Charge transfer sensitivity and dose efficiency with pixilated detectors and ptychographic phase contrast imaging in STEM

Abstract

Ptychography provides a sophisticated means of retrieving the complex object function via coherent diffractive imaging. It has become successfully established in the x‐ray and visible light communities as a means of lensless imaging and for its super‐resolution capability. Super‐resolution was also the original use of the method in electron microscopy [1]. However the technique did not become popular in the high resolution electron microscopy community due to the difficulty of acquiring and processing the four dimensional datasets required. Recent advances in detector technology however have resulted in a resurgence of interest in the method. As aberration correction now provides atomic resolution in hardware without the need for super‐resolution techniques, interest in ptychography in scanning transmission electron microscopy (STEM) has shifted towards achieving efficient phase contrast imaging. STEM provides sensitivity to atomic number via Z‐contrast annular dark field (ADF) imaging. The approximately quadratic variation of the intensity in ADF images with atomic number provides relatively facile compositional interpretability as compared to phase contrast imaging. However a relatively small proportion of the beam current is scattered out to the high angles sampled by ADF detectors, particularly for thin samples composed of light elements. Most of the transmitted electrons are contained within the bright field (BF) disk. Ptychography has recently been shown to be more efficient than other phase contrast imaging methods used in STEM, including conventional BF, annular bright field (ABF), and differential phase contrast (DPC) [2,3]. It has also proven superior to these modes at revealing the positions of light elements hidden by the scattering of heavy elements in the ADF signal [4]. Furthermore, ptychographic phase imaging requires no aberrations to achieve contrast, meaning the electron probe can be tuned to maximum capability of the aberration corrector. Here we investigate the sensitivity of STEM ptychography for two different applications. The first makes use of the sensitivity of phase contrast imaging to electromagnetic fields to detect charge transfer. Such charge transfer sensitivity was demonstrated in conventional TEM by Meyer et. al. by making use of lens aberrations to reveal contrast changes in N‐doped graphene and hexagonal boron nitride (hBN) that only matched with simulations based on potentials including the effects of charge transfer produced by density functional theory (DFT) and not the neutral atom potentials. We will present the results of testing charge transfer sensitivity in STEM with ptychography and various low dimensional materials. Figure 1 compares the projected potentials of (hBN) simulated with and without charge transfer. Figure 2 shows an example of simultaneously acquired ADF and ptychographic phase images of a region of single layer hBN surrounded by a double layer taken with the microscope fully tuned with the aberration corrector. As residual aberrations can affect phase images, we will also investigate the use of post acquisition aberration quantification and correction applied to ptychographic datasets of samples with the relatively subtle contrast effects of charge transfer. The second application of the sensitivity of STEM ptychography is its use for beam sensitive samples. We will assess the dose effectiveness of the method through simulations of varies samples, including biological samples frozen in amorphous ice, and compare to conventional TEM imaging. Consideration will be made of the pixelated detector technologies currently available, as the sensitivity and speed of the detector directly influence the dose effectiveness of the ptychographic phase images.