Big Ball Scars

Buckyballs are weird. They’re little soccer balls that collect electrons—one of the few materials used as electron acceptors in solar cells. They’re the largest object to show particle–wave duality, and that’s quite a feat for sixty carbon atoms. If an alkali-metal is shoved in the center, the whole thing acts metallic and, in some cases, superconducting. They even have a toy named after them. And we can’t forget that the 1996 Nobel Prize in Chemistry went to the discoverers of fullerene. Needless to say (of course, I’m about to say it anyways), these large spherical molecules are a hot topic.

Some researchers are moving away from the “small” fullerenes, C60 and C70, to larger versions. But as the size grows so do the problems. A recent report in ACS Nano by David Wales of the University of Cambridge details the defects that arise in larger configurations (sorry for the paywall link). This work is entirely theoretical, as causing defects in a specific arrangement is as difficult (or maybe more so) than forming defect-free molecules. The molecule can get a “scar” when the pentagon–hexagon–pentagon structure is dislocated to a pentagon–heptagon–pentagon structure.


C860 and C1160 fullerenes with red scars.

Defects are unavoidable in large spheres and the configuration of them constitutes a Thomson problem, the location of repulsive points on a sphere, which is one of eighteen unsolved mathematical problems on the list of Smale’s problems for the 21st century. While the Thomson model was previously used to describe the configuration of electrons in an electron shell (and has since been abandoned since it’s failure to do so), the problem is now being applied to fullerenes.


Defects in different sized fullerenes.


The paper also looked at funnels, or outward curving structures, and showed that defects can occur there, too. Interestingly, the fourth to last structure below (which is akin to a carbon nanotube) shows no defects (at least in their picture, they didn’t discuss the lack of defects in the text).


Defects in curved surfaces.

So what does this mean? What can defects do? Well, they can disrupt aromaticity and conjugation which would change the electronic structure of the molecule, diminishing conductivity and widening the band gap so that they’d be unsuitable for solar cells or other electronic applications. (That was a mouthful… fingerful?) But it’s not all bad. A paper from last year in the Journal of Physical Chemistry C (paywall again) states that defects in a graphene surface can lead to reactive sites. In other words, chemistry can happen. I don’t think it’s too far fetched to say there would be a similar situation with fullerenes. After all, they’re just balled-up graphene.


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