Study Of Temperature Dependence Research Articles

Studies of temperature-dependent Raman spectra of thin PtSe2 layers on Al2O3 substrate

The results of temperature-dependent Raman spectroscopy studies of thin layers of PtSe2 (1–10 monolayers) deposited on an Al2O3 substrate are disscused in this paper. A redshift of the vibrational Raman modes (Eg and A1g) is observed when the thickness of PtSe2 and temperature increase. The results allow for determining the thickness of the PtSe2 layer deposited on the Al2O3 substrate by analysing the Raman shift of the PtSe2 modes (Eg and A1g) and the screening effects on the surface vibration mode (A1g). The other original result is the determination of the stresses and doping effects in PtSe2 for the considered range of layer thicknesses by analyses of the correlative plot. Finally, the thermal dependences of the Raman spectra are discussed regarding Raman shifts and intensity. The atomic force microscopy measurements show the presence of residual contamination with surface densities varying between samples.

Luminescence properties of Tb3+/Eu3+ ions activated LiLaSiO4 phosphors for solid-state lighting and flexible display applications

The synthesis and luminescence properties of Tb3+ and Eu3+ single- and co-doped LiLaSiO4 (LLSO) phosphors with energy transfer mechanisms were reported. The rare-earth ions doped LLSO phosphors were synthesized via a solid-state reaction method and their physical characterizations such as phase purity, surface morphology, and elemental analysis were investigated systematically. For the Tb3+ and Eu3+ single- and co-doped LLSO phosphors, their photoluminescence (PL) properties were studied in detail. The LLSO:Tb3+ and LLSO:Eu3+ single-doped phosphors showed superior red and green emissions due to 5D4→7F5 and 5D0→7F2 in the optimized characteristic electronic transitions under ultraviolet (UV) irradiation, respectively. The luminescence performances for both the LLSO:Tb3+ and LLSO:Eu3+ phosphors were optimized at 0.125 mol of doping ion concentration. Under UV excitation, by adjusting the Eu3+ ion concentration in LLSO:0.125Tb3+/Eu3+ phosphors, the tunable emissions from green to red via yellow-orange were obtained. Thus, the tunable emission spectra and luminescence decay lifetimes with increasing the doping content of acceptor ions (Eu3+) confirmed an efficient energy transfer from Tb3+ to Eu3+ ions in the co-doped samples. The critical distance between Tb3+ and Eu3+ was determined to be 13.26 Å and the energy transfer mechanism from Tb3+ to Eu3+ ions was demonstrated to be a dipole-dipole mechanism interaction. In addition, the temperature-dependent PL study was carried out for the LLSO:0.125Tb3+/0.025Eu3+ co-doped phosphor sample and it showed better thermal stability against the temperature quenching. The Tb3+ and Eu3+ single- and co-doped LLSO/polydimethylsiloxane (PDMS) flexible composite films were fabricated by mixing the phosphor powders and PDMS solution. Benefiting from the effective protection of the PDMS matrix, the luminescence stability of the composite films was greatly enhanced. Based on the excellent properties of the synthesized phosphors, Tb3+ and Eu3+ single-doped and Tb3+/Eu3+ co-doped LLSO phosphors could serve as a single-phase multicolor-emitting phosphor under UV excitations.

Investigation of Peak-to-Valley Current Ratio of GaN/AlN Resonant Tunneling Diodes

Peak-to-valley current ratios (PVCRs) achie-ved in GaN/AlN resonant tunneling diodes (RTDs) are less than 2, significantly less than their counterparts, such as InGaAs/AlAs RTDs (e.g., PVCR <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 12). This has hindered their high-speed applications because the maximum self-oscillation frequency and minimum switching time generally increase and decrease with PVCR. In this article, we investigate the problem of low PVCR in GaN/AlN RTDs with both modeling and experiments. Firstly, we developed a Schrödinger equation-based solver for RTD simulations. While the solver gives reasonable agreement with the experimental current–voltage ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) characteristics, such as the positions of resonant peaks, it predicts a much higher PVCR. Then, we conducted temperature dependence studies on GaN/AlN RTDs and compared them with InGaAs/AlAs RTDs. The similarities observed suggest that sequential tunneling with phase randomization, resulting from scattering processes such as optical phonon and interface roughness scattering, is responsible for the large valley currents in GaN/AlN RTDs.

