Structural, vibrational and thermodynamic properties ofAgnCu34−n nanoparticles

Abstract

We report results of a systematic study of structural, vibrational andthermodynamical properties of 34-atom bimetallic nanoparticles from theAgnCu34−n family using model interaction potentials as derived from the embedded atommethod and invoking the harmonic approximation of lattice dynamics. Systematictrends in the bond length and dynamical properties can be explained largelyfrom arguments based on local coordination and elemental environment. Thusan increase in the number of silver atoms in a given neighborhood introduces amonotonic increase in bond length, while an increase of the copper content doesthe reverse. Moreover, for the bond lengths of the lowest-coordinated (six andeight) copper atoms with their nearest neighbors (Cu atoms), we find that thenanoparticles divide into two groups with the average bond length either close to(∼2.58 Å) or smallerthan (∼2.48 Å) that in bulk copper, accompanied by characteristic features in their vibrational density ofstates. For the entire set of nanoparticles, we find vibrational modes above the bulk bandsof copper/silver. We trace a blue shift in the high-frequency end of the spectrum thatoccurs as the number of copper atoms increases in the nanoparticles, leading to shrinkageof the bond lengths from those in the bulk. The vibrational densities of states at thelow-frequency end of the spectrum scale linearly with frequency as for single-elementnanoparticles, with a more pronounced effect for these nanoalloys. The Debyetemperature is found to be about one-third of that of the bulk for pure copper and silvernanoparticles, with a non-linear increase as copper atoms increase in the nanoalloy.