Atomic-Level Measurements of Rough Surfaces

Researchers use surface-sensitive signals to atomically resolve the structure of a rough surface.

Advanced electron microscopy technique permits the simultaneous collection of both signals: secondary electron (that are sensitive to the surface) and transmitted electron. — Image courtesy of Laurence Marks, Northwestern University

The Science

Electron microscopy provides information on the structure and composition of matter from the interactions of energetic electrons with the atoms that make up the material. A particular challenge is the characterization of rough surfaces that are found in materials used in many applications. For the first time, scientists have resolved the atomic structure of a surface by combining the information from electrons coming out of the surface with the information from electrons that pass through the material. This new technique, called high-resolution secondary electron microscopy (HRSEM), achieved atomic-scale imaging of even rough surfaces, using a state-of-the-art electron microscope equipped with a surface sensitive detector coupled with advanced theory.

The Impact

Every material interacts with the world through its surface, whether it is via the catalyst used to clean the emissions from your car, formation of rust on bridges used in the Nation’s infrastructure, or the internal surfaces found in multilayered electronic materials. Thus having an accurate understanding of the structure of surfaces and how they evolve is critical. In demonstrating this new microscopy technique, scientists not only showed a new way to gather detailed information, but also provided the correct surface structure for strontium titanate, a model system for substrates for electrocatalysis and low power electronics.

Summary

The atomic structures of surfaces control the majority of the chemical properties of materials. Precise characterization of the surface structure is a challenge as the surface is often very different from the bulk structure. In addition, it is experimentally difficult to determine the atomic structure of the rough surfaces typical for many technologically relevant materials. For the first time, the atomic structure of the surface of strontium titanate, a material whose surface is very important because it serves as a substrate for growth of other materials for electronics, has been resolved with high-resolution secondary electron microscopy. This new technique couples atomically resolved images of the surface and bulk with advanced theory. In an atomic-resolution electron microscope, a low-energy electron signal produced by interactions of the electrons with the surface of the sample was collected with a secondary electron detector, while simultaneously collecting images of the bulk structure formed by the electrons that passed through the sample. Coupling these images with advanced theory permitted atomically resolved reconstructions of the surface and bulk structures. The research resulted in a correction to a previously published structure for this material. There was excellent agreement between experimental images and simulations, confirming the promise of the new technique for the determination of the atomic structure of surfaces.

Contact

Laurence Marks
Department of Materials Science and Engineering, Northwestern University
[email protected]

Funding

This work was primarily supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (L.D.M., Y.L., and YZ), including use of the Molecular Foundry, an Office of Science User Facility. Additional support was provided by the National Science Foundation (P.K.) and the Australian Research Council (Discovery Projects and Discovery Early Career Researcher Awards).

Publications

J. Ciston, H.G. Brown, A.J. D’Alfonso, P. Koirala, C. Ophus, Y. Lin, Y. Suzuki, H. Inada, Y. Zhu, L.J. Allen, and L.D. Marks, “Surface determination through atomically resolved secondary-electron imaging.” Nature Communications 6, 7358 (2015). [DOI: 10.1038/ncomms8358].