Semiconducting CdS quantum dots as a guest in a four ring bent-core nematic medium: modified dielectric and elastic properties

Abstract The nematic phase of the bent-core family proves to be particularly fascinating due to its distinct properties in comparison to its calamitic counterpart. Here we revealed after the dispersion of semiconducting CdS QD an achiral unsymmetrical (4′-fluoro phenyl azo) phenyl-4-yl 3-[N-(4′-n-hecyloxy two hydroxybenzylidene)amino]-2-methylbenzoate (6-2M-F) a bent-core nematic (BCN) liquid-crystalline medium made of bent-shaped molecules with short cores and low bend angles. These molecules are on the cusp between conventional bent-core molecules and rod-like molecules, with characteristics somewhere in between, resembling hockey sticks. The threshold voltage dielectric permittivity and elastic constants of pure BCN have been considerably influenced by semiconductor nanoparticles. The threshold voltage of nano-disperse BCN has decreased with QD doping, and we know that the low operation voltage is one of the key considerations in the development of mobile liquid crystal display technology. Similar to other BCNs with smectic-like clusters previously described, the elastic anisotropy for 6-2M-F is negative; however, the insertion of CdS QDs causes the anisotropy to become very positive (bend elastic constant > splay elastic constant ). While the parallel component of permittivity has declined with doping, the perpendicular component of permittivity has increased, leading to a considerable reduction in dielectric anisotropy in the nanocomposite, which implies the effect on nematic ordering due to the interaction between CdS QDs and the LC molecules. Graphical Abstract


Introduction
Bent-core liquid crystals (BCLCs) have been the topic of substantial investigation since they have shown many of the same phases as calamitic liquid crystals (LCs), such as columnar (B1) [1,2], ferroelectric with smectic phases (SMX) [3,4], biaxial nematic [5], etc.However, the bent-core nematic (BCN) phase is less prevalent than the calamitic nematic (N) phase, which could be attributed to the twisted structure allowing for less translational mobility.The use of N-phase BCLCs in displays and other electro-optic devices has sparked a flurry of research.The electric transport properties (dielectric constant and conductivity) and the elastic properties of bent-core nematic liquid crystals (BCNLCs) are responsible for this occurrence.According to a study of electric fieldinduced instabilities (electro-convection), the creation of patterns in nematic liquid crystals (N LCs) is dependent on dielectric permittivity, electrical conductivity, and the initial direct orientation of the LC [6].Nematic liquid crystals (N LCs) display numerous electro-convection patterns as seen in the standard model of electro-convection due to a strong connection between the orientational degrees of freedom, flow field, and an induced electric field.However, BCNLC molecules exhibit only three nonstandard electro-hydrodynamic instabilities that are exempt from the standard model [7,8].Ions in LCs have also been shown to cause various instability patterns [9].As a result, many research projects have been undertaken to investigate the electro-optic phenomena displayed by BCNLCs and their composites.
Because of their size-tunable features, quantum dots (QDs) are contributing significantly to nanotechnology and nanoscience research in practically all fields.The dispersion of micro-or nanosized guest particles in an LC host has gotten a lot of attention among non-synthetic processes, especially because of its potential to tailor different physical properties of LCs for creating new and superior devices [10,11].Using carbon nanotubes (CNTs)-doped BCNLCs, the difficulties of forming sticky pictures in pure LC displays were substantially alleviated.The residual dc according to hysteresis investigations of voltage-dependent transmittance and capacitance under ac and dc electric fields is associated with an issue with images sticking on liquid crystal displays, which was significantly reduced as a result of the ion trapping by CNTs [12].The external field has been used to regulate the direction of doped CNTs using the self-organizing and self-reorientation capabilities of LCs [13].These guest particles have been shown to impact dielectric anisotropy [14], and LC phase transition temperatures [15], boost optical diffraction or beam coupling efficiencies [16,17], and affect the Freedericksz transition properties [18].Thus, the doping method is favorable to both LCs and doped nanoparticles, resulting in a wide range of applications in sensors, light-steered electric switches, electrically controlled in-plane switches, and other areas.
