Advancing EELS Applications by Combining New Scan Strategies with Direct Detection Cameras

Advancements in electron optics have pushed the spatial resolution of electron energy loss spectroscopy. This has enabled scientists to study chemical and electronic structure changes at the atomic scale [1]. Traditionally, spectrum images were acquired in one single pass. A typical probe current of 100-200pA and dwell times on the order of 5-10ms were needed to obtain a sufficient signal-to-noise ratio to resolve the features in an EELS spectrum image (SI), leading to a total dose on the order of 10⁷ e^-/Ų. Such a large dose is needed due to the relatively small inelastic cross sections of most ionization edges of interest, and the poor collection efficiency of the previous generation CCD cameras. These large doses have prohibited achieving similar resolutions in dose-senstive samples such as Zeolites, which has a total dose thresh hold of &lt;3000 e^-/Ų. In extremely dose-sensitive samples such as polymers and biological samples, whose total dose threshold is between 40-100 e^-/Ų, the spatial resolution is limited to a range of 10’s nm. Such a dose-constrained resolution prevents the investigation of soft-mater interfaces and potential insight into chemical changes in biological samples that occur on the nm scale.<br/><br/>The Continuum GIF, combined with Gatan's Direct Detection Cameras, enables SI pixel dwell times of ~100s of μsec. due to the increased frame rate of the cameras and collection efficiency of the spectrometer’s optics. An increase in speed inherently changes the mode of SI acquisition from a single pass to acquiring the SI in multiple rapidly acquired passes. This has been shown to reduce or eliminate sample degradation through dose fractionation, where a series of fast spectrum image passes spread the total dose over the same accumulated time as a single pass [2,3]. Compared to the traditional raster pattern, alternative scan patterns have also been shown to reduce the level of sample degradation in ADF STEM images by reducing the effect of damage delocalization caused by multiple scattering to the surrounding sample volume [4]. These new scan strategies are now being implemented with Gatan's Digital Micrograph and DigiScan 3. The fast read-out speed, the ability to fractionate the dose over multiple passes, and the reduction of damage delocalization has enabled the advancement of EELS SI into materials applications that were otherwise not thought possible.<br/><br/>We demonstrate how the combination of these new scan strategies enabled by direct direction cameras can push EELS spectrum imaging into new applications. We show that atomic resolution Spectrum imaging can be achieved on a ZSM-5 Zeolite sample. For multipass spectrum image acquisition, we use Digital Micrographs in-situ SI tool to individually save each SI and pass. This allows us to play back the sequence of passes post-acquisition to monitor degradation in the sample. We hope to show that these new scan strategies, combined with direct detection cameras, can help push EELS into new applications, including biological samples.<br/><br/>[1] Muller D.A. <i>et. al., Science, </i><b>319</b> (2008) DOI: 10.1126/science.1148820<br/>[2] Jones L. <i>et. al.</i>, <i>Microscopy</i>, <b>67</b> (2018) DOI: 10.1093/jmicro/dfx125<br/>[3] Johnston-Peck A.C. <i>et. al.</i>, <i>Ultramicroscopy</i>, <b>170</b> (2016) DOI : 10.1016/j.ultramic.2016.07.002.<br/>[4] Velazco A. <i>et. al., Ultramicroscopy</i>, <b>232</b> (2022) doi.org/10.1016/j.ultramic.2021.113398.