Manipulation Of Ferroelectric Domain And Structure And Mechanical Writing Of BiFeO₃ Ultrathin Films

BiFeO₃?BFO?is the only-known room-temperature single-phase multiferroic material,which has large ferroelectric remnant polarization,G-type antiferromagnetism,and relatively small band gap.The excellent properties of BFO thin film make it have broad application prospects in the fields of magnetoelectric coupling,ferroelectric photovoltaic effect,ferroelectric photocatalysis,ferroelectric and multiferroelectric tunnel junction,etc.The as-grown ferroelectric polarization state and structure are critical to the study of ferroelectric tunnel junction and megnetoelectric coupling performance,and the band gap has a direct effect on the light absorption efficiency and further affects the photovoltaic and photocatalysis performance.Therefore,it is of great significance for the research and application of BFO thin film to effectively regulate the polarization state and improve the physical properties.In this dissertation,we take BFO as research object to study the influence of nonstoichiometry of the thin films on the polarization state,structure,band gap and other properties,as well as the effect of mechanical force on the polarization switching through flexoelectric effect.The main research results are stated as follows:By controlling the Bi nonstoichiometry in BFO ultrathin films from Bi-excess to Bi-deficient,the modulation of ferroelectric domain states of the films has been realized by obtaining domain states from upward polarization to downward polarization,and meanwhile the crystal structure of the BFO ultrathin films evolves from monoclinic phase to tetragonal phase.By controlling the composition ratios of the targets,a series of pure-phase epitaxial BFO ultrathin films with different Bi nonstoichiometry from Bi-excess to Bi-deficient were successfully prepared using laser molecular beam epitaxy technology on the La_0.7Sr_0.3MnO₃/SrTiO₃?LSMO/STO?substrates.Piezoresponse force microscope?PFM?results show that the Bi-excess films have single domain state with as-grown polarization completely upward,and then mixed domains with randomly upward and downward polarizations gradually appear in the films with decreasing Bi/Fe ratio,and finally the single domain with polarization completely downward is obtained in the most Bi-deficient film.It is found by atomically resolved energy dispersive X-ray spectroscopy?EDXS?that the interface atomic configuration of the BFO/LSMO heterojunctions can be changed by the elemental ratio of the BFO films.The interface atomic configuration of the most Bi-deficient film is-?La,Sr?O-FeO₂-,whereas the Bi-excess film shows-MnO₂-BiO-interface.Interface dipoles with opposite directions form at different interface configurations,and thus lead different polarization orientations of the BFO layer.In addition,high resolution scanning transmission electron microscope?STEM?,second harmonic generation and Raman spectra results indicate that the Bi-deficient and Bi-excess thin films have different crystal structures,i.e.the Bi-deficient thin films have tetragonal structure,whereas the Bi-excess thin films show monoclinic structure.Spectroscopic ellipsometry measurments show that the nonstoichiometry of the films can effectively modulate the optical band gap.The sample closest to stoichiometry has the maximum band gap of 2.65 eV,and the band gap gradually narrow with the increase of Bi deficiency or Bi excess.These results show that the nonstoichiometry of BFO thin films has an important influence on their ferroelectric domain,phase structure,band gap,and other physical properties,and provide a new avenue to artificially design and manipulate the related properties of BFO film.Applying mechanical force on the surface of BFO ultrathin films by AFM tip,the ferroelectric domains can be reversed via flexoelectric effect induced by the strain gradient,and the effect of uniform strain via piezoelectric effect and strain gradient via flexoelectric effect on the ferroelectric polarization reversal has also been investigated.Moreover,the tunneling electroresistance effect of the BFO ultrathin film has been observed.Strain and strain gradient can be generated by AFM tip applying mechanical force on the BFO thin film.When the mechanical force exceeds a certain threshold,the ferroelectric domain can be reversed due to the flexoelectric effect,therefore the mechanical writing of ferroelectric domain in the BFO ultrathin film has been realized.In addition,the combined effect of the stress and electric field on domain switching has been studied.For polarization reversal from upward to downward,the mechanical force can effectively reduce coercive electric field,which is ascribed to the combined effect of flexoelectric effect and piezoelectric effect.In the case of polarization reversal from downward to upward,the mechanical force has no obvious effect on the coercive field,because of the opposite contribution of flexoelectric effect and piezoelectric effect.Surface potential results show that the surface potential of the mechanically written domain is lower than that of the electrically written domain,demonstrating that the surface screening charge density of the mechanically written domain is lower than that of the electrically written domain.At last,the Conductive-AFM observations show the tunneling electroresistance effect of the BFO ultrathin film with both mechanically and electrically written domains.