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The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Sicore-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studiedwithin Density Functional Theory. Effects induced by quantum confinement andstrain on the near-band-edge states localization, as well as the band-offsetproperties between Si and Ge regions, are investigated in detail. On the onehand, we prove that Si(core)/Ge(shell) nanocrystals always show a type IIband-offset alignment, with the HOMO mainly localized on the Ge shell regionand the LUMO mainly localized on the Si core region. On the other hand, ourresults point out that a type II offset cannot be observed in small (diameterless than 3 nm) Ge(core)/Si(shell) nanocrystals. In these systems, quantumconfinement and strain drive the near-band-edge states to be mainly localizedon Ge atoms inducing a type I alignment. In larger Ge(core)/Si(shell)nanocrystals, instead, the formation of a type II offset can be engineered byplaying with both core and shell thickness. The conditions that favor thetransition from a type I to a type II alignment for Ge(core)/Si(shell)nanocrystals are discussed in detail.

Interplay of quantum confinement and strain effects in type I to type II transition in Ge/Si core-shell nanocrystals