Fast and Low-dose Electron Ptychography

Abstract

In recent years, there have been many significant developments made in scanning transmission electron microscopy (STEM), notably the development of aberration correctors and complementary electron optical components such as monochromators and high brightness guns [1]. These advances have made it possible to obtain a 0.5Å resolution at 300kV for radiation resistant materials. However, spatial resolution is still limited for beam-sensitive specimen such as organics, biological specimens, zeolites and ceramics due to radiation damage. Beam-sensitive specimens have varying tolerances to electron dose due to different damage mechanisms. Although it is possible to reduce the dose in the STEM geometry by decreasing the pixel dwell time or the probe current density, a large pixel array is still needed for atomic resolution imaging and in addition manual adjustment of residual low order aberrations further increases the dose at the sample.Single shot coherent diffraction imaging (CDI) and ptychography has been widely used in light and Xray optics. The advantage of ptychography over traditional CDI is that it does not need prior information about the probe function and also overcomes some of the other issues of CDI, such as non-unique solutions and a limited field of view [2]. In electron microscopy, it has also attracted considerable interest due to its potential for super resolution [3], high phase sensitivity [5], three-dimensional [6] and low-dose [7] capabilities. One of the major developments that has advanced this field is the availability of a new generation of direct detection cameras that are particularly suited to ptychographic data acquisition with new modes of operation, such as electron counting and fast acquisition [8,9]. These new cameras dramatically also increase the detective quantum efficiency (DQE) and hence significantly improve the signal to noise ratio (SNR) of the recorded far field diffraction patterns (DP). Hence lower signals at higher scattering angles can be captured in each DP in a ptychograhic dataset which facilitates higher resolution in ptychographical reconstructions, even within the constraints of low electron dose work as required for beam sensitive samples.In this paper, we will firstly review our previous work on the capabilities of defocused probe ptychography to achieve a 2D phase reconstruction of a nanocrystal at sub-Å resolution [4] and a 3D reconstruction of nanostructured materials [7] using traditional CCD camera. Subsequently we will show results from focused and defocused electron ptychography using a fast direct electron detector to reconstruct the wavefunction of various low dimensional materials under different low dose conditions.

Imaging Beam‐Sensitive Materials by Electron Microscopy

“…The ptychographically reconstructed image has improved contrast transfer and better overall SNR than DPC or ABF/ADF,267 even under low dose conditions 291. This point can be further improved once a pixelated detector has higher sensitivity and dynamic range, because those weak signals at high scattering angles that contain high‐resolution information are collected with higher SNR 292. Sagawa et al developed a pixelated STEM detector coupled with a direct electron CCD image sensor under the ADF detector, allowing the simultaneous collection of ADF‐STEM images and 4D datasets under low‐dose conditions 293.…”

“…Muller and co‐workers reported a new type of electron microscopy pixel‐array detector (EMPAD) that allows deep sub‐Ångström resolution and significantly improved contrast for imaging point defects in 2D materials 294. An alternative low‐dose ptychography strategy is to use a defocused scanning probe, which allows post‐acquisition focusing and effectively overcomes the problems associated with long acquisition, high dose, specimen drift and unmanageable volume of data via only a few tens of highly overlapped probe positions, to achieve atomic‐resolution ptychographic reconstruction 292,295. Wang and co‐workers carried out low‐dose ptychography on MoS 2 using a direct detection camera and such a defocused probe 296.…”

Atomic Resolution Defocused Electron Ptychography at Low Dose with a Fast, Direct Electron Detector

“…A 17pA probe current was used corresponding to a dose of 1.78 × 10 2 e − /Å 2 per diffraction pattern. Since multiple DPs were recorded at each probe position, datasets could be synthesized post-experimentally by integrating different numbers of DPs at each probe position to give different doses 42 . Summations of eight, four, two, and one DPs provided four datasets with exposure times per DP equivalent to 16 ms, 8 ms, 4, ms, and 2 ms, respectively, as shown in Fig.…”