Ordered Semiconducting Nitrogen-Graphene Alloys

Abstract

The interaction between substitutional nitrogen atoms in graphene is studied by performing first-principles calculations. The effective nearest-neighbor interaction between nitrogen dopants is found to be highly repulsive because of the strong electrostatic repulsion between nitrogen atoms. This interaction prevents the full nitrogen-carbon phase separation in nitrogen-doped graphene. Interestingly, there are two relatively stable nitrogen-nitrogen pair configurations, whose stability can be attributed to the anisotropy in the charge redistribution induced by nitrogen doping. We reveal two stable, ordered, semiconducting N-doped graphene structures, C3N and C12N , through the cluster-expansion technique and particle-swarm optimization method. In particular, we show that C12N has a direct band gap of 0.98 eV. The heterojunctions between C12N and graphene nanoribbons might be a promising basis for organic solar cells.

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Plain Language Summary

Monolayer graphene, a semimetal, conducts electricity on a par with copper. Adding to this property graphene’s atomic-sheet structure, remarkable mechanical stability, and high thermal conductivity, you have a list of physical properties that are highly desired for high-performance electronics. But, pure graphene lacks an essential feature needed for controlled and reliable transistor operations: Being a semiconductor. Many efforts of deriving semiconductors from pure monolayer graphene have been made using different strategies of modifications. One of them is to chemically modify monolayer graphene by substituting some of the carbon atoms with a different atomic species, a process called substitutional doping. Substitutional doping of graphene by nitrogen has become possible only very recently, opening new possibilities for investigating and exploiting the structural and electronic properties of chemically modified graphene. Taking a position on this new front, this theoretical paper predicts two new nitrogen-doped graphene structures that are semiconducting and thermodynamically stable for all practical purposes.

Performing first-principles calculations first, we reveal the nature of the effective interaction between two substitutional nitrogen atoms in a monolayer graphene: The interaction depends on the interatom distance in an almost monotonic way, but showing a number of relative minima; and it is repulsive, as a result of the strong electrostatic repulsion arising from the negative charge distributions of the nitrogen dopant. The presence of the relative minima and the effective repulsive interaction, which keeps substitutional nitrogen atoms from aggregating together, provide essential microscopic mechanisms for generating modified graphenes which are stable and in which nitrogen dopants are well dispersed.

Indeed, our investigation of the structural and electronic properties of graphene substitutionally doped by nitrogen at a range of macroscopic concentrations predicts two stable nitrogen-doped graphene structures: One, C12N , corresponds to a doping concentration of 7.7%, and the other, C3N , 25%. In both structures, nitrogen atoms are distributed in a particular order. And even more interesting and important, the electron-rich nitrogen atoms in these structures do not simply donate their electrons to the native carbon atoms, unlike what an isolated nitrogen atom does. Both these structures are semiconducting. In fact, C12N has a semiconductor band gap of approximately 1 eV, making absorption of light in the optical range possible—and calling to mind the possibility of C12N acting as a charge donor in organic solar cells.

Our work indicates that substitutional nitrogen doping of graphene holds enough promise to be further explored as a route to engineering graphene for applications in electronics and photovoltaics.