Browsing by Author "Ravindra Dhar"

Now showing 1 - 12 of 12

Behaviour of ionic liquid adsorbed on the surface of nano silica particles and in confined system of silica matrices
(Elsevier B.V., 2020) Yogendra Lal Verma; Manish Pratap Singh; Santosh Kumar; Ravindra Dhar; Rajendra Kumar Singh
This paper presents studies on behaviour of a solid ionic liquid (IL); 1-butyl-2,3-dimethylimidazolium hexafluorophosphate [BMMIM][PF6] based ionogels obtained by two different techniques, first by physical confinement of IL into nano porous silica matrix and second by adsorption of IL on the outer surface of nano silica particles. The properties of samples have been investigated using various experimental techniques like N2-sorption measurements, TEM, DSC, TGA, and FTIR. It has been observed that the properties of IL have been affected more in confined geometry of silica pores as compared to the adsorption on the surface of nano silica particles. Loading of IL initially increases size and volume of pores while decrease the surface area by ~ 23 %. However, further increase in loading of IL does not produce significant change. This might be explained on basis of π-π stacking of imidazolium ring and an additional π-π stacking association between imidazolium cations. The Fourier transform infrared (FTIR) results show that stronger interaction of IL with the silica pore wall surface in confined matrix while weaker interaction due to the interfacial interaction of nano silica particles with the ions of the IL in surface adsorbed IL. The interactions and method of confinement of IL have been found to be the key parameters that determine the properties of confined and adsorbed IL. © 2020 Elsevier B.V.
Charge carrier generation in a polymer by using n-type doping for the improvement of electrical properties
(VBRI Press, 2016) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari; Suresh Chand
In this paper, the charge carrier generation in polymer blends by chemical doping has studied. In these studies, we employed n-type dopant molecule decamethylcobaltocene (DMC) which exhibit very strong electron donating nature. We have demonstrated that such type of doping favours the formation of charge transfer complex (CTC) and reduce the recombination probability. We have confirmed the CTC formation form the absorption spectroscopy. Further we have used transient photoluminescence spectroscopy to reveal the reduced initial recombination of charge transfer exciton. We interpret our results based on a reduced formation of emissive charge transfer excitons in doped blends, induced by state filling of immobile tail states in the polymer HOMO. © 2016 VBRI Press.
Dielectric and electro-optical properties of zinc ferrite nanoparticles dispersed nematic liquid crystal 4’-Heptyl-4-biphenylcarbonnitrile
(Taylor and Francis Ltd., 2020) Fanindra Pati Pandey; Ayushi Rastogi; Rajiv Manohar; Ravindra Dhar; Shri Singh
In this work, comprehensive results on dielectric and electro-optical properties of pristine and zinc ferrite (ZnFe2O4) nanoparticles (NPs) dispersed in host nematic 4ʹ-Heptyl-4-biphenylcarbonnitrile (7CB) systems are reported. The dielectric and EO properties of pristine and NPs dispersed (concentrations 0.05, 0.1, 0.15, 0.2, 0.3 and 0.5 wt%) into the host nematic system have been studied. We have seen uniform NPs dispersion into nematic 7CB up to 0.15 wt% concentration of NPs by the POM. The NPs aggregation starts at 0.2 wt% dispersion. It is shown that the dielectric and electro-optical properties are strongly influenced by the concentration of NPs in host nematic and applied electric field. The dielectric anisotropy is maximum for the NPs dispersed system of 0.15 wt% concentration. The Birefringence, parallel and perpendicular components of permittivity and conductivity are found to be dependent on temperature and concentration of NPs into host matrix. In pure nematic material, small memory is seen but in NPs dispersed systems noticeable memory is found. In particular, pronounced memory is found for the dispersed system of 0.3 wt% concentration of NPs. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
Effect of doping on the electron transport in polyfluorene
(American Institute of Physics Inc., 2016) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari
In this paper, electron transport of pure and DMC doped polyfluorne (PF) films have been studied at various doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J-V characteristics of doped PF were ohmic at low voltages due to thermally released carriers from dopant states. At higher voltages the current density increases nonlinearly due to field dependent mobility and carrier concentration thereby filling of tail states of HOMO of the host. The conductivity of doped fims were analyzed using the Unified Gaussian Disorder Model (UGDM). The carrier concentration obtained from the fitting show a non-linear dependence on doping concentration which may be due to a combined effect of thermally activated carrier generation and increased carrier mobility. © 2016 Author(s).
