Theoretical Study of Cu/Mg Core-shell Nanocluster Formation

In a recently reported helium droplet-mediated deposition experiment to produce copper-coated magnesium core-shell nanoclusters, structural inversion was observed, which resulted in copper in the nanocluster interior, surrounded by oxidized magnesium on the copper surface. This study utilizes density functional theory methods to model the migration of copper atoms into the interior of a magnesium nanocluster to probe the energetics of this process and to compare it to the complementary process of magnesium atom migration into the interior of a copper nanocluster. Potential energy surfaces describing the forced migration of copper (magnesium) atoms into the interior of a 30-atom magnesium (copper) cluster were generated using the B3PW91 hybrid generalized gradient approximation functional with the augmented correlation consistent core-valence polarized triple-ζ basis set for magnesium and a pseudopotential plus valence-only basis set for copper. The estimated barrier for atomic copper to penetrate the surface of Mg₃₀ is 0.6 kcal mol^-1. In contrast, the migration of atomic magnesium into the interior of Cu₃₀ crosses an estimated barrier of 6 kcal mol^-1. These results are qualitatively consistent with the observed structural inversion of copper-coated magnesium nanoclusters and also suggest that inversion of a magnesium-coated copper cluster is less likely to occur.