Variable Temperature Crystal Structure of Cu4TiTe4

AbstractA variable temperature single crystal X‐ray diffraction study of the ternary compound, Cu4TiTe4 is reported between 200–400 K. The temperature dependent study indicates the compound undergoes an order‐disorder phase transition at ∼320 K. Below the transition temperature, the phase that crystallizes in the primitive cubic space group P 3 m with lattice parameter of a≈5.9 Å, transforms into the two‐fold superstructure. It is modeled as a fourfold twinned rhombohedral structure. The transition is reversible. To relate the phases, a group‐subgroup symmetry relations are established. Upon decreasing temperature, the atomic ordering is noticed in the occupationally disordered Cu‐sites of Cu4TiTe4. This characteristic structural rearrangement may have a significant impact on the electronic structure of Cu4TiTe4. Hence, the detailed structural characterization is supposed to be useful for further understanding of thermoelectric transport properties in the Cu4TiTe4.

Frequency dependent impedance response analysis of nanocrystalline ZnO chemiresistors

ZnO is a widely studied gas sensor material and is used in many commercial sensor devices. However, selectivity towards any particular gas remains an issue due to lack of complete knowledge of the gas sensing mechanism of oxide surfaces. In this paper, we have studied the frequency dependent gas sensor response of ZnO nanoparticles of a diameter of nearly 30 nm. A small rise of synthesis temperature from 85 °C to 95 °C in the solvothermal process, shows coarsening by joining and thereby distinct loss of grain boundaries as seen from transmission electron micrographs. This leads to a substantial reduction in impedance, Z (GΩ to MΩ), and rises in resonance frequency f res (from 1 to 10 Hz) at room temperature. From temperature dependent studies it is observed that the grain boundaries show a Correlated Barrier Hopping mechanism of transport and the hopping range in the grain boundary region is typically 1 nm with a hopping energy of 153 meV. On the other hand, within the grain, it shows a change of transport type from low temperature tunneling to beyond 300 °C as polaron hopping. The presence of disorder (defects) as the hopping sites. The temperature dependence of f res agrees with different predicted oxygen chemisorbed species between 200 °C to 400 °C. As opposed to the traditional DC response, the AC response in the imaginary part of (Z″) shows gas specific resonance frequencies for each gas, such as NO2, ethanol, and H2. Among the two reducing gases, ethanol and hydrogen; the former shows good dependence on concentration in Z″ whereas the latter shows a good response in f res as well as capacitance. Thus, the results of frequency dependent response allow us to investigate greater details of the gas sensing mechanism in ZnO, which may be exploited for selective gas sensing.

Nigella sativa-Manganese Ferrite-Reduced Graphene Oxide-Based Nanomaterial: A Novel Adsorbent for Water Treatment.

In this study, a novel nanohybrid composite was fabricated via the incorporation of manganese ferrite (MnFe2O4) nanoparticles into the integrated surface of reduced graphene oxide (rGO) and black cumin seeds (BC). The nanohybrid composite was prepared by a simple co-precipitation method and characterized by several spectroscopic and microscopic techniques. The characterization analysis revealed that the rGO-BC surface was decorated with the MnFe2O4. The strong chemical interaction (via electrostatic and H-bonding) between the integrated surface of rGO-BC and MnFe2O4 nanoparticles has been reported. The prepared composite was highly porous with a heterogeneous surface. The average size of the prepared composite was reported in the ranges of 2.6-7.0 nm. The specific surface area of the prepared composite was calculated to be 50.3 m2/g with a pore volume of 0.061 cc/g and a half pore width of 8.4 Å. As well, many functional sites on the nanohybrid composite surface were also found. This results in the excellent adsorption properties of nanohybrid composite and the effectual elimination of methylene blue dye from water. The nanohybrid was tested for various linear isotherms, such as Langmuir and Freundlich, for the adsorption of methylene blue dye. The Freundlich isotherm was the well-fitted model, proving the adsorption is multilayer. The maximum Langmuir adsorption capacity of nanohybrid composite for methylene blue was reported to be 74.627 mg/g at 27 °C. The adsorption kinetics followed the pseudo-second-order recommended surface interaction between the dye and nanohybrid composite. The interaction between methylene blue and the nanohybrid composite was also confirmed from the FTIR spectrum of the methylene blue-loaded adsorbent. The rate-determining step for the present study was intraparticle diffusion. Temperature-dependent studies of methylene blue adsorption were also carried out to estimate adsorption's free energy, enthalpy, and entropy. The methylene blue adsorption was feasible, spontaneous, and endothermic. A comparison study revealed that the present materials could be successfully prepared and used for wastewater treatment.

