Transmission Electron Microscopy Study On The Moiré Structures And Phase Transitions Of Layered Nano-materials

Transmission electron microscope?TEM?is a powerful tool for characterizing the microcosmic structure of materials.Particularly,researchers prefer TEM with aberration corrector for its superior atomic resolution and ability to detect directly the elements and valence states in materials.In the last 20 years,the invention and improvement of the aberration corrector have made the spatial resolution of the TEM critically improved.At present,the lateral resolution of TEM has reached several tens of picometers,which can compare to the diameter of an atom.It is worth noting that the vertical resolution of the scanning transmission electron microscope?STEM?has also been greatly improved through introducing the spherical aberration corrector,and has achieved the nanometer level.As indicated by the theoretical simulations,the vertical resolution will also reach the atomic level with the development of technology in the near future.The improvement of vertical resolution has explored more potential applications of STEM,making it possible to acquire the three-dimensional information of the sample and characterize complex internal structures faster.In addition,another advancement is the in-situ technology based on TEM,which provides a platform for manipulating and measuring samples at nanoscale.It enable us to observe the structural evolutions of microscopic regions in a variety of external fields in real time,and can measure the dynamic changes of physical and chemical properties such as mechanical,thermal,optical,electrical,electrochemical and catalytic properties at the same time.The combination of aberration-corrected?S?TEM and in-situ technology can achieve atomic-level structural characterization,multi-field manipulation and property measurement simultaneously,realizing that“seeing is believing”.Based on state-of-the-art aberration-corrected?S?TEM and in-situ technology,this thesis studied the structures of several layered nanomaterials,and explored the relationships between structural phase transitions and physical properties.Here,our studies mainly focused on the moirépatterns in Cu₂Se nanoplates;the electro-induced phase transition and resistance change effect in Cu₂Se material;the structural phase transition and optical property change phenomenon caused by Mo S₂ lithiation.The main results of this thesis are as follows:1. A method was proposed for distinguishing moirépatterns and modulated crystal structure patterns in nanomaterials.We used electron beam optical depth sectioning method with spherical aberration-corrected STEM to deeply explore a class of abnormal long-period patterns?LPP?in Cu₂Se nanoplates.The atomic resolution images of the two superposed crystals are separately recognized for the first time,revealing that the LPP is a moirépattern that caused by the twisted stacking configuration.This work solves the problem to distinguish the moirépattern and the modulated crystal structure,which is difficult to confirm in nanoscale materials in the previous reports.Moreover,it removes the obstacles for further exploring the structures.2. The crystal structure of thermoelectric material Cu₂Se nanoplates was characterized by various means.The aberration-corrected TEM and rotational electron diffraction?RED?three-dimensional reconstruction method were used to reveal the stacking faults in the solvothermally synthesized Cu₂Se nanoplates,which explained the cause of the abnormally elongated reciprocal rods.In addition,the reversible electro-induced phase transition and resistance change phenomenon in superionic material Cu₂Se were studied by using the home-made in-situ electrical testing system.And the nanoparticle precipitates induced by electric field were studied on its composition,mechanical properties and quantum confinement effect.The in-situ electrical results provide an idea for explaining the decay of the performance of thermoelectric materials under electric field.3. The in-situ lithiation process and optical properties of Mo S₂nanosheets were studied by using aberration-corrected?S?TEM combined with in-situ photo-electro-probe technology.Cathodoluminescence?CL?spectra of Mo S₂nanosheets before and after lithiation were detected.It was found that the peak of band-edge emission was quenched after lithiation.The various 1T'phase superstructures produced during the lithiation process were observed on the atomic scale,and the stability of the superstructures was explored.