The distinctive colors of colloidal gold and silver are due
to a phenomenon known as plasmon absorbance. Incident
light creates oscillations in conduction electrons on the surface
of the nanoparticles and electromagnetic radiation is absorbed.
The spectrum of the clear yellow colloidal silver from
the synthesis above is shown in Figure 2. To adjust the absorption
maximum between 0.5 and 0.7, the product sol was
diluted with distilled water. The plasmon resonance produces
a peak near 400 nm with PWHM of 50 to 70 nm.
The wavelength of the plasmon absorption maximum
in a given solvent can be used to indicate particle size. Silver
nanoparticles that produced the spectrum in Figure 3 (λmax= 400 nm) were examined using transmission electron microscopy
(TEM). A sample of silver nanoparticles from a
freshly synthesized clear yellow sol was prepared by drying a
small drop on a carbon-coated 200-mesh copper grid. The
TEM image of a region of the sample is shown in Figure 3,
and the size distribution is shown in Figure 4. In general, as