wavelength, characteristic of the aspect ratio. Ellipsoid particles with an axis ratio
close to 1 lead to a broadening of the absorption band.
Figure 9.29a and b display the resonance wavelength for surface plasmons of gold
and silver spheroids with different axis ratios. It can be seen that the transversal
modes are almost independent of the particle size, whereas the longitudinal modes
depend on particle diameter and axis ratio. In Figure 9.29, spherical particles are
characterized with an axis ratio of 1. As mentioned above, for particles up to
dimensions of less than 50 nm, the wavelength of the absorption maximum is
almost independent of the particle size.
Figure 9.29 Wavelength of the absorption
maximum of surface plasmons of silver and
gold spheroids with different sizes and axis
ratios [21]. (a) Wavelength of the surface
plasmons of spheroidal silver nanoparticles
with different sizes and axis ratios. The
transversal modes are, up to a diameter of at
least 20 nm, completely independent of the
particle diameter. (b) Position of the wavelength
of surface plasmons of spheroidal gold
nanoparticles as a function of particle diameter
and axis ratio. Up to a particle diameter of
around 40 nm, the transversal modes are
independent of particle diameter.
9.5 Metallic and Semiconducting Nanoparticles Isolated and in Transparent Matrices j233