Green synthesis of Oscimum Sanctum mediated silver nanoparticles to fabricate sensors for hydrogen peroxide detection
(Elsevier B.V., 2024) Sonam Mishra; Jay Singh; Bishnu Kumar Pandey; Ravindra Dhar
In this study, silver nanoparticles (Ag NPs) of an average size of ∼15 nm have been produced using the green synthesis technique with Oscimum Sanctum leaf extract. Further, these nanoparticles have been used to prepare electrodes for electrochemical sensors to monitor hydrogen peroxide. Hydrogen peroxide sensors have many applications in different sectors, such as medical, agriculture, and industry. The optical and structural characteristics of the Ag NPs have been investigated by UV–visible absorption spectroscopy and XRD patterns. Particle size, shape, and morphology of as-synthesized Ag NPs have been analyzed using transmission and scanning electron microscopic techniques. These nanoparticles were deposited onto an indium-tin-oxide glass substrate using the electrophoretic technique, which was used as an electrode to assess the electro-oxidation of Ag NPs by cyclic voltammetry. The sensitivity of the sensor has been estimated to be 4.16 µA/mM-cm2. The limit of detection of the sensor is estimated to be 40 µM within the linear range of 5–28 mM. The lattice parameter, interplanar spacing, and crystallite size of Ag NPs have been quantified and estimated against pH variation. © 2024 Elsevier B.V.
Ionic liquid mediated nano-composite polymer gel electrolyte for rechargeable battery application
(Taylor and Francis Inc., 2020) Shalu Kataria; Yogendra L. Verma; Himani Gupta; Shishir K. Singh; Nitin Srivastava; Ravindra Dhar; Rajendra Kumar Singh
Nano-composite polymer gel electrolytes (NPGEs) based on polymer poly(vinylidene fluoride-co-hexafluoropropylene) PVdF-HFP, ionic liquid, 1-butyl-3- methylimidazolium bis(trifluoromethanesulfonyl)imide BMIMTFSI, Li-salt along with the addition of SiO2 nanoparticles have been synthesized and characterized by various techniques. Prepared NPGEs show high room temperature ionic conductivity (~10−3 S/cm) and have a wide electrochemical window (ECW) (~3.3–3.5 V). The galvanostatic charge/discharge profile was studied by sandwiching best performing NPGEs between a LiFePO4 cathode and lithium metal anode. The specific discharge capacity of the cell (Li/NPGE/LiFePO4) room temperature at 0.1C rate is found to be 138 mAh/g. © 2020, © 2020 Taylor & Francis.
Momentous past and key advancements in ionic liquid mediated polymer electrolyte for application in energy storage
(John Wiley and Sons Ltd, 2021) Shalu; Rajendra Kumar Singh; Ravindra Dhar
This review article discusses the recent advances of ionic liquid-based polymer electrolytes (ILPEs) to create innovative polymer electrolytes (PEs) and their applications in energy generation and storage. Ionic liquids (ILs) possess high ionic conductivity and are thermally, chemically, and electrochemically stable which enables their uses in electrochemical gadgets. Despite of these above-mentioned advantages, liquidus nature of ILs is the major curse for their practical application because of spillage and compactness issues. Thus, there is ever increasing need to immobilize ILs into some organic/inorganic frameworks that furnish great mechanical steadiness alongside saving the fundamental properties of ILs that can altogether give a huge scope of utilization to these materials. The ILPEs thus formed have been reported to have advantageous properties (such as high ionic conductivity, thermal, chemical, and electrochemical stability) as compared to conventionally used PEs. In this review, we have not only discussed the key advancement in the field of ILPEs but also exemplified how the adaptability of these technologically important materials especially in energy storage (rechargeable batteries) has shown momentous past and is riding toward a promising future. © 2021 John Wiley & Sons Ltd.
P-Type doping of tetrafluorotetracynoquinodimethane (F4TCNQ) in poly(para-phenylene vinylene) (PPV) derivative "super Yellow" (SY)
(Royal Society of Chemistry, 2014) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari; Suresh Chand
In this paper, we report a case of hole transport in tetrafluorotetracynoquinodimethane doped poly(para-phenylene vinylene) derivative "Super Yellow". The hole mobility of pristine and doped polymer thin films was determined by impedance spectroscopy. The increase in hole mobility upon doping was also verified by current density-voltage measurements. It was found to be increased by two orders of magnitude upon doping. The increase in hole mobility upon p-type doping was explained on the basis of extended Gaussian disorder model by measuring current density-voltage characteristics in the same devices. At low bias, J-V characteristics exhibit clear space charge limited conduction in pristine state but ohmic behaviour when doped. Further, at higher voltages the current density increases non-linearly due to the field-dependent mobility and carrier concentration by filling of tail states of highest occupied molecular orbital of the host material. The room temperature J-V characteristics were well described with the single value of free hole density at low fields. At higher fields it becomes field-dependent followed by a field enhancement factor γ. This journal is © the Partner Organisations 2014.
