Faculty Publications
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Item Route to achieving giant magnetoelectric coupling in BaTiO3/Sr2CoO3 F perovskite heterostructures(American Physical Society revtex@aps.org, 2018) Reddy, I.R.; Oppeneer, P.M.; Tarafder, K.Polarization-induced spin switching of atoms in magnetic materials opens the possibilities to design and develop advanced spintronic devices, in particular, storage devices where the magnetic state can be controlled by an electric field. We employ density functional theory calculations to study the magnetic properties of a perovskite strontium cobalt oxyfluoride Sr2CoO3F (SCOF) in a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectic BaTiO3 (BTO) layers. Our calculations show that the spin state of the central Co atom in SCOF can be controlled by altering the polarization direction of the BTO, specifically, to switch from a high-spin state to a low-spin state by changing the relative orientation of the ferroelectric polarization of BTO with respect to SCOF, leading to an unexpected, giant magnetoelectric coupling, ?s?21×10-10Gcm2/V. © 2018 American Physical Society.Item Salt/Cocrystal of Anti-Fibrinolytic Hemostatic Drug Tranexamic acid: Structural, DFT, and Stability Study of Salt/Cocrystal with GRAS Molecules(American Chemical Society service@acs.org, 2019) Nechipadappu, S.K.; Reddy, I.R.; Tarafder, K.; Trivedi, D.R.Tranexamic acid (TXA) is an important and essential medicine needed in a health system and is approved by the US FDA for the treatment of excessive blood loss from trauma, postpartum bleeding, surgery, tooth removal, nosebleeds, and heavy menstruation. One of the notable disadvantages of the TXA drug is that has low absorption (â35-40%) in the gastrointestinal tract, possibly due to its amphoteric nature. In the present work, nine molecular salts and two cocrystals of the TXA molecule have been synthesized by a simple water-mediated solvent evaporation method. The coformers/counterions used were salicylic acid (SAL), 3-hydroxybenzoic acid (3HBA), 2,4-dihydroxybenzoic acid (2,4HBA), 2,5-dihydroxybenzoic acid (2,5HBA), 2,6-dihydroxybenzoic acid (2,6HBA), gallic acid (GAA), oxalic acid (TXA), tartaric acid (TTA), fumaric acid (FUM), succinic acid (SUA), and crotonic acid (CRA). The synthesized salts/cocrystals were characterized by various spectroscopic, thermal, and XRD techniques. The crystal structures of all of the molecular adducts were determined by SC-XRD techniques. In the synthesized salts, charge-assisted acid···amine heterosynthons and O-H···O hydrogen bonds between the acid group of TXA and the coformer are favored, and the salts TXA-FUM and TXA-SUA were found to be isostructural on the basis of the isostructural parameters Ï€ and Î? . In the cocrystal, molecules interacted through the acid group of the coformer with the carboxyl group of the TXA molecule. Further, these salts/cocrystals were found to be stable for a period of 6 months under ambient conditions (â25-30 °C, â60-65% RH). Furthermore, density functional theory (DFT) calculations were carried out to better understand the geometric structure of the molecules presented in our study. The interaction energies of the molecular salts and cocrystals were calculated, and they supported the reported structure of the crystalline adducts. The cocrystal formation in the case of TXA-GAA and TXA-CRA has been confirmed by a DFT calculation study, as the salt formation in these cases resulted in a higher interaction energy in comparison to the cocrystal. Consequently, these molecular salts offer promise for the development of new drug products of TXA, and a few salts, namely TXA-SAL and TXA-2,5HBA, offer the possibility of development of combination drugs. © 2018 American Chemical Society.Item Interfacial spin manipulation of nickel-quinonoid complex adsorbed on co(001) substrate(MDPI, 2019) Reddy, I.R.; Oppeneer, P.M.; Tarafder, K.We studied the structural, electronic, and magnetic properties of a recently synthesized Ni(II)-quinonoid complex upon adsorption on a magnetic Co(001) substrate. Our density functional theory +U (DFT+U) calculations predict that the molecule undergoes a spin-state switching from low-spin S = 0 in the gas phase to high-spin S ? 1 when adsorbed on the Co(001) surface. A strong covalent interaction of the quinonoid rings and surface atoms leads to an increase of the Ni–O(N) bond lengths in the chemisorbed molecule that support the spin-state switching. Our DFT+U calculations show that the molecule is ferromagnetically coupled to the substrate. The Co surface–Ni center exchange mechanism was carefully investigated. We identified an indirect exchange interaction via the quinonoid ligands that stabilizes the molecule’s spin moment in ferromagnetic alignment with the Co surface magnetization. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Item Theoretical Investigations of Electronic Structure and Magnetic and Optical Properties of Transition-Metal Dinuclear Molecules(American Chemical Society service@acs.org, 2020) Reddy, I.R.; Tarafder, K.In this work, we have reported the electronic structure, spin state, and optical properties of a new class of transition-metal (TM) dinuclear molecules (TM = Cr, Mn, Fe, Co, and Ni). The stability of these molecules has been analyzed from the vibration spectra obtained by using density functional theory (DFT) calculations. The ground-state spin configuration of the tetra-coordinated TM atom in each molecule has been predicted from the relative total energy differences in different spin states of the molecule. The DFT + U method has been used to investigate the precise ground-state spin configuration of each molecule. We further performed time-dependent DFT calculations to study the optical properties of these molecules. The planar geometric structure