In-situ Dynamic Characterization And Repairment Of Intrinsic Defects In Low-dimensional Structures

Low-dimensional structures have been the subject of rapidly growing research motivated by their diverse technological applications due to the extraordinary physical and chemical properties. However, full realization of their applications in future devices requires a comprehensive understanding of the mechanical properties, which are closely realted with the intrinsic defects. This thesis focuses on employing the electron beam (e-beam) to characterize and repair the intrinsic defects in low-dimensional structures.The main findings regarding the in-situ dynamic characterization of intrinsic defects are summarized as follows:1. Through in-situ high-resolution transmission electron microscopy (HRTEM) observations, the deformation processes in sub-10-nm-sized gold crystal were directly visualized. Our results indicate that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominated the deformation of gold (Au) nanocrystals;(2) the crystallographic orientation (schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibited a phase transformation from a face-centered cubic to a body-centered tetragonal structure due to the surface stress after failure. These findings provide direct experimental evidence for the vast amount of theoretical modeling on the deformation mechanisms of nanomaterials that have appeared in recent years. The corresponding results are published in Nat. Commun.2010,144(1):1-8.2. The formation and vanishing processes of a low angle grain boundary (GB) in nanosized Au during tension and release of stress, respectively, were observed by in-situ high resolution transmission electron microscopy. The nucleation of perfect dislocations led to the formation of a15°low angle GB inside an Au nanocrystal upon off-axial tensile loading (coupled uniaxial tensile and bending stress). Strikingly, the dislocations were completely annihilated accompanied with the disappearance of the GB after the removal of external stress, indicating that plastic bending is recoverable in the nanocrystal. The back force and surface stress played important roles in such a pseudo-elastic behavior. This transient GB dynamics cannot be captured in ex-situ experimental investigations. Such pseudo-elastic bending deformation in nanosized crystals will have an important impact on the designing of nanomechanical devices with ultrahigh bending capability. The related results have been published in Phys. Rev. Lett.,2012,109(22):225501.3. Tensile deformation behavior in silver nanowire with single twin structure was directly monitored by in-situ TEM. Our observation revealed that the initial stage of plastic deformation was dominated by surface-mediated partial dislocation activity. Strikingly, for the first time, the void formation and growth was shown to govern the final period of plasticity, leading to the ductile fracture in the nanowires.The works concerning the e-beam induced repairment of intrinsic defects are summarized as follows:1. The polycrystalline Li2O nanowires sustained an enhanced elongation (from80%to176%) under low dose e-beam irradiation near room temperature as compared with that (from51%to57%) without e-beam irradiation. The extremely high deformability could be understood by the fast Li2O diffusion under e-beam irradiation and tensile stress condition, which enables the "healing" of vacancies and cracks. The results are presented in Sci. Rep.,2012,542(2):1-4.2. We have successfully fabricated the nanopores in Mg alloy by focused e-beam. Employing the in-situ high resolution transmission electron microscopy technique, we presented the unambiguous evidence that layer-by-layer growth of atomic planes at the nanopore periphery occurred when e-beam was spread out, leading to the shrinkage and eventual disappearance of nanopores. Such healing process results from the e-beam induced anisotropic diffusion of Mg atoms at the vicinity of nanopore edges. Our results constitute the first experimental investigation of nanopores in Mg alloy and have extended the potential application of Mg alloy in biological and material society. Besides, our work indicates that the TEM might develop as an alternative candidate to join other techniques, such as scanning tunneling microscopy, in the race towards comprehensive investigation of surface atom diffusion process, which might as well affect the mechanical performance of nanoscale materials.