Due to its substantial direct band gap, high optical absorption, and outstanding stability, cadmium sulfide (CdS) is one of the fundamental technical materials employed as a group II-VI semiconductor compound [19,20].Solar cells [21,22], photosensors [23], DNA sensors [24], gas sensors [25,26], and other electronic and photonic devices are just a few of the numerous potential uses for CdS nanoparticles.A superior photovoltaic (PV) performance has been shown by the mixed coating of poly(3-hexylthiophene) (P3HT) and CdS-covered cellulose acetate (CA) fibers that dry quickly [27].The potential for CdS nanoparticles as a humidity sensor has been further explored due to their electrical response being highly sensitive to changes in relative humidity [28].The quantum confinement effect is used to explain why the dielectric constant of CdS nanoparticles is substantially higher than that of bulk CdS [29].The recent development of photoelectrochemical (PEC) detection techniques based on CdS NPs, particularly those related to the direct and indirect interactions of NPs with target analytes, is highlighted in a great review article by Ibrahim et al. [30].Chitosan-coated CdS NPs are excellent for in vitro, straightforward bio-imaging applications since fluorescence microscopy investigations have shown that they glow in the right amounts [31].
The semiconductor nanoparticles must now be incorporated into novel electric and optoelectronic devices to fully realize their benefit for future practical applications.The driving voltage of the twisted nematic liquid crystal display (TN-LCD) devices made from CdS-doped 5CB decreased with the degree of CdS nanoparticle doping increased [32].For the use of one-dimensional semiconductor nanostructures in optical switches, integrated photonic devices, and electrochromatic devices CdS nanorods embedded LC cells open a new route [33].The threshold voltage and doping concentration under LCD configuration were predicted by computer simulations based on the liquid crystal elastic continuum theory and the hypothesized model that nanoparticle doping produces the lower-order parameter (S) in liquid crystal bulk materials and it has also been verified experimentally [34].Using aromatic azomethine monomers and CdS nanoparticles, researchers have created CdS nanocomposites and predict that when the concentration of CdS nanoparticles increases, the photoluminescence of these nanocomposites would rise [35].For CdS-doped ferroelectric liquid crystals (FLC), an anti-ferroelectric to ferroelectric transition is expected rather than a ferroelectric to paraelectric transition [36].The development of more effective holographic materials for dynamic data processing applications resulted from the influence of CdS and CdSe QDs on N LCs [37].The newly proposed CdS nanoparticle-based random lasing devices demonstrate significant advancements in lasing and should spur revitalized interest in CdS NPs as an emitting material [38].By spreading a BCLC compound, it was possible to manipulate electrical and optoelectronic devices at the nanoscale thanks to the semiconductor CdS nanowires' tailored physical properties [39].The homogenous dispersion of various nanomaterials as a dopant into liquid-crystalline compounds yielded the LC-based hybrid nanocomposite materials, which appear to be particularly interesting for use in smart switchable display systems [40][41][42][43].
In this document, we examined how bent-core nematic liquid crystals' (BCNLCs) dielectric and elastic characteristics (Frank constants) are affected by the dispersion of a semiconductor quantum dot (CdS QD).The host BCN sample is a short core, reduced bend angle possessing polar fluoro substituent in the longitudinal direction and methyl group in bent direction i.e., (4 0 -fluoro phenyl azo) phenyl-4-yl 3-[N-(4 0 -n-hecyloxy 2hydroxybenzylidene) amino] À 2-methylbenzoate (6-2M-F).The effect of a doped single-wall carbon nanotube (SWCNT) on the viscosity, dielectric and elastic properties of the same BCN LC has been analyzed in one of the previous reports [44].To effectively use these LCs in displays and other nanotechnology-related fields, the effect of the quantum confinement property of CdS QD on them is examined here.Dielectric relaxation spectroscopy, polarization optical microscopy (POM), and electro-optic method have been used to characterize the BCN LC.The blend of 0.1 wt.% BCN þ CdS QDs has decreased the threshold voltage for the Freedericksz transition and dielectric anisotropy reduced after nanodispersion.With the 0.1 wt.% nano mix, the elastic anisotropy, which is negative for pure BCN, is now positive.

