Atomic Scale TEM In-situ Characterization of Novel Two Dimensional Oxide Films and Functional Devices

Author:Cai Zuo Zuo

Supervisor:wang peng pan xiao qing


Degree Year:2019





Transmission electron microscopy(TEM)is one of the most powerful techniques for nano and atomic scale structural characterization.Thanks to the development of aberration correction technology,the spatial resolution of advanced TEM has been improved to sub-atomic level.The structures of functional materials determine their possible features.Also each unique property has a structural origin.A complete understanding of a nano-structure’s dynamic behaviors under actual operation conditions is the foundation of mechanism exploration.In-situ TEM technology works by applying external effects to samples inside the TEM chamber,so samples’ dynamic behaviors under the applied stimulus can be recorded simultaneously.The ability to observe the effect of external stimuli on materials under working conditions would provide a more detailed and accurate understanding on how materials behave dynamically at nano-scale.This makes it possible to build a relationship between micro-structures and device performances.The fast development of photoelectric functional materials and devices has attracted much attention.But there still lacks a reliable solution for atomic resolution TEM in-situ opto-electronic characterization.In this work,we designed a chip-based micro-electro-mechanical-system(MEMS)for sample illumination and electrical test inside TEM.All in-situ functions were integrated onto a single MEMS chip.To realize controllable optical illumination,a micro-LED was utilized as the light source.Micro electrodes were also reserved for electrical connection.The entire in-situ test platform consists of in-situ opto-electronic MEMS chip,TEM holder,source measure unit(SMU)and control computer,which allows simultaneous in situ illumination and electrical measurements.Compared with traditional techniques,this platform has several important advantages such as high stability,high efficiency and high response speed,which are crucial for sub-atomic resolution characterization and spectroscopic analysis.Based on this in-situ test platform,we carried out in-situ TEM researches on working mechanisms of new devices and materials.We found the Graphene/MoS2-xOx/Graphene van der waals(vdW)heterostructure stacked by fully layered two-dimensional materials exhibits excellent memristive performances.What’s more,its ultra-high thermal stability with a max operating temperature up to 340℃ is much beyond traditional memristors.In order to explore the working mechanism of this unique device,we fabricated cross-sectional sample of this memristor on in-situ chip.With the help of in-situ test platform’s electrical test channel,we successfully controlled the ON/OFF states of this memristor sample inside the TEM by applying corresponding voltages.We found the actual position of conduction channel in MoS2-xOx film when device turns ON(low resistance state)by scanning transmission electron microscopy(STEM)characterization.EDS mapping also revealed the proportion change of oxygen and sulfur atoms in conduction channel,indicated a well-defined conduction channel and a switching mechanism based on oxygen ions migration induced hole carrier concentration change.The layered structures of MoS2-xOx and Graphene were found to be well maintained during resistive switching processes,which plays a crucial role in the robustness of this device.Ferroelectric memory is a type of nonvolatile memory that has many possibilities for use as next-generation semiconductor memory.In this work,we carried out in-situ STEM characterization on ferroelectric polarization switching process and interfacial coupling effect on PZT/LSMO heterojunction sample using in-situ test platform.By accurately controlling the intensity of applied electrical field,the moving speed of ferroelectric domain wall in polarization switching process can be decreased to less than 0.54 nm/s,making it possible for atomic scale STEM direct observation of domain wall’s dynamic behavior.The polarization mapping results on atomic resolution STEM high angle annular dark field(HAADF)images of dynamic domain wall reveals the existence of polarization gradient at domain wall region.Polarization switching was realized by reduction of spontaneous polarization and reverse enhancement through several unit cells.We also found the shape of domain wall is not straight during the side expansion process.This slant domain wall consists of several nano-scale steps,each step comprises several unit cells along<-101>,<001>and<100>directions.This phenomenon indicates the existence of several unexpanded nucleation points on the dynamic domain wall.Combined with large scale STEM-BF characterization on expansion behaviors of dynamic domain wall,these results not only provide an experimental support to diffuse boundary model of ferroelectric polarization switching,but also reveals the unique creeping mechanism of ferroelectric domain wall at an extremely low speed.EELS analysis on Mn valence state at PZT/LSMO interface also verified the modulation effect of ferroelectric polarization state on ferromagnetic state of LSMO.Advances in high-quality freestanding perovskite oxide films provide more possibilities for the study of low-dimensional strong correlation systems and the application of perovskite oxide films.In order to explore the influence to films’ nano structure by freestanding state,we carried out a series of atomic resolution cross-sectional STEM-HAADF characterization on freestanding SrTiO3 and BiFeO3 films.In freestanding SrTiO3 films,stable crystalline state can be found even at an ultra-thin thickness of only 2 unit cells.This experimental result exceeds the 5 unit cells limitation of freestanding perovskite oxide films proposed by previous work.The reduction of thickness also has dramatic influences on physical properties of freestanding perovskite oxide films.When thickness is reduced to less than 5 unit cells,a stretch of c axis lattice spacing combined with polarization variation happens in ferroelectric freestanding BiFeO3 film.Freestanding perovskite oxide thin films also exhibit unique mechanical properties.The atomic resolution STEM-HAADF characterization result on freestanding BiFeO3 film under 3%strain state indicates the lattice structure can afford an over 30%distortion.Strain gradient induced flexoelectric effect can also influence the polarization state of freestanding BiFeO3 film.This result directly verifies the theoretical model of flexoelectric effect at atomic scale.The development of this in-situ opto-electronic test platform may provide a new technical solution for in-situ TEM researches on functional materials and devices.In-situ experiments on two-dimensional memristor and multi-ferroic oxide heterojunction give a deeper understanding on nano-scale working mechanisms of electronic memory devices.These results will also benefit the development and application of in-situ TEM technique.Works on freestanding perovskite oxide thin films reveal their unique properties and provide an important foundation for future applications of this novel low-dimensional material system.