First principles study of magnetism in nanographenes

TL;DR

This study uses first principles calculations to explore how magnetism emerges in nanographenes, revealing size-dependent antiferromagnetic phases linked to zigzag edges, with implications for synthesis and understanding experimental magnetism.

Contribution

It demonstrates the size-dependent emergence of antiferromagnetic phases in nanographenes and connects theoretical findings to experimental observations of magnetism in activated carbon fibers.

Findings

Antiferromagnetic phase appears at a certain nanographene size.

Magnetism originates from localized electronic states at zigzag edges.

Larger nanographenes approach properties of infinite graphene ribbons.

Abstract

Magnetism in nanographenes (also know as polycyclic aromatic hydrocarbons, or PAHs) are studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. The smallest PAH still having an AFM ground state is identified. With increased length of the zigzag edge, PAHs approach an infinitely long ribbon in terms of (1) the energetic ordering and difference among the AFM, ferromagnetic (FM), and nonmagnetic (NM) phases and (2) the average local magnetic moment at the zigzag edges. These PAHs serve as ideal targets for chemical synthesis of nanographenes that possess magnetic properties. Moreover, our calculations support the interpretation that experimentally observed magnetism in activated carbon fibers originates from the zigzag edges of the nanographenes.