Experiment
The BCN sample employed in the experiment is (4 0 -fluoro phenyl azo) phenyl-4-yl 3-[N-(4 0 -n-hecyloxy 2-hydroxybenzylidene) amino]-2-methylbenzoate and has denoted by 6-2M-F.The synthesis and detailed characterization of this compound 6-2M-F are reported elsewhere [45].It consists of bent-shaped molecules with short cores, a reduced bend angle of 144 possessing polar fluoro substituent in the longitudinal direction, and methyl group in the bent direction.With a heating/cooling rate of 5 C=min, the phase transition temperatures and corresponding enthalpies of 6-2M-F have been measured using differential scanning calorimetry (Perkin-Elmer Pyris-1 system; Perkin-Elmer (India) Pvt Ltd., Kolkata, India) (Figure S1).Enantiotropic nematic phase with a thermal range of $ 31 and $ 80 C is observed in the heating and cooling cycles respectively.The molecular structure, phase transition temperature, and nematic phase range of the host BCNLC is shown in Table 1.
The size of the CdS QDs has confirmed by taking one drop of the thoroughly sonicated solution on a carbon-coated copper grid to view the morphology using JEOL-JEM, 2010, Transmission Electron Microscope (TEM) at 200 kV (Figure S2).The absorbance spectra of the used CdS QDs were obtained using a Shimadzu 2550 UV À vis spectrophotometer (Figure S2).The synthesis and detailed characterization of this compound CdS QDs are reported elsewhere [46].The following describes how CdS quantum dots were dispersed in a 6-2M-F liquid crystal.A non-polar solvent, such as chloroform, is used first to disperse the quantum particles.This solution is added to the LC in a weight ratio of 0:1%: The mixture is then subjected to an ultrasonic treatment that lasts around three hours at a temperature above the clearing point.This leads to the creation of a uniform dispersion.The solvent is then forced out of the composite by placing it in a vacuum for 24-48h: When ready for characterization, the composite is subsequently placed into the cell.
Planar cells with a thickness of 5 lm and an indium tin oxide coating measuring 5 mm by 5 mm with an antiparallel rubbing surface (Instec Inc., Boulder, CO, USA) were used to fill the BCN-CdS composite and pure BCN.The filling was carried out by capillary action at a temperature significantly higher than the host BCNLC's nematicisotropic transition temperature.Leica DMLP polarizing microscope was used to take optical measurements.With the help of the Instec HCS-302 hot stage, the temperature of the cells is carefully controlled (60:1 C).We employed an impedance gain analyzer E4990A (Keysight Technologies) with a frequency range of 20 Hz to 10 MHz for dielectric analysis.The dielectric and C-V characteristics of pure BCN and nanocomposite were examined to research the impact of doping QDs in the liquid crystal medium.For C-V characterization, the impedance analyzer has used a DC bias voltage with an AC voltage of 1 V, 5 kHz.By doing so, the ionic contribution is removed and a strong enough electric field is created for molecular reorientation.From 0 to 25 V, the bias voltage was adjusted in 0.2 V steps.Temperatures between 95 and 145 C were used to calculate capacitance values with voltage parameters.Capacitance and conductance variations as functions of frequency were measured at each temperature throughout the liquid crystalline range at a step of 5 C in order to characterize the dielectric.Due to the overwhelming influence of the ITO coating's finite sheet resistance on the glass plates, the measurements in the high-frequency range have been restricted to 10 MHz.

