3D Electron Microscopy Characterization of Nanoscale Materials for Catalysis and Optoelectronics Under Realistic Conditions

Electron tomography is a powerful tool to explore the morphology, 3D structure, and composition of a broad range of (nano)materials. Although these experiments are already at the state-of-the-art, several open questions remain. These questions are often related to the fact that 3D characterization by TEM is typically performed using the conventional conditions of a TEM: ultrahigh vacuum and room temperature. Since it is known that the morphology and consequently, the activity of nanomaterials will transform at higher temperatures or pressures, this poses a fundamental limitation. It is therefore not surprising that much effort has been devoted to monitoring nanoparticle transformations upon application of external stimuli by TEM.<br/><br/><i>In</i><i> situ</i> TEM characterization can be performed either using a dedicated environmental TEM or through a wide variety of holders based on MEMS devices. In this manner, we were able to characterize strong metal support interaction for catalytic Ni nanoparticles on TiO₂ supports under operando conditions [1]. However, understanding the complex changes for anisotropic nanosystems in 3D rather than in 2D remains very challenging and conventional electron tomography techniques are no longer applicable. By combining aberration corrected electron microscopy with a quantitative interpretation and modelling approaches [2], we can perform quantitative measurements of the coordination numbers of the surface atoms of catalytic nanoparticles at high temperatures and in gaseous environments.<br/><br/>An additional challenge is related to the beam sensitivity of specific nanostructured materials. For example, metal halide nanocrystals are extremely sensitive to the electron beam. In order to distinguish degradation effects caused by the electron beam from those related with external triggers, new low-dose electron microscopy techniques need to be applied. One such technique exploits the use of four-dimensional scanning transmission electron microscopy, a technique during which for every scan position a diffraction pattern is collected. In this manner, we were able to investigate FAPbBr₃ nanocrystals and reveal an elongation of the projected Br atomic columns. This is caused by an anisotropic displacement of the Br anions perpendicular to the Pb-Br-Pb bonds, leading to local distortions in an on-average cubic crystal structure. Finally, we also used low-dose in situ electron microscopy to understand the role of defects in the stabilization processes of doped metal halide perovskites.<br/><br/>References<br/>[1] Monai M, Jenkinson K, Melcherts AEM, Louwen JN, Irmak EA, Van Aert S, Altantzis T, Vogt C, van der Stam W, Duchon T, Smid B, Groeneveld E, Berben P, Bals S, Weckhuysen BM. Science 380 (2023) 644-651<br/>[2] De Bakcer A, Bals S, Van Aert S. Ultramicroscopy 274 (2023) 113702