Experimental and Simulation Methods in Scanning Electron Nanobeam Diffraction

Abstract

Traditional scanning transmission electron microscopy (STEM) detectors are large, single pixels that integrate a subset of the transmitted electron beam signal scattered from each electron probe position. These transmitted signals are extremely rich in information, containing localized information on sample structure, composition, phonon spectra, three-dimensional defect crystallography and more. Conventional STEM imaging experiments record only 1-2 values per probe position, throwing away most of the diffracted signal information. With the introduction of extremely high speed direct electron detectors, we can now record a full image (2D data) of the diffracted electron probe scanned over the sample (2D grid of positions), producing a four-dimensional dataset we refer to as a 4D-STEM experiment. In this tutorial, we will describe the challenges and opportunities created by 4D-STEM, and explain in detail the nuts and bolts of some 4D-STEM experiments and simulation methods. Figure 1a shows the basic geometry of a 4D-STEM experiment. A converged electron probe with a size ranging from sub-Ångstrom to multiple nanometers is focused upon the sample surface. The probe scatters from the sample volume, forming a convergent beam electron diffraction (CBED) pattern. This pattern is recorded with a high speed direct electron detector (a pixelated camera), and then the probe is moved to the next position on the sample surface. Electrons scattered to higher angles are often also recorded using an annular detector, generating another signal channel typically referred to as a high angle annular dark field (HAADF) image.