Quantitative Characterizations Of Ferroelectric Materials By Piezoresponse Force Microscopy

The micro structure and its related properties in ferroelectric material has been under intensive study recently,and many important breakthroughs occur in this area in recent few years.These fruitful results are closely related to the development and applications of new techniques on scanning probe microscopy,especially piezoelectric force microscopy(PFM)and conductive atomic force microscopy(c-AFM).Particularly,PFM as nondestructive tool,with its high resolution(?nm)and simple sample preparations,has become a maintrean tool in ferroelectric domain/domain wall characterizations.However,many current works that apply PFM to study the micro structure dependent properties,are largely relying on researchers' intuitive analysis.Moreover,there is no commonly accepted solutions for simultaneous quantitative vertical PFM(VPFM)and lateral PFM(LPFM)measurements.Here we aim to address these issues.Here as our first effort we attempt to understand the PFM response through finite element method(FEM)simulations under the quasi-static and decoupled framework.By using FEM simulations,we analyzed the mechanisms of LPFM response and confirmed its piezoelectric origin,rather than the electrostatic mechanism.Furthermore,by FEM simulations on PFM scanning across a LiNbO3180° domain wall,the existence of a nonzero 90° LPFM response is predicted.This is in fact the first theoretical support since its inital experimental report on this 90°LPFM line-profile.Based on the quantitative PFM analysis on LiNbO3 from FEM simulations,we analyzed the relationship between the effective piezoelectric coefficients and a material's intrinsic piezoelectric coefficients.For example,the 90° LPFM response near the wall is found to be dominated by the piezoelectric coefficient d22.Finally,we further extend the current analytical theory and provide important improvements on its applications,making it able to predict the 90° LPFM line-profile correctly.In this thesis,we further proposed a decoupling solution to the as-measured VPFM signals,whch are typically from a coupled cantilever bending and bulkling motions.Through PFM scanning across a LiNbO3 180° domain wall,we demonstrated the experimental applications of this idea.Through geometry calibration approach,for the first time the two decoupled PFM response curves from the original VPFM measurements and the original LPFM response curves are put together to form a three-dimensional quantitative PFM measurement.This method is simple and convenient,and it can be easily generalized to the three-dimensional quantitative PFM analysis on other ferroelectric domain and domain wall systems.Additionally,we propose a reliable way to estimate the contact radius for general contact AFM scanning.Based on the two aforementioned works,here by doing PFM scanning across a LiNbO3 180° domain wall,we further showed the demonstration of the first realization of simultaneous VPFM and LPFM calibrations on one single sample.This is an important step towards PFM quantitative characterizations.With this calibration approach,we demonstrated the quantitative PFM measurements of the effective piezoelectric coefficients d33eff and d35eff on a(001)-orientated PbTiO3 single crystal,and dicussed their relationship with this material's intrince piezoelectric coefficients.Based on the finite element simulations,we further discussed the capability of PFM as tool in fully capturing a material's piezoelectric coefficients.As a complimentary tool,PFM is found to provide an additional approach for material property characterizations.This result suggests a higher piezoelectric anisotropy in PbTiO3 beyond all previous reports.Lastly,we introduced our recent result on a material system LiRTiO4(R=La,Nd,Sm,Eu,Gd,Dy,Y and Ho).This series of materials were known to be centrosymmetric,according to known literatures.However,according to our current first principle calculations,LiRTiO4 with R= Sm,Eu,Gd,Dy,Y and Ho are curenlty predicted to be noncentrosymmetric,distinct from previous literature results,while LiRTiO4 with R=La and Nd are predicted to be centro symmetric,consistent with knows results.Our synchrotron X-ray diffraction,neutron diffraction,optical second harmonic generation and PFM tools provide a comprehensive study on this series of materials.Experiment results are consistent with theory prediction.The effective piezoelectric coefficient d35eff measured by quantitative PFM provide important insight on material's intrinsic piezoelectric coefficients for LiRTiO4 series.