First-Principles Study of the Electronic and Magnetic Properties of Defects in Carbon Nanostructures

TL;DR

This paper reviews theoretical density functional studies on how defects like transition metal substitutions and covalent functionalizations induce and control magnetic properties in graphene-based nanostructures, with implications for spintronics.

Contribution

It provides a comprehensive understanding of defect-induced magnetism in graphene, including models for metal impurity hybridization and methods to control magnetic moments via strain and functionalization.

Findings

Hybridization model explains metal impurity magnetism.

Chemical functionalization induces a universal spin moment of 1.0 μB.

Strain and curvature can activate magnetism in otherwise non-magnetic defects.

Abstract

Understanding the magnetic properties of graphenic nanostructures is instrumental in future spintronics applications. These magnetic properties are known to depend crucially on the presence of defects. Here we review our recent theoretical studies using density functional calculations on two types of defects in carbon nanostructures: Substitutional doping with transition metals, and sp$^3$-type defects created by covalent functionalization with organic and inorganic molecules. We focus on such defects because they can be used to create and control magnetism in graphene-based materials. Our main results are summarized as follows: i)Substitutional metal impurities are fully understood using a model based on the hybridization between the $d$ states of the metal atom and the defect levels associated with an unreconstructed D$_{3h}$ carbon vacancy. We identify three different regimes, associated with the occupation of distinct hybridization levels, which determine the magnetic properties obtained with this type of doping; ii) A spin moment of 1.0 $\mu_B$ is always induced by chemical functionalization when a molecule chemisorbs on a graphene layer via a single C-C (or other weakly polar) covalent bond. The magnetic coupling between adsorbates shows a key dependence on the sublattice adsorption site. This effect is similar to that of H adsorption, however, with universal character; iii) The spin moment of substitutional metal impurities can be controlled using strain. In particular, we show that although Ni substitutionals are non-magnetic in flat and unstrained graphene, the magnetism of these defects can be activated by applying either uniaxial strain or curvature to the graphene layer. All these results provide key information about formation and control of defect-induced magnetism in graphene and related materials.