Séminaire Aseem Rajan KSHIRSAGAR

Part 1: Light tunable gas adsorption in functionalised metal-organic frameworks: insights from ab initio methods

Metal-organic frameworks (MOFs), crystalline hybrid porous materials made up of organic linkers and metal nodes, constitute an important family of candidate materials for carbon capture. MOFs functionalized with azobenzene, a photo-isomerising molecule, are capable of light stimulated capture and release of CO2. The use of light as a stimulus, in this case, can reduce greatly the energy costs of the carbon capture. Using atomistic modeling, we reveal the microscopic mechanism behind light-tunable gas uptake in azobenzene functionalised MOF-5 to be the blocking and unblocking of the metal-node, by distinct geometric configurations of azobenzene. Further, we shed light on the electronic excitations in the functionalised MOF to propose strategies for achieving high yields of photo-isomerisation. Our study also shows that electronic excitations in prototype MOF-5 give rise to strongly bound states of electron-hole pair, analogous to organic insulators.

Part 2: Modulation of magnetization in BiFeO3 using circularly polarized light

BiFeO3 is a multiferroic material featuring ferroelectricity and noncollinear antiferromagnetism. Dynamic and efficient control of the characteristic spin texture of BiFeO3 is attractive for emerging quantum devices. Crystal-field d → d excitations localised on Fe atomic sites in BiFeO3 induce a complex interplay among the spin, charge and lattice degrees of freedom, making them relevant for manipulation of the spin texture. In this work, ab initio methods based on the GW approximation and the Bethe-Salpeter equation are used to characterize localised spin-flip excitations within Fe-3d shell. These excitations are strongly bound and appear deep within the electronic gap. Their spin-content and strong localisation are protected by the d5 antiferromagnetic ordering. The underlying crystal symmetry gives rise to chiral spin-flip exciton states localized on distinct Fe centers which possess net angular momentum of ±h/2π. These chiral excitons couple selectively to light of a particular circular polarisation and are confined to a particular Fe magnetic sub-lattice. As a consequence, net spin-magnetisation can be achieved using circularly polarised light coupling with exciton of desired chirality, thereby modulating the antiferromagnetic texture and giving rise to transient ferrimagnetism.