Spectral analysis of oxidation on localized surface plasmon resonance of copper nanoparticles thin film.

Copper nanoparticles (CuNPs) possess localized surface plasmon resonance (LSPR) effect. Cu thin films composed of individual CuNPs exhibit stronger LSPR than the individual CuNPs due to the LSPR coupling among CuNPs. However, CuNPs are easy to be oxidized, which results in the rapid LSPR damping of the CuNPs thin films. Simulation of the variations of the coupled LSPR of two adjacent CuNPs with the thickness of oxide shells formed during oxidation is of great importance for understanding the mechanisms of the strong LSPR of CuNPs thin films and its rapid attenuation. In this paper, Discrete-dipole approximation method is used to simulate the extinction spectra of two adjacent spherical CuNPs as a function of the shell thickness (t), the ambient refractive index (n), the diameter (D) of the CuNPs, and the inter-nanoparticle spacing (L). The calculation is validated by experimental results. According to our model, for a definite CuNPs thin films, the oxide shell thickness of CuNPs can be calculated only if the extinction spectra and the morphology are provided. Further, it is found when the oxide shell thickness is small (t/R< 0.3), increasing n and decreasing L/D have an obvious synergistic effect on enhancing the coupled LSPR, but this synergistic effect weakens with the deepening of oxidation, and disappeared when t/R > 0.5. This study provides a calculation method for coupled core-shell nanoparticles and throws light on the role of oxidation on the rapid damped LSPR of CuNPs thin films.