Faculty Publications
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Item Insulator-to-metal-like transition in thin films of a biological metal-organic framework(Nature Research, 2023) Sindhu, P.; Ananthram, K.S.; Jain, A.; Tarafder, K.; Ballav, N.Temperature-induced insulator-to-metal transitions (IMTs) where the electrical resistivity can be altered by over tens of orders of magnitude are most often accompanied by structural phase transition in the system. Here, we demonstrate an insulator-to-metal-like transition (IMLT) at 333 K in thin films of a biological metal-organic framework (bio-MOF) which was generated upon an extended coordination of the cystine (dimer of amino acid cysteine) ligand with cupric ion (spin-1/2 system) – without appreciable change in the structure. Bio-MOFs are crystalline porous solids and a subclass of conventional MOFs where physiological functionalities of bio-molecular ligands along with the structural diversity can primarily be utilized for various biomedical applications. MOFs are usually electrical insulators (so as our expectation with bio-MOFs) and can be bestowed with reasonable electrical conductivity by the design. This discovery of electronically driven IMLT opens new opportunities for bio-MOFs, to emerge as strongly correlated reticular materials with thin film device functionalities. © 2023, The Author(s).Item An Intricate Balance of Ionicity and Covalency: Metal-Like Conduction in All-Inorganic Halide Double Perovskite Cs2AgSbCl6(American Chemical Society, 2025) Kalyani, M.; Ananthram, K.S.; Saha, S.; Ninawe, P.; Tarafder, K.; Ballav, N.Halide perovskites have recently evolved as attractive materials with enormous technological significance due to synthetic control over the structure-property relationship. Halide perovskites are often realized to be either electrical insulators or semiconductors. We present an unusual metal-like conduction (thermally deactivated) in a Pb-free all-inorganic halide double perovskite, Cs2AgSbCl6. The experimental results were understood using density functional theory studies, combined with molecular dynamics simulations and electron localization function calculations, revealing retention of the predominant ionicity of the Ag-Cl bond and an increase in the covalency of the Sb-Cl bond at an elevated temperature, which resulted in a significant change of the electronic band structure, including the density of states, thereby exhibiting an intricate balance of ionicity and covalency. A significant modulation of the electrical conductivity (more than 3 orders of magnitude) without any noticeable structural change will stimulate the investigation of hitherto unknown electronic phase transitions in halide double perovskites. Additionally, light-induced unidirectional rectification of current in Cs2AgSbCl6 was ascribed to a dynamic internal polarization effect. © 2025 American Chemical Society.Item 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.Item Rotational Flexibility in Dication Drives Ambient Temperature Ferroelectricity in an Organic–Inorganic Hybrid Halide(John Wiley and Sons Inc, 2025) Hassan, N.; Panday, R.; Chandru, P.G.; Ananthram, K.S.; Jose, T.M.; Bhoi, U.; Sieradzki, A.; Zar?ba, J.K.; Boomishankar, R.; Tarafder, K.; Ballav, N.Organic–inorganic hybrid halides (OIHHs) have gained attention as potential ferroelectric materials due to structure-property synergy of the organic and inorganic constituents. This study introduces an unusual Ag(I)-based ternary OIHH, (4,4?-bpy)Ag2Br4, featuring rotational flexibility in the organic dication to induce asymmetry into the structure. The compound crystallizes in a monoclinic crystal system with a non-centrosymmetric polar P21 space group at room-temperature and undergoes a structural phase transition to a centrosymmetric phase (P21/c) at Curie temperature (Tc) of 330 K which was further supported by differential scanning calorimetry (DSC), second harmonic generation (SHG) signals, dielectric anomaly, current-voltage (I–V) profiles, and X-ray photoelectron spectroscopy (XPS) data. Ferroelectricity is confirmed through polarization–electric field (P–E) hysteresis loops and piezoresponse force microscopy (PFM), exhibiting switchable polar domains. Density functional theory (DFT) calculations revealed electronic structures of the ferroelectric and paraelectric phases, identified the (?-AgBr2)nn? inorganic anionic chain contributing to the net polarization, and in general, complemented the experimental results. Comparative studies with structurally analogous Ag(I)-based OIHHs lacking dication rotational freedom endorse the critical role of organic flexibility in driving ferroelectricity. This study provides insights into the role of organic dications in controlling ferroelectric behavior and offers a promising pathway for developing coinage metal-based OIHH ferroelectric materials. © 2025 Wiley-VCH GmbH.