The Creation of New Forms of Carbon with a Charged Atomic Needle

 by Harry C

Carbon’s unique stability and valency allows it to take many complex forms, ranging from C60 'buckyballs’ to nanotubes and diamond. The usefulness and complexity of these molecules has established a new field of chemistry, focusing on synthetic carbon allotropes. Many potential carbon structures have been proposed as thermodynamically stable compounds (i.e. can exist without spontaneously decomposing/rearranging). One such suggested family of molecular carbon allotropes are rings of 2-coordinate carbon atoms called cyclo[n]carbons (Cn). 

Carbon allotropes (left) & cyclo[18]carbon (right)

Large cyclocarbons, particularly C18 have been a topic of debate among chemists. Not only how they would exist, but also how they could even be made. While the second question proved elusive, the first question can be somewhat answered using current theories. A distinctive feature of these carbon allotropes is that they contain two perpendicular π-conjugated electron systems. Hückel’s rule suggests that an aromatic structure will occur due to the delocalization of two scaffolds of 18 π electrons (shown below): 18πin and perpendicular to it 18πout. And so it was theorised that cyclo[18]carbon would occur in either a polyynic form (consisting of alternating single and triple bonds of different lengths) or cumulenic form (consisting of successive double bonds). 

Up to its synthesis, different models of computational chemistry gave conflicting results concerning the bond structure of C18. Møller-Plesset perturbation theory and density functional theory calculations suggested that the lowest energy form was its cumulene form. While the Hartree-Fock method predicted a conversely polyyne state. Another model even predicted a combination of the two: a polyyne structure as a ground state which then distorts through a cumulene structure into its inverted polyyne form (i.e. the triple and single bonds switch places through the double-double form). 

Furthermore, the arrangement of these bond forms was another unknown. Two forms were settled on for each of the polyyne and cumulene forms. For cumulene, the two proposed were D18h (a regular 18-sided polygon) or D9h (a regular nonagon). For polyyne structures, the two proposed were D9h with all interior angles the same, or C9h where each side of the nonagon consisted of one linear triple-to-single bond edge. While many theories could provide plausible explanations, going into analysis, the two most pressing questions were the nature of C18’s bonding, and what geometry this bond system would exhibit.

C18’s potential forms (top) and their electron structures (bottom)

Before its first stable synthesis, many attempts on C18’s creation had been made, with most involving the masking of alkynes (carbon-carbon triple bonds surrounded by protective molecules to limit their reactivity) which were then unmasked through a variety of methods. This provided but fleeting glimpses at C18’s existence in a gas phase before it would coalesce into larger fullerenes. However thanks to advances in the practice of atomic force microscopy (AFM), the idea for a radical new method of synthesis was brought forward by an Oxford research team led by Professor Harry L. Anderson in conjunction with a research team at IBM. He proposed that, instead of conducting traditional larger scale reactions, cyclo[18]carbon could instead be formed at the individual atomic level via individual atom manipulation.

For context, AFM works by moving a probe just above a sample surface (1-100nm) which then repels a cantilever. This causes miniscule fluctuations in the cantilever height, which can be measured by firing a laser at the probe that reflects into a photodetector. Recently, breakthroughs have occurred not only increasing the resolution of these scans, but also allowing functionalization of the tip. 

And so the team sublimed precursor molecules of C24O6 onto a smooth copper surface coated with laser smoothed sodium chloride (salt) crystals. The crystal was cooled to ≈10 K (-263°C) providing an inert surface for these compounds to be imaged and for individual molecules to be manipulated. The tip of the AFM was placed in the vicinity of a few nanometres of the molecule and then a potential difference of +3V was applied for a few seconds. The high energy from the electrons was able to knock off either 2,4 or 6 carbonyl (C=O) moieties thanks to the high reactivity of cyclo[18] carbon and its oxides, forming the cyclic C18.

The synthetic routes to C18

The resulting molecule was then imaged (shown below) using AFM both close up R and further away Q showing a cyclic molecule containing nine bright lobes. The nine bright lobes corresponded to simulations of the polyyne form f & h (i.e the presence of alternating single and triple bonds) but could not distinguish between D9h and C9h bond angles. And although one side was brighter, this was attributed to the molecules not being parallel to the surface, and not some strange exotic structural arrangement.

Simulations of C18 potential forms (left) & real AFM scans of C18 (right)

But what’s the point of making a few nanometre sized rings on a freezing crystal? Beyond simply pure academic curiosity, C18’s high bond conjugation implies it could facilitate the movement of electrons similarly to what’s seen in graphene and other allotropes. This combined with the fact that C18 tends to spontaneously react with itself to form long ring chains could make C18 a unique candidate for molecular wires in nanoscale electronic devices. 

Furthermore, C18 is extremely reactive and could be functionalized via further reaction allowing for custom nanoelectronic components to be made. Thinking at an even larger scale, C18 could be used as a building block for larger carbon superstructures with novel properties and applications in materials science.