Faculty Publications
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Item Charge-transfer interface of insulating metal-organic frameworks with metallic conduction(Nature Research, 2022) Sindhu, P.; Ananthram, K.S.; Jain, A.; Tarafder, K.; Ballav, N.Downsizing materials into hetero-structured thin film configurations is an important avenue to capture various interfacial phenomena. Metallic conduction at the interfaces of insulating transition metal oxides and organic molecules are notable examples, though, it remained elusive in the domain of coordination polymers including metal-organic frameworks (MOFs). MOFs are comprised of metal centers connected to organic linkers with an extended coordination geometry and potential void space. Poor orbitals overlap often makes these crystalline solids electrical insulators. Herein, we have fabricated hetero-structured thin film of a Mott and a band insulating MOFs via layer-by-layer method. Electrical transport measurements across the thin film evidenced an interfacial metallic conduction. The origin of such an unusual observation was understood by the first-principles density functional theory calculations; specifically, Bader charge analysis revealed significant accumulation and percolation of charge across the interface. We anticipate similar interfacial effects in other rationally designed hetero-structured thin films of MOFs. © 2022, The Author(s).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.