Dynamic Analysis Of Scanning Probe Cantilever And Its Application In CIPS Electromechanical Coupling Effect

With the increasing demand for miniaturization and integration of electronics,nanoscience and technology has developed rapidly in various fields.Guided by the technical requirements in new nanomaterial design and preparation,there is an urgent need for rapid,quantitative,stable,and high-resolution measurement methods at nanoscale.Scanning Probe Microscope（SPM）is a key invention in the field of microscopic property characterization giving rise to the thrive of nanoscience.Since its birth in the 1980s,with all kinds of innovations in related methods and practices,SPM gradually developed into an extended family with rounded application fields and a wide variety of functions.Its application in nanomaterials has led to a paradigm shift in the understanding and perception of matter at nanometer or even atomic level.With the deepening of our understanding in the complexity of material properties new challenges have been put forward in quantitative characterization and manipulation of multi-field coupling effects.Understandings of cantilever dynamics of the scanning probe and the analysis of the microscopic electro-mechanical coupling between SPM tip and sample are of great significance to the study of the multi-field coupling properties of material.In this thesis,the mechanical and electrical coupling properties of bulk ferroelectric materials（PPLN,PMN-PT）and new two-dimensional ferroelectric materials（CIPS）are investigated by SPM.The research work mainly includes four aspects,1)analysis of high-order modal dynamic stiffness of SPM cantilever,2)quantitative decoupling analysis of piezoelectric and electrostatic responses,3)correlation analysis of ferroelectric polarization and ion strain,4)manipulation of ferroelectric polarization based on flexoelectric effect.The main research contents and results are as follows:（1）The electromechanical coupling between the probe and the sample may come from a variety of complex sources,such as inverse piezoelectric effect,electrostrictive effect,flexoelectric effect,Vegard strain,and electrostatic forces.Among these,the electrostatic interaction between the sample surface and the probe cantilever has strong influence on the piezoelectric signal,which is easy to be confused with the real piezoelectric signal in the piezoelectric measurement.Therefore,from the theoretical model of cantilever dynamics,we use the three-dimensional finite element method to simulate and analyze the dynamic frequency characteristics,modal shape,modal stiffness,amplitude and driving forces of the multi-order response of probe cantilever,respectively.The dependence of the electrostatic response and the piezoelectric response on the dynamic stiffness of the probe of different order modes were compared.The results illustrate that the high-order modes of cantilever have higher dynamic stiffness thus restraining the electrostatic response obviously.（2）Electrostatic effect appears universally in piezoelectric force microscope（PFM）,which is a main problem in the quantitative measurement of piezoelectric properties.Based on the response difference between electrostatic sand piezoelectric response under different orders of eigenmode stiffnesses of the probe,a two-mode response decoupling method is proposed in this work.Through PFM test under two different resonant orders,the quantitative decoupling of piezoelectric and electrostatic response signals can be realized.Firstly,the two-mode decoupling method was simulated by constructing a piezoelectric-electrostatic coupling finite element model.Then,the two-mode decoupling method was experimentally verified on the periodically polarized lithium niobate（PPLN）and lead magnesium niobate-Lead titanate（PMN-PT）single crystal samples by using a custom-developed two-mode sequential excitation piezoelectric measurement model.The two-mode decoupling method of piezoelectric electrostatic response provides a new solution for quantitative PFM testing.（3）Cu In P₂S₆（CIPS）,as one of the few two-dimensional layered van der Waals ferroelectric materials with room-temperature out-of-plane ferroelectric properties,has wide application prospect in the field of two-dimensional ferroelectric devices.Both ferroelectric polarization and ionic conductivity properties in CIPS are closely related to the motion and distribution of Cu ions,the strong coupling effect between the two phenomena has received extensive research interest.In this work,the ferroelectric polarization and ion migration in CIPS were characterized by PFM,conductive atomic force microscopy（c AFM）and low-frequency strain measurement techniques,respectively.The ferroelectric polarization switching characteristics in CIPS were studied,as well as the self-rectification and Vegard strain phenomena caused by the ion migration process.（4）CIPS has both ferroelectric and ion conductivity.When the polarization of CIPS is reversed under the action of external electric field,it is accompanied by the long-range migration of copper ions and may lead to the destruction of lattice structure,which restricts its application and development as iron electrical devices.Based on the flexoelectric effect in CIPS,we modulate the polarization in CIPS by local strain gradients.The results of phase field simulation found the critical curvature radius of ferroelectric polarization switching induced by flexoelectric effect in CIPS is about 5μm.Ferroelectric polarization manipulation based on controllable strain gradient is realized by transferring CIPS to silver nanofibers and periodic gratings.Continuous modulation of CIPS polarization by strain gradient caused by SPM tip was studied,which provides theoretical and experimental support for the design of novel multistate polarization devices of CIPS.