In Situ TEM Study Of Electron Radiation Induced Evolution Of Nanoparticles

Nanoparticle(NP)is an essential part in nanoscience and nanotechnology research field;it is also one of the foundamental components in nano matrerials design and fabrication.The high-performance new functional materials constructed from nanoparticles have potential in applications of various fields,including physics,chemistry,biology,energy,environment and medicine,which have great development prospects in the future.Compared with macroscopic bulk materials,nanoparticles not only have small size,but also bring great changes in properties and behaviors due to size reduction.These changes can improve the performance of materials and expand new functions.However,they also bring uncertainties in the processes of preparation and applications.Therefore,the mastery of structures,properties and evolutionary rules of nanoparticles will help modify the preparation of nanomaterials,improve their performance and find new functions,which are of crucial importance for the future development of nanomaterials.In this thesis,the evolution behavior and mechanism of nanoparticle under electron beam irradiation are investigated by in situ transmission electron microscopy(TEM);and some significant results have achived.The study is divided into three parts in electron beam induced NPs evolution: small-scale stability limit,atomic structural evolution and self-assembly behavior in liquid-phase environment.The main contents include:1.Our study provides a new experimental method for measuring the stability of materials at the nanometer scale,and observes the evolutionary behavior of small size effects at the sub-nanometer scale.The copper nanoparticle thinning process caused by electron beam sputtering has been studied,which explains the relationship between TEM observation and actual results.Since graphene can realize self-repaired by electron beam irradiation under heating conditions,moderately heated graphene is very suitable as an ultra-thin substrate for in-situ TEM experiments that requires long-time electron beam irradiation.In the experiment,the complete process of nanoparticles being sputtered until disappeared has been observed.This process is divided into two sections by an obvious turning point.The change of material properties in the second section is caused by the small size effect of nanoparticles.Electron beam induced sputtering results in anisotropic atomic loss.We have proposed a simplified sputtering rate distribution model to quantitatively describe the dynamic process of the three-dimensional(3D)morphology evolution of nanoparticles.As the results,the 2D TEM images can be used to explain the 3D evolutionary dynamics of nanoparticles.Our model agrees well with the experimental results and gives the stable limit dimensions of copper at the sub-nanometer level in the thickness direction.This model illustrates the relationship between the stability limit of materials and the projection result of TEM,which will serve as a proof for the TEM study of very small-scale objects.2.The transformation from nanoparticles or atomic clusters into monatomic layer under electron beam irradiation has been studied.Two novel single-atomic-thick copper-oxygen nanostrucurre have been discovered,which are cubic and hexagonal,respectively.Through experiments and theoretical calculations,it has been determined that the two 2D copper-oxygen structures have different electrical properties with bulk materials.The cubic copper-oxygen 2D atomic structure can achieve the conversation between indirect band gap and direct band gap by adjusting the proportion of copper and oxygen atoms.By analyzing the electron beam induced transition process from nanoparticle to 2D monatomic layer,we have found that in the six-membered ring structure,the zigzag edge of the copper atom can be stable rather than the oxygen edge with lower energy under electron beam irradiation.The copper-oxygen nanostructures are codetermined by the energy of stable structure and the ability to withstand the destruction of the electron beam.This study has uncoverd novel types of 2D materials and revealed their growth mechanism induced by electron beam irradiation.This mechanism also explains that the two different structures with cubic and hexagonal rings are obtained through different electron beam energies.3.The formation of cobalt oxide nanoparticle rings has been investigated by using TEM liquid cell.The nanoparticles are assembled into a necklace-like annular structure along the edges of the droplets attached to the substrate.The size and shape of the ring are determined by the droplet template which can be varied over a wide range of sizes,while the width of the nanoring depends on the average diameter of the nanoparticle which is around 4 nm.We have found that the aspherical profile of the nanodroplets can be responsible for the formation of the nanoparticle rings: First,the nanoparticles preferentially nucleate and grow in the region of the lowest curvature of the droplet edge;then,the attraction of the droplet surface to the nanoparticle is graded,pointing to the edge of the droplet,the nanoparticles on the droplet are moved and collected onto the perimeter of the droplet.This study proposes a novel scheme for the bottom-up fabrication of nanostructures,which may be extended to a broader range of material systems in the future.