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Advancements in Atomic Imaging: Understanding Surface Structures

The characteristics of surfaces are crucial in various chemical processes, notably in catalysis and material degradation. For engineers and chemists alike, comprehending the atomic layout of a functional material’s surface is vital. A team from Nagoya University in Japan has harnessed atomic-resolution secondary electron (SE) imaging to investigate the atomic structure of surface layers in materials, revealing differences relative to their subsurface layers. Their findings were detailed in the journal Microscopy.

Many materials demonstrate a phenomenon known as ‘surface reconstruction,’ where the arrangement of atoms at the surface diverges from that within the bulk. To investigate this effectively at an atomic scale, specialized surface-sensitive techniques are required.

Scanning electron microscopy (SEM) has conventionally served as a powerful tool for examining nanoscale features. SEM operates by directing a focused electron beam onto a sample and collecting the secondary electrons emitted from the surface. However, because secondary electrons originate from a shallow subsurface area, visualizing surface reconstruction can be challenging, especially when only a single atomic layer is involved.

To address this limitation, the researchers from Nagoya University employed a straightforward yet effective model: a two-layered molybdenum disulfide (MoS₂) sample. This setup enabled them to discern the nuances of how much information SE imaging can reveal about both surface and subsurface layers.

They discovered that atomic-resolution SE imaging significantly enhances the identification of atomic arrangements on surfaces, exhibiting a remarkable sensitivity. Their results showed that the intensity of SE images captured from the surface layer was approximately three times greater than that from the second layer, underscoring the method’s effectiveness.

In the atomic-resolution SE images of a single-layer MoS₂ sample, the researchers observed intricate honeycomb-like formations composed of molybdenum and sulfur atoms. These images not only provide visual intrigue but also displayed overlapping patterns that suggest distinct atomic arrangements between the surface and subsurface layers.

“Most notably, the SE yield from the surface layer was about three times greater than from the second layer,” noted Koh Saitoh, lead researcher at Nagoya University’s Institute of Materials and Systems Sustainability (IMASS). “This finding implies that the surface layer either absorbs or disperses SEs emitted from the second layer, thereby enhancing the method’s sensitivity to depth.”

The overarching goal of this research group is to leverage atomic-resolution SE imaging to elucidate surface structures at the atomic level, including surface reconstruction and other unique surface formations. Such understanding is critical for controlling the development, fabrication, and consequent electronic and mechanical properties of nanomaterials.

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