The Fabrication And In-Situ Transmission Electron Microscopy Investigation Of Magnesium Nanopores

Solid-state nanopores hold the powerful application prospects in many fields, especially in nanopore based sensors and gene sequencing aspects as a tool to detect and characterize biomolecular polymer, due to their special structural characteristics, and the corresponding nanopore technology has been one of the most popular research hotspots. Controllable fabrication and effective tune of the structure of nanopoers are the most fundamental prerequisite for the integration of the nanopores with device, which has a vital role in the application of the nanopore-based devices. This thesis focuses on employing in-situ high resolution transmission electron microscopy (HRTEM) technique to characterize the structures of nanopores fabricated in crystal Mg, explore the healing mechanism of nanopores under electron beam (e-beam) irradiation, supervise and research the shape-dependent atomic diffusion behaviors and oxide growth dynamics of metal Mg at the atomic scale. The main findings are summarized as follows:1. Different shaped nanopores in Mg substrate were fabricated by convergent e-beam inside transmission electron microscope (TEM). Employing in-situ HRTEM technique, we characterized the two-dimensional shapes of nanopores. The results indicate that:(1) the shapes of fabricated nanopores present obvious edge features surrounded by several sets of parallel edges; (2) the shapes of nanopores are related to the e-beam irradiation directions. Our results have extended the diversity of the nanopore shapes and laid the foundation for the related applications; (3) the results provide a feasible reference for the nanopatterning of metallic materials, and promote the research and understanding of the controllable fabrication of "nanomold".2. Three-dimensional (3D) structures of Mg nanopores with approximately hexagonal shape were researched by in-situ HRTEM and log-ratio method. We found that:(1) the 3D structures of nanopores were mainly two kinds:hexagonal prism shaped and hourglass-shaped; (2) crystal surfaces with relatively low surface energy {1101},{1100}, and {0001} should play important roles in managing the geometry morphologies of nanopores; (3) the 3D structures of nanopores are found to depend on the widest nanopore diameter-to-thickness ratio (D/t). Our results have important guiding significance for the controllable fabrication nanopores in size and structure.3. Through in-situ HRTEM technique, the dynamics of healing process for Mg nanopores under e-beam irradiation were tracked in real time. We found that the healing phenomenon is related to nanopore initial size while has nothing to do with the shapes. Moreover, the healing process is realized by discontinuous layer-by-layer atomic planes growth at the edge of the nanopores, which results from the directly observed surface atomic diffusion at the vicinity of nanopore edges. Besides, further researches shown that the healing priority at the nanopore sidewalls was related to the shapes of nanopore. The experimental results provide a reference for the further application of metal Mg and its alloys under irradiation conditions.4. The atomic-scale deformation of nanopores was realized by manipulating the focused e-beam, which indicated that the metal nanopores with specific shapes may be obtained. The results enriched the diversity of the nanopore shape. Moreover, employing HRTEM technique, the atomic diffusion behaviors at the vicinity of different shaped nanopores under e-beam irradiation were observed. Furthermore, the unambiguous evidences about the dependence of atom diffusion behaviors on nanopore shapes were obtained.5. In-situ observation of the atomic-scale oxidation process of metal Mg was achieved inside TEM. The oxidation process is characterized by the layer-by-layer growth of magnesium oxide (MgO) nanocrystal via the adatom process. Consistently, the nucleated MgO crystals exhibit faceted surface morphology as enclosed by {200}MgO lattice planes. These results facilitate the understanding of the nanoscale oxide growth mechanism that will have an important impact on the development of Mg or Mg alloys with improved resistance to oxidation.