Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials

TL;DR

This study investigates the vibrational density of states in nanoscale materials, revealing how surface, grain boundaries, and capillary pressures influence vibrational properties in nanoparticles and nanocrystalline structures.

Contribution

It provides a detailed analysis of vibrational density of states in nanoclusters and nanocrystalline materials, highlighting the roles of surfaces, grain boundaries, and capillary pressure effects.

Findings

VDOS inside nanoclusters resembles bulk compressed by capillary pressure

Surface atoms show enhanced low-energy VDOS and surface-like structures

Grain boundaries significantly contribute to VDOS enhancements in nanocrystalline materials

Abstract

The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.