remains intact in most of the cases; hence, these molecules are expected to be well adsorbed and self-assembled on metal substrates. In addition, the optical characterization of these molecules indicates that the absorption spectra have a large peak in the blue-light wavelength range; therefore, it could be suitable for advanced optoelectronic device applications. Our work promotes further computational and experimental studies on TM dinuclear molecules in the field of molecular spintronics and optoelectronics. © © 2020 American Chemical Society.Item Pressure-driven structural and spin-state transition in a Hofmann clathrate coordination polymer(Elsevier B.V., 2021) Reddy, I.R.; Oppeneer, P.M.; Tarafder, K.Hofmann-type organometallic frameworks are well known for their porous crystal structure, exhibiting interesting electronic, optical, and magnetic properties, and are therefore considered as promising materials for various technological applications. Here, using density functional theory+U (DFT+U) calculations, we investigate the spin-state transition in a newly synthesized Hofmann clathrate, namely the Fe{OS(CH3)2}2{Ag(CN)2}2 complex, by applying hydrostatic pressure as an external perturbation. Our study reveals that under a relatively low isotropic hydrostatic pressure, the complex exhibits a reversible spin switching, whereas it undergoes a structural phase transition when the pressure is larger and anisotropic. The spin state of the Fe atom in the Hofmann clathrate complex transforms from high spin to intermediate spin state under anisotropic compression of the lattice parameters. The coordination polymer complex remains a magnetic semiconductor after the pressure-driven structural transformation. © 2020 Elsevier B.V.Item Understanding and tuning of spinterface for chemisorbed Ni-dinuclear quinonoid on Co(001) substrate(IOP Publishing Ltd, 2021) Reddy, I.R.; Tarafder, K.Planar magnetic molecules are of great research interest in the past few years because of their possible application in molecular spintronics. Microscopic understanding of the adsorption and magnetic exchange interaction of these molecules to the metallic/magnetic surfaces may pave the way in developing efficient molecular spin switching devices. Herein, using density functional theory + U calculations, we have studied the structural, electronic, and magnetic properties of a Ni-dinuclear molecule chemically adsorbed on a Co(001) substrate. Switching of the spin and oxidation state of the Ni atom present in the molecule was observed due to the adsorption. We report a strong antiferromagnetic coupling between the spins of the Ni-dinuclear molecule to the ferromagnetic Co(001) substrate. The study reveals an indirect exchange interaction between the magnetic center of the molecule and the substrate Co atoms. The exchange interaction is mediated through the ligands of the molecule that stabilizes the spin moment of the molecule in an antiferromagnetic alignment to the substrate magnetization. Our study also shows that the spin state and strength of MAE of the adsorbed molecule can be tailored through the magneto-chemical method by adding the Cl atom as an axial ligand to the magnetic center of the molecule. © 2021 IOP Publishing Ltd.Item Low field-cooled induced large exchange bias effect and DFT calculations in ferromagnetic Sm2CoMnO6(Elsevier Ltd, 2024) Nayak, A.; Prashanth, C.H.; Bala, D.; Reddy, I.R.; Tarafder, K.; Adyam, V.; Jyothinagaram, K.In the present report, we study the large exchange bias effect in Sm2CoMnO6 (SCMO) polycrystalline samples synthesized with the presence of two crystallographic phases: ordered-phase (monoclinic; P21/n) and disordered-phase (orthorhombic; Pnma). X-ray photoelectron spectroscopy study revealed the presence of mixed valence states for Co (2+ & 3+) and Mn (4+ & 3+). M(T) data exhibits an inhomogeneous magnetic state with the presence of ferromagnetic ordering at TC ∼128 K due to the super-exchange interactions of Co2+-O2--Mn4+ and antiferromagnetic-like spin correlations for T < 50 K, attributed to Co3+-O2--Co3+, and Mn3+-O2--Mn3+ interactions. M(H) loop shift with a conventional exchange bias (EB) effect of 10 kOe for a field-cooled (HFC) of 10 kOe at 2 K was observed. Such a large value of the EB effect for low HFC in SCMO is comparable to that of large EB compounds, such as La1·5Sr0·5CoMnO6 and NiFe2O4/CoO nanocomposites. The zero-field cooled asymmetry in the M(H) loop is termed a spontaneous exchange bias effect (SEB) observed for T < 20 K. The systematic study of EB effects like HEB and MEB with T(K) and HFC was explained qualitatively by the presence of unidirectional anisotropy formed at the interface of inhomogeneous magnetic phases. Further, density functional theory (DFT) calculations validate the ferromagnetic ground state of SCMO with Co and Mn networks. Moreover, the semiconductor characteristics of SCMO are established with a band gap of 1.3 eV. © 2023Item Graphene Straintronics by Molecular Trapping(American Chemical Society, 2025) Srivastava, P.K.; Khandelwal, V.; Reddy, I.R.; Tarafder, K.; Ghosh, S.Here, we report on controlling strain in graphene by trapping molecules at the graphene–substrate interface and leveraging molecular dipole moments. Spectroscopic and transport measurements reveal that strain correlates with the dipole moments of trapped molecules extending beyond their molecular sizes, where values ranging from 1.5 to 4.9D lead to a 50-fold increase in strain and a significant rise in residual carrier density. This has been possible by charge transfer between graphene and trapped molecules, altering the C?C bond length and causing biaxial strain. First-principles density functional theory calculations confirm a consistent dependence of the bending height on molecular dipole moments. © 2025 American Chemical Society