A combined theoretical-experimental investigation of paramagnetic centres in chemically exfoliated graphene nanoribbons

Abstract

A combined experimental and theoretical study of the origin of paramagnetic centres in graphene nanoribbons (GNRs) is presented. GNRs were prepared from multi-wall carbon nanotubes by an oxidative method at various temperatures. Increasing the oxidation temperature led to GNRs of shorter length with no noticeable effect on the width. Electron spin resonance showed that the ribbon sizes influence both the spin density and type of paramagnetism, with longer ribbons being more prone to form localized paramagnetic centres and shorter ribbons exhibiting a significant paramagnetic contribution from extended states. The density of states for GNRs was calculated with varying dimensions and chiralities using a Hückel tight-binding method. The formation energies of zigzag edges and vacancies, which are expected to be responsible for paramagnetic centres, were evaluated. Our results indicate that longer GNRs favour formation of vacancies while shorter structures favour zigzag edges. This analysis explains the existence of localized paramagnetic centres in longer GNRs and paramagnetism due to electronic states delocalized along zigzag edges in shorter GNRs.