimental data. Investigation of dielectric properties demonstrates that relaxation frequency is decreased because of the dispersal of Ni(II)Pc and CdSeS/ZnS QD under UV exposure. The electro-optical investigation also shows that UV illumination and dispersal of Ni(II)Pc and CdSeS/ZnS QD cause a decrease in threshold voltages. Overall, results indicate that Ni(II)Pc, CdSeS/ZnS QD and UV light enhanced the electro-optical and dielectric properties of 5CB nematic LC.

Study of ferro- and anti-ferroelectric polar order in mesophases exhibited by bent-core mesogens

Ferroelectric nature in the NCybC phase and anti-ferroelectric nature in the SmC(I) and SmC(II) phases.

Design and electro-optic investigations of de Vries chiral smectic liquid crystals for exhibiting broad temperature ranges of SmA* and SmC* phases and fast electro-optic switching

Two new ferroelectric liquid crystalline compounds using 5-phenyl-pyrimidine core terminated by achiral trisiloxane chain, and chiral (S) alkyl chain on the opposite ends of their respective mesogens are designed with better characteristics of switching speed and broader temperature range.

Cybotactic nematic phase of achiral unsymmetrical bent-core liquid crystals – Quelling of polar ordering and the influence of terminal substituent moiety

Nematic phase of bent-core liquid crystals (LCs) exhibiting cybotactic clusters (NCyb) have gained significant importance owing to its promising ability to demonstrate macroscopic biaxiality and the ferronematic phase. In this context, three achiral unsymmetrical four-ring bent-core LC compounds, bearing a long alkyloxy chain and differing only in the terminal substituent moiety (methyl, chloro, nitro), are designed and synthesized followed by their optical, dielectric, electro-optic and structural investigations. The presence of NCyb in the methyl and chloro substituted compounds was confirmed via dielectric spectroscopy and X-ray diffraction observations. The absence of ferroelectric behaviour in any of these compounds, even in the cybotactic nematic phase and in presence of polar substituent moieties (chloro and nitro), is attributed to the increased alkyloxy chain length and antiparallel molecular arrangement. The density functional theory (DFT) optimized molecular structure along with the experimental observations further substantiates these findings. The study establishes that cybotactic clusters and polar end moiety, although being a prerequisite for ferroelectric-like nature, do not necessarily result in a ferronematic phase.

Observation of disordered mesomorphism in three-ring-based highly polar bent-core molecules: design, synthesis and characterisation

ABSTRACTResearch on low-temperature polar bent-core nematogens having lower molecular weight has gathered appreciable momentum by virtue of their significance in potential applications. However, the lack of availability and easy-to-perform synthesis processes appears to be the bottleneck towards their fabrication and thereby limiting their possible device applications. Hence, we have designed a new class of achiral symmetrical three-ring-based-bent shaped molecules incorporating an imine and ester linkage at the molecular bend with highly polar nitro/cyano terminal moiety exhibiting low-temperature nematic mesmorphism. The occurrence of disordered nematic mesomorphism has been confirmed by optical texture, differential scanning calorimetry scan and X-ray diffraction measurement. Dielectric spectroscopy and electro-optical investigation has also been carried out intending towards the potential applicability of the materials. Density functional theory analyses at the molecular level provide valuable information regarding the formation of the nematic mesophase and various parameters of the molecular spatial arrangement. Polarising optical microscopy study reveals the easy of alignment of these types of polar bent-core materials upon glass surface suitable for liquid-crystal-based sensing applications. Formation of mesophase with such a small bent molecule is rather difficult but we have successfully demonstrated the existence of disordered nematic mesomorphism at relatively low temperature.

Effects of molecular chirality on self-assembly and switching in liquid crystals at the cross-over between rod-like and bent shapes.

A bent-core compound derived from a 4-cyanoresorcinol core unit with two terephthalate based rod-like wings and carrying chiral 3,7-dimethyloctyloxy side chains has been synthesized in racemic and enantiomerically pure form and characterized by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical investigations to study the influence of molecular chirality on the superstructural chirality and polar order in lamellar liquid crystalline phases. Herein we demonstrate that the coupling of molecular chirality with superstructural layer chirality in SmCsPF domain phases (forming energetically distinct diastereomeric pairs) can fix the tilt direction and thus stabilize synpolar order, leading to bistable ferroelectric switching in the SmC* phases of the (S)-enantiomer, whereas tristable modes determine the switching of the racemate. Moreover, the mechanism of electric field induced molecular reorganization changes from a rotation around the molecular long axis in the racemate to a rotation on the tilt-cone for the (S)-enantiomer. At high temperature the enantiomer behaves like a rod-like molecule with a chirality induced ferroelectric SmC* phase and an electroclinic effect in the SmA'* phase. At reduced temperature sterically induced polarization, due to the bent molecular shape, becomes dominating, leading to much higher polarization values, thus providing access to high polarization ferroelectric materials with weakly bent compounds having only "weakly chiral" stereogenic units. Moreover, the field induced alignment of the SmCsPF(()*()) domains gives rise to a special kind of electroclinic effect appearing even in the absence of molecular chirality. Comparison with related compounds indicates that the strongest effects of chirality appear for weakly bent molecules with a relatively short coherence length of polar order, whereas for smectic phases with long range polar order the effects of the interlayer interfaces can override the chirality effects.

