Optical Characterization of Thin Colloidal Gold Films by Spectroscopic Ellipsometry

Spectroscopic ellipsometry and atomic force microscopy (AFM) experiments are employed to characterize nanocolloidal gold films, self-assembled at APTES-derivatized Si/SiO2 surfaces. X-ray fluorescence measurements after deposition confirm that AFM provides a representative means to probe the absolute surface coverage. Optical properties of gold nanocrystal assemblies are investigated both ex situ after drying and in situ prior to evaporation of the solvent. Quantitative optical characterization of these highly inhomogeneous systems is not unambiquous. Conventional effective medium approximations are not applicable to these systems. To enable an accurate analysis, extinction measurements on colloidal suspensions are performed. The limited particle size in relation to the electron mean free path leads to a modification of the dielectric function at longer wavelengths. Ellipsometry spectra of the colloidal gold films are analyzed qualitatively using an optical invariant and principal component analyses. Quantitative results are obtained using a theory which treats the nanocrystals as polarizabilities at the Fresnel interface. Above approximately 20%, the coverages determined from optical spectra are in agreement with what is found from AFM images. At lower coverages, the optical results seem to overestimate the actual nanocrystal density. The discrepancies are discussed in terms of image charge effects arising from the proximity of the substrate.