| A wide range of functional properties have been found in ferroelectric materials,including ferroelectricity,dielectricity,piezoelectricity,pyroelectricity,photoelectric effect,nonlinear optical effect and so on.These characteristics make ferroelectric materials widely applicable to various electronic,optical,and electromechanical devices,such as sensors,infrared detectors,and micro-motors.Most importantly,ferroelectric materials can be used in high-density non-volatile memory due to the ability to switch between different polarization states by applying an electric field.In order to meet the development trend of miniaturization of devices,ferroelectric materials need to be applied to devices in the form of thin films.In the ferroelectric thin film,the domain structure and the stability of the domain in the applied field are important factors that affect the performance and prospect application of the ferroelectrics.In this paper,focusing on the PbTiO3 thin film system and use the aberration-corrected transmission electron microscopy to carry out in-depth research and analysis.Many current technologies using ferroelectric thin films need to control the domain configuration.For example,the a1/a2 domains in ferroelectric thin films are suitable for high dielectric capacitance devices in dynamic random access devices.In order to achieve effective control of the domain configuration,it is necessary to conduct an in-depth study of the evolution of the domain structure.Here,using the Pulsed Laser Deposition(PLD)method to grow a PbTiO3 ferroelectric thin film with a certain thickness on the orthogonal NdScO3(110)o substrate,and control the domain configuration by controlling the cooling rate,and study influence of the physical properties of the film.Combining XRD spectroscopy and Selected Area Electron Diffraction(SAED)analysis,found that the domain configuration of the film is mainly a-domain.Further from the TEM diffraction analysis,found that the a1/a2 domain formed in the film.The distribution characteristics of the domain in the film show changes in the size,distribution orientation,and morphological characteristics with the change of the cooling rate.Using advanced aberration-corrected transmission electron microscopy to study the structural details of a1/a2 domains in atomic scale,it was found that when the cooling rate is low,the a1/a2 domain size is larger,and the domain walls are narrow and uncharged;when the cooling rate is high,the domain size of a1/a2 is small,and the domain wall is wide and charged.PFM piezoelectric performance characterize shows that the film with high cooling rate has high piezoelectric performance.Based on these experimental results,believe that the domain configuration and physical properties of the ferroelectric thin film can be adjusted by growth parameters(such as cooling rate).The results of this study not only help us understand the dynamic and thermodynamic effects on the domain configuration of the thin-film,but also provide ideas for exploring the reasonable control of the domain configuration of ferroelectric thin-films,and are of great significance for the development of nanodevices based on ferroelectric materials.Since most utilities of ferroelectric materials are related to domain configuration and domain switching,understanding the dynamic characteristics of ferroelectric domains is essential for controlling functional characteristics and developing new applications.In-situ transmission electron microscopy is a powerful tool for studying the details of ferroelectric phase transitions in real time.It has been reported that in-situ electron microscopy can directly study the specific process of domain nucleation and switching process of ferroelectric thin film under the action of external electric field or force There has been a few experimental report on the excitation of domain transition by electron beam.For this reason,have grown a certain thickness of PbTiO3 ferroelectric thin film on GdScO3(110)o substrate and covered it with a thin layer of SrTiO3 thin film.The electron beam in TEM is used as the irradiation source to irradiate the thin film and perform real-time in-situ observation research.It found that the a1/a2 domains in the film under electron beam irradiation are transformed into flux-closure domains in two ways:One is transformed into fully closed domains by nucleation,growth,and expansion in the a1/a2 matrix;The other is in the area where a1/a2 and fully flux-closure coexist.The a1/a2 domain completes the transformation through the form of "dislocations gliding".Different ways of transformation depend on the local topography.The phase-field simulation results show that this transition is due to the change in relative dielectric constant caused by the accumulation charges,which causes the depolarization energy changing.This study can be used to study the possible phases transitions between different phases with respect to the stability method of domain configuration under external field excitation,and provide a new idea for the design of potential high piezoresponse ferroelectric thin films in practical applications.It considered that the a1/a2 domain configuration has no critical thickness in the ferroelectric thin film because it not affect by the depolarization field.In order to verify the prediction and study the evolution law of a1/a2 domains with film thickness,grown PbTiO3 ultrathin films on NdScO3(110)?substrates and studied the domain configuration in the films in details.From our previous work,know that under tensile stress,the domains in the film arranged as a1/a2 domains.From a series of films with different thickness,we find that the formation of a1/a2 domains has no critical thickness,and its morphology evolution significantly as the thickness increases(2nm-5nm-8nm-20nm-30nm).The main distribution orientation of the domains changes from isotropy to anisotropy,and there are two differences critical thickness range in the transformation process:One is that when the domain distribution changes from 2nm(domain distribution along[110]and[110],isotropic distribution)to 5nm and 8nm(domain distribution is mainly along the[110]direction,anisotropic distribution);the other is that when the domain distribution changes from 8nm to 20nm and 30nm,the domain distribution is mainly along[110],which shows that the preferred direction changes.The polarization distribution of a1/a2 domain studied by advanced aberration corrected transmission electron microscopy.It was found that the distribution of domain in thick(30nm)films is regular,the domain wall is sharp and the width is very small,and the polarization along the[100]or[010]direction.As the decrease of the thickness(8nm,5nm to 2nm),the domain width small,and density increase,the domain wall wide,some areas diffuse.The polarization direction changed from[100]in the typical tetragonal phase to[100]and[110]coexisted,which indicates that there may be a low symmetry phase different from the tetragonal phase in the ultra-thin film.On this basis,propose that the change of thickness of ferroelectric film can induce the low symmetry phase.This provides important information for the coexistence of phases in ultra-thin films,and has important practical value for the development of ultra-thin nano ferroelectric devices. |
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