Electro-optical and charge injection investigations of the donor-π-acceptor triphenylamine, oligocene–thiophene–pyrimidine and cyanoacetic acid based multifunctional dyes

The corner stone of present study is to tune the electro-optical and charge transport properties of donor-bridge-acceptor (D-π-A) triphenylamine (TPA) derivatives. In the present investigation, an electron deficient moiety (pyrimidine), electron-rich moiety (thiophene) and oligocene (benzene, naphthalene, anthracene, tetracene and pentacene) have been incorporated as π-spacer between the donor TPA unit and cyanoacetic acid acceptor and anchoring group. The elongation of bridge usually affects the energy levels, i.e., higher the highest occupied molecular orbital (HOMO) while lower the lowest unoccupied molecular orbital (LUMO) thus reduces the HOMO–LUMO energy gap. The lowered LUMO energy levels of cyano-{2-[6-(4-diphenylamino-phenyl)-pyrimidin-4-yl]-tetraceno[2,3-b]thiophen-8-yl}-acetic acid (TPA-PTT4) and cyano-{2-[6-(4-diphenylamino-phenyl)-pyrimidin-4-yl]-pentaceno[2,3-b]thiophen-9-yl}-acetic acid (TPA-PPT5) dyes revealed that electron injected from dye to semiconductor surface might be auxiliary stable resulting in impediment of quenching. The broken co-planarity between the π-spacer conceiving LUMO and the TPA moiety would help to impede the recombination process. Moreover, it is expected that TPA derivatives with the tetracenothiophene and pentacenothiophene moieties as π-bridge would show better photovoltaic performance due to lowered LUMO energy level, higher electronic coupling constant, light harvesting efficiency and electron injection values.

Electrically tunable birefringence of a polymer composite with long-range orientational ordering of liquid crystals.

We report an optical film with electrically tunable birefringence in which the liquid crystals (LCs), mixed with the host polymer, form long-range ordering. The film was prepared through polymerization without phase separation between the LCs and polymers. Driving voltage below 30 V for full switching of birefringence is achieved in a 6 μm-thick film. Electro-optical investigations for the film suggest that the long-range ordering of the LCs mixed in the film caused by polymerization lead to rotations of the LCs as well as optical anisotropy in the film. These films with electrically tunable birefringence could have applications as flexible light modulators and phase retardation films for 2D-3D image switching.

Plasmonics in Nanostructures

Plasmonics has developed into one of the rapidly growing research topics for nanophotonics. With advanced nanofabrication techniques, a broad variety of nanostructures can be designed and fabricated for plasmonic devices at nanoscale. Fundamental properties for both surface plasmon polaritons (SPP) and localized surface plasmons (LSP) arise a new insight and understanding for the electro-optical device investigations, such as plasmonic nanofocusing, low-loss plasmon waveguide and active plasmonic detectors for energy harvesting. Here, we review some typical functional plasmonic nanostructures and nanosmart devices emerging from our individual and collaborative research works.

Electro-optic and dielectric investigations of a perfluorinated compound showing orthoconic antiferroelectric liquid crystal

We observed microsecond order response ranging from 100 to 600μs with respect to temperature for a single component fluorinated rigid core antiferroelectric liquid crystals having highly tilted phases. In the antiferroelectric region, we observed V-shaped electro-optic switching in a 4μm indium tin oxide coated homogeneously aligned cell having PS, ranging from 100 to 200nC/cm2. Dielectric studies have revealed an interesting phase that exists during heating in the temperature region between 74°C and 84°C. From dielectric results studied we calculated activation energies associated with different relaxation modes using Arrhenius law.

Reduction of optical response time for fluorescent dye doped ferroelectric liquid crystal

The electro-optical and dielectric investigation of the fluorescent dye doped ferroelectric liquid crystal (FLC) system was performed for the entire range of SmC* phase. The doping of the dichroic fluorescent dye (Benzo 2,1,3 Thiadiazole) in the pure FLC improves dielectric and electro-optical parameters. It is found that the presence of the fluorescent dye in the pure FLC changes the characteristics of pure geometry which causes the alteration in the physical parameters of the pure FLC system. The studies on the temperature dependence of different parameters for pure FLC and sample doped with fluorescent dye have been presented. In addition to this, a better optical (slow) response has also been observed for this composite system which may be useful for potential applications as slow optical response time has been a critical issue with the doped system.

Electrical and optical investigations on Pb1 − 3x/2LaxZr0.2Ti0.8O3 thin films obtained by radiofrequency assisted pulsed laser deposition

Thin films of ferroelectric relaxor Pb1−3x/2LaxZr0.2Ti0.8O3, x=0.22 have been integrated in an oxidic heterostructure for electro-optical investigations. The quadratic electro-optic behavior and optical properties have been studied by means of variable angle spectroscopic ellipsometry method in reflection mode. Birefringence values up to δΔ=0.17° have been obtained for quadratic compositions at λ=540nm and 65° angle of incidence. Structural, chemical and morphologic properties of Pb1-3x/2LaxZr0.2Ti0.8O3 (x=0.22) thin films have been investigated by x-ray diffraction and atomic force microscopy techniques. The dielectric and ferroelectric behavior has been investigated using dielectric spectroscopy and a ferroelectric test system.