Results and discussions
With the help of visual inspection, using a polarized optical microscope the alignment of the LC and the dispersion of QDs was verified (Figure 1a, b).With nanodispersions of 0.1 wt.%, no QD aggregation could be seen (Figure 1b).Interpreting the variance in the dielectric characteristics of the pristine and QD-doped samples at various The enthalpies (DH in kJ mol À1 ) and entropies (DS) are presented in parentheses.
temperatures and frequencies allows for further study.The fluctuation of the threshold voltage (V th ) with regard to temperature was investigated to evaluate the permittivity curve of the sample.The threshold voltage was calculated using extrapolation of the C-V curve at each temperature at the Freedericksz transition (Figure S3). Figure 2a illustrates how the threshold voltage (V th ) of pure BCN and its nanocomposite rises quickly with decreasing temperature throughout the temperature range and exhibits a decreasing character close to the nematic-crystal (N-Cr) transition temperature.However, compared to pure BCN, nanocomposite has a lower V th : Key elements that affect the threshold voltage are the order parameter (S).We predicted that the local order parameter would be impacted by the nanoparticles, potentially decreasing significantly after doping.The average order parameter of doped LC materials is thereby decreased from a macroscopic perspective [32,34,47].The interaction between the CdS QD's dipole moment and the nearby LC molecules, which results in a local electric field that encourages reorientation of the LC molecules and lowers V th , maybe a possible explanation for this behavior [48].When the threshold voltage is not reached, the LC molecules are aligned perpendicular to the applied electric field and along the rubbing direction, which is known as the homogenous condition.Therefore, the capacitance and permittivity that come from this geometry have corresponding perpendicular components (C ?, e ? ).The parallel components (C k , e k ) are obtained by extrapolating the C-V plot to a higher voltage range (20-25 V), where a strong electric field causes LC molecules to reorient into a homeotropic state (Figure S3).Where De ¼ e k À e ? is used to calculate dielectric anisotropy.According to Figure 2b, pure and doped BCN both  experience increases in e k and e ?upon cooling from the isotropic transition temperature due to an increase in the order parameter (S) of the LC, but there is a modest drop in e k at the N-Cr transition temperature.As previously investigated this distinctive trait is caused by "Cybotactic clusters," or the nano-sized aggregation at lower temperatures [49,50].Such clusters' dipole moments exhibit a variety of arrangements in both the longitudinal and transverse directions.In smectic layers, the head-to-tail arrangement causes the longitudinal component of the dipole moment to align anti-parallelly while the transverse component stays in the same direction.Thus, when it cools, e k decreases while e ?increases.
As a result of doping, e ?rises while e k falls, and as a result of the cooling both the dielectric permittivity increases with decreasing temperature due to an increase in the order parameter (S) as already discussed.The dielectric permittivity of the CdS QDs generally increases with decreasing temperature at a particular frequency [29].The increasing and decreasing values of e ?and e k , respectively, for the nano disperse system as compared to the pure BCN are quite interesting from the fundamental point of view.The electric response of the semiconducting CdS QDs at different applied field strengths is may responsible for this anomalous behavior and will be a future direction for our study to understand the underlying physics behind this interesting phenomenon.The decreasing nature of e k near the N-Cr transition for the nano-dispersed system with temperature is again can be explained by the formation and growth of the cybotactic clusters by lowering the temperature.According to Figure 2c, for both pure and doped BCNLC, the dielectric anisotropy changes with temperature in the following ways: it first rises on cooling at high temperature (145-135 C), stabilizes, and then rapidly falls around N-Cr transition temperature (105-95 C).The increase in De upon cooling shows that the system's overall ordering has increased as it transitions from the isotropic phase into the deep nematic phase.However, the presence of cybotactic clusters that resemble smectic structures in the nematic phase can account for the drop in below 105 C: Cybotactic clusters increase in number and size as the sample cools.The transverse separation within these clusters is larger in comparison to the longitudinal direction, as a result, the longitudinal dipole moments prefer anti-parallel alignment leading to a decrease in e k , whereas the parallel alignment of the transverse dipole enhances the e ?: After nanodispersion, throughout the temperature range De takes a lower value as compared to the pure BCN, and signifies that the overall order of the system has disturbed.Since the measurement of De involves e k and e ?, the anomalous behavior of these parameters is already explained earlier which also explains the overall decrease of De for the nano-disperse system as compared to the pure BCN.Moreover, the isotropic dielectric properties of the spherical CdS QDs cause random disorder in the nematic media.By including these particles, the amount of LC molecules per unit volume is reduced, which lowers the dielectric anisotropy.
