Study Of Polarization Effects In Sodium Niobate Nanowires By Transmission Electron Microscopy

With the rapid development of science and technology and the increasing demands of human society,various functional materials have become hot topics.Ferroelectric materials,as a kind of functional materials with high photoelectromechanical thermal coupling effect,are widely used in pressure sensing,ferroelectric storage,photovoltaic and other related fields.Lead zirconate titanate(PZT)ferroelectric ceramics and thin films are the best performance and most widely used ferroelectric materials.However,the environmental pollution of lead is very serious,and the lead-free ferroelectric materials have gradually become the focus of research in recent years.Potassium-sodium niobate(KNN)is a kind of ferroelectric material with perovskite structure,which is one of the potential ferroelectric materials for future replacement of PZT.In this paper,KNN nanowires are carefully studied as to the crystal structures and related polarization effects.In this study,the hydrothermal method is applied to grow potassium-sodium niobate nanowire arrays on strontium titanate(STO)substrate,and then the interfacial region is cut and lifted out by focused ion beam,and characterized by spherical aberration corrected transmission electron microscopy.The high resolution TEM images and electron energy dispersive X-ray spectra reveal the interfacial crystal structures and chemical information,respectively.Through select area electron diffraction(SAED)and geometric phase analysis(GPA),the interfacial lattice mismatch rate is obtained to be ~3.24%.For in situ heating and electrical biasing TEM characterizations,the corresponding the SAED and HRTEM of KNN nanowires are obtained relating to the changing environment.It is found that the electron beam would introduce serious damage to the crystalline structure of the nanowires at high temperature.Using aberration-corrected scanning transmission electron microscopy(STEM)technique,the high-angle annular-dark-field(HAADF)images and two-dimensional elemental maps are obtained at the KNN-STO interfacial region.An edge dislocation is also discovered at the interface.The accurate positions of the atom columns in the HAADF image are determined by two-dimensional gaussian fitting method,and through the python program,we obtained the displacements of the positions of the Nb atoms relative to the surrounding K/Na atomic centers.By calculating the unit cell size and c/a ratio of KNN before and after 800 ? annealing,the a axis of the unit cell is found to be larger,and the c axis is smaller after 800 ? annealing.Thus,it is very possible that KNN crystal phase transition happens before 800 ?,and KNN is near the morphotropic phase boundary,which explains the piezoelectric polarization performance enhancements of KNN after 800 ? annealing.We also observe nanoscale ferroelectric domain structure in the KNN nanowires for the first time,which turns out to be a 90° domain wall between the adjacent domain structures.