Technology at the nanoscale has emerged as one of the primary challenges in science, as new physical and chemical effects can be manipulated at will. As advancements in materials science push the boundaries of physics and chemistry, it becomes crucial to understand the origins of these unique properties and how they relate to changes at the atomic scale, particularly those linked to structural alterations in materials, often associated with crystal defects or surface terminations. This understanding is especially vital for low-dimensional materials designed for energy and environmental applications, where crystallography and the distribution of atomic species play a critical role in determining their physical properties, thereby enhancing their performance, including efficiency and selectivity in specific reactions.

In this presentation, I will demonstrate how the combination of advanced electron microscopy imaging and electron spectroscopy, utilizing aberration-corrected scanning transmission electron microscopy (STEM), enables us to investigate elemental composition and structure with unprecedented spatial resolution. Multimodal analytical techniques can be now combined to extract information down to the atomic scale.[1] This approach allows us to determine growth mechanisms and correlate structural properties with performance.

Typically, materials intended for physical applications require perfect crystallinity, free from defects, disorder or inhomogeneities. However, I will illustrate how, in the context of chemistry applications such as catalysis, defects can sometimes enhance the reactivity of certain nanomaterials, particularly in two-dimensional systems. This shift in perspective reveals that defects such as inhomogeneous grain boundaries [2], presence of vacancies [3,4] or even amorphization [5] can lead to the formation of free radicals or an increased density of dangling bonds at the surface, which may improve catalytic properties for selective molecules. I will show how in-situ reactions can now be conducted within the transmission electron microscope (TEM), facilitating precise visualization of active sites.[6,7] Finally, I will discuss how artificial intelligence can assist in automating the analysis of defects and vacancies, enabling statistically significant evaluations of our systems and reactions.[8-10]