Surfaces are crucial in a variety of chemical reactions, such as catalysis and corrosion. For engineers and chemists, grasping the atomic layout of the surface of a material is vital. Researchers from Nagoya University in Japan harnessed atomic-resolution secondary electron (SE) imaging to analyze the atomic structure of the outermost layer of materials, helping them identify how it differs from the layers beneath. Their discoveries were shared in the journal Microscopy.
Some materials undergo a phenomenon known as ‘surface reconstruction’, where the arrangement of surface atoms differs from that of the atoms inside the material. Observing this at the atomic level requires surface-sensitive methods.
Traditionally, scanning electron microscopy (SEM) has proven effective for examining nanoscale structures. SEM operates by directing a focused electron beam onto a sample and detecting the SEs emitted from the surface. These SEs originate from just beneath the surface, which can hinder the observation of surface reconstruction, especially when only a single atomic layer is involved.
The researchers from Nagoya University addressed this challenge by utilizing a simple two-layer system of molybdenum disulfide (MoS2) to assess how much information SE imaging could yield about the surface and subsurface layers. By layering two sheets of MoSâ‚‚, they were able to differentiate the outer layer from the one below it using this technique.
The team discovered that atomic-resolution SE imaging was effective in mapping surface atomic configurations with remarkable sensitivity. Their results showed that the intensity of SE images from the surface layer was approximately three times higher than that of the second layer, strongly indicating the method’s sensitivity.
Atomic-resolution SE images of a single-layer MoSâ‚‚ sample displayed striking honeycomb-like patterns formed by molybdenum and sulfur atoms. In addition to its aesthetic appeal, SE imaging unveiled overlapping patterns that suggested unique atomic configurations within the surface and subsurface layers.
“Notably, the SE output from the surface layer was about three times greater than that from the second layer,” stated Koh Saitoh, the lead researcher at Nagoya University’s Institute of Materials and Systems Sustainability (IMASS). “This indicates that the surface layer either absorbs or scatters SEs from the second layer, contributing to the method’s depth sensitivity.”
The research group’s goal is to use atomic-resolution SE imaging to disclose the atomic-level surface structure, including surface reconstruction and other distinctive formations on surfaces. Understanding these aspects is crucial for managing the growth, development, and electronic and mechanical characteristics of nanomaterials.
