Browsing by Author "Nagaraja, S."

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    Excitonic cuprophilic interactions in one-dimensional hybrid organic-inorganic crystals
    (Royal Society of Chemistry, 2024) Hassan, N.; Nagaraja, S.; Saha, S.; Tarafder, K.; Ballav, N.
    The everlasting pursuit of hybrid organic-inorganic lead-free semiconductors has directed the focus towards eco-friendly copper-based systems, perhaps because of the diversity in chemistry, controlling the structure-property relationship. In this work, we report single crystals of a Cu(i) halide-based perovskite-like organic-inorganic hybrid, (TMA)Cu2Br3, (TMA = tetramethylammonium), consisting of unusual one-dimensional inorganic anionic chains of -(Cu2Br3)-, electrostatically stabilized by organic cations, and the Cu(i)-Cu(i) distance of 2.775 Å indicates the possibility of cuprophilic interactions. X-ray photoelectron spectroscopy measurements further confirmed the presence of exclusive Cu(i) in (TMA)Cu2Br3 and electronic structure calculations based on density functional theory suggested a direct bandgap value of 2.50 eV. The crystal device demonstrated an impressive bulk photovoltaic effect due to the emergence of excitonic Cu(i)-Cu(i) interactions, as was clearly visualized in the charge-density plot as well as in the Raman spectroscopic analysis. The single crystals of a silver analogue, (TMA)Ag2Br3, have also been synthesized revealing a Ag(i)-Ag(i) distance of 3.048 Å (signature of an argentophilic interaction). Unlike (TMA)Cu2Br3, where more density of states from Cu compared to Br near the Fermi level was observed, (TMA)Ag2Br3 exhibited the opposite trend, possibly due to variation in the ionic potential influencing the overall bonding scenario. © 2024 The Royal Society of Chemistry.
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    High Thermoelectric Figure of Merit (zT) in ?-Ag2Se via Aliovalent Doping
    (John Wiley and Sons Inc, 2025) Acharya, A.; Nagaraja, S.; Hassan, N.; Tarafder, K.; Ballav, N.
    High-performance thermoelectric materials are essential for efficient low-temperature (300–400 K) heat energy harvesting, with n-type Ag2Se being a promising candidate. To further enhance the thermoelectric figure of merit (zT) of Ag2Se, aliovalent doping has emerged as a key strategy. However, achieving wet-chemical aliovalent doping of Ag2Se at ambient temperature has proven challenging. In this work, a high zTmax of 1.57 at 398 K is reported for an optimally Cd(II)-doped Ag2Se sample, specifically in the structurally phase-pure Ag1.98Cd0.02Se, which is successfully synthesized via an aqueous-based method at room-temperature (300 K). The Ag1.98Cd0.02Se sample also exhibits an impressive average zTavg of 1.12 over the temperature range of 315–400 K. Density functional theory (DFT) calculations for both the pristine and doped samples reveal significant changes in the electronic band structures, including notable modulations in the density of states near the Fermi energy, particularly for the Ag-3d states. The remarkable thermoelectric performance of Ag1.98Cd0.02Se is attributed to an optimization of charge carrier induced by the Cd(II)-doping. © 2025 Wiley-VCH GmbH.
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    Revealing the Microscopic Picture of the Charge Transfer Mechanism between Graphene and Dopant Molecules
    (American Chemical Society, 2023) Khandelwal, V.; Srivastava, P.K.; Nagaraja, S.; Yadav, P.; Tarafder, K.; Ghosh, S.
    It is generally recognized that the dipole moment of the adsorbed molecules is a crucial factor in determining the charge-transfer interaction between molecules and graphene. However, the microscopic details of this process have remained elusive. In this study, we experimentally investigate the charge-transfer interaction between adsorbed molecules and graphene, which holds great promise for achieving controllable doping. By trapping various molecules at the graphene-substrate interface, our results emphasize that the doping effect primarily depends on the reactivity of the constituent atoms in the attached molecules rather than just their dipole moment. Observation of (i) the emergence of the Raman D peak exclusively at the edges for trapped molecules without reactive atoms, and throughout the entire basal plane for those with reactive atoms, and (ii) variations in the density of attached molecules (with and without reactive atoms) to graphene with their respective dipole moments provides compelling evidence to support our claim. These findings are well-supported by experimental results and first-principles density functional theory calculations. © 2023 American Chemical Society.
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    Ultralow thermal conductivity and thermally-deactivated electrical transport in a 1D silver array with alternating δ-bonds
    (Royal Society of Chemistry, 2024) Hassan, N.; Nagaraja, S.; Saha, S.; Tarafder, K.; Ballav, N.
    We report the synthesis of a (TMA)AgBr2 (TMA = tetramethylammonium) crystal, which comprises inorganic anionic chains of -(AgBr2)∝- stabilized by columnar stacks of organic TMA cations with a periodic arrangement of shorter and longer Ag(i)⋯Ag(i) bonds, even though all the Ag(i) ions are chemically equivalent. The presence of two chemically non-equivalent bridging Br ions is attributed to the primary cause of such an unusual arrangement, as clearly visualized in the charge density plot of (TMA)AgBr2 extracted from the theoretical calculations based on density functional theory. Remarkably, we identified from the orbital-projected density of states the existence of alternate δ-like bonding involving dxy orbitals of 4d10 Ag(i), which was attributed to the cause for ultralow thermal conductivity and thermally-deactivated electrical transport in (TMA)AgBr2. Barring the energetics, our observations on the existence of a δ-bond will shed new light in understanding the nature of metal-metal chemical bonding and its unprecedented implications. © 2024 The Royal Society of Chemistry.