Faculty Publications
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Item Blue emitting 1,8-naphthalimides with electron transport properties for organic light emitting diode applications(Elsevier B.V., 2017) Ulla, H.; Raveendra Kiran, M.R.; Garudachari, B.; Ahipa, T.N.; Tarafder, K.; Vasudeva Adhikari, A.; Umesh, G.; Satyanarayan, M.N.In this article, the synthesis, characterization and use of two novel naphthalimides as electron-transporting emitter materials for organic light emitting diode (OLED) applications are reported. The molecules were obtained by substituting electron donating chloro-phenoxy group at the C-4 position. A detailed optical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical calculations (DFT) were performed to get a better understanding of the electronic structures. The synthesized molecules were used as electron transporters and emitters in OLEDs with three different device configurations. The devices with the molecules showed blue emission with efficiencies of 1.89 cdA-1, 0.98 lmW?1, 0.71% at 100 cdm-2. The phosphorescent devices with naphthalimides as electron transport materials displayed better performance in comparison to the device without any electron transporting material and were analogous with the device using standard electron transporting material, Alq3. The results demonstrate that the naphthalimides could play a significant part in the progress of OLEDs. © 2017 Elsevier B.V.Item Route to achieving enhanced quantum capacitance in functionalized graphene based supercapacitor electrodes(Institute of Physics Publishing helen.craven@iop.org, 2019) Sruthi, T.; Tarafder, K.We have investigated the quantum capacitance (CQ) in functionalized graphene modified with ad-atoms from different groups in the periodic table. Changes in the electronic band structure of graphene upon functionalization and subsequently the CQ of the modified graphene were systematically analyzed using density functional theory (DFT) calculations. We observed that the CQ can be enhanced significantly by means of controlled doping of N, Cl and P ad-atoms in the pristine graphene surface. These ad-atoms are behaving as magnetic impurities in the system, generating a localized density of states near the Fermi energy which, in turn, increases charge (electron/hole) carrier density in the system. As a result, a very high quantum capacitance was observed. Finally, the temperature dependent study of CQ for Cl and N functionalized graphene shows that the CQ remains very high in a wide range of temperatures near room temperature. © 2019 Institute of Physics Publishing. All rights reserved.Item Enhanced photocatalytic efficiency of layered CdS/CdSe heterostructures: Insights from first principles electronic structure calculations(Institute of Physics Publishing helen.craven@iop.org, 2020) Shenoy, S.; Tarafder, K.Metal sulfides are emerging as an important class of materials for photocatalytic applications, because of their high photo responsive nature in the wide visible light range. In this class of materials, CdS with a direct band gap of 2.4 eV, has gained special attention due to the relative position of its conduction band minimum, which is very close to the energies of the reduced protons. However, the photogenerated holes in the valence band of CdS are prone to oxidation and destroy its structure during photocatalysis. Thus constructing a CdS based heterostructure would be an effective strategy for improving the photocatalytic performance. In this work we have done a detail theoretical investigation based on hybrid density functional theory calculation to get insight into the energy band structure, mobility and charge transfer across the CdS/CdSe heterojunction. The results indicate that CdS/CdSe forms type-II heterostructure that has several advantages in improving the photocatalytic efficiency under visible light irradiation. © 2020 IOP Publishing Ltd.Item Spin transport through metal-dichalcogenides layers: a study from first-principles calculations(IOP Publishing Ltd, 2020) Devaraj, N.; Tarafder, K.Spin transport through monolayer and trilayers of molybdenum dichalcogenides were studied considering Co as leads. Detailed investigations of the electronic structure of the Co/MoS2 interface and magnetic tri-junctions are carried out by using density functional theory calculations to understand transport behavior. The study revealed that new spin-polarized hybridized states appeared at the Fermi level due to the formation of Co/MoS2 interface that effectively acted as a spin filter and enhanced the spin injection efficiency of the systems. Spin-polarized current through the system as well as the magnetoresistance (MR) was estimated at different applied bias voltages. Large MR up to 78% was calculated for the trilayer MoS2 system at a relatively high applied bias voltage. The MR values are further improved by tuning the structure of the scattering region. A very large MR of 123% for MoS2/MoSe2/MoS2 trilayer at an applied bias 0.8 V was observed, which is much higher than the previously reported bias dependent MR values in similar systems. © 2020 IOP Publishing Ltd.Item Enhanced quantum capacitance in chemically modified graphene electrodes: Insights from first principles electronic structures calculations(Elsevier B.V., 2021) Sruthi, T.; Tarafder, K.We have carried out a systematic study of quantum capacitance in functionalized graphenes by using DFT calculations. The graphene functionalization has been done by doping with different aliphatic and aromatic molecules and their radicals. The quantum capacitance of functionalized graphenes was estimated from the accurate electronic band structures of the system obtained by using DFT calculations. Our theoretical investigation reveals that aromatic and aliphatic radicals introduce localized density of states near the Fermi level of the functionalized systems, due to a charge localization. As a result, a very high quantum capacitance (>230?F?cm2) was observed in the system. The effects of atomic dislocation and the vacancy defect on graphene during functionalization has also been incorporated in our investigation. Our study suggests an effective way to synthesize highly efficient graphene-based supercapacitor electrode materials by using aromatic and aliphatic molecule/ radical functionalization of graphene. © 2020 Elsevier B.V.Item Theoretical investigation of quantum capacitance in the functionalized MoS2-monolayer(IOP Publishing Ltd, 2021) Sruthi, T.; Devaraj, N.; Tarafder, K.In this work, we investigated the electronic structure and the quantum capacitance of a set of functionalized MoS2 monolayers. The functionalizations have been done by using different ad-atom adsorption on MoS2 monolayer. Density functional theory calculations are performed to obtain an accurate electronic structure of ad-atom doped MoS2 monolayer with a varying degree of doping concentration. Subsequently, the quantum capacitance in each functionalized system was estimated. A marked quantum capacitance above 200 ?F cm-2 has been observed. Our calculations show that the quantum capacitance of MoS2 monolayer is significantly enhanced with substitutional doping of Mo with transition metal ad-atoms. The microscopic origin of such enhancement in quantum capacitance in this system has been analyzed. Our DFT-based calculation reveals that the generation of new electronic states at the proximity of the band-edge and the shift of Fermi level caused by the ad-atom adsorption results in a very high quantum capacitance in the system. © 2021 Institute of Physics Publishing. All rights reserved.Item Photo- and Electrocatalytic Reduction of CO2 over Metal-Organic Frameworks and Their Derived Oxides: A Correlation of the Reaction Mechanism with the Electronic Structure(American Chemical Society, 2022) Payra, S.; Ray, S.; Sharma, R.; Tarafder, K.; Mohanty, P.; Roy, S.A Ce/Ti-based bimetallic 2-aminoterephthalate metal-organic framework (MOF) was synthesized and evaluated for photocatalytic reduction of CO2 in comparison with an isoreticular pristine monometallic Ce-terephthalate MOF. Owing to highly selective CO2 adsorption capability, optimized band gaps, higher flux of photogenerated electron-hole pairs, and a lower rate of recombination, this material exhibited better photocatalytic reduction of CO2 and lower hydrogen evolution compared to Ce-terephthalate. Thorough probing of the surface and electronic structure inferred that the reducibility of Ce4+ to Ce3+ was due to the introduction of an amine functional group into the linker, and low-lying Ti(3d) orbitals in Ce/Ti-2-aminoterephthalate facilitated the photoreduction reaction. Both the MOFs were calcined to their respective oxides of Ce1-xTixO2 and CeO2, and the electrocatalytic reduction of CO2 was performed over the oxidic materials. In contrast to the photocatalytic reaction mechanism, the lattice substitution of Ti in the CeO2 fluorite cubic structure showed a better hydrogen evolution reaction and consequently, poorer electroreduction of CO2 compared to pristine CeO2. Density functional theory calculations of the competitive hydrogen evolution reaction on the MOF and the oxide surfaces corroborated the experimental findings. © 2022 American Chemical Society.Item Unprecedented Electroreduction of CO2over Metal Organic Framework-Derived Intermetallic Nano-Alloy Cu0.85Ni0.15/C(American