Quantification of light elements in ABF and 4D STEM electron ptychography

Session

Stream 1: EMAG - 4D-STEM

Authors

Dr Emanuela Liberti (1, 2, 4), Dr Colum M. O'Leary (2, 3), Mr Kevin P. Treder (2), Dr Judy S. Kim (1, 2, 4), Prof Peter D. Nellist (2), Prof Angus I. Kirkland (1, 2, 4)

Affiliations

1. The Rosalind Franklin Institute
2. University of Oxford, Department of Materials
3. University of California
4. electron Physical Science Imaging Centre (ePSIC)

Keywords

Quantification, STEM, 4D STEM, oxygen, atom counting, ABF, electron ptychography 

Abstract text

Annular-dark field imaging is one of the most readily interpretable techniques in scanning transmission electron microscopy (STEM). The ADF detector integrates the intensity of incoherent, high-angle scattered electrons providing images with strong atomic number sensitivity that can be directly interpreted in terms of the sample’s atomic structure. The ADF contrast increases monotonically with the number of atoms in projection. This allows direct atomic counting to solve three-dimensional (3D) structures or determine composition at a given thickness [1 – 3]. However, ADF STEM is not efficient for light elements detection because they scatter less strongly at high scattering angles. More suitable methods to image both light and heavy atoms are based on bright-field or phase-contrast techniques that collect low-angle scattered electrons. However, because these electrons scatter coherently, image quantification, in this case, is non-trivial.

This talk will discuss quantification methods for two main (quasi-)coherent STEM imaging modes: annular-bright field (ABF) imaging and 4D STEM electron ptychography.

In the case of ABF, image quantification is applied to the characterisation of the oxygen framework in lithium-rich Li_1.2Ni_0.2Mn_0.6O₂ (LNMO) layered cathodes [4]. In this material, short-range oxygen sublattice distortions are evidence for oxygen participation in charge compensation mechanisms. These movements are of the order of picometres and, to be measured, require well-designed quantification methods. However, achieving picometre accuracy and precision is challenging in these materials because of the limited signal-to-noise ratio of the images imposed by the electron dose [5]. The first part of this talk discusses how to achieve high-quality ABF quantification using a combination of experimental design and computational data processing including simultaneous ADF atom counting and multi-slice image simulations. 

The second part of the talk shows how these methods can be extended to the quantification of 4D STEM electron ptychography data using fast electron detectors [6]. A four-dimensional dataset consist of a two-dimensional (2D) convergent beam electron diffraction pattern (CBED) recorded at every pixel of a 2D scan array. The exit wave resulting from the electron-specimen interaction can be restored from this dataset using electron ptychography. In analogy with the contrast in ADF, the exit wave phase is fully quantitative and is highly sensitive to the number of atoms in the sample and their position along the column [7]. Here, phase and ADF quantification are used to count the number of light and heavy elements simultaneously. The technique is applied to determine the composition in a reduced CeO_2-x nanoparticle by simultaneous counting the number of Ce and O atoms.

Experimental design is fundamental for any quantification technique. The last part of this talk discusses design strategies for comparing ptychographic techniques at low dose and imaging of light biological materials. Also, it examines how to efficiently compare restored data based on the effect of the contrast transfer function for the recovered spatial frequencies [8,9].

 

References

[1] L. Jones et al., Nano Lett. 14 (2014) 6336.

[2] S. Van Aert et al., Phys. Rev. B 87 (2013) 064107.

[3] A. Rosenauer et al., Ultramicroscopy 111 (2011) 1316.

[4] E. Liberti et al., Ultramicroscopy 210 (2020) 112914.

[5] J. G. Lozano et al., Nano Lett. 18 (2018) 6850.

[6] H. Yang et al., Nat. Comm. 7 (2016) 12532.

[7] D. Van Dyck et al., Nature 486 (2012) 243.

[8] C. O’ Leary et al., Ultramicroscopy 221 (2021) 113189.

[9] L. Zhou et al. Nat. Comm. 11 (2020) 2773.