Energy saving, transparency changing thermochromism in dye-doped cholesteric liquid crystals for smart windows

An electrically switchable smart window is made from a dye-doped Cholesteric liquid crystal (CLC). In this work, the impact of coumarin dye (0.3 and 1 wt%) on the structural and electro-optical properties of CLCs that reflect IR and blue light (Blue) was explored. The results suggest that high transmittance in Visible and NIR useful to preserve interior transmittance as well as heat suited for all seasons. Temperature-dependent microscopic studies confirmed the homogenous solubility of the dye in CLCs. The fabricated smart window was observed to show three switchable modes, comprising planar (P), focal-conic (FC), and homeotropic (H) states, depending on the applied voltages (1 kHz). The structural observations from microscopic studies and the transmission spectra of pure and dye-doped composites with and without the application of voltage are consistent. The visual transmittance of dye doped CLC increased from 84% in the planar state (OFF state) to 18% in the focal conic state (3 V/µm) and 92% in the homeotropic state (5 V/µm) with a short response time (<1ms). Using dye and CLCs, we demonstrated the functionality of the device as smart windows for energy saving, privacy windows as well as for authenticated/unauthenticated QR codes. To investigate the impact of both pure CLC and dye-doped CLC devices on interior temperature management, a solar simulator device was used. The findings demonstrate that the device can be used as a smart window to regulate the internal temperature throughout the year, cutting down on the expense of operating artificial lighting, heating, and cooling systems for buildings. In comparison to pure CLCs, coumarin dye-doped CLCs showed high transmittance in the transparent and homeotropic state and a low transmittance in the scattering state when an electric field is applied and it also exhibited a good stability over repeated switching cycles.

Structural and lattice dynamics investigation of double luminescent nanosized hybrid Gd2O3

A series of cubic Yb3+/Eu3+ co-doped Gd2O3 using melamine as a complexing ligand have been prepared by low-temperature treatment at 200 °C. Structural and lattice dynamics have been done using XRD and Raman analysis. Photoluminescence analysis of the co-doped Gd2O3 samples is performed at 220, 275, and 980 nm excitations and the plausible mechanism is discussed in detail. The Eu3+ ions asymmetric ratio is calculated as 2.254 in Yb3+ co-doped Gd2O3 lattice. Luminescence lifetimes oscillate between 3.83 and 739 µs. Spectral parameters and J-O intensity parameters are calculated for all co-doped Gd2O3 NPs. Electric-multipole interaction is the energy transfer mechanism responsible for concentration quenching with a critical distance of 11.45 Å in Gd2O3: Yb3+(20 mol%), Eu3+(5 mol%) nanoparticles (NPs) on 980 nm excitation. The sample emits near-white, blue, and orange-red light emissions and can be utilized for bio-labeling applications. Temperature-dependent studies on Gd2O3: Yb3+ (20 mol%), Eu3+ (5 mol%) NPs are performed in the temperature range of 303–478 K. The developed NPs exhibit paramagnetism with a value of 1.580 emu/g owing to Gd3+ intrinsic magnetic moments.

Study of energy gaps and their temperature-dependent modulation in LaCrO3: A theoretical and experimental approach

In the present study, the origin of three energy gaps of lanthanum chromium oxide, one arises due to the charge-transfer gap between O-2p and Cr-3d and two arises from the d–d transitions, is analyzed in detail using the diffuse reflectance spectroscopy technique. The spin allowed transitions, such as 4T1(P)→4A2, 4T1(F)→4A2, and 4T2→4A2, and the spin and parity forbidden 2E →4A2 transitions are depicted using a Tanabe–Sugano (T–S) diagram and an absorption spectrum. The high crystal field strength of 3.27 obtained from the T–S diagram provides high directional emission and makes the system suitable for near infrared lasing applications. Moreover, the investigations into the variation of a charge-transfer gap with temperature will provide insights into the modification of numerous optical properties toward development of optoelectronic devices. Using this temperature-dependent diffuse reflectance spectroscopy studies, we have obtained the important optical parameters, such as Urbach energy (Eu) and Urbach focus. Furthermore, first-principles calculations are carried out in order to validate the experimental findings on LaCrO3. The experimental results are in consonance with the charge-transfer gap obtained from the theoretical calculations. Furthermore, the bandgap variation with temperature is fitted using Varshni's relation, and the Debye temperature is calculated.