The following expression was used to calculate the splay elastic constant (K 11 ) of pure materials and nanocomposites at various temperatures [51][52][53]: where, e 0 ¼ free space permittivity, De ¼ dielectric anisotropy and V th ¼ threshold voltage.
Bend elastic constant K 33 is extracted following Uchida's approach [54] by fitting the C-V curve well above V th with the expression: where, n ¼ e jj e ?and k ¼ ð K 33 K 11 À 1Þ: Figure 3a-c depicts the fluctuation of the elastic constants for splay (K 11 ) and bend (K 33 ), as well as elastic anisotropy (DK ¼ K 33 À K 11 ), with respect to temperature.Due to the improved ordering in the nematic medium, both elastic constants for the pure BCN rise with cooling, become saturated, and begin to fall somewhat at the N-Cr transition (Figure 3a, b).Further investigation is necessary to understand the decrease in K 11 and K 33 below 105 C, which may be caused by the emergence of the twist-bend nematic phase.According to past reports on bent-core nematics, the elastic anisotropy is negative for pure bent-core samples [55][56][57][58].Temperature-dependent of splay elastic constants for the nano-dispersed system is almost similar to that of the pure BCN.But the splay distortion of the LC medium is quite easy after the nano-dispersion, which reduced the overall K 11 values throughout the temperature range (Figure 3a).As we discussed earlier that the dispersion of QDs may influence the local ordering of the nematic medium (Moreover, K 11 / S 2 , where S is the order parameter of the system.)which results in an overall reduction of threshold voltage and dielectric anisotropy of the nanocomposite.And from Equation (1) we can see that the lower value of threshold voltage and dielectric anisotropy reduced the value of the splay elastic constant for the nanocomposite.On the other hand, the bend elastic constant shows an increasing behavior with decreasing temperature with a decreasing nature near the N-Cr transition.The higher and extremely high values (at low-temperature region) of K 33 for the nanodispersed system as compared to the pure BCN signify that the bend distortion is not favorable after nano-dispersion (Figure 3b).The weakening in the coupling of the molecules' bent shapes with bend distortion, which leads to an increase in bend stress, is thought to be the cause of the surge in K 33 : As a result of nano-dispersion elastic anisotropy (DK) alter its sign and takes extraordinarily high positive values at low-temperature region (Figure 3c).So, the anomalous and surprising values of both the elastic constant after nano-dispersion are interesting and also need further analysis.The dielectric relaxation of both pure and BCN-CdS composite was investigated (Figure 4a), and the spectrum had two relaxation modes, one at low frequency (P1) and the other at high frequency (P2), as was clear from previously studied LC composite [59].The polar arrangement of bent core molecules within the ferroelectric clusters causes the low frequency peak (P1) to arise.It happens at about the same frequency as ferroelectriclike switching in this compound, and the polarization that is seen depends on frequency [49].The rotation of the molecules along the short axis causes the high-frequency peak P2 (Figure 4a).The ratio of sample capacitance to air capacitance served as the basis for calculating the real part of the dielectric permittivity (e 0 ), while the product of e 0 and the dielectric loss (r=2pfC) (where C ¼ Capacitance, r ¼ Conductance, and f ¼ frequency) served as the basis for calculating the imaginary component (e 00 ).The superposition of the conductivity contribution and the Havriliak -Negami fit function [60] describes the frequency dependence of the complex dielectric permittivity under the given conditions.The following extended Havriliak-Negami function equation was used to obtain characteristic dielectric parameters such as relaxation frequency and dielectric strength from the dielectric data: where e 0 is the vacuum permittivity (8.854 pF/m), r 0 is the conduction parameter, x is the angular frequency, e k is the dielectric strength, s k is the relaxation time of each individual process k engaged in dielectric relaxation.Exponents