Gold Nanoparticles Growing in a Polymer Matrix: What Can We Learn from Spectroscopic Imaging Ellipsometry?

Despite an increasing number of studies and the apparent simplicity of these model systems, the mechanisms of the growth of metal nanoparticles in a polymer matrix and, in particular, that of gold nanoparticles is still not fully understood. Usually, reported results concern global (ca. surface-averaged) measurements. Furthermore, the optical properties of plasmonic nanocomposites are difficult to investigate when the metal volume fraction is very low, typically less than 1%. This is especially true in the case of gold for which the localized plasmon resonance is less sharp than in the case of silver and, therefore, less easy to probe. In this article, the optical properties of gold-doped nanocomposites have been studied at the (sub)micrometer scale using spatially resolved spectroscopic ellipsometry. At low gold volume fraction, the thermal annealing of the composite leading to the in situ growth of the gold nanoparticles induces a local inhomogeneity of the Ψ and Δ ellipsometric images that can be analyzed in terms of heterogeneity of the gold fraction. Spectroscopic imaging ellipsometry confirms the existence of gold-depleted regions in the vicinity of the largest gold particles.