Secondary Electron Induced Current in Scanning Transmission Electron Microscopy—An Alternative Way to Visualize the Morphology of Nanoparticles

Electron microscopy is a useful tool to perform a detailed characterization at the level of individual nanoparticles. Although a plethora of electron microscopy imaging modes are available, a rough distinction can be made between scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The difference between both approaches is related to the fact that in SEM mode one is predominantly probing the surface structure of a sample under investigation, whereas in TEM mode, a projection of the entire sample is measured. SEM is quite user-friendly and often accessible in a scientific environment, but the resolution of a typical SEM instrument is on the order of 1–20 nm. On the other hand, (scanning) transmission electron microscopy ((S)TEM) yields (atomic resolution) information on both the structure and composition of a broad variety of nanomaterials, eventually along with signatures of their electronic and optical properties. However, TEM images conventionally correspond only to a two-dimensional (2D) projection of a three-dimensional (3D) object, which often hampers a clear understanding of the morphology of nanoparticles (NPs).<br/><br/>Electron tomography (ET) enables one to determine the 3D structures of nanomaterials from 2D images. These 2D projection images are acquired over a large tilt range and combined in a 3D reconstruction of the structure of interest through a mathematical algorithm. During past decades, ET in high-angle annular dark-field STEM (HAADF-STEM) mode has become a popular technique to investigate the overall morphology of nanomaterials, to determine the nature of surface facets, and even to characterize the atomic structure in 3D. Unfortunately, the acquisition of a conventional tilt series for ET is a time-consuming process that requires at least 1 h for a standard experiment. In addition, after the acquisition, a postprocess reconstruction step is required to evaluate the final 3D shape of the nanomaterial. Consequently, one can typically analyze approximately 10 NPs in a time frame of 1 day. This restriction further limits a thorough understanding of the structure–property relations, especially because the properties of nanomaterials are mostly measured by ensemble techniques.<br/><br/>We thus aimed to increase significantly the throughput of structural investigation of nanoparticles, for which we decided to exploit imaging by secondary electron-based electron beam-induced current (SEEBIC) in STEM [1,2]. This technique uses the generation of secondary electrons (SEs) in a TEM and can be considered as an unusual modification of the electron beam-induced current (EBIC) setup. The measured current in SE-based EBIC (SEEBIC current) arises from holes generated by the emission of SEs from the sample, upon interaction with the primary beam. This measured current is equal and opposite to the generated SE signal and can be mapped pixel-by-pixel to produce an image. Since SEs originate from near-surface regions of the samples, the SEEBIC image intensity is sensitive to variations in surface topography [2,3].<br/><br/>In this contribution, we will show that SEEBIC can be considered an attractive approach to imaging the morphology of nanomaterials with shorter acquisition and processing times in comparison to ET and superior resolution in comparison to SEM. We will discuss the importance of using a closed membrane to minimize imaging artifacts. Direct access to surface morphology obtainable on the order of minutes opens up the possibility to use SEEBIC for high-throughput analysis, e.g. of chiral NPs and to combine 3D imaging with <i>in situ</i> stimuli.<br/><br/>References<br/>[1] Hubbard, W. A.; Mecklenburg, M.; Chan, H. L.; Regan, B. C. <i>Phys. Rev. Appl.</i> 2018, <i>10</i>, 044066<br/>[2] Vlasov, E.; Skorikov, A.; Sánchez-Iglesias, A.; Liz-Marzán, L. M.; Verbeeck, J.; Bals, S. <i>ACS Mater. Lett.</i> 2023, <i>5</i>, 1916<br/>[3] Vlasov, E.; Heyvaert, W.; Bing, N.; Van Gordon, K.; Girod, R.; Verbeeck, J.; Liz-Marzán, L. M.; Bals, S. <i>ACS Nano </i>2024<i> 18, </i>12010