a and b are empirical fit parameters that, respectively, represent a symmetric broadening and a nonsymmetric broadening of the relaxation peaks.Free charge carriers' mobility in the sample is described by the first term on the right side of Equation (3).The dc conductivity resulting from charge accumulation at the interfacial layers is nonlinear, and this nonlinearity is determined by the exponent s of the angular frequency.In the case of an Ohmic behavior (s ¼ 1), r 0 is the Ohmic conductivity of the smectic material.The liquid crystal is modeled as a lattice of molecules limited by a potential field imposed by its nearby molecules.The volume of the entire mass in an LC medium that is not taken up by the LC molecules themselves is referred to as free volume.Generally speaking, it can be described as the space or pores inhabited between the LC molecules.When QDs are added, the material becomes more amorphous and the tightly packed pure BCN molecules are subsequently less tightly packed.Due to the greater free volume that is available in this environment, polar cluster movements accelerate, which results in the low-frequency peak changes to the high-frequency side (Figure 4a).The low-frequency peak on the CdS quantum dot dispersion shifts to the high frequency side, and the high-frequency peak moves to the low frequency side.This peculiar behavior may be brought on by the exponent s or by the presence of CdS, a semiconductor with a lengthy relaxation period that increases the conductivity of the composite.For peak P1, cooling causes a gradual decline in the dielectric strength and relaxation frequency (Figure 4b).This could result from the compound 6-2M-F becoming more rotationally viscous as it cools from isotropic temperature, which prevents the clusters of molecules from moving collectively.

Conclusions and future aspects
The effects of CdS-QD dispersion in a non-symmetric fluorinated four-ring bent-core nematic liquid crystal (BCNLC) are discussed.Doped with inorganic semiconducting quantum dots has an impact on the anisotropic properties of this BCNLC.The dielectric properties of the composite are a result of the dipolar, space charge polarization that these QDs exhibit in the presence of an external field.While the threshold voltage (V th ) and dielectric anisotropy (De) decrease with doping due to the decrease of the average order parameter of doped LC materials, on the other hand, the elastic anisotropy (DK) rises and becomes positive.Since the nanoparticles serve as filler materials, bending deformation is more challenging in nano-dispersed systems.On the other hand, splay deformation is quite favorable for the nano-disperse system which helps to alter the sign of DK from negative to positive.This fundamental inquiry reveals the influence of semiconductor quantum dots dispersion in bent-core nematic media and the ensuing alteration in elastic and dielectric characteristics, both of which have never been explored before.This simple investigation has identified significant changes in the host's elastic, dielectric, electro-optical, and ionic characteristics.Memory effect, electrooptical switching, and polar or chiral features on the nanoscale in composite combinations of BCN þ CdS QDs are some of the new resources that could combine to be beneficial and spark new technological interest.The usage of bent-core materials for creating innovative functional materials would be made easier by modulating the elastic characteristics of BCN by doping it with a very small amount of CdS QDs.

Table 1 .
Molecular structure and phase sequence of compounds 6-2M-F.Phase transition temperatures ( C) and nematic phase thermal range of the compound 6-2M-F were recorded for second heating (first row) and second cooling (second row) cycles at 5 C=min from DSC and confirmed by polarized optical microscopy.CompoundPhase transition temperatures ( C)

Figure 2 .
Figure 2. Temperature dependence of (a) threshold voltage, (b) dielectric constant, and (c) dielectric anisotropy for pure BCN and its nanocomposites.

Figure 4 .
Figure 4. (a) Dielectric spectra at 115 C and (b) temperature dependence of dielectric strength (de 1 ) and relaxation frequency (f R1 ) of peak P1.