Optimal Structural Determination with Focused Probe Electron Ptychography

Over the past decade advances in camera technology have greatly facilitated 4D scanning transmission electron microscopy (STEM) and electron ptychography. A wealth of information on the probe position dependence of the scattering can be discerned from 4D STEM data, which electron ptychography uses to provide an especially useful means of directly imaging atomic structures. In essence ptychography solves the so called phase problem in electron microscopy, determining the relative phases of the scattered beams. As the phase of the beam electrons is altered by even the weakest of specimen potentials, efficient detection of phase provides a very efficient form of imaging. The dose efficiency of ptychography far exceeds that of Z-contrast annular dark field (ADF), now a mainstay for local atomic structure determination in materials science [1]. However ptychography can also significantly exceed the dose efficiency of phase contrast imaging in plane wave illumination TEM [2], the mainstay technique for the most beam sensitive materials such as proteins. Such TEM based phase contrast provides intensity images rather than actual images of the phase. Furthermore TEM phase contrast images are often complex and difficult to interpret being the result of using the aberration function as a virtual phase plate, with the resulting contrast transfer function generally oscillating to positive and negative values. Ptychography provides a simple single signed transfer function controlled by the convergence angle without the need for aberrations, which would be detrimental to simultaneous ADF imaging.<br/><br/>In addition to pure dose efficiency ptychography also fills another important deficiency of ADF imaging. Light atoms are often hidden in the strong scattering from neighboring heavy elements in the ADF signal. Addressing this gave rise to the popularity of the annular bright field (ABF) modality. Recently centre of mass (CoM) based methods such as integrated CoM have become more popular as they provide superior dose efficiency compared to ABF. However the dose efficiency of ptychography exceeds all of these, and allows heavy and light elements to be seen most clearly. Similarly, the greater dose efficiency should allow more sensitive examination of charge density for looking at effects from bonding, for instance. The ability to correct for the residual aberrations post collection is also a clear advantage for ptychography compared to CoM methods, and is indeed very important for separating true effects of charge density from those of aberrations.<br/><br/>Phase imaging with electron ptychography is however not entirely free from complications. It can be more difficult to discern different elements than with ADF Z-contrast. Furthermore, as sample thickness increases the projected potentials cause increasingly large changes in the phase of the scattered beams, which eventually wrap around because phase values exist only in the range of 0 to 2π. Thus complex contrast reversals can also occur in ptychography. However these can be simply compensated for by a small defocus. Ensuring that the structures reflect reality is greatly facilitated by simultaneous Z-contrast imaging, easily available with focused probe ptychography. Thus with focused probe ptychography one can have the best of both worlds, Z-contrast and highly efficient phase imaging. With event driven camera technology this can be performed without any loss of speed compared to conventional rapid scan STEM [3].<br/><br/>[1] T. J. Pennycook et al., Ultramicroscopy 151, 160-167 (2015)<br/>[2] T. J. Pennycook et al., Ultramicroscopy 196, 131-135 (2019)<br/>[3] D. Jannis et al., Ultramicroscopy 233, 113423 (2022).<br/>[4] We acknowledge funding from European Research Council grant nos. 802123-HDEM and 823717–ESTEEM3, European Union’s Horizon 2020 Research and Innovation Programme grant No. 101017720 and FWO projects G042920N and G042820N.