Electron Beam Induced Structure Transitions In MWCNT And Au-MWCNT

Carbon nanotubes(CNTs)are as indispensable and efficient as employed in nanodevices,nanotechnology,and other nanoscaled systems,fabricated through numerous methods and techniques.Their outstanding elasticity and exceptional optoelectronic properties with high aspect ratio make them a potential material candidate for countless applications.Nevertheless,confronting with the complexities in their synthesis and assembling,the shapes,performances,and functionalities of the as-fabricated or assembled devices and systems are not preferable.Thus,modification in the shapes,dimensions,and configuration of CNTs or other nanostructures,at the targeted locations is highly enviable,specifically in an individual nanostructure.To meet such challenges,numerous significant routes have been adopted to tailor the structural properties of nanostructures or CNTs or devices or systems,along with the advancement in the current fabrication techniques.Nevertheless,modification in their structures at particular or targeted sites is challenging so far.New openings arise,when the nanostructures or CNTs in the aforementioned challenge are nanoscaled and energetic e-beams of transmission electron microscope(TEM)can be exploited to irradiate them.TEM offers adequate resolution(angstrom or even atomic level precision)and it can be directly used to characterize and tailor local structural properties of nanostructures,in addition,it lets in-situ monitoring of local structural evolution of nanostructures under electron beam(e-beam)irradiation.To be specific,in this dissertation,multi-walled carbon nanotube(MWCNT)structural transformation with novel passivation effect of gold nanoparticles(Au NPs)is studied under electron beam(e-beam)irradiation at room temperature(RT).An experimental series carried out with in-situ e-beam in TEM leads towards intriguing findings,for example,at the first instance,e-beam induced structural transformation in fixed-ended MWCNTs with/without metal nanoparticles is evaluated.The study indicates that MWCNT structural evolution induced by energetic e-beam irradiation leads towards a faster shrinkage,as revealed via in-situ transmission electron microscopy,while MWCNT surface modification with Au NPs(Au-MWCNT)slows down the shrinkage by impeding structural evolution process for a prolonged time under the same irradiation conditions.The new relationship between MWCNT and Au-MWCNT shrinking radii and irradiation time illustrates that MWCNT shrinkage rate is faster compared with the theoretical predictions as well as Au-MWCNT.As compared with the outer surface energy(positive curvature),the inner surface energy(negative curvature)of the MWCNT contributes more to the tube wall atoms to evaporate athermally,leading to the structural instability and shrinkage under e-beam irradiation.Conversely,Au NPs possess only the outer surface energy(positive curvature)compared with the MWCNT.Their presence on MWCNT surface retards the dynamics of MWCNT structural evolution via slowing down the evaporation process of carbon atoms,thus restricting Au-MWCNT to shrink.Au NPs interaction and growth evolved athermally on the MWCNT surface exhibit increase in their size and associates this mechanism with the coalescence induced by e-beam activated electronic excitations.Despite their growth,Au NPs show extreme structural stability and remain crystalline under prolonged irradiation.It is proposed that surface energy of MWCNT and Au NPs together with e-beam activated soft modes or lattice instability effect govern all the above structural evolutions predominantly.Secondly,non-uniform structural transformation in fixed-ended MWCNTs with/without metal nanoparticles under focused e-beam is investigated.The main findings disclose that elongation and length contraction in multi-walled carbon nanotubes with/without metal(Au)nanoparticles under non-uniform electron beam irradiation is in-situ studied experimentally at RT with transmission electron microscopy.It is observed that the plastic flow and direct evaporation of carbon atoms strongly rely on the curvatures and surface energies of the nanotubes.The multi-walled carbon nanotubes without metal nanoparticles elongate and shrink by the plastic flow of carbon atoms along the shells in the tube axis direction induced by inward diffusion(self contraction)of shells.In contrast,multi-walled carbon nanotubes with metal nanoparticles reduce their lengths and diameters by direct evaporation(sputtering)of carbon atoms into the free space passivated by metal nanoparticles.Thus,experimental verification is provided that the non-uniform structural modification(evolution)process in multi-walled carbon nanotubes fleetly induced by the nanoscaled negative(positive)curvature effect under athermal effect of electron beam passivates by metal nanoparticles.Finally,e-beam activated tip instability effect in free-ended multi-walled carbon nanotubes with/without metal nanoparticles is examined.The core results intend at expatiating tip instability effect and metal passivation effect in normal-free-ended-multi walled carbon nanotube(MWCNT)and volcano-free-ended-MWCNT induced by electron beam(e-beam)irradiation at RT.In-situ TEM observations illustrate that under e-beam,faster length reduction in normal-free-ended-MVWCNT induced by pronounced tip instability is proceeded without any change in the tip diameter.On contrary,faster length reduction in gold nanoparticles(Au NPs)coated normal-free-ended-MWCNT(normal-free-ended-Au-MWCNT)is not only being retarded and extended over much prolonged irradiation time but also its tip diameter is increased appreciably under the same e-beam.In-situ TEM examination also demonstrate that though,volcano-free-ended-MWCNT tip is remained more instable than the normal-free-ended-MWCNT tip under the same e-beam,length reduction in volcano-free-ended-MWCNT is remained slower and its tip diameter is decreased,which is associated with its less instable sidewalls.In contrast to volcano-free-ended-MWCNT,a vast retardation to the length reduction with noticeable increase in tip diameter of Au NPs modified volcano-free-ended-MWCNT(volcano-free-ended-Au-MWCNT)is observed under the same e-beam.Overall,structural evolution rates can be sequenced as;normal-free-ended-MWCNT>volcano-free-ended-MWCNT>normal-free-ended-Au-MWCNT>volcano-free-ended-Au-MWCNT.The study proposes that tip instability driven unidirectional carbon atom diffusion from tip to the sidewalls and carbon atom clustering within the tips escort these nanoscale structural transformations or evolutions.The characteristic features of these intriguing phenomena lie in negative curvature effect and e-beam activated soft mode and lattice instability effect.