Efficient phase contrast imaging in STEM using a pixelated detector. Part 1: Experimental demonstration at atomic resolution

Abstract

We demonstrate a method to achieve high efficiency phase contrast imaging in aberration corrected scanning transmission electron microscopy (STEM) with a pixelated detector. The pixelated detector is used to record the Ronchigram as a function of probe position which is then analyzed with ptychography. Ptychography has previously been used to provide super-resolution beyond the diffraction limit of the optics, alongside numerically correcting for spherical aberration. Here we rely on a hardware aberration corrector to eliminate aberrations, but use the pixelated detector data set to utilize the largest possible volume of Fourier space to create high efficiency phase contrast images. The use of ptychography to diagnose the effects of chromatic aberration is also demonstrated. Finally, the four dimensional dataset is used to compare different bright field detector configurations from the same scan for a sample of bilayer graphene. Our method of high efficiency ptychography produces the clearest images, while annular bright field produces almost no contrast for an in-focus aberration-corrected probe.

Practical aspects of diffractive imaging using an atomic-scale coherent electron probe

“…As presented above, DPC offers a simple and fast constructive algorithm for phase reconstruction, but it is not the only approach to phase reconstruction from 4D data, the most established alternative in the STEM geometry perhaps being ptychography. Constructive algorithms exist for ptychography from weakly scattering objects, which have been shown to constitute an optimally electron-efficient processing of 4D data [7,9]. For strong phase objects, ptychography tends to involve iterative approaches [36][37][38].…”

“…Though position-resolved electron diffraction has a long history [1][2][3], the slow readout speed of the conventional CCD camera, limited data transfer speed and limited storage space have restricted acquisition to a small number of frames, making atomic resolution diffractive imaging extremely challenging [4]. Recent developments in fast-readout pixel detectors and powerful computers significantly improve the speed of data acquisition and transfer, and make possible the acquisition of two-dimensional CBED patterns with two dimensional probe scanning positions -a four dimensional (4D) dataset -at atomic resolution [5][6][7]. Such 4D datasets have been shown to allow synthesizing multiple imaging modes [4,8], differential phase contrast imaging [6] and ptychographic phase reconstruction [7,9].…”

“…Recent developments in fast-readout pixel detectors and powerful computers significantly improve the speed of data acquisition and transfer, and make possible the acquisition of two-dimensional CBED patterns with two dimensional probe scanning positions -a four dimensional (4D) dataset -at atomic resolution [5][6][7]. Such 4D datasets have been shown to allow synthesizing multiple imaging modes [4,8], differential phase contrast imaging [6] and ptychographic phase reconstruction [7,9]. However, the full potential of 4D datasets for quantitative analysis has yet to be established.…”

4D-Data Acquisition in Scanning Confocal Electron Microscopy for Depth-Sectioned Imaging

“…4D-STEM can deliver much more structural information [1,2] than conventional STEM where only integrated electron intensities are acquired. The 4D dataset of CBED patterns can be utilized for structural analysis such as ptychographic reconstruction [3][4][5][6][7][8][9], strain mapping [10][11][12], electric and magnetic fields imaging using differential phase contrast [13,14], and composition and thickness measurements [15] with position-averaged CBED (PACBED) [16].…”

Atomic resolution ptychographic phase contrast imaging of polar-ordered structures in functional oxides

“…In recent years, however, there has been a resurgence in the use of low angle scattered electrons using modes like annular bright field imaging [3] and standard bright field imaging [4,5]. This allows the direct imaging of oxygen atoms, which can be used for a full quantification of the local polarization [5], as has previously been performed using negative C S imaging using high resolution TEM [6].In principle, however, it is possible to reconstruct the phase at atomic resolution using ptychographic methods from suitable diffraction patterns with overlapping diffraction discs recorded at each scan point in a STEM image [7]. However, to do so practically requires the rapid recording of diffraction data, both to allow the acquisition of images with reasonable numbers of pixels in a sensible time scale, as well as to minimize the distortion images in real-space due to sample-drift.…”