Electron microscopy reveals colours of outermost electron layer

Surfaces play a key role in catalysis and corrosion; understanding the atomic structure of a surface of a functional material is necessary for engineers and chemists. Now, researchers from Nagoya University have used atomic-resolution secondary electron (SE) imaging to capture the atomic structure of the top layer of materials to better understand the differences from lower layers. The research findings were published in the journal Microscopy.

Some materials exhibit ‘surface reconstruction’, where the surface atoms are organised differently from the interior atoms. To observe this, especially at the atomic level, surface-sensitive techniques are needed.

Traditionally, scanning electron microscopy (SEM) has been an effective tool to examine nanoscale structures. SEM scans a sample with a focused electron beam and captures the SEs emitted from the surface. SEs are typically emitted from a shallow depth below the surface, making it difficult to observe phenomena like surface reconstruction, especially if only a single atomic layer is involved.

The researchers from Nagoya University addressed this challenge by using a two-layered molybdenum disulphide (MoS₂) sample, to measure how much information SE imaging can extract from the surface and subsurface layers. By stacking two layers of MoS₂, they distinguished the surface layer from the second layer using the technique.

The researchers found that atomic resolution SE imaging is effective in identifying surface atomic arrangements with high surface sensitivity. Their findings revealed that the intensity of SE images from the surface layer was approximately three times higher than from the second layer, providing evidence of the method’s sensitivity.

Atomic-resolution SE images of a single-layer MoS₂ sample revealed honeycomb-like structures composed of molybdenum and sulfur atoms. SE imaging also revealed overlapping patterns, indicating distinct atomic arrangements in the surface and second layers.

Koh Saitoh, lead researcher from Nagoya University, said the SE yield from the surface layer was about three times greater than from the second layer. “This result suggests that the surface layer absorbs or scatters SEs from the second layer. This absorption contributes to the method’s depth sensitivity,” Saitoh said.

The researchers aim to use atomic resolution SE imaging to reveal the surface structure at the atomic level, including surface reconstruction and other unique structures formed on surfaces. To control the growth, fabrication, and electronic and mechanical properties of nanomaterials, understanding these processes is essential.

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