Development Of In Situ Platform And 4D-STEM Imaging Techniques For High-Resolution Characterization Of Novel 2D Materials

With excellent properties such as ferroelectricity,ferromagnetism,piezoelectricity,photoelectricity,and so on,ABO₃perovskite oxide films have rich applications in information storage,sensing,energy conversion,and many other fields.In recent years,an epitaxial growth method using the sacrificial layers has made it possible for the perovskite oxide film with a minimum thickness of 2 u.c.to be released from a substrate and exist stably.This freestanding thin film shows good mechanical properties,such as bending resistance like traditional 2D materials,which enables it to be well integrated with silicon devices.What's more,with the decrease of film thickness,it exhibits a significant enhancement of the spontaneous polarization due to the size effect,which is essential for physical research and potential applications.It is of great importance and irreplaceable value to reveal the structure of this material system under the external electric field by using high-resolution transmission electron microscopy?TEM?for understanding its complex properties and guiding material designs.The rapid development of TEM in situ technologies in the late years allowed researchers to apply real-time optical,electrical,mechanical,thermal and other stimulations inside the microscope,while it usually requires complex modifications to the chamber or sample holders of the TEM and can rarely meet the characterization demands of subatomic resolution.Therefore,in this study,a high-precision TEM in situ platform is developed using a self-designed photoelectric MEMS chip and in situ control program written in Lab VIEW.At the same time,a scanning transmission electron beam induced current analysis system based on the same platform is designed,and a quantitative analysis software for in situ data is programmed in Matlab.This platform is applied to Si C nanowires,and the experimental results show that:?i?the in situ platform has the advantages of high stability,wide application range,low modification difficulty,and simple operation;?ii?the electron beam induced current analysis system can effectively reduce scanning time and irradiation damage;?iii?the quantitative analysis program can extract the information of chemistry,polarization and stress in materials quickly and accurately.Furthermore,in order to explore micro-mechanism of the size effects of novel 2D materials on the in situ platform,4D-STEM,a state-of-the-art characterization method is used in this paper to systematically compare the key factors of a variety of new imaging methods used to image conventional 2D material Mo S₂,such as resolution and signal-to-noise ratio?SNR?.Compared with the simulation results,the advantages and disadvantages of different imaging conditions in characterizing 2D materials are explored.The optimal imaging conditions of four imaging methods are found,and it also shows that:?i?d DPC image,with its contrast representing the charge density distribution,is most sensitive to light elements;?ii?i DPC image,with its contrast representing projection potential distribution,has the unique advantages of being sensitive to defects and low-frequency information while improving the SNR;?iii?the ptychography image,with its contrast representing projection potential distribution,is a phase image,which has the comprehensive advantages of super-resolution,higher SNR,and high sensitivity to light elements.Finally,in the field of ultra-high-resolution characterization of nanomaterials such as perovskite oxide films with the external electric field,the study of internal potential distribution has often been transformed into a high-resolution characterization of space charge transfer between atoms.4D-STEM experiments have shown that the phase image of ptychography has the characteristics of super-high-resolution and weak phase sensitivity to light elements.However,the potential of this technique for the characterization of small changes in local space charge transfer of samples has not yet been widely demonstrated and applied in charge transfer characterization.In this part,density functional theory?DFT?and ptychography simulation are combined for the first time to simulate and demonstrate the ability of ptychography in characterizing charge transfer.We use WIEN2k software to calculate the difference of projection potential between IAM and DFT models of the oxygen doped graphene structure,and use the ptychography method to obtain the corresponding ptychography phase image.By comparing the charge transfer characterization of these two models near the doping site,results show that:?i?the ptychography method has the ability of characterizing charge transfer in subatomic resolution;?ii?the contrast change caused by charge transfer cannot be ignored;?iii?compared with IAM model,DFT model can more accurately reflect the real situation of materials under the external field and be used for the analysis of experimental phenomena.In summary,this thesis presents three work:establishing a new in situ photoelectric platform;comparing the differences of a variety of new imaging methods in ultra-high-resolution applications;verifying the ability of the ptychography method to characterize the charge transfer effects by combining DFT.They are all designed to provide in situ multi-scale ultra-high-resolution characterization for freestanding oxide films and other materials.It can be expected that the combination of these new TEM characterization techniques will be of great significance for the extension of high-resolution characterization methods of 2D materials and for the in situ experimental design of various functional materials.