Carbon can be arranged in a number of configurations. When each of its atoms is bonded to three other carbon atoms, it is relatively soft graphite. Add just one more link and it becomes one of the hardest minerals known, diamond. Chuck 60 carbon atoms together in football form and boom, boomers.
But a ring of carbon atoms, in which each atom is connected to only two others and nothing else? This has eluded scientists for 50 years. Their best efforts have led to a gaseous carbon ring that quickly dissipates.
So it's a big deal that a team of researchers at Oxford University and IBM Research have created a stable carbon ring.
The ring-shaped carbon compound, called cyclocarbon, is made of 18 carbon atoms, the smallest cyclic carbon that is predicted to be thermodynamically stable. And modern microscopy techniques have provided images of it.
So far, studies on the structure of cyclocarbon suggest that it acts as a semiconductor, meaning that it has potential use in electronics. And the very property that made cyclocarbons so difficult to isolate in the first place – their high reactivity – means that they can be used to create other carbon allotropes and carbon-rich materials.
To withdraw from this feat, researchers began by synthesizing triangular cyclocarbon monoxide C24Oh6, These are the 18 carbon atoms bonded to six carbon monoxide molecules, two groups at each of the three angles of a triangle.
They transferred this mixture into a layer of sodium chloride on a copper plate cooled in a vacuum chamber to just above absolute zero. This provides an inert surface that keeps the structure stable.
Then, using the tip of an atomic force microscope, the team removed the carbon monoxide (CO) molecules from the structure, leaving only the ring of carbon atoms behind.
Sounds a lot easier than it was, of course. Researchers cannot always break down all COs without breaking the ring structure; in many cases they instead produce molecules such as C22Oh4 and C.20Oh2 (see image above).
"We have removed all six parts of CO from C24Oh6, yielding 13 percent, which usually results in circular molecules, ”the researchers wrote in their paper.
Interestingly, the atoms in the cyclocarbon form what is known as a polyline structure, with alternating triple and single bonds. It has long been a question of whether one-dimensional carbon will produce this or a cumulative structure with multiple double bonds.
This alternating structure is thought to produce semiconductivity – and suggests that carbon chains would also be semiconductor.
Now what? Well, that's still a little unclear. The process of building up hydrocarbons will need to be refined to produce more reliable yields, for starters. Currently, the rings can only be built one at a time, so the team plans to find ways to build several hydrocarbons at once.
And now that stable cyclic hydrocarbons have been produced, scientists can begin to experiment with applications – to understand how semiconductivity can be used, for example, or to study cycling carbon properties as a major building block for even more complex molecules.
"Our results," the researchers wrote, "provide direct experimental studies on the structure of cyclocarbon and pave the way for the creation of other elusive carbon-rich molecules through the manipulation of atoms."
The study was published in science,