  Conducting submicron particles are well-suited as filler particles in
non-conducting polymer matrices to obtain a conducting composite with a low
percolation threshold. Going to nanometer-sized filler particles imposes a
restriction to the conductivity of the composite, due to the reduction of the
density of states involved in the hopping process between the particles,
compared to its value within the crystallites. We show how those microscopic
parameters that govern the charge-transport processes across many decades of
length scales, can accurately and consistently be determined by a range of
dielectric-spectroscopy techniques from a few Hz to infrared frequencies. The
method, which is suited for a variety of systems with restricted geometries, is
applied to densely packed 7-nm-sized tin-oxide crystalline particles with
various degree of antimony doping and the quantitative results unambiguously
show the role of the nanocrystal charging energy in limiting the hopping
process.
