Mapping of Physical Properties and Structure of Nanomaterials by Dose Controlled Electron Holography and Precession Electron Diffraction
Novel nanostructures and their striking physicochemical properties, have the potential to revolutionize all engineering fields. Prior their incorporation into materials and devices, these nanostructures require reliable characterization methods to test and drive results. For that reason, imaging and diffraction phenomena using different irradiation sources (photons, neutrons and electrons) results essential. Transmission electron microscopy (TEM) and related analytical techniques have played a canonical role to study the structure of materials. The TEM progress in the last two decades has been spectacular due to aberration-corrected microscopy, which yields sub-angstrom resolution images, setting an unprecedented way to understand and engineer materials at the atomic scale. The extraction of physical quantities via electron-matter interaction consists in the use of TEM analytical techniques. However, some of these properties remain unattained as they are contained in the projected crystal potential of the recording data. The image forming process of the transmitted beam, implies in general the interpretation of the amplitude of the electron wave and the phase of electrons. In this way, the doctoral work incorporates both aspects, the analysis of the amplitude and phase retrieval, using precession electron diffraction (PED) and off-axis electron holography (EH), respectively.
The beginning of the doctoral research project, was focused on the acquisition of diffraction patterns by implementing an innovative methodology to acquire data in fast scanning mode (few milliseconds), using a sensitive axial CMOS detector under reduced electron dosage. Since there is a compromise between the current density of electrons (related to probe size) and the interaction volume of the nanoparticles; the intensity of the reflections needs to be postprocessed with imaging algorithms part of the doctoral research work. It should be noted that the developed methodology is capable to detect spatial oscillations of the clusters caused by the angular momentum induced by the drift velocity of electrons through the sample. The stability of the collected diffraction patterns validates our procedure to provide a systematic and precise way to unveil the structure of atomic clusters without detrimental structural damage. The automated correlation of diffraction patterns by PED is not limited to the formation of crystal orientation maps. As variations in composition or segregation can take place, the collected diffraction patters can be used to track those changes and represent them on phase maps. On this dissertation, phase maps have been constructed to investigate the growth mechanism and structural defects in the cubic and hexagonal phases of InN on GaAs substrates, using them as a tool to visualize their quality, distribution and crystallinity.
The rest of the doctoral research project, is based on a combination of PED and phase reconstruction of electron holograms to correlate the crystalline structure with magnetic properties of metallic ferromagnetic nanowires. Off-axis EH provides the most direct and reliable access to the image phase. Reconstructed phase contains, among others, the contribution of magnetic and electric behavior of specimens. Shape anisotropy is associated to the shape of the sample; however, local variations of the magnetic field depend on the uniaxial anisotropy in specific crystalline orientations of the polycrystalline materials. In this way, the doctoral work covered the combined PED and EH in different crystals and demonstrated the importance of consider not only the magnetic behavior over individual nanowires but also the inter and intra magnetic interactions on arrangements of multiple nanowires. Magnetic measurements were performed using EH both under Lorentz conditions and under a magnetized state by exciting the microscope's objective lens. The novelty of this work relies in the measurement of local magnetometry at individual nanowires and 2D arrays prepared with a focused ion beam. Quantitative measurements via phase retrieval require separate electric and magnetic contributions. Strategies followed to separate them were in-situ TEM heating above the Curie temperature and by manipulating two phase images recorded at different accelerating voltages.