Giant dielectric constant, magnetocaloric effect and spin-phonon coupling in EuTbCoMnO6 semiconductor

Transport, dielectric, magnetic and Raman studies of polycrystalline double perovskite EuTbCoMnO6 have been performed. Temperature-dependent resistivity follows the variable range and small polaron hopping mechanism. The temperature-dependent dielectric study shows the usual frequency-dependent step-like behavior. It also shows thermally activated relaxor peak in loss curves with giant dielectric constant near room temperature. The temperature-dependent magnetization demonstrates a second-order phase transition around 113 K. The critical behavior near the magnetic transition has been systematically investigated. The Widom scaling relation and magnetic entropy change calculation have verified the reliability of critical parameters. Maximum magnetic entropy change around the magnetic transition temperature shows the magnetocaloric effect. The estimated values of critical parameters are close to the value of the theoretical mean-field model. Temperature-dependent Raman study shows spin-phonon coupling in the system. The appearance of the magnetocaloric effect and colossal dielectric constant shows that these materials are interesting for real-life applications.

Effect of Sm3+ and Yb3+ doping on the structural and magnetic properties of La2−xRExNiMnO6 (x = 0.1–0.7) double perovskites

The doping effect of samarium (Sm) and ytterbium (Yb) at the La sites of La2NiMnO6 (LMNO) double perovskite on the structural and magnetic properties was studied. The rare-earth (RE = Sm, Yb) ion-doped La2-x RExNiMnO6 (x = 0.1–0.7) double perovskites were synthesized using sol-gel auto combustion route. The X-ray diffraction patterns of the Sm- and Yb-doped samples indicated single-phase monoclinic symmetry (P21/n space group). The Raman spectra provided complementary information of structural phase stability. The antisymmetric (∼528 cm−1) and symmetric (∼670 cm−1) stretching modes of vibration and a shoulder at wavenumber ∼250 cm−1 of the Raman spectra were consistent with a monoclinic phase. The X-ray photoelectron spectroscopy revealed the presence of La3+, Sm3+, Yb3+, Ni3+, Ni2+, Mn3+, and Mn4+ ions and improvement of the ordering of Ni2+ and Mn4+ ions in monoclinic structure of the Sm- and Yb-doped double perovskite structure. The temperature and magnetic field dependent magnetization studies revealed ferromagnetic nature with Curie temperature (TC) in the range of 205–265 K. The increase of Sm and Yb doping at the La sites of LMNO decreased TC and the presence of an additional weak magnetic transition at low-temperature range (∼100–150 K). The downturn feature of magnetization curves for a low-temperature regime (≤50 K) indicated a decrease of ferromagnetic exchange interactions and was a signature of spin freezing behavior in Sm- and Yb-doped LNMO samples.

Spectral fingerprints of individual Mn2+ impurities and Mn2+ pairs in magic-sized nanoclusters.

The chemical synthesis of (CdSe)13 magic-sized clusters (MSCs) allows the replacement of host atoms by individual transition metals such as Mn. By analyzing the spectral fingerprints of the Mn2+ photoluminescence (PL) in MSCs with different dopant concentrations, we are able to distinguish between single Mn2+ ions and coupled Mn2+ pairs. In case of Mn2+ pair emission, temperature-dependent studies show a pronounced red shift, followed by a distinct blue shift of the PL energy upon heating. This is related to a spin ladder formation of the ground and excited states due to Mn2+-Mn2+ exchange interaction at cryogenic temperatures, which is assumed to vanish at higher temperatures. In contrast, single Mn2+ ion PL exhibits a unique redshift with increasing temperature, which can be attributed to a particularly strong coupling to vibronic modes due to the ultimate small size of the MSCs.

