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Characterization and modelling

Characterization and modelling

Scanning probe techniques measure up

19 May 2019 Anna Demming
afm

If a picture is worth a thousand words, pictures that have even higher resolution or that record functional properties besides structural topology will surely speak volumes. It is hard to escape the role scanning probe techniques play in materials science, providing structural insights that help to explain and even predict material behaviour. Although it is over 30 years since these characterization techniques hit the scene, a constant stream of innovations and reinventions that improve their performance means they are still making headlines – the single-molecule-terminated tip developments reported this week are a case in point.

The first scanning probe technique was the scanning tunnelling microscope, a technique that can trace the topology of a surface with atomic resolution by monitoring the strength of the highly distance-dependent tunnelling current between the surface and a nanoscopically sharp tip. The first sight of atomic-scale features was so awe-inspiring that the inventors of scanning tunnelling microscopy (STM) Gerd Binnig and Heinrich Rohrer won the 1986 Nobel Prize for Physics, alongside Ernst Ruska, for the development of electron microscopy. While STM brought jaw-dropping images of conducting surfaces, there were still researchers hungry for the same resolution for insulating surfaces as well. I was lucky enough to speak to Christoph Gerber, co-inventor of the atomic force microscope alongside Gerd Binnig and Calvin Quate, about how they developed a technique that would bring high resolution to a broader range of surfaces. You can hear him describe the discovery as well as his colleagues and others working in the field in our series of short movies celebrating the invention and subsequent developments.

As Belle Dumé reports, a recent gear change in the resolution atomic force microscopy can achieve was the development of bond-imaging using a tip with a single carbon monoxide molecule at its tip. Her research update highlights work by researchers at Justus Liebig University Giessen in Germany to achieve the same resolution for 3D molecules – where the depth range had been problematic – by incorporating scanning tunnelling measurements to track the changes in topology. At the University of California at Irvine, researchers have also been extending the functionality of scanning tunnelling techniques with a single-molecule terminated tip, this time with (Ni(cyclopentadienyl)2). The magnetic properties of the (Ni(cyclopentadienyl)2) detect spin–spin interactions with magnetic features on the surface that affect the tunnelling current, providing uniquely high-resolution magnetic imaging.

These nanoscale images are possible thanks to techniques that allow ultrasensitive measurements of interactions between tip and surface. Measurement science is an often unsung hero in materials science but not on 20 May when people all around the world will be celebrating the progress in measurement instrumentation and even the units we use for measurements on World Metrology Day.

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