Molecular functionalization of ferroelectric oxide surfaces

Resumen del póster presentado al 4th Scientific Meeting of BNC-b Students (JPhD), celebrado en Bellatera (España) del 6 al 7 de junio de 2019.
Properties of ferroelectric thin films are intrinsically entangled to their surface chemistry, essentially mediated by screening mechanisms trying to compensate bound polarization charges, which produce an electric field opposing the bulk polarization. In general, the orientation of polarization greatly determines the composition of adsorbates present on a ferroelectric surface as well as their sticking coefficients. In the case of thin ferroelectric films, interfacial electrochemical effects can become dominant, creating permanent imprints to bulk ferroelectric polarization or fixing polarization direction. Up to now, focus has been placed on the interplay of adsorbates that normally exist in ambient conditions on ferroelectric surfaces, such as water molecules and carbonates, known to play a critical role as external screening agents as dipole molecules and also by providing ionic species after dissociation. Still, the composition of adsorbates layers in ambient conditions shows a low level of control and reproducibility since it is strongly dependent on the history and storage conditions of the samples and moreover, it leads to remarkable aging of ferroelectric polarization. An alternative to prevent environmental damage of ferroelectric surfaces is to gain control on the adsorbate layer by the molecular functionalization of surfaces, providing the right chemical stability while keeping enough electrochemical reversibility to allow ferroelectric polarization switching. In this contribution we will show our initial results on BaTiO3 thin films functionalized with self-assembled layers (SAMs) of organic and electronically active molecules. The combined use of Piezoelectric Force Microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM) enabled us to study ferroelectric properties of thin films and their surface composition respectively. In this sense, we were able to probe the presence of the molecules on the surfaces on the base of KPFM measurements that revealed the difference in work function between functionalized and bare BaTiO3 thin films. Local hysteresis loops were probed by Switching Spectroscopy PFM (SS-PFM) measurements, whereby an increment of coercive fields of about three times was observed upon molecular deposition. We observed that organic molecules tend to protect the polarization of BaTiO3 thin films in both directions, while keeping ferroelectric switchability.