Magnetism of graphene with defects: Vancancies, substitional metals, and covalent functionalization

Abstract

Magnetic properties of graphenic nanostructures, relevant for future spintronics applications, depend crucially on doping and on the presence of defects. Here we present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of defects such as: substitutional doping with transition metals, vacancies, chemical functionalization with organic and inorganic molecules, light atoms, and polymers. We have found that such defects can be used to create and control magnetism in graphene-based materials. Our main results can be summarized as follows: (a) Substitutional metallic impurities can be fully understood using a model based on the hybridization between the states of the metal atom, particularly the d electrons, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of the different electronic levels between hybridized graphene-metals which determines all the magnetic properties obtained by the doping; (b) In chemical functionalization, independently of the particular adsorbate, a spin moment of 1.0 Bohr is induced when a molecule chemisorbs on a graphene layer via a single C-C covalent bond. This effect is similar to H adsorption, which saturates one pz orbitals creating an effect on the electronic structure that resembles a single vacancy in a π-tight-binding, however with universal character. The magnetic coupling between adsorbates was also studied and showed a key dependence on the sublattice adsorption site; (c) Monovacancies under isotropic strain display a rich phase diagram of spin solutions with the geometry configuration. Stretching increases the moment in different spin phases and compression reduces or even kills the magnetic moment. The transition to the non-magnetic solutions can be traced to changes in the local structure of graphene that are associated with the global rippling of the layer. All these results provide key information about defects in the magnetism of graphene.