Development of Quantitative In Situ Transmission Electron Microscopy for Nanoindentation and Cold-Field Emission

Abstract : This thesis has focused on in situ transmission electron microscopy (TEM) techniques and especially quantitative in situ TEM. We have used a special TEM nano-probing sample holder, which combines local electrical biasing and micromechanical testing. Finite element method (FEM) modeling was used to compare with the experimental results. Different electron holography techniques have been used to measure electric fields and strains. The first part of this thesis addresses cold-field emission of a carbon cone nanotip (CCnT). This novel type of carbon structure may be used as an alternative to W-based cold-field emission guns (C-FEG), which are the most advanced electron guns used in TEMs today. When a sufficiently strong electric field is applied to the CCnT, electrons can tunnel through the energy barrier with the vacuum, which corresponds to the phenomenon of cold-field emission. The important parameters are the local electric field around the tip and the exit work function of the material. The experiment was realized by applying, inside the TEM holder, a potential to an anode facing the CCnT. By approaching the CCnT to the anode and increasing the bias, the electric field increased until field emission began. The electrons in the imaging beam of the TEM, arriving perpendicular to the electrons emitted from the CCnT, acquire a phase shift when traveling through the strong electric field. A map of the relative phase shift was obtained using off-axis electron holography. Combining the results with FEM, a quantitative value of the critical local electric field around the tip was obtained for the CCnT emission (2.5 V/nm). Finally, using this information together with one of the Fowler-Nordheim equations, which describes the field emission process, a value of the exit work function of the CCnT is determined (4.8±0.3 eV). We have also measured the charges on the CCnT, before and after the onset of field emission. The second part of the thesis focuses on the plastic deformation of an Al thin film deposited on an oxidized substrate to test dislocation-interface interactions. Here, we used a diamond-equipped microelectromechanical system (MEMS) sensor, to measure the force transmitted to a cross-sectional type sample. This configuration allows the simultaneous observation of moving dislocations processes in the sample and a measure of the applied force. Nanoindentation of a thin sample causes it to bend, and impede a stable image formation. Here, focused ion beam (FIB) was used to sculpture electron transparent sample windows in an H-bar configuration, which provides support for the sample. FEM was used to find the optimum window size that is a good balance between the stiffness provided by the H-bar shape and the side effects generated from the bulk part of the sample. According to dislocation theory, a dislocation close to an interface with a stiffer material should be repelled by it. The force being inversely proportional to the distance, a dislocation under an applied stress should be stationary at a certain distance from the interface. Here, we find that dislocations moving towards the oxidized interface are absorbed by this stiffer interface at room temperature. The stress at which this absorption occurs is derived from a combination of load-cell measurements and FEM calculations, and compared with supposed image force. This extends the findings of dislocation absorption at Al/SiO2 interfaces made at higher temperatures. Finally, a first try to combine in situ indentation and dark-field electron holography is reported. The goal there is to acquire a strain map of the indented sample directly from phase analysis. In addition of being a unique tool to see mechanisms unraveling in materials, in situ TEM techniques can nowadays provide quantitative information. This is achieved both by the development of sensor equipped TEM holders and by expanding previously static imaging techniques, modeling and analysis.
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Ludvig de Knoop. Development of Quantitative In Situ Transmission Electron Microscopy for Nanoindentation and Cold-Field Emission. Materials Science [cond-mat.mtrl-sci]. I3EM, CEMES-CNRS (UPR 8011), Université Toulouse 3 Paul Sabatier, 2014. English. ⟨tel-01786884⟩

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