Effects of Ferro-Electric Polarization Direction on Surface Properties

New two-dimensional materials and hetero-structures – especially structures including metal-organic coordination networks – offer a promising route towards new micro-electronics applications. However, such applications require the ability to control micro-scale electronic or magnetic properties. One avenue through which this goal is being pursued is the use of ferroelectrics – materials whose unit cell has an electric dipole that can readily switch direction in response to an applied electric field – as substrates on which two-dimensional materials of interest can be placed. This project considers two types of ferroelectric substrate – the two-dimensional α-In3Se2 and the prototypical perovskite BaTiO3 – and examines their influence on two-dimensional adsorbates. Ab initio atomistic calculations using density-functional theory are performed on a system consisting of a single Ni adatom on a heterostructure of graphene over α-In2Se3. The magnetic moment of the Ni adatom was found to depend on the direction of the ferroelectric polarization, changing by 0.6 μ_B upon polarization reversal. Modelling the surface of a bulk ferroelectric such as BaTiO3 is more complex, due to the computational cost of making slabs thick enough to capture surface effects accurately when periodic boundary conditions are applied. To overcome this, we examine the possibility of using an efficient passivation scheme to minimize the required slab thickness. By examining the density of states and electrostatic potential we find that passivation can be used to conduct efficient calculations. Finally, calculations were performed on a system consisting of a single TCNE admolecule on the non-ferroelectric phase of BaTiO3. This points the way towards future work where this passivation technique can be used to simulate the adsorption of TCNE on ferroelectric BaTiO3 efficiently.
Based on the results obtained here, we expect that a two-dimensional metal-organic coordination network will be affected by the electric polarization of a ferroelectric substrate.