First principles absorption spectra of Au nanoparticles: from quantum to classical

TL;DR

This study compares quantum and classical methods for calculating absorption spectra of gold nanoparticles, revealing quantum effects dominate at small sizes and classical models fail to capture key spectral features.

Contribution

It demonstrates the limitations of classical FDTD methods in predicting spectral features of small gold nanoparticles, highlighting the importance of quantum effects.

Findings

Classical models fit quantum results at 3.1 nm but fail at smaller sizes.

Quantum effects cause peak splitting and blue shifts in spectra.

Classical plasmon resonances transition to individual electron excitations in small nanoparticles.

Abstract

Absorption cross-section spectra for gold nanoparticles were calculated using fully quantum Stochastic Density Functional Theory and a classical Finite-Difference Time Domain (FDTD) Maxwell solver. Spectral shifts were monitored as a function of size (1.3-3.1 nm) and shape (octahedron, cubeoctahedron, and truncated cube). Even though the classical approach is forced to fit the quantum TDDFT at 3.1nm, at smaller sizes there is a significant deviation as the classical theory is unable to account for peak splitting and spectral blue shifts even after quantum spectral corrections. We attribute the failure of classical methods at predicting these features to quantum effects and low density of states in small nanoparticles. Classically, plasmon resonances are modeled as collective conduction electron excitations, but at small nanoparticle size these excitations transition to few or even individual conductive electron excitations, as indicated by our results.