Optical Properties of Multilayer Colloids

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Abstract
The theory of Aden and Kerker on the scattering of electromagnetic waves from two concentric spheres has been applied to the interaction of light with two‐layer metallic colloids. Starting with gold nuclei 59 Å in ``diameter,'' silver layers 5 to 220 Å in thickness have been grown epitaxially. The optical absorption spectra of these hydrosols containing two‐layer particles are compared with the spectra calculated from the theory of Aden and Kerker. The calculated and experimentally observed spectra are in relatively good agreement in the wavelength range 380–700 mμ. Large discrepancies occur between 300 and 380 mμ for hydrosols containing particles with silver layers less than 15 Å. This is attributed to a deviation of the silver refractive index from that of bulk silver for these layer thicknesses. The experimental absorption peaks are generally found to be broader than the calculated results. This is due in part to size and shape anisotropy. Other possible factors are lattice imperfections and trapped ions at the gold—silver interface.

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