Ferroelectric materials are polar materials with a permanent electric dipole below a certain transition temperature. This polarization can be controlled and switched by externally applied electric fields, for example to build thin-film ferroelectric memories. At the surface of ferroelectric materials, polarization suffers from serious fundamental and practical challenges such as depolarization fields and surface charge screening. Uncompensated surface charges due to the discontinuity of the normal polarization component result in depolarization fields that strongly affect polarization states. The ultimate stability of ferroelectric phases is determined by a balance between bulk thermodynamics and the screening mechanisms for polarization, which can be internal (domain formation or charge carriers migration within the bulk) or external (chemical environment or adsorbates). Understanding the interplay between ferroelectric phase stability, screening, and atomistic processes at the surface is key to control low-dimensional ferroelectricity. The interplay between polarization and surface adsorbates works in both directions: adsorbates influence polarization, but the orientation of the polarization also determines the type of adsorbates that bond at the surfaces, and it has been demonstrated that ferroelectric surfaces with opposite polarity can have different behavior toward molecules adsorption.
In this research line we focus our interest in studying surface screening mechanisms on ferroelectric materials using Piezoresponse Force Microscopy (PFM) and Ambient Pressure Photoelectron Spectroscopy (AP-XPS). AP-XPS allow us to obtain direct information of the different chemical species that form at the ferrolectric surface in contact with water vapor and other gases present in the ambient were devices containing ferrolectric materials are expected to work.