Influence of plasmon-excitation electrons on ptychography phase imaging

Abstract text (incl. figure legends and references)

Introduction

Electron ptychography is a phase retrieval method in transmission electron microscopes (TEM) in which the phase of exit wavefunction is reconstructed from electron diffraction patterns[1]. It is used for a wide range of specimens for atomic-scale imaging, including sensitive battery materials[2] and biological structures[3]. As a method of coherent imaging, contrast of electron ptychography is based on coherence of electron beam. However, inelastic scattering that happens in the interaction between beam and specimen introduces an energy distribution of the exit beam. With increased thickness of the sample, the proportion of inelastic scattered electrons (most of which are plasmon-excited electrons) increases.

Objective

In this work, we investigate the influence of inelastic electrons, especially plasmon-excited electrons on ptychographic reconstruction.

Materials & Methods

To explore the influence of inelastic scattered electrons, in an aberration-corrected TEM at 200kV, 4D-STEM datasets with and without energy filter were recorded at different defocus conditions on a sample of gold nanoparticles and carbon film. The convergence angle of the electron beam is 22 mrad. Then ptychography reconstructions were carried out on recorded 4D-STEM datasets using the ePIE algorithm[4].

Results

The reconstruction results shows that phase contrast images for atomic-scale resolution are stable whether or not the inelastic scattered electrons are filtered. This result is in line with the prediction based on characteristic scattering angle for inelastic scattering. For this experiment, the characteristic scatter angle of plasmon-exited electrons (energy loss: 20 eV) is 0.005 mrad. However, the pixel size of 4D-STEM detector used in the experiment is 0.25 mrad, which is far bigger than the characteristic scatter angle. This means that statistically, most electrons will still be detected by the same pixel whether or not they are inelastic scattered along with the image-forming elastic scattering. Therefore, the normalized intensity distribution of recorded diffraction patterns with or without energy filter are almost the same.

However, if a smaller pixel size is used, i.e. a larger camera length, for a beam with a smaller convergence angle, the situation is different. For example, if a convergence angle of 1~2 mrad is used for large field-of-view ptychographic imaging[3], the pixel size on detector may be comparable to characteristic scatter angle under a similar experimental condition. In this situation, the influence of inelastic scattered electrons on electron ptychography should not be ignored.

Conclusion

In summary, plasmon-excitation electrons, which are the majority amount of inelastically scattered electrons, have a negligible effect on atomic resolution ptychography.

Reference

[1] Nellist, P., McCallum, B. & Rodenburg, J. M. Nature 374, 630-632 (1995).

[2] Lozano, J. G., Martinez, G. T., Jin, L., Nellist, P. D. & Bruce, P. G. Nano letters 18, 6850-6855 (2018).

[3] Zhou, L. et al. Nature communications 11, 1-9 (2020).

[4] Maiden, A. M. & Rodenburg, J. M. Ultramicroscopy 109, 1256-1262 (2009).

figure legends:

Figure 1: Ptychographic phase reconstructions (a) without energy filter, i.e. elastic and inelastic scattered electrons are both recorded, and (b) with energy filter, i.e. only elastic scattered electrons are recorded. Defocus value is -14 nm.