The use of electron ptychography methods for phase imaging in the scanning transmission electron microscope (STEM) have gained renewed interest due to the recent developments of fast pixelated detectors. One of those being the pnCCD (S)TEM camera [1] developed by PNSensor and PNDetector, it allows the recording of a 4D-dataset that consists of the full convergent beam electron diffraction pattern for each probe position that is scanned over the sample. This dataset contains all the scattering information generated by the electron sample interaction in the STEM experiment. The technique has allowed improved resolution beyond conventional limits [2] and phase contrast imaging at atomic resolution of different materials [3, 4]. In combination with other STEM techniques, it enables imaging of heavy and light elements of radiation-sensitive materials at the atomic scale [4].
In this work, we present an analysis and discussion of two different phase reconstruction algorithms, such as the Single Side Band (SSB) [3, 5, 6] and the Wigner-Distribution Deconvolution (WDD) [7] methods, in terms of their robustness to dynamical effects. The main difference of both methods is that the phase difference retrieved by the SSB method is still affected by the probe aberrations, meanwhile the WDD method allows to correct for this [4, 7]. However, in both cases a multiplicative interaction between the specimen function and the electron wave is assumed, which complicates the interpretation of the reconstructed wave when dynamical effects start to play a major role. Figures 1 and 2 show experimental results of retrieving the modulus (a) and the phase difference (b) of a gold nanoparticle measured with the pnCCD (S)TEM camera and using the discussed ptychography methods. It can be observed that by correcting the probe aberrations [3] while using the WDD method, the quality of the reconstructed image is dramatically improved. Moreover, a ring-like shape of the reconstructed phase difference for some atomic columns is observed. This behavior is studied in detail by image simulations (c) and discussed in terms of how dynamical effects influence the reconstruction algorithm. Furthermore, we explore the comparison of this method with other phase imaging techniques.
[1] H. Ryll, M. Simson, R. Hartmann, P.Holl, M. Huth, S. Ihle, Y. Kondo, P. Kotula, A. Liebel, K. Müller-Caspary, A. Rosenauer, R. Sagawa, J. Schmidt, H. Soltau, L. Strüder, manuscript accepted at Journal of Instrumentation
[2] P.D. Nellist, B.C. McCallum and J.M. Rodenburg, Nature 374 (1995) 630-632.
[3] T. J. Pennycook, A. R. Lupini, H. Yang, M. F. Murfitt, L. Jones, P. D. Nellist, Ultramicroscopy 151 (2015) 160 – 167.
[4] H Yang, R.N. Rutte, L. Jones, M. Simson, R. Sagawa, H. Ryll, M. Huth, T.J. Pennycook, H. Soltau, Y. Kondo, B.D. Davis, P.D. Nellist, manuscript submitted.
[5] J. M. Rodenburg, B. C. McCallum, P. D. Nellist, Ultramicroscopy 48 (1993) 304 – 314.
[6] H. Yang, T. J. Pennycook, P. D. Nellist, Ultramicroscopy 151 (2015) 232 – 239.
[7] J. M. Rodenburg and R. H. Bates, Phil. Trans. R. Soc. Lond. A. 339 (1992) 521 – 553.
Acknowledgement
The research leading to these results has received funding from the EPSRC (EP/M010708/1).
Figures:
Figure 1: Single Side Band method. Reconstruction of Au nanoparticle: a) Modulus (intensity bar from 5.5 to 6.9 x 10^6). b) Phase difference (intensity bar from -0.15 to 0.25 rad). Scale bar = 1 nm. c) Reconstruction of simulated images for 1 to 15 atoms. First row shows modulus (intensity bar from 0.5 to 1), second row shows phase difference (intensity bar from -0.05 to 0.4 rad).
Figure 2: Wigner Distribution Deconvolution method. Reconstruction of Au nanoparticle: a) Modulus (intensity bar from 0 to 0.03). b) Phase difference (intensity bar from -0.4 to 1.5 rad). Scale bar = 1 nm. c) Reconstruction of simulated images for 1 to 15 atoms. First row shows modulus (intensity bar from 0 to 0.001), second row shows phase difference (intensity bar from -0.05 to 1.5 rad).
To cite this abstract:
Gerardo T Martinez, Hao Yang, Lewys Jones, Martin Simson, Martin Huth, Heike Soltau, Lothar Strüder, Ryuusuke Sagawa, Yukihito Kondo, Peter D Nellist; Study on the robustness of electron ptychography for phase imaging in the STEM using fast pixelated detectors. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/study-on-the-robustness-of-electron-ptychography-for-phase-imaging-in-the-stem-using-fast-pixelated-detectors/. Accessed: December 2, 2023« Back to The 16th European Microscopy Congress 2016
EMC Abstracts - https://emc-proceedings.com/abstract/study-on-the-robustness-of-electron-ptychography-for-phase-imaging-in-the-stem-using-fast-pixelated-detectors/