Study On Nanoinstability Of Nanowire As Athermally Induced By Electron Beam Irradiation And Related Physics Issues

With the rapid development of nanoscience and nanotechnology,we are increasingly motivated to reveal structure and property of materials not only at nanometer scale(or under extremely small space limitation)but also at nano-,pisco-,or femto-second scale(or under extremely short time limitation).In current literature,there are some scattered and unsystematic reports on structure changes of and interactions such as welding between crystalline Si,ZnO and metal nanowires.Besides,with the assistance of electron beam(e-beam)irradiation,the external force-induced elongation and bending of SiO2,Li2O and Si nanowires have also been reported.Nevertheless,there are few systematic investigations into the structural instabilities of nanowires and the underlying physics issues from nonequilibrium,highly localized,and ultrafast aspects.As a result,the fundamental issues of nanoscience such as nanocurvature-related nanospace effect of one dimensional nanowires and athermal activation-related nanotime effect of ultrafast e-beam were not addressed in these literature.Based on the above considerations,in this thesis,the beam-induced structure changes(or so-called nanoinstability or nanoprocessing)of SiOx nanowire,Au nanoparticles-modified SiOx(Au@SiOx)nanowire and Si nanowire and related physics issues are studied from nonequilibrium,highly localized,and ultrafast aspects via in-situ e-beam irradiation in a field-emission transmission electron micorscope(TEM).The main results are summarized into three parts and listed as follows:In the first part,the nanoinstability of amorphous SiOx nanowire is studied.Firstly,we compare the nanoinstability of SiOx nanowire by changing the wire surface morphology.Two athermal mechanisms of the beam-induced atomic transport,that is,directional diffusion(including massive atomic plastic flow)and selective or preferential atomic evaporation(or ablation)are herein proposed.Secondly,we compare nanoinstabilities of the SiOx nanowire under irradiation with different beam current density or energy deposition rate.It is found that atom diffusion dominates at low current density whereas atom evaporation dominates at high current density.Thirdly,we realize local bending,uniform elongation and accelerated radial shrinkage of the SiOx nanowire,which respectively demonstrate an effect of surface nanocurvature and an effect of beam-induced athermal activation experimentally.Lastly,we perform the welding of SiOx nanowires,which reveals the interaction details and diffusion processes between two nanowires.In the second part,the passivation effect of crystalline Au nanoparticles on the structure changes of amorphous SiOx nanowires are further studied.It is found that after a modification of the Au nanoparticles,the structure changes of the nanowire such as uniform elongation,accelerated radial shrinkage,necking and cutting of SiOx nanowires are greatly retarded and the s-shape deformation of the wire is even totally restrained.All of these demonstrate a pronounced passivation effect of the Au nanoparticles.Meanwhile,the main atomic transport modes such as diffusion,evaporation and plastic flow along with their contributions to structure changes are changed by the passivation effect.By comparing the effects of surface nanocurvature and beam-induced athermal activation between the crystalline Au nanoparticle and the amorphous SiOx nanowire,an overall and appropriate explanation for the passivation effect of Au nanoparticles is thus given.In the third part,the nanoinstability of single crystal Si nanowire is studied.It was observed that a series of intriguing phenomena within the irradiated area such as preferential amorphization from the nanowire surface and formation of a local coaxial structure as induced by focused e-beam irradiation.Meanwhile,in the center of the Si nanowire the amorphization seemed to be non-uniform and much more difficult accompanying with rotation of crystal grains and compression of d-spacing.Furthermore,the findings reveal a difference between the amorphization process of Si nanowires and that of Si films under focused e-beam irradiation.The above study on the nanoinstability of SiOx,Au@SiOx and Si nanowires and related physics under energetic e-beam irradiation are very important to performance stability,structure fabrication and nanoprocessing of future SiOx and Si nanowires-based nanotechnology and devices.More importantly,the above study reveals the novel surface nanocurvature effect and the beam-induced athermal activation effect in nanowire,which are normally neglected in current literature but have been proven to be able to universally explain phenomena of nanoinstability and nanoprocessing of low dimensional nanostructure in general.