Since the first experimental charge coupled device was reported in 1982 [1], there have been a series of major developments in digital imaging techniques for transmission electron microscopy (TEM). These include use of complementary metal-oxide semiconductor (CMOS) devices, which resulted in improvements in camera sensitivity, detective quantum efficiency (DQE) and speed. Here we will present how such developments can benefit some common TEM based experiments, such as electron tomography (ET) and four dimensional scanning TEM (4D-STEM) diffraction.

ET consists of acquisition of a series of images of the specimen in different viewing directions and is used for three dimensional (3D) studies of nanoscale materials in a TEM. The tilt range and tilt increment in an ET experiment directly affects the resolution of the 3D reconstruction.  In cases where the specimen is electron sensitive, the number of projections that can be recorded is typically limited as the sample is repeatedly exposed to the beam. Leveraging the advantages of a high speed camera can also benefit low dose ET and 3D time-resolved studies of dynamic processes in a TEM. Here high speed ET datasets will be presented that were collected using a high speed CMOS camera while the TEM goniometer was continuously tilting. Such an approach improves the resolution of 3D reconstruction for thicker specimens by reducing the tilt increment from several degrees to a small fraction of a degree, and reduces the data acquisition time from several tens of minutes to a few minutes, simultaneously improving angular resolution and potentially reducing beam damage to the specimen.

STEM diffraction imaging is a common analytical method to collect specimen structure, strain and texture. Here either a convergent or parallel electron beam is used to produce diffraction patterns, which can be used to characterize defects, interfaces and small nanostructures and allow accurate measurements of strain and crystal orientation. 4D-STEM diffraction is done by collecting a diffraction pattern pixel by pixel, as the electron beam is scanned on the specimen. Limited data collection speed (i.e., frame rate of the sensor) has been one of the main challenges of this technique. Conventional CCD cameras were limited to up to 30 frames per second (fps), which restricted the number of diffraction patterns collected in a given amount of time. This can be even more challenging in the cases of beam sensitive specimens, or when drift exists. We will present 4D-STEM datasets collected with high speed CMOS cameras and will show how these new systems with superior DQE and speed can benefit STEM diffraction imaging experiments.

Figure 1a below shows 2 images from a high speed tomography experiment on an array of Au nanoparticles. These two images are approximately 60 degrees apart and it was collected in 120 degree tilt range in 110 seconds. The reduction of such a data stream as a tomogram will be presented. And, Figure 1b shows CBED patterns from inside and outside of a vacancy dislocation loop in a Cu specimen.

Reference:

[1] PTE Roberts, JN Chapman and AM MacLeod, Ultramicroscopy 8 (1982), p. 385.

Figures:

Figure 1. (a) Pair of images 60 degrees apart in high speed tilt experiment. Latex spheres are 250 nm in diameter (b) CBED patterns from a prismatic dislocation loop in a copper foil. Image width is 100 nm.

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EMC Abstracts - https://emc-proceedings.com/abstract/new-approaches-to-multi-dimensional-experiments-in-stem-application-of-high-speed-cameras/