Graphite has diverse applications, ranging from pencils to electronic devices and nuclear reactors. Defects — displacements of carbon atoms — can occur in its structure, which may be beneficial in electronics, but could be dangerous in old-style, air-cooled nuclear reactors. There, the defects store energy and can lead to fire. Quantum-mechanical computer simulations by Rob Telling and colleagues (Nature Materials doi:10.1038/nmat876; 2003) now show that these defects may be structurally more complex than previously thought.
The structure of graphite itself is essentially simple. It consists of layered sheets known as graphene, each sheet being formed from a planar array of carbon atoms. Defects can form, for example, through the irradiation of graphite in nuclear reactors. This can induce a carbon atom to leave a sheet, forming a 'vacancy defect'. Until now, it had been assumed that the remaining atoms in the sheet are unaffected, and retain a planar configuration.
But the simulations by Telling and colleagues show that the planar state would actually be unstable, and that the atoms surrounding the vacancy are more likely to be displaced out of the plane, very unlike the situation in a flawless sheet. The authors propose that if displaced atoms in two sheets were near to each other, a covalent bond could form between them, effectively bridging the gap.
A vacancy defect can also create another type of flaw, in which the removed carbon atom positions itself between two neighbouring sheets, forming an 'interstitial'. Again, covalent bonds could form. Because the interaction between sheets is usually quite weak — the distance between them is 3.35 Å, some two-and-a-half times greater than the distance between atoms within a sheet — bonding between them has implications for the properties of the structure.
Although these simulations challenge current ideas about the nature of graphite defects, they are not inconsistent with some of the accepted evidence; they simply provide another explanation for the results. Further research is needed to corroborate Telling and colleagues' theory, but, if validated, the new understanding may help to make the decommissioning of old nuclear reactors safer, and could pave the way to a whole new set of materials based on carbon nanotubes — effectively, rolled-up graphene sheets — which are already provoking great interest and a wealth of research.
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Applied Physics Letters (2017)
Journal of Applied Physics (2013)
Surface Science (2011)
The Journal of Physical Chemistry C (2010)