Mesomorphism and luminescence in coordination compounds and ionic salts based on pyridine-functionalized β-diketones. Influence of the pyridine nitrogen position

Pyridine-functionalized β-diketones with the pyridine nitrogen atom at 3 or 4 position (L3N and L4N, respectively) have been used as building blocks to obtain two new families of compounds with mesomorphic and luminescent behaviors. The N-pyridine coordination of the β-diketones to zinc(II), palladium(II) and platinum(II) metal centers has allowed to synthesize neutral coordination compounds of the type [MCl2(LZN)2] (M = Zn(II), Pd(II), Pt(II); Z = 3, 4). Particularly, coordination of L4N towards Pt(II) gives rise to the cationic species [Pt(NH3)2(L4N)2]2+. Otherwise, protonation of the pyridine moiety was strategically used to obtain a novel series of pyridinium ionic salts of the type (HLZN)nA (n = 1, A = Cl; n = 2, A = MCl4, M = Zn(II), Pd(II), Pt(II); Z = 3, 4). The neutral or ionic nature of the complexes, as well as the β-diketone ligands and the metal center in both the neutral and anionic complexes, has a great influence on the mesomorphic behavior. Thus, columnar or smectic mesophases can be selectively formed. Single-crystal and temperature-dependent powder X-ray diffraction studies have been performed to analyze the molecular packing in the crystal and the supramolecular ordering in the mesophase. All the aforementioned factors also play a key role on the luminescence properties, the Zn(II) coordination compounds and ionic salts being the best candidates as bifunctional materials.

Temperature and frequency-dependent electrical transport studies of manganese-doped zinc ferrite nanoparticles

Manganese substituted zinc ferrite nanoparticles (Zn1-xMnxFe2O4, x ≤ 10%) have been successfully synthesized by the co-precipitation technique. The x-ray diffraction technique was employed to determine the structure of nano-ferrites at room temperature. SEM, EDX, and FT-IR spectroscopy were used to establish surface morphologies, elemental composition, and phase formation. Impedance spectral analysis was carried out at various temperature ranges varying from 600 K to 870 K in the frequency range of 100 Hz to 2 MHz. The dielectric parameter measurements were used to compute the types of conduction in the synthesized samples. The significance of the distribution of cations between A and B sites was analyzed using frequency and temperature-dependent ac electrical transport studies of the synthesized nanoparticles. The conduction mechanism, overlapping large polaron tunneling (OLPT) was found in Zn1-xMnxFe2O4 nano-ferrite system.

Dynamic memory tuned by frequency in a homologous thermotropic liquid crystal.

With scaling of dynamic RAM and NAND memory technologies reaching a limit, there is a need for dynamic memory with high density. In this work, an investigation on existence of dynamic memory feature in a frequency tuned homologous series of thermotropic liquid crystals has been carried out. A homologues series of five thermotropic liquid crystalline compounds comprising of 4-butyl benzoic acid and various alkyloxy benzoic acids are prepared, and all these mesogens exhibit only nematic phase. Liquid crystal dynamic memory storage setup consists of a conducting transparent glass cell with two indium tin oxide coated transparent glass plates acting as electrical electrodes in which the selected thermotropic liquid crystal is filled by capillary action. The temperature dependent dielectric relaxation studies enable to elucidate the relaxation frequency of each of these mesogens in nematic phase. The liquid crystal at different temperatures is excited with the relaxation frequency at various chosen fields, and the dielectric hysteresis is recorded. The magnitude of the hysteresis loop is directly proportional to the memory storage capacity. The main objective of this work is observation of dynamic memory storage in liquid crystals exhibiting nematic phase, at different frequencies in a thermotropic liquid crystal as the ingredient in a conducting polyamide buffed glass cell excited by an external electrical dc stimulus. The variation of the hysteresis loop with varying field; temperature and frequency are also studied and reported in this work.

Insights into the Strong Emission Enhancement of Molecular Rotor Thioflavin T in Aqueous Cellulose Nanocrystal Dispersion: White Light Generation in Protein and Micellar Media.

Thioflavin t (THT) is a well-known molecular rotor extensively used to detect amyloid-like structures. But THT shows very weak emission in water. In this article, we have found that THT shows very strong emission in the presence of cellulose nanocrystals (CNCs). Steady-state and time-resolved emission techniques have been used to study the strong emission of THT in aqueous CNC dispersion. The time-resolved study showed that in the presence of CNCs, the lifetime increased by ∼1500 fold compared to pure water (<1 ps). To know the nature of interaction and also the reason for this increase in emission zeta potential, stimuli-dependent and temperature-dependent studies have been carried out. These studies proposed that electrostatic interaction is the main factor for this binding of THT with CNCs. Further, the addition of another anionic lipophilic dye, merocyanine 540 (MC540), with CNCs-THT in both BSA protein (CIE: 0.33, 0.32) and TX-100 micellar (4.5 mM) (CIE: 0.32, 0.30) solutions produced excellent white light emission. Lifetime decay and absorption studies proposed a possible fluorescence resonance energy transfer mechanism in this generation of white light emission.

