Abstract
Crystalline materials used in technological applications are often complex assemblies composed of multiple phases and differently oriented grains. Robust identification of the phases and orientation relationships from these samples is crucial, but the information extracted from the diffraction condition probed by an electron beam is often incomplete. We have developed an automated crystal orientation mapping (ACOM) procedure which uses a converged electron probe to collect diffraction patterns from multiple locations across a complex sample. We provide an algorithm to determine the orientation of each diffraction pattern based on a fast sparse correlation method. We demonstrate the speed and accuracy of our method by indexing diffraction patterns generated using both kinematical and dynamical simulations. We have also measured orientation maps from an experimental dataset consisting of a complex polycrystalline twisted helical AuAgPd nanowire. From these maps we identify twin planes between adjacent grains, which may be responsible for the twisted helical structure. All of our methods are made freely available as open source code, including tutorials which can be easily adapted to perform ACOM measurements on diffraction pattern datasets.
Highlights
The two primary tools used to study the orientation of polycrystalline materials are electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM), and transmission electron microscopy (TEM)
To generate a spatially-resolved orientation map, we can focus a scanning TEM (STEM) probe down to dimensions of 0.5 to 50 nm, scan it over the sample surface, and record the diffraction pattern for each probe position, a technique referred to as nanobeam electron diffraction (NBED) (Ozdol et al, 2015), scanning electron nanobeam diffraction (SEND) (Tao et al, 2009), or four dimensional-scanning transmission electron microscopy (4D-STEM) due to the 4D shape of the collected data (Bustillo et al, 2021). 4D-STEM experiments are increasingly enabled by fast direct electron detectors, as these cameras allow for much faster recording and much larger fields of view (Ophus, 2019; Nord et al, 2020; Paterson et al, 2020)
We have introduced an efficient and accurate method to perform automated crystal orientation mapping, using a sparse correlation matching procedure
Summary
The two primary tools used to study the orientation of polycrystalline materials are electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM), and transmission electron microscopy (TEM). We can directly measure the atomic-scale structure and the orientation of polycrystalline grains, either by using plane wave imaging in TEM (Li et al, 2020), or by focusing the probe down to sub-atomic dimensions and scanning over the sample surface in scanning TEM (STEM) (Peter et al, 2018). Howeve,r strictly limits the achievable field-of-view, and requires relatively thin samples, and is primarily suited for measuring polycrystalline grain orientations of 2D materials (Ophus et al, 2015; Qi et al, 2020). Another approach to orientation mapping in TEM is to use diffraction space measurements. To generate a spatially-resolved orientation map, we can focus a STEM probe down to dimensions of 0.5 to 50 nm, scan it over the sample surface, and record the diffraction pattern for each probe position, a technique referred to as nanobeam electron diffraction (NBED) (Ozdol et al, 2015), scanning electron nanobeam diffraction (SEND) (Tao et al, 2009), or four dimensional-scanning transmission electron microscopy (4D-STEM) (we choose this nomenclature for this text) due to the 4D shape of the collected data (Bustillo et al, 2021). 4D-STEM experiments are increasingly enabled by fast direct electron detectors, as these cameras allow for much faster recording and much larger fields of view (Ophus, 2019; Nord et al, 2020; Paterson et al, 2020)
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