Effects of morphology on phonons of nanoscopic silver grains

TL;DR

This study investigates how the shape and surface features of nanoscopic silver grains influence their vibrational phonon properties, revealing complex dependencies on morphology through atomistic simulations.

Contribution

It provides a detailed analysis of phonon behavior in various nanograin morphologies, highlighting effects of edges, strain, and disorder on vibrational spectra.

Findings

Single-crystalline grains show torsional and radial phonons at low frequencies.

Surface protrusions lower the acoustic gap and increase vibrational DOS.

Irregular grains exhibit a low-frequency DOS proportional to v^2 in 1-2 THz range.

Abstract

The morphology of nanoscopic Ag grains significantly affects the phonons. Atomistic simulations show that realistic nanograin models display complex vibrational properties. (1) Single-crystalline grains. Nearly-pure torsional and radial phonons appear at low frequencies. For low-energy, faceted models, the breathing mode and acoustic gap (lowest frequency) are about 10% lower than predicted by elasticity theory (ET) for a continuum sphere of the same volume. The sharp edges and the atomic lattice split the ET-acoustic-gap quintet into a doublet and triplet. The surface protrusions associated with nearly spherical, high-energy models produce a smaller acoustic gap and a higher vibrational density of states (DOS) at frequencies \nu<2 THz. (2) Twined icosahedra. In contrast to the single-crystal case, the inherent strain produce a larger acoustic gap, while the core atoms yield a DOS tail extending beyond the highest frequency of single-crystalline grains. (3) Mark's decahedra, in contrast to (1) and (2), do not have a breathing mode; although twined and strained, do not exhibit a high-frequency tail in the DOS. (4) Irregular nanograins. Grain boundaries and surface disorder yield non-degenerate phonon frequencies, and significantly smaller acoustic gap. Only these nanograins exhibit a low-frequency \nu^2 DOS in the interval 1-2 THz.