Celebrating another 30 year anniversary in the SPM world….
Who in the microscopy world isn’t a high resolution junkie? There is even an unspoken competition about which methods provide the highest resolution. Without too much of an inferiority complex, I can comfortably say that TEM is the winner in that category. With sub-angstrom resolution, high resolution TEM routinely images defects and single atoms. In the SPM world, STM has historically been at the forefront of high resolution studies, but AFM is giving it a run for its money.
What is also amazing about the SPM field is that the probe provides a unique and powerful tool to manipulate atoms. Thirty years ago in November 1989, Don Eigler and Erhard Schweizer (of IBM Almaden in California) wowed the scientific community by writing out “IBM” with Xenon atoms on a nickel surface using an STM tip.
Xenon atoms on a nickel surface, courtesy of IBM
Combining both “superpowers” of the SPM in terms of high resolution and atom manipulation, the AFM group out of IBM Research in Zurich, led by Leo Gross, has been amazing us for a decade now with truly impressive images of molecules and even chemical reactions. In 2017, I highlighted some of this work in a blog entitled “Seeing molecules with AFM”. This amazing team has now accomplished another goal that takes advantage of AFM’s atomic and molecular manipulation powers: they have synthesized a long-predicted allotrope of carbon using an AFM tip, a fantastic way to celebrate the 30th anniversary of Eigler and Schweizer’s milestone.
An allotrope is simply a different structural form of an element. Carbon has several different allotropes such as graphite, diamond, graphene, and fullerene. Oxygen also has allotropes – the common O2 (dioxygen) and ozone (O3). Chemists have been trying to synthesize this particular 18-member carbon allotrope for years. In the early 1980’s, a precursor molecule was synthesized for this purpose, but the full synthesis of cyclo  carbon remained elusive. That precursor was then brought back with this AFM study.
Carbon 18 Precursor, courtesy of IBM Research
The precursor molecule was a multi-ring cyclo-carbon oxide molecule C24O6 adsorbed onto a sodium chloride surface at 5K. What author Katharina Kaiser did was then pick off the carbon monoxide molecules that are hanging off the ends of the precursor to ultimately result in the 18 carbon cyclical molecule. Of course now that they had the molecule on the surface, they were able to image its structure with very high resolution. The molecule could have been a string of continuous double bonds, but also it could be alternating single and triple bonds. Thanks to the high resolution AFM image, the structure was determined to be the latter.
Carbon 18 allotrope, courtesy of IBM Research
What I think is really amazing is that these kinds of high resolution studies have been going on for a decade. The fact that they have been going on for so many years does not yet change the fundamental fact that these kind of high resolution measurements are incredibly challenging and can still only be done by a handful of group with homebuilt instrumentation. That being said, it gives us something to strive for….and provides pretty good competition for HRTEM! And here’s to more atomic manipulation in the pursuit of chemical synthesis– I can’t wait to see what they’ll accomplish next.
Dalia Yablon, Ph.D.