Abstract text (incl. figure legends and references)

(Scanning) transmission electron microscopy ((S)TEM) is a rapidly developing field with a large variety of techniques widely used in modern material science. Signals generated within TEM during electron-matter interaction provide information about the composition, crystallography, mass-thickness, and local electromagnetic fields [1]. Secondary electrons (SEs) are electrons ejected from a sample after electron beam irradiation. SEs originate from the near-surface region of the sample, so these electrons are often exploited in scanning electron microscopy to form images that are sensitive to the surface topology. Topographic information from SEs would greatly aid the interpretation of conventional 2-dimensional STEM images [2], but unfortunately, SE imaging is not available on most conventional STEM instruments. In this work, we demonstrate the capabilities and benefits of SE imaging in STEM as an alternative 3D characterization method.

SE images were obtained using an aberration-corrected "cubed" Thermo Fisher Themis Z instrument operated at an acceleration voltage of 200 kV and a beam current of 250 pA. We used a recently proposed methodology to detect SEs using electron beam-induced current (SEEBIC) [3]. A homemade transimpedance amplifier (TIA), electrically connected to the sample via a DENS Solutions Wildfire holder, was used to convert the SEEBIC signal into a voltage signal digitized by the Gatan Digiscan II unit along with the HAADF-STEM detector signal.

SE images contain topography information, so they enable revealing the information hidden in conventional HAADF-STEM. We demonstrate this effect on self-assemblies of rounded Fe_xO/CoFe₂O₄ nanocubes in spherical confinement [4]. Figure 1 shows the five-fold symmetry of an icosahedron, and demonstrates the presence of stacking faults. It should be noted that retrieving the information about such complex morphologies from a single or a few images is very important when the acquisition of tilt series for electron tomography is not possible due to various factors. These approaches are inspired by techniques used in scanning electron microscopy for surface metrology. We demonstrate our first attempts to retrieve 3D information using the shape-from-shading and structure-from-motion algorithms. Another important direction is the investigation of surface facets. The surface nature of the SE signal can greatly aid the understanding of the microscopy data.

Figure 1. (a) HAADF-STEM and (b) SEEBIC images of self-assemblies of Fe_xO/CoFe₂O₄ nanocubes.

In conclusion, we have demonstrated the versatility of SE imaging in modern material science for various purposes. The surface origin of the SE contrast enables to extract morphological information that can be hidden in conventional STEM images. The use of novel techniques for 3D reconstructions was discussed as an alternative to conventional electron tomography.

The project has received funding from European Research Council (ERC Consolidator Grant 815128, REALNANO).

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