Researches On Traditional And Novel Ferroelectric Materials Based On Scanning Probe Microscopy

Ferroelectric material is an important functional material where a spontaneous electric polarization can be switched by an applied external electric field.It has a widely application in the field of non-volatile random-access memories,sensors,optical electronics and so on due to its high density,low energy and ultrafast response speed etc.With the rapid progress in microelectronics,the demands of nano-feature size and highly integration of materials make ferroelectrics at the nanoscale much attractive.Scanning Probe Microscopy?SPM?has emerged as one of powerful tools for high-resolution characterization of lowdimensional ferroelectric materials in recent years.Except for classical characterization of ferroelectric properties,observation of in-situ electrical transport,surface charges and potentials etc.make SPM become a multi-functional technique,largely widening the scope of studies.In this dissertation,we majorly demonstrate three parts deeply including identification of ferroelectric and ferroelectric-like behaviors in different material systems by SPM,mediation in BiMnO₃ second-order synaptic emulator and modulation of lowdimensional conductive domain walls with the function of 2D materials.?.Novel methods to identify ferroelectric and ferroelectric-like behaviors by SPM.???Obvious phase contrasts and domain walls' signals is the basic way to differentiate ferroelectric and ferroelectric-like behaviors.???Keep property is an important feature in ferroelectric materials.???Phase loops behaves different in inorganic ferroelectric materials compared with organic ferroelectrics and nonferroelectrics when V_ac is increased to larger than the coercive field.???Phase loops give different responses in ferroelectric and non-ferroelectric materials if DC voltage is smaller than their coercive field with a pretty low frequency of DC waves.?.Mediation in the BMO second-order synaptic emulator with C-AFM.The filament evolution dynamics including processes of forming and spontaneous decay are directly observed by C-AFM.The size of conductive filaments is the firstorder variation in the second order memristor,deciding the high or low resistance of BMO device.A competition between migration and diffusion of oxygen vacancies is regarded as the second-order variation and the internal mechanism of spontaneous decay.By controlling amplitude,width and interval of voltage spikes,a transform from short-term plasticity to long-term potentiation and from second order to first-order memristor,are achieved.?.Modulation of low-dimensional conductive domains walls in ferroelectric materials with the function of 2D materials.The combination of 2H-MoS₂ and BFO thin films scale down the size of ferroelectric materials with detected conductive domain walls.We observed conductive domain walls in 30 nm and 18 nm BFO thin films with the function of 2H-MoS₂ while no conductive domains walls could be obtained in bare BFO thin films.Besides,the application of voltage pulses could lead to complete switch of polarization in the whole volume of BFO underneath the 2H-MoS₂ flake due to the enhanced conductivity of 2H-MoS₂ when the polarization direction of as-grown BiFeO₃?BFO?thin film is upward.In contrast,application of voltage pulses would result in local switching underneath the SPM tip because of the decreased conductivity of 2H-MoS₂ when the polarization of as grown BFO is downward.