Center for Functional Nanomaterials Seminar

The question of the stability of polar interfaces is closely related to that of polar surfaces of ionic solids: the apparent presence of an electric dipole moment in the unit cell perpendicular to the surface/interface leads to an “electrostatic catastrophe” in classical theory. The accepted solution for this problem was that polar oxide surfaces must facet to neutral planes to avoid diverging potentials, and that polar interfaces cannot be abrupt for the same reasons. Hence, much of catalysis was modeled with neutral oxide surfaces only, and polar interfaces were avoided in devices. Having shown that the proverbial polar MgO(111) surface can be stabilized by reconstructions and by hydrogen adsorption, our recent work focuses on polar interfaces. We study model interface systems prepared by epitaxial growth of polar oxide and nitride films on differently stabilized oxide substrates. Electron microscopy and diffraction studies were used, in reflection and transmission geometry, to reveal the atomic structure of polar surfaces and interfaces. Multislice simulations and density functional calculations were performed to determine the atomic and electronic structure in an integrated fashion. Our goal is to tease out the physics behind the exotic behaviors that we find when polar oxides meet face to face.