Atomic-Level EELS Mapping Using High-Energy Edges in Dualeels™ Mode

Abstract

With advancements in aberration correction, the spatial resolution of scanning transmission electron microscopy (STEM) has been enormously improved. In addition to the reduction of the STEM probe size, a dramatic increase in the STEM probe current has been realized, leading to the routine acquisition of high-resolution elemental and chemical maps using electron energy loss spectrometry (EELS). Using EELS combined with these advanced STEM instruments, atomic-level resolution information can be obtained from various types of materials, revealing the nature of interfaces, elemental distribution, presence of defects, and much more. In addition to simple elemental composition distributions, EELS is capable of delivering information about the chemical bonding, local atomic coordination, oxidation states, band gaps, and chemical phases of a broad range of materials at the fundamental resolution limit of the property being probed. Atomic-level EELS maps of these fundamental material properties can now be obtained with the acquisition time, to a large extent, limited only by the speed of the EELS spectrometer and not by the signal being measured. The availability of fast EELS spectrometers with large angular collection efficiencies has closed the gap between the rate of signal generation in the specimen and the speed at which this signal can be detected. This significantly increases the amount of information that can be acquired using EELS. Using the most recent generation of spectrometers, EELS data can be acquired at well over 1,000 spectra per second with a high-duty cycle. Fifth-order spectral aberration correction in this generation of spectrometers allows the use of the large collection angles needed to match the increased convergence angle that Cs-probe-corrected systems present, improving collection efficiency while maintaining energy resolution. These advances, when taken together, result in a well matched source/detector system capable of recording high-energy EELS edges at atomic resolution at a rate fast enough to limit electron beam damage to the sample.