Sampling Optical Modes and Electronic States with Fast, Monochromated EELS

In recent years, the field of monochromated STEM-based electron energy-loss spectroscopy (EELS) has undergone significant technological advances, bringing a sub-10 meV spectral resolution that has opened up new research possibilities, such as the mapping of phonon modes, and isotope discrimination. In comparison, here we present the experimental avenues enabled by other instrumental developments, as offered by the monochromated high brightness X-FEG source of a double aberration-corrected FEI Titan Themis 60-300. This source enables STEM-EELS analyses with a subnm electron probe that combines an energy resolution of ~90–110 meV with high beam currents of 200–250 pA. By allying this probe with data acquisition using a Gatan GIF Quantum ERS spectrometer, we achieve a rapid, high spatial statistics mapping of optical mode excitations in the 0.4–3 eV energy loss range (e.g. 800 x 800 px in < 10 minutes). Equally, using a relatively high beam energy of 300 keV reduces the proportion of optical loss signal that is convolved with bulk plasmon scattering, thereby improving the signal to noise ratio when measuring the projected loss intensity across the body of nanophotonic structures.