Shape and composition effects on the Debye temperature and breathing frequency of metal nanoparticles †

Abstract

The vibrational properties of metal nanoparticles with dimensions 1-10 nm are computationally explored at the atomistic level of details, without resorting to lattice dynamics in the harmonic approximation. Using molecular dynamics simulations, power spectra of the atomic velocities and a measure of the particle volume are obtained, providing estimates for the vibrational density of states (VDOS) and the breathing frequency, respectively. The Debye temperature is inferred directly from the heat capacity resulting from the VDOS. For monometallic particles made of pure gold or iron, the influence of the shape around the cube reference is scrutinized either by varying the sharpness of their edges, or by varying their anisotropy from flat to elongated particles, keeping the number of atoms as approximately constant. Bimetallic silver-gold particles are also revisited, focusing on the role of chemical ordering (core-shell versus alloy), composition, and size. While the Debye temperature exhibits relatively modest variations throughout all cases investigated, the breathing frequency shows marked differences upon changes in the shape anisotropy or the chemical ordering, especially in coreshell nanoparticles with 20% composition in the core. Competing breathing modes arise in weakly deformed iron nanoparticles.