High-energy resolution monochromated STEM-EELS mapping across large areas

Scanning transmission electron microscopy (STEM) allows for high spatial-resolution analysis of materials and, when coupled with electron energy loss spectroscopy (EELS), becomes capable of providing substantial insight into both chemical and optical properties. In recent years, focus has moved towards understanding material properties at the atomic level using EELS. However, there are still significant barriers when attempting to perform high-energy resolution monochromated STEM-EELS analysis on large structures. Off-axis distortions cause additional aberrations to couple into the spectrometer when scanning across large regions. This often limits STEM-EELS mapping to small areas to maintain the energy resolution or requires sacrificing this resolution to spectrum maps spanning multiple microns. We propose here a methodology enabling low-loss STEM-EELS spectrum mapping to be performed over tens to hundreds of microns while maintaining high energy-resolution through modification of the EELS collection conditions. This is accomplished not only through careful alignment of the scan/descan coils, but, more importantly, through implementation of elongated camera lengths that effectively magnify the object over the EELS entrance aperture, cutting out higher order aberrations and reducing shifts on the spectrometer.