Second-Hyperpolarizability (γ) Enhancement in Metal-Decorated Zigzag Graphene Flakes and Ribbons: The Size Effect

A study about the size-dependence of the hyperpolarizability-enhancement observed in metal-functionalized finite graphene species is presented. The reported ab initio and density functional results suggest that edge-passivation of graphene fragments with metallic agents (in this case Li) triggers an impressive enhancement of the second hyperpolarizability at the static limit. However, such a trend holds only in small graphene-type fragments. Strong evidence is provided showing that the specific effect drastically weakens with increasing the size of the graphene fragments regardless of their shape. The observed hyperpolarizability-enhancement in small systems and its severe decrease with increasing the size of the graphene species is qualitatively explained in terms of their charge transfer polarization mechanism.