In Situ Study On Suface And Interface Structural Regulation And Evolution Of Nanomaterials

The surface and interface of materials have unique physical and chemical properties that are different from the interior,which has a decisive impact on the overall performance of the material.Consequently,studying the structure evolution of the surface and interface under various external fields is helpful to understand and construct the relationship between material structure and performance,and is of great significance for the design of and devices.Based on this,in this thesis,the evolution and regulation process of the surface and interface structure of silver and related materials under different external fields were studied at the atomic scale by means of in situ transmission electron microscopy.The main research contents are as follows:1.In situ tailoring of Ag/Te-Ag₂Te heterostructures by electric field.By applying a forward bias of Ag to Te,the Te atoms are controlled to migrate to the Ag nanowires by surface diffusion,forming a Ag@Ag₂Te core-shell heterostructure;a reverse bias voltage was applied to control the migration of Ag atoms to the Te nanowire by bulk diffusion,forming a Ag₂Te-Te axially segmented heterostructure.It is found that the direction of atomic diffusion is controlled by the direction of the electric field,while the diffusion mode is greatly affected by the local temperature near the Ag-Te interface.The diffusion of Te at the Ag nanowire is dominated by surface diffusion because the temperature at the Ag side is relatively low due to its high thermal conductivity;while the temperature at the Te side is high,bulk diffusion plays a nonnegligible role.On this basis,a method is proposed to control atomic diffusion with electric field to realize the construction of different types of heterointerface structures in the same material system.2.In situ regulation of Ag surface structure under electron beam irradiation.The layer-by-layer etching process of the flat surface of Ag crystals and the repair process of nanopores were observed in real time at the atomic scale.The etching occurred preferentially at the defect site,while the undamaged atomic layer was relatively stable.The layered growth at the edge of the hole enables the repair of small defects and obtains high-quality crystals.Besides,the combined action of electron beam irradiation and stress can stimulate the phase transformation of Ag from face-centered cubic phase to hexagonal phase.The residual stress at the edge after repair can lead to the expansion of the amorphous structure and the formation of hexagonal phase nuclei;then,the combined effect of electron beam irradiation and stress can make the phase transition reaction continue.3.In situ regulation of the Ag-Ag₂O heterointerface under electron beam irradiation.The oxidation behavior of Ag crystal and the growth process of Ag₂O nanocrystals were observed in real time at the atomic scale.Atomic oxygen and ionic oxygen produced by electron beam radiolysis of oxygen-containing species lead to the oxidation of Ag crystals.The growth of Ag₂O includes the layered growth on the surface of Ag₂O and the migration of Ag/Ag₂O interface into the silver crystal,which are dominated by the outwards diffusion of Ag atoms and the inwards diffusion of O elements,respectively.In addition,the oscillatory growth of Ag₂O atomic layers caused by the double action of electron beam irradiation was also observed at the atomic scale.