DETERMINATIONOF SIZE SHAPE AND CONCENTRATION OF SPHEROIDAL SILVER COLLOIDS EMBEDDED IN GLASS BY VIS-SPECTROSCOPY

Spherical silver colloids (silver clusters about 10 nm in diameter) in glass show a strong bell-shaped extinction band in the visible region of light (peak at about 410 nm wavelength). In the case of spheroidal silver colloids this band splits into two. One band tends to shorter wavelengths due to the absorption of light polarized parallel to the short axis of the ellipsoid. The other band shifts to longer wavelengths due to the absorption of light polarized parallel to the long axis of the ellipsoid. Calculations show that the peak positions of these two bands only depend on the particle shape (eccentricity of the ellipsoid) but not on the particle size. In contrast, the half-bandwidths of the bands only depend on the particle size but not on its shape. A further result is that the volume concentration of spheroidal silver colloids in glass is proportional to the area beneath an extinction curve. Electron micrographs of prolate silver colloids, made by heating and stretching of glass containing spherical silver colloids, show very good agreement with geometrical data drawn by this spectroscopic method.