Enhanced Resolution from Full-Field Ptychography with an Electron Microscope Pixel Array Detector

Abstract

In STEM, complete information about the scattering potential of a sample is encoded in the distribution of transmitted electrons. To measure the entire diffraction pattern, a new generation of high-speed, momentum-resolved detectors has been developed, enabling new imaging modes. Unfortunately, most of the new detectors were designed for low-dose x-ray and biological imaging. Their pulse counting architectures are limited to beam currents of less than ~0.1 pA/pixel, making them poorly suited to high-speed scanning electron diffraction applications. Recently, we developed electron microscope pixel array detector (EMPAD) [1] that has a high dynamic range (1,000,000:1) while preserving single electron sensitivity. The camera has fast readout speed (0.86 ms/frame) and can record all the scattered electrons for atomic-resolution probes. These properties open the doors for new quantitative imaging applications including strain fields in 2D materials [2] and polarization vortices in ferroelectrics [3]. Here we show that applying electron ptychography to full diffraction patterns obtained by EMPAD leads to higher resolution reconstructions than ADF-STEM or prior bright-field ptychography techniques. Figure 1(a) shows a diffraction pattern extracted from a single molybdenum atom from a 4D data set of a monolayer MoS 2 recorded on an aberration-corrected FEI Titan at 80 keV and 7 pA. We used the super-resolution ePIE algorithm [4] and simultaneously reconstructed the transmission function and probe functions with a pixel size of 0.11 angstrom, roughly half of the scan step size. Compared with annular dark field (Figure 1b) and integrated center-of-mass images (Figure 1c), the ptychography reconstruction (Figure 1d) achieves a higher resolution and is more robust to noise and aberrations in the probe. The sulfur monovacancy (indicated by red arrows) is also well-resolved, as shown in Figure 3c. We explored the role of higher angle diffraction information by applying a mask to each CBED pattern before ptychographic reconstruction. The cutoffs are chosen to be integer multiples of the convergence semi-angle (a=21.4 mrad). When only using the central bright-field disk, the reconstructed phase (Figure 2a) is similar in resolution to iCoM and ADF. As the cutoff increases, atoms become sharper until about 3a (Figure 2b-d). However, the probe's shape is similar at different cutoffs (Figure 3a), indicating resolution improvement is mainly due to the electrons collected at high scattering angles. References: [1] M. Tate et al, Microscopy and Microanalysis 22