Interplay between ferroelectricity and surface chemistry in BaTiO3

Abstract

Ferroelectric materials possess permanent electric polarization, which can be reoriented between possible equilibrium states by the application of external electric field of proper orientation and magnitude. Due to the finite size of ferroelectric materials, a discontinuity of polarization arises at the interface between ferroelectric and surrounding medium, giving rise to the bound charge and a depolarizing field. In some cases, this field might be sufficiently high to jeopardize ferroelectric stability if not properly screened. Necessary screening charges can be supplied internally, in the form of free charge carriers, externally by metallic electrodes or, alternatively in the case of bare (electrode-free samples), by different adsorbate species such as atmospheric water and its dissociation products. Driven by the depolarization field screening, both internal and external mechanisms play important roles in defining surface chemistry of ferroelectrics. In this sense, ferroelectric materials are able to develop polarization-specific surface chemical states (e.g. stoichiometry, electronic and structural configurations), which further govern adsorption, desorption and barriers for chemical reactions of adsorbate molecules on polarized ferroelectric surfaces. Moreover, the relation between surface chemistry and ferroelectricity is bidirectional: any change in the surface-chemical composition leads to a change in surface boundary conditions and depolarizing fields, that altogether may have profound effect on the dipolar interactions and ferroelectric properties of thin films, where surface-to-volume ratios are high. In turn, this opens new pathways towards chemically controlled ferroelectricity, where by the tuning of surface chemistry desired ferroelectric properties could be achieved or, vice versa, the modification of polarization by external fields can be exploited in (ferro)catalytic processes. However, in reality these are challenging tasks due to severe limitations in our experimental knowledge of the chemical interactions at the surfaces of ferroelectric thin films. These limitations include both the actual chemical species involved, and the dynamics of the electrochemical screening process - which, in turn, affect domain size (which would be the ‘bits’ in ferroelectric memories), coercive voltage, and maximum speed at which information can be encoded in ferroelectric thin films, being of practical interest for information storage applications. This doctoral thesis sets out to explore these aspects. Starting from the preparation of high quality epitaxial and ferroelectric BaTiO3 thin films, used as the main material in this thesis, the presented work will encompass study of interplay between ferroelectricity and surface chemistry from different perspectives. The polarization- and environmental exposure- dependent surface chemistry of BaTiO3 thin films, and furthermore its evolution during dynamic polarization modulation across the Curie transition temperature in water vapor conditions, relevant in the frame of catalytic processes on ferroelectric materials, is studied by conventional and ambient pressure X-Ray photoelectron spectroscopy (AP-XPS). The effect of adsorbed water will be further assessed in the frame of ferroelectric lithography experiments and Piezoresponse Force Microscopy (PFM) studies in controlled humidity conditions, by investigating effect of relative humidity on the maximum speed of domain writing, domain size and domain switching dynamics. Lastly, influence of molecules different from water, achieved by deliberate chemical functionalization with aromatic para-aminobenzoic acid (pABA) molecules, will be studied with respect to ferroelectric polarization stability.

Date of Award	10 Jun 2022
Original language	English
Awarding Institution	Universitat Autònoma de Barcelona (UAB)
Supervisor	Jordi García-Antón Aviñó (Tutor), Domingo Marimon, Neus (Director) & Catalán Bernabé, Gustau (Director)