Quantitative Electron Microscopy
For nanoscale particles it is critical to know their size, structure, shape, and composition. Nanometrology, however, currently falls short of this goal. Counting atoms in clusters and particles with STEM was severely limited by a calibration of image intensities that depends on the microscope operating conditions, i.e. the illumination intensity and contrast/brightness settings. We are interested in developing methods to remove the specific microscope conditions from the equation to enable measurement of universal calibration curves. Combined with EELS and EDS, the composition of particles can be analyzed, supplying the information necessary to determine the type of and number of atoms within each particle. Through application of these techniques, our interest is understand the structure of catalytic particles and explore how the 3D distribution of elements affects observed conversion efficiency. Now that quantitative imaging has been achieved, the LeBeau group is interested in applying and developing these techniques to extract more information from STEM images than ever before.
Phonon Band Structure
Characterizing phonon band structure is key to understanding many material properties, from ferroelectricity to superconductivity. One way to quantify this is by using X-ray and neutron diffuse scattering techniques, which has been indispensable for studying bulk materials. Until recently, these techniques were inaccessible to electron microscopy as the EELS energy resolution was too poor and the diffraction cameras were insufficiently sensitive. Overcoming the detection issue, the Electron Microscopy Pixel Array Detector (EMPAD) can simultaneously capture both Bragg and diffuse scattering on a fully quantifiable scale. This is enabling direct comparison between experiment and electron scattering theory. In comparison to X-ray and neutron scattering, the electron diffraction approach is simpler to set up, has shorter acquisition times, and less material restrictions. In combination, we are exploring the use of electron spectroscopy, diffuse scattering, and STEM image intensities as complementary tools to quantify local thermal transport.
Magnetism in Oxide Thin Films
Atomic resolution electron microscopy and electron spectroscopy play a key role in understanding the structural underpinnings of magnetic properties in oxide thin films. For example, using manganese and chromium based perovskite thin films and superlattices as a testbed, we are studying the relationship between octahedral tilting/distortion, chemical homogeneity, and electronic structure vis a combination of STEM imaging, diffraction, and spectroscopy techniques. This work is being extended to other oxide systems, where we continue to study not only the materials, but also develop microscopy and data analysis techniques.
Automating Scanning Transmission Electron Microscopy
The development of the electron microscopy field has reached the point where microscopists can readily acquire atomic level information on materials, but the required human input for each image limits the quantity and reproducibility of the data being acquired. Automated control of the electron microscope enables statistically significant data sets to be collected in a systematic way. Using Python code, the LeBeau group develops and utilizes software to control every aspect of the electron microscope. These scripts are universal and can be applied to any microscope. Further advancements in automated image acquisition will incorporate machine learning to search for specific features of interest in a sample. The enhanced capabilities will open new opportunities to study infrequent features such as defects and dopants in a statistically meaningful way.