Chemical Society, 2022) Payra, S.; Devaraj, N.; Tarafder, K.; Roy, S.Designing suitable catalysts for efficient and selective electrocatalytic reduction of CO2 is a need of the hour, and in this regard, the well-defined, highly dispersed active metal centers can be a trendsetting research endeavor toward CO2 electroreduction due to the maximum atom utilization and unique electronic structure. This study describes the synthesis and electrocatalytic CO2 reduction activity of atomistically dispersed Cu/C and Ni/C and the intermetallic nano-alloy Cu0.85 Ni0.15 /C. The catalysts were synthesized from the corresponding MOF precursors. The successful synthesis of the monometallic and intermetallic nano-alloys was established from structural, surface morphological, and electronic properties. Cu0.85 Ni0.15 /C exhibited an unprecedented electrocatalytic reduction of CO2 with a high selectivity and high faradaic efficiency toward CH3 OH. The kinetic studies and the first-principles calculations elucidated the intricate mechanism and the superior activity of electrocatalytic reduction of CO2 over the intermetallic Cu0.85 Ni0.15 /C catalyst. © 2022 American Chemical Society. All rights reserved.Item Magnetic complexity, magnetodielectric effect and DFT calculations on correlation driven Gd2CoMnO6 insulator(Elsevier B.V., 2022) Prashanth, C.H.; Reddy, I.; Tarafder, K.; Chandrasekhar Kakarla, D.; Yang, H.D.; Adyam, V.; Jyothinagaram, K.In the family of Re2CoMnO6 manganite double perovskites, in contrast to parent La2CoMnO6 compound, Gd2CoMnO6 exhibits multiple magnetic transitions; ferromagnetic (FM) ordering, TC ∼ 112 K followed by AFM transition, TN ∼ 47 K, Gd spins ordering for T < 10 K and large isothermal entropy changes. A study of DC field-superimposed AC magnetic susceptibility measurements revealed the field-induced magnetic glassy behavior below TC and enhancement of FM correlations above TC. From the analysis of Almeida-Thouless behavior and dynamical power-law fit to frequency dependent AC susceptibility, Gd2CoMnO6 exhibits a volume spin glass-like nature below the freezing temperature, Tf ∼ 117.5 K. The isothermal field-dependent magnetic and dielectric permittivity data and temperature dependent Raman measurements (reported in ref. R. X. Silva et al., J, Appl. Phys. 114 194,102 (2013)) confirms the spin-phonon coupling induced magnetodielectric effect. Further, the ground-state electronic structure and magnetic properties of Gd2CoMnO6 are investigated using DFT + U formalism with Vienna Ab-initio Simulation Package (VASP) code and predicted the material to be a correlation-driven insulator. The correlation value of the Hubbard U parameter at the 4f-Gd elements changes the stability of the magnetic state from Ferri to FM spin alignment for Ueff ≥ 3 eV and is correlated to the experimentally observed field-induced transformation of the short-range-order FiM/spin-glass-like phase into the long-range ordered FM phase. © 2022Item Investigation of CdSe and ZnSe as Potential Back Surface Field Layers for CdTe-Based Solar Cells: A Study from First Principles Calculations(John Wiley and Sons Inc, 2023) Ray, S.; Tarafder, K.A class of II–VI semiconductors, especially CdTe, is a highly photo-reactive compound that would be suitable for photovoltaic applications. However, being a highly resistive material, CdTe produces considerable contact resistance and drastically reduces the efficiency of photovoltaic devices. Introducing a back surface field layer at the contact region may significantly improve the device's performance. This work investigates the suitability of using ZnSe and CdSe layer as a back-surface-field layer in CdTe-based solar cells through accurate electronic structure calculations using the hybrid-density functional theory method. The calculations show that both ZnSe/CdTe and CdSe/CdTe behave as type-II heterojunctions with band gaps of 2.0 and 1.1 eV, respectively. The Mulliken electronegativity method is used to determine the correct band edge positions concerning the vacuum level for all the pristine semiconductors and their interfaces. Calculation shows that a significant charge redistribution in the interface leads to the formation of an effective local field near the contact region for both ZnSe/CdTe and CdSe/CdTe heterostructures. This local field may help to separate the photogenerated electron–hole pairs in the active layer by pushing the opposite charges into the two different sections of the heterojunction. Additionally, the heterojunctions also exhibit better light-absorption characteristics in the visible light range. © 2023 The Authors. Advanced Theory and Simulations published by Wiley-VCH GmbH.