Here’s a surprise: a report of a completely new (and rather unusual) allotrope of carbon. There doesn’t appear to be a manuscript out there yet, but the results were presented earlier this month at a conference in Richmond and earlier this year at the APS meeting, and caused a stir. Weirdly, this one appears to have properties that are best described as semi-metallic: it’s harder than stainless steel (although much lighter), electrically conductive, reflects light like a mirror, and is ferromagnetic. Those properties could turn it into a very useful industrial substance, depending on how it can be produced and worked.
This work was revealed by Joel Therrien of UMass-Lowell. He was trying to produce “pentagraphene“, a predicted-but-never-observed form of carbon consisting of nothing but irregular five-membered rings. (That would be a rather odd substance, too, although there are arguments about whether it would actually be a stable form).
. . .He placed a catalyst, a sheet of copper foil, on a pedestal in the center of a chemical vapor deposition (CVD) oven and heated it to about 800°C. But rather than pumping in a feed gas of the usual small hydrocarbons, such as methane, he injected a more complex precursor: 2,2 di-methylbutane, a cheap petrochemical that is available by the ton. . .
. . .Therrien says that on 13 November 2017, after teaching an evening class, he returned to his lab to check on his oven and noticed the smell of tar. The inside of the furnace was caked in black pitch. But the copper foil was covered in something that looked like polished silver. “I just stopped dead,” he says. “I literally spent half an hour just staring at this, trying to figure out what on Earth had happened.”
I have to admit, I immediately grinned and burst out laughing when I got to that part. I know exactly that feeling, and I think many readers will, too, because it’s one of the reasons that we’re scientists! It comes over you when you’re suddenly faced with a new result that shows you just how strange and unexpected the physical universe can be. At first, all you can do is just stand there and wonder what the hell just happened (and whether you can do it again!) The feeling is an intense version of the amazement you feel at watching a particularly good magic trick, intense because it’s not a trick.
Therrien and co-workers can indeed do it again. They’ve been at it for a couple of years now and can produce micron-thick films that are centimeters across. So far, the best hypothesis from the spectra data is that the structure consists of six- and twelve-membered carbon rings that form wavy layers which have bonds also bridging the layers; it has a mixture of sp2 and sp3 bonds as well. What I would like is a good drawing of how that is supposed to look, but I haven’t found one yet! It forms on both copper and aluminum oxide substrates, and anneals to a much more homogeneous form (which is more of a semiconductor) if heated above 1100C. That would suggest the formation of larger crystalline domains, and my first thought would be that this would be a terrific candidate for electron diffraction crystallography – you could perhaps find small regions of the solid under the electron microscope that would provide solid diffraction data for a structure. I’m sure this has occurred to the discoverers as well; I hope it works!
The light reflection is >90%, even for a 50 nM layer, and as a telescope owner I have to say I like the idea of a reflective coating that doesn’t degrade in air: telescope mirrors have to be resilvered (well, re-aluminumed) eventually, especially instruments that get exposed to the elements like portable amateur ones do. The ferromagnetism is apparently still present even at elevated temperatures, which also sounds very useful. I would expect to see quite a bit of work following up on this in the nanoscale community, and I look forward to seeing where it goes!
