Advancing Atomic-Scale Imaging with Ultra-Low Dose Cryo-EELS Mapping

Despite the use of modern, high-brightness probes and extremely efficient energy loss spectrometers, beam-sensitive materials pose a challenge for atomic-resolution electron energy loss spectroscopy (EELS) in scanning transmission electron microscopes.

These materials cannot survive the high probe currents commonly used in an analytical STEM. Cryogenic cooling of the specimen enhances dosage sensitivity, but it introduces the difficulty of specimen drift and instability.

Multi-frame spectrum imaging (SI) summation was proposed and proven effective in enhancing STEM image quality and signal-to-noise ratio (SNR).¹

Scintillator-based CMOS and CCD detectors are insufficient for multi-frame SI at low dosage and fast speeds in EELS due to the adverse effects of read noise. Direct detection and counting cameras have practically noise-free readout, making them excellent for multi-frame EELS SI capture at low dose.²

This article presents some of the advantages of this technology for cryo-EELS mapping of beam-sensitive material at atomic resolution.

Materials and Methods

The data was collected at -170 °C with a 20 pA probe through a GIF Continuum^® K3^® IS electron counting EELS system placed on an uncorrected 200 kV cFEG (S)TEM. Sr₂RuO₄ (SRO) grown on [110] DyScO₃ (DSO) was selected as a model system due to the combination of radiation-hard (DSO) and beam-sensitive (SRO) on the same sample.

To control the dosage rate and reduce drift, separate 360 x 50-pixel 2D array SI passes were recorded with a 339 µs dwell time, resulting in a 6.1 s duration for each pass. A total of 234 passes were aligned and averaged, resulting in a net integration time of 79 ms/pixel.

Figure 1 shows a comparison of annular dark field (ADF) and raw EELS signal intensity maps for a single SI pass and the entire 234-pass summation. The raw data exhibits periodic contrast with a spacing of ~4 Å, requiring no denoising.

The ADF intensity minima and maxima are consistent with cation locations in all elemental maps, as well as the expected A- and B-site cation occupancy for film and substrate crystallographic structures.

Figure 1. 360 x 50-pixel 2D array dose fractionated EELS SI acquired from SRO/[110] DSO with a 339 μs pixel dwell time. Each individual SI pass is acquired in 6.1 s. ADF images and raw EELS signal intensity maps for an individual pass are shown on the left. Equivalent results for the 234-pass summation are shown on the right. Line integrals from the summation data show atomic resolution is achieved up to 3,000 eV energy loss, despite a short-accumulated pixel time of 79 ms and ultra-low probe current of 20 pA. An individual K3 electron counted spectrum and summation are shown overlaid in the bottom left. All major and minor edges for SRO and DSO are resolved at high energy resolution (0.9 eV), high SNR, and in a single spectral range of 3,110 eV. This is only possible due to the narrow PSF and high sensitivity of the K3 camera. Sample courtesy of B. Goodge, Max Planck Institute for Chemical Physics of Solids, Dresden, Germany. Image Credit: Gatan, Inc.

Summary

Dose-fractionated EELS spectrum images of SRO grown on [110] DSO were taken under cryogenic conditions.

The K3 electron counting camera generated atomic resolution maps of ionization edges up to 3,000 eV with dwell lengths of ~79 ms and probe current of 20 pA, resulting in less beam damage on both the DSO and the more sensitive SRO.

References

1. Jones, L., et al. (2018). Managing dose-, damage- and data-rates in multi-frame spectrum-imaging. Microscopy, 67(suppl_1), pp.i98–i113. DOI: 10.1093/jmicro/dfx125. https://academic.oup.com/jmicro/article/67/suppl_1/i98/4797531.
2. Goodge, B.H., Baek, D.J. and Kourkoutis, L.F. (2020). Atomic-resolution elemental mapping at cryogenic temperatures enabled by direct electron detection. arXiv (Cornell University). DOI: 10.48550/arxiv.2007.09747. https://arxiv.org/abs/2007.09747.

This information has been sourced, reviewed and adapted from materials provided by Gatan, Inc.

For more information on this source, please visit Gatan, Inc.