  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.
