Effects of bond disorder and surface amorphization on optical phonon lifetimes and Raman peak shape in crystalline nanoparticles

TL;DR

This paper investigates how bond disorder and surface amorphization affect optical phonon lifetimes and Raman spectra in crystalline nanoparticles, revealing regimes of vibrational behavior and a core-shell model explaining experimental observations.

Contribution

It introduces a core-shell model for strongly disordered nanoparticles and analyzes the impact of surface amorphization on phonon modes and Raman spectra.

Findings

Weak disorder leads to separated and overlapped vibrational levels.

Strong surface disorder causes phonons to repel from the disordered shell.

Raman spectra show narrow peaks from the core and a noisy background from the shell.

Abstract

Optical phonons in nanoparticles with the randomness of interatomic bonds are considered both analytically and numerically. For weak dilute disorder two qualitatively different regimes of separated and overlapped levels are observed, resembling the case of random atomic masses investigated previously. At stronger and/or more dense disorder, the particles become essentially inhomogeneous, thus constituting the minimal model to describe an amorphous phase, where the picture of vibrational modes becomes more subtle. We concentrate here on the experimentally relevant case of strong disorder located near the particle surface and formulate the core-shell model aimed to describe the ubiquitous phenomenon of particle surface amorphization. We observe a peculiar effect of volume optical phonons "repelling" from the disordered shell. It results in the Raman spectrum in the form of a combination of narrow well-resolved peaks stemming from the quantized modes of a pure particle core (red-shifted due to its effective smaller size), and the noisy background signal from the disordered shell placed primarily to the right from the main Raman peak.