Ultrafast 4D-STEM with dwell times less than 100 ns
4D-STEM relies on scanning the electron beam over a 2-dimensional sample plane. Each scanning point provides 2-dimensional diffraction data. Here is why the acronym 4D. Contrarily to conventional STEM detectors which record only single intensity values, hybrid pixel detectors maintain the spatial information of the reciprocal-space dataset, because of the 2-dimensionality of the pixel array.
What makes this technique so exciting is its versatility. 4D-STEM includes: virtual imaging (BF, DF, ADF, HAADF), phase mapping, orientation mapping, strain mapping, differential phase contrast and ptychography. These techniques are powerful tools in order to study materials composition, morphology, crystallinity, local displacements, lattice parameters and ultimately atoms with sub-Ångström resolution.
Ptychography in Electron Microscopy is a game changer and will replace conventional imaging. Recently, Dave Muller et. Al. showed the resolution increase by factor of 3 to 0.39 Ångström. Furthermore, the group of Angus Kirkland showed a tremendous contrast increase for cryo samples based on HI-Virus and Rotavirus with the usage of 4D-STEM and Ptychography. It was possible to see protein structure with a relative low dose (20 Electrons per Ångström²). The limit for this method is still the readout speed of the frame-based detector and a detector with a speed of 100,000 frames per second (10 µs dwell time) is needed.
The CheeTah T3 based on Timepix3 technology solves this problem with speeds up to 640,000,000 frames per second due to the event-based acquisition. Another advantage of the event-based data acquisition is the sparsity of the data packages.
Verbeeck et. Al. showed a low-dose application with an event-based Timepix3 detector for zeolites to avoid unacceptable damage. During the experiments, the defocused electron beam was rastered through the sample with short dwell times of 100 ns for 1024 x 1024 positions on the samples. Due to the fast speed and the sparse data, the Timepix3 has an acquisition rate for the complete sample (1024×1024) of less than one second. This method is especially suited for low-dose imaging and reducing unaccepted drift of the sample. This opens up the possibility to always perform 4D-STEM instead of conventional STEM and gain from the higher information content about the sample.
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