An ellipsometric analysis to model the order-disorder transition in Au-SiO2 nano-granular thin films induced by thermal annealing

Abstract

We present an ellipsometric analysis to model the order-disorder transition in nano-granular gold embedded in a silica matrix (Au-SiO2). The optical measurements were performed by means of Reflection Spectroscopic Ellipsometry (RSE) over the spectral wavelength range 300–2500 nm. Upon thermal annealing of Au-SiO2 in the temperature range of 200 to 800 °C, a clear order-disorder textural transition in the film is exhibited. Analyzing the surface roughness by atomic force microscopy (AFM), the as-prepared film shows a narrow height distribution peak around ~3 nm, which reveals a good uniformity of the particles size. When the annealing temperature increases, the particles increasingly tend to coalesce into bigger sizes leading to non-homogeneous films showing increasingly larger surface roughness. We systematically determined the evolution of the film surface roughness parameters by using the AFM cumulative height distribution, which was introduced in RSE in order to evaluate the effective optical constants of the Au–SiO2 films, and to numerically fit the recorded Ψ, Δ and T spectra using a multiple Gaussian oscillators model. As a result, a reliable determination of the Au-SiO2 effective thicknesses and optical constants was achieved. Additionally, we fitted the experimental data to the Maxwell-Garnett (MG) effective medium approximation (EMA). It is shown that MG-EMA does not fit for the lower annealing temperature due to the particle self-organization. However, when the particles disorder takes place, as a consequence of the annealing temperature increase, the MG-EMA fit is increasingly improved. All in all, a clear evidence of the order-disorder transition in the Au-SiO2 nano-granular system induced by thermal annealing is provided.