Metal nanoparticles (gold, silver, copper) that are embedded in an insulating organic
host material exhibit optical plasma resonance absorption in the visible and near-infrared region. The spectral position, the half width and the intensity
of the plasma resonance absorption all depend on the particle size, the particle shape,
and the optical behavior of the cluster and the host material.
The optical extinction of various gold, silver or copper particle assemblies embedded in
plasma polymer or gelatin was measured at 4.2 K and 1.2 K as well as at room temperature.
The packing density of several samples was high enough to
resolve a reversible increase of the plasma resonance absorption
intensity towards lower temperatures. Additionally, at larger silver particles
$D_{\rm m} > 50$ nm a significant blue shift of the plasma resonance absorption
was measured.
Particle size and shape distribution were determined by transmission
electron microscopy (TEM).
For the first time, simultaneous measurements of the electrical and optical properties
at one and the same particle assembly were performed at low temperatures.
Contrary to the increasing optical extinction, the d.c. conductivity decreased
to two orders of magnitude. At silver particles embedded in a plasma polymer made from
thiophene a significant photocurrent was measured.