PhD: Nona Mirzamohammadi

Directors:

Directors: ICREA Prof. Gustau Catalán and Group Leader, Dr David Pesquera and Dr Kumara Cordero-Edwards, Oxide Nanophysics group at ICN2.

Short Abstract:

Downscaling ferroelectric materials to nanometer and atomic dimensions is limited by depolarizing fields, which destabilize polarization, suppress switchability, and reduce retention. This thesis investigates para-aminobenzoic acid (pABA) molecular layers as adaptive molecular electrodes for compensating polarization charge in ultrathin BaTiO₃ (BTO) films. First, epitaxial BTO/SrRuO₃ (SRO) heterostructures are optimized by RF sputtering to obtain smooth, reproducible ultrathin films. Piezoresponse Force Microscopy measurements then show that pABA functionalization significantly enhances polarization switching, enabling stable domain writing at higher scan speeds than bare films, indicating faster and more effective screening than atmospheric adsorbates.

In the ultrathin regime, pABA restores stable polarization reversal and retention even below the conventional critical thickness. Polarization-dependent tunneling measurements further demonstrate reproducible tunneling electroresistance in 2-unit-cell BTO films, confirmed by local CAFM and macroscopic current–voltage measurements. Overall, this work establishes molecular screening as a route to stable, switchable, atomic-scale ferroelectric tunnel junctions.

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