Phase diagrams and morphology of polymer-dispersed liquid crystals: An analysis
(2012) Jagdeesh Kumar Srivastava; Rajendra Kumar Singh; Ravindra Dhar; Shri Singh
Thermal and morphological studies of polymer-dispersed liquid crystals (PDLCs) for various compositions of liquid crystalline material 4-undecyloxybenzoic acid (UDBA) in two polymer matrices, polyvinylidenefluoride-co-hexafluoropropylene P(VdF-HFP) or polyethylene oxide (PEO), have been carried out using differential scanning calorimetry (DSC) and polarising optical microscopy (POM). Phase diagrams for different series of PDLCs have been analysed using the Flory-Huggins theory of isotropic mixing and the Maier-Saupe-McMillan theory, to include the anisotropic contributions. Mesogenic transitions of UDBA are observed to be greatly influenced when dispersed in these polymers. The morphologies and miscibility studies of these PDLCs suggest that UDBA is highly miscible in PEO, only partially miscible in poly(methyl methacrylate) (PMMA), but almost immiscible in P(VdF-HFP). © 2012 Copyright Taylor and Francis Group, LLC.
Role of reduced pi-pi stacking in the charge transport in polyfluorene
(Elsevier Ltd, 2016) Manisha Bajpai; Ritu Srivastava; Ravindra Dhar; R.S. Tiwari
We investigated the effect of blending of a bulky copolymer on the hole transport properties of poly (9,9-dihexyl fluorenyl-2,7-diyl)(PFO) thin films (∼120 nm) at different temperatures by obtaining the current density-voltage measurement. The poly (phenylenevinylene) based copolymer ‘‘Super Yellow” (SY) has been used as a blending component. Such polymer blending matrix is useful as it combines the host polymer (PFO) with the guest polymer of bulky side groups where pi-pi stacking is hindered by bulky side groups. This makes polyflorene much interesting and versatile material for lighting application. It has been found that the current density decreases on blending for a given applied bias. We demonstrated that the current conduction mechanism is bulk limited. It has been analyzed by trapped charge limited currents (TCLC) with electric field. We conclude that the relatively low hole mobility in blends as compared to pristine PFO would result due to increase in the pi-pi stacking distance and disorder. © 2016 Elsevier B.V.
Studies of dispersed liquid crystals in binary mixtures with ionic liquid and their excitation by electric signals
(Royal Society of Chemistry, 2015) Jagdeesh Kumar Srivastava; Rajendra Kumar Singh; Ravindra Dhar; Shri Singh
We report the results of dispersion studies of two nematogens (i) 6CHBT (4-(trans-4-n-hexylcyclohexyl)isothiocyanatobenzene) and (ii) PCH5 (4-(trans-4-pentyl-cyclohexyl)benzonitrile) in their binary mixtures with phosphonium based ionic liquid [P6,6,6,14][Ntf2] (trihexyl(tetradecyl)phosphoniumbis(trifluoromethylsulfonyl)amide). The morphology and properties of these mixtures have been investigated by using differential scanning calorimetry (DSC), polarizing optical microscopy (POM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and, their excitations by electric signals. For both the liquid crystals, nematic droplets are found to be dispersed in the ionic liquid having a particular anion, i.e., [Ntf2]. These droplets closely resemble the common droplet morphologies observed in polymer dispersed liquid crystals. Thermal and morphological investigations on the mixtures show a decrease in the peak value of the isotropic to nematic transition and broadening of the associated peak with a decrease in the compositions of nematogens in mixtures. As a result, nematic droplets in a wide temperature range ∼20 °C (i.e., from ∼16 °C to ∼36 °C) are observed for various mixtures. Electrical excitations of the mixtures show significant changes in the droplet size and nematic director orientation inside the droplets due to change in the anchoring conditions with the application of external electric pulses. The influence of the electric field on these nematic droplets and their stability over a wide temperature range suggests their potential for various device development applications. © The Royal Society of Chemistry 2015.
Thermal and morphological studies of liquid crystalline materials dispersed in a polymer matrix
(2011) Jagdeesh Kumar; Rajendra Kumar Singh; Ravindra Dhar; Shri Singh
Polymer dispersed liquid crystal (PDLC) films have been prepared by a combination of solvent-induced phase separation and thermal-induced phase separation techniques, for three mesogenic materials: 4-(undecyloxy) ben-zoic acid (UDBA), 4-(decyloxy) benzoic acid (DBA) and 4-(dodecyloxy) benzoic acid (DDBA) in poly(methyl methacrylate) (PMMA) as polymer matrix and tetrahydrofuran (THF) as common solvent. Differential scanning calorimetry and polarising optical microscopy are used for thermal and morphological investigations of prepared PDLCs, and findings are explained with the help of a suitable phase diagram. It is found that isotropic liquid-ne-matic transition temperature (TIN) for these mesogens in the PMMA matrix decreases by about 2.8-5.6C as compared with the pure mesogens. Methods based on the solubility parameter and the fraction of liquid crystal (LC) in LC-rich domains inside the PMMA matrix at different concentrations have been adopted to achieve an optimised condition for uniform dispersion of the film over a larger area. Remarkable changes in the droplet morphology of the mesogens confined in PMMA matrix as compared with their bulk counterparts have also been observed. Bipolar droplets and Maltese-type crosses are observed by POM for 75 and 30 wt% UDBA in the PMMA matrix. © 2011 Taylor & Francis.