Predicted penta-graphene falls flat
An international group of scientists has ruled out the possibility that penta-graphene, a two-dimensional carbon layer made exclusively from pentagons, will ever exist. The team has also produced a set of criteria that all future carbon allotrope predictions should be measured against.
Earlier this year, scientists from China predicted the existence of penta-graphene. Qian Wang and her colleagues from Peking University used computer simulations to create the exotic allotrope and suggested it could be synthesised from single T12-carbon sheets.
But Christopher Ewels from the University of Nantes, France, and his colleagues, including the Nobel prize winner Harry Kroto, have cast doubt on Wang’s prediction. Using density functional calculations, the team analysed penta-graphene for its chemical stability and whether it could be distinguished from isomers with similar energies. According to the simulation, penta-graphene would rapidly transform into the more stable hexagonal form in the presence of any impurities.
C P Ewels et al, PNAS, 2015, DOI: 10.1073/pnas.1520402112
In recent years, a plethora of theoretical carbon allotropes have been proposed, none of which has been experimentally isolated. We discuss here criteria that should be met for a new phase to be potentially experimentally viable. We take as examples Haeckelites, 2D networks of sp2-carbon–containing pentagons and heptagons, and “penta-graphene,” consisting of a layer of pentagons constructed from a mixture of sp2- and sp3-coordinated carbon atoms. In 2D projection appearing as the “Cairo pattern,” penta-graphene is elegant and aesthetically pleasing. However, we dispute the author’s claims of its potential stability and experimental relevance.
We describe criteria that should be applied when evaluating whether theoretically proposed carbon allotropes may be experimentally isolated. We discuss the importance of energetic isomeric “funnels” centered on a stable allotropic form, the role of defects in catalyzing structural transformations to lower energy isomers, and chemical stability. This is demonstrated with literature examples such as C60 and B80. We apply these criteria to a recently proposed carbon allotrope, penta-graphene, demonstrating with the aid of density functional calculations that it will not be experimentally attainable. A second example, Haeckelites, are unlikely to be experimentally achievable when neutral but may be stabilized through significant charge transfer. The principals discussed here are general and can be applied to any theoretically proposed materials.