Mechanical and electromechanical properties of ferroelectrics at the nanoscale studied by Atomic Force Microscopy

Abstract

Piezoresponse force microscopy (PFM) is a powerful tool for the electromechanical characterization of ferroelectric materials. In this atomic force microscopy (AFM) mode, an external ac voltage excites the sample and its piezoelectric response is measured, providing information about the magnitude, orientation and sign of ferroelectric polarization. However, as I show in this thesis, the PFM contact resonance frequency signal does not depend only on the electromechanical properties of ferroelectrics, but is also affected by the mechanical properties of the material at the nanoscale. The research developed in this thesis focuses on the identification and quantification of mechanical response of ferroelectric materials and their contribution on the total electromechanical response. This thesis shows how the mechanical response of ferroelectrics can be detected and quantified in a non-destructive way using Contact Resonance Atomic Force Microscopy (CR-AFM). This technique has been used to quantify the mechanical response of domains, which depends both on the orientation and on the sign of their polarization. This mechanical asymmetry between oppositely-polarized domains enables the concept of mechanical reading of ferroelectric polarization. In addition to ferroelectric domains, domain walls were also investigated. The mechanical properties of 180º ferroelectric domain walls are found to be distinctly different -softer- than the adjacent domains. After having measured the mechanical properties both of ferroelectric domains and domain walls using CR-AFM, Band Excitation AFM techniques are used to compare mechanical and electromechanical measurements on ferroelectric materials. The results show that the contact resonance frequency signal of PFM measurements can be used to extract information about the purely mechanical response of ferroelectric materials, concluding that it is possible to distinguish mechanical properties of ferroelectrics through electromechanical measurements.

Date of Award	24 May 2022
Original language	English
Supervisor	Jordi Sort Viñas (Tutor), Domingo Marimon, Neus (Director) & Catalán Bernabé, Gustau (Director)