Condensed-Matter Physics & Materials Science Seminar

Scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) has proven to be a powerful technique to study structural, compositional, and electronic information of materials at the atomic scale. With the recent addition of 3rd-order and now 5th-order aberration correction, the numerical aperture can be opened up by a factor of 2-3, allowing sub-Angstrom resolution to be achieved in a STEM. Additionally, the enlarged numerical aperture couple with the use of a cold-field-emission gun provides a factor of 100x increase in the usable current for probing inelastic scattering events, while still maintaining an Angstrom beam size. This allows for the acquisition of 2-D compositional and bonding maps of both bulk and nanostructured materials at atomic resolution. In addition, the depth of focus of a convergent electron beam has a more rapidly-diminishing inverse squared relationship with the numerical aperture. This shortened depth of focus of an aberration-corrected STEM (~3-6nm) could potentially enable 3-D reconstruction of nano-objects by recording through-focal series in a similar fashion to optical microscopy. Here, I will give a background on our techniques and will present the study of the chemical microstructures of a statistically significant ensemble of Pt-Co fuel cell nanocatalysts before and after annealing, acid leaching and operational aging. I will also discuss the current and future prospects of 3-D imaging in an aberration-corrected electron microscope.