Study of temperature and frequency dependences of quartz glass in the microwave range

The use of glass in microwave devices requires study of the dielectric properties of the material. We present the results of studying temperature and frequency dependences of the dielectric properties of quartz glass in the microwave region. The frequency dependence of the dielectric permittivity and dielectric loss tangent were determined using high-precision devices based on resonant cavity at a fixed frequency within a range of 8 – 26 GHz under a normal temperature. The temperature dependence was obtained using resonant cavity made of platinum coated quartz glass with a low thermal-expansion coefficient and insignificant change of intrinsic parameters under heating up to 1200°C. Experimental temperature and frequency dependences of the dielectric permittivity and dielectric loss tangent were obtained using classical conduction mechanisms taking into account the structural peculiarities of quartz glass. The effect of ionic conductivity was also evaluated in quartz glass in the microwave region. The revealed changes in the temperature dependence of the permittivity do not exceed the measurement error. The results obtained can be used for practical application of quartz glass in radio-engineering devices.

Binding of CsPbBr3 Nanocrystals to MOF-5 for the Detection of Cadmium Ions in Aqueous Media

CsPbBr3 perovskite nanocrystals (CNCs) have emerged as promising candidates for optoelectronics, although instability issues have retarded their progress toward commercialization. As a solution to this core issue, herein CNCs are bound to the metal–organic framework MOF-5 to form a stable composite MOF:CNC for sensing applications in aqueous media. The electrostatic interaction of Pb2+ and Br– ions of CNCs with the terephthalate and Zn2+ ions of MOF-5 do not disturb the structural integrity of the MOFs and the photoluminescence (PL) properties of the CNCs but renders more stability to both the MOFs and CNCs in a water medium. As a proof-of-concept experiment, this water-resistant MOF:CNC probe was successfully validated for the detection of Cd2+ in a water medium. The mechanism of fluorescence enhancement during the sensing of Cd2+ is elucidated using X-ray photoelectron spectroscopy, temperature-dependent, and time-resolved PL studies. The high stability and photoefficiency exhibited by this MOF:CNC composite proves that it is a viable system for applications in aqueous media.

Investigation of dielectric relaxor-like anomaly behaviour of non-ferroelectric BaZrO3 polycrystalline ceramic

The dielectric responses and ac conductivity of non-ferroelectric BaZrO3 polycrystalline ceramic were examined in the wide ranges of frequency and temperature. The colossal dielectric behaviour was detected in the dielectric spectra, accompanied by two relaxor-like anomalies. The activation energy associated with the relaxor behaviour at low temperature regions was estimated to be 0.85 eV. The bulk relaxation and Maxwell-Wagner relaxation effect are responsible for the oxygen-vacancy induced pseudo-relaxor behaviour at low-temperature dielectric anomalies. The dielectric anomaly at 585 °C was confirmed to be a paraelectric to ferroelectric phase transition induced by the process of disordered to ordered state of oxygen vacancies. A modified empirical Cole-Cole model was proposed to explain the frequency-dependent permittivity and dielectric loss spectra. The two humps at low and high-temperature regions associated with oxygen-vacancy induced pseudo-relaxor and paraelectric to the ferroelectric phase transition, respectively, were observed in the temperature-dependent ac conductivity study.

Nematic Phase of Polar Unsymmetrical Bent-Core Molecules.

In liquid crystalline systems, the presence of polar groups at lateral or terminal positions is fundamentally and technologically important. Bent-core nematics composed of polar molecules with short rigid cores usually exhibit highly disordered mesomorphism with some ordered clusters that favourably nucleate within. Herein, we have systematically designed and synthesized two new series of highly polar bent-core compounds comprised of two unsymmetrical wings, highly electronegative -CN and -NO2 groups at one end, and flexible alkyl chains at the other end. All the compounds showed a wide range of nematic phases composed of cybotactic clusters of smectic-type (Ncyb ). The birefringent microscopic textures of the nematic phase were accompanied by dark regions. Further, the cybotactic clustering in the nematic phase was characterized via temperature-dependent XRD studies and dielectric spectroscopy. Besides, the birefringence measurements demonstrated the ordering of the molecules in the cybotactic clusters upon lowering the temperature. DFT calculations illustrated the favourable antiparallel arrangement of these polar bent-core molecules as it minimizes the large net dipole moment of the system.