Electron enrichment of zigzag edges of armchair–oriented graphene nano–ribbons increases their stability and induces pinning of Fermi level

Abstract

Zigzag edges of neutral armchair–oriented Graphene Nano–Ribbons show states strongly localized at those edges. They behave as free radicals that can capture electrons during processing, increasing ribbon's stability. Thus, charging and its consequences should be investigated. Total energy calculations of finite ribbons using spin–polarized Density Functional Theory (DFT) show that ribbon's charging is feasible. Energies for Pariser-Parr-Pople (PPP) model Hamiltonian are compatible with DFT allowing the study of larger systems. Results for neutral ribbons indicate: i) the fundamental gap of spin–polarized (non–polarized) solutions is larger (smaller) than experimental data, ii) the ground state is spin–polarized, a characteristic still not observed experimentally. Total energy of GNRs decreases with the number of captured electrons reaching a minimum for a number that mainly depends on zigzag–edges size. The following changes with respect to neutral GNRs are noted: i) the ground state is not spin–polarized, ii) fundamental gap is in-between that of spin–polarized and non–polarized solutions of neutral ribbons, iii) while in neutral ribbons valence and conduction band onsets vs. the fundamental gap, linearly and symmetrically approach mid–gap with slope 0.5, charging induces Fermi level pinning, i.e., the slopes of the valence and conduction bands being about 0.1 and 0.9, in agreement with experiment.