Research Projects

Interface is the device – originally referred to thin films of conventional inorganic semiconductors has been realized over the decades to be equally true to electrically insulating transition metal oxides (TMOs) and organic molecules (OMs). The study of interfaces in crystalline materials, particularly in thin film hetero-structures of TMOs, revealed unique electronic properties which are not intrinsic of the bulk parents. These interfaces often exhibit incomplete valence-states and dangling bonds, leading to unusual charge distribution and polar discontinuity. A notable discovery was the highly mobile electron gas at the LaAlO3/SrTiO3 hetero-structure, which showed extraordinary electron mobility along with superconductivity emerging at low-temperatures. Further studies revealed that a combination of electrically insulating and metallic layers, such as La2CuO4 and La1.55Sr0.45CuO4, could produce superconductivity at the interface, with transition-temperature (Tc) reaching up to 50 K depending on the design of the hetero-structure. Also, charge-transfer interfaces of electrically insulating OMs, tetrathiofulvalene (TTF) and tetracyanoquinodimethane (TCNQ), exhibited metallic conduction. Structurally, TMOs resembles metal-organic frameworks (MOFs) in the sense that smaller oxygen ions in the former system are replaced by larger organic molecular ions (ligands) in the latter system, which brings permanent porosity in the crystalline solid framework. Also, MOFs are often found to be electrical insulators likewise TMOs. Our initial investigations from IISER Pune on hetero-structured thin films of MOFs, Cu-BTC/Cu-TCNQ and Cu-TCNQ/Cu-BTC (BTC = benzenetrocarboxylate), demonstrated current rectification akin to commercial Si-diodes, indicating the formation of p-n junctions. Further exploration revealed bistable interfaces in Mott-like insulators where switching between rectifying and non-rectifying states was achieved through thermal energy. Finally, electrical transport across the Cu-BPyDC/Cu-TCNQ (BPyDC = bipyridenedicarboxylate) interfaces showed metallic conduction. However, superconducting interface in hetero-structured thin films of coordination polymers (CPs) including MOFs remained elusive. This proposal aims to take up an extreme challenge: developing various hetero-structured thin films of electrically conducting and insulating MOFs by judicious choice of the ligands and metal ions including variation in the structural dimensionality of MOFs, followed by searching the superconducting interfaces upon thoroughly investigating the electrical transport (possibly in the presence of some external magnetic field) across the hetero-structured thin films of MOFs in the temperature range 4 K to 400 K.