  We have fabricated Pt-containing granular metals by focused electron beam
induced deposition from the $(CH_3)_3CH_3C_5H_4Pt$ precursor gas. The granular
metals are made of platinum nanocrystallites embedded in a carbonaceous matrix.
We have exposed the as-grown nanocomposites to low energy electron beam
irradiation and we have measured the electrical conductivity as a function of
the irradiation dose. Postgrowth electron beam irradiation transforms the
matrix microstructure and thus the strength of the tunneling coupling between
Pt nanocrystallites. For as-grown samples (weak tunnel coupling regime) we find
that the temperature dependence of the electrical conductivity follows the
stretched exponential behavior characteristic of the correlated variable-range
hopping transport regime. For briefly irradiated samples (strong tunnel
coupling regime) the electrical conductivity is tuned across the
metal-insulator transition. For long-time irradiated samples the electrical
conductivity behaves like that of a metal. In order to further analyze changes
of the microstructure as a function of the electron irradiation dose we have
carried out transmission electron microscope (TEM), micro-Raman and atomic
force microscopy (AFM) investigations. TEM pictures reveal that the
crystallites' size of long-time irradiated samples is larger than that of
as-grown samples. Furthermore we do not have evidence of microstructural
changes in briefly irradiated samples. By means of micro-Raman we find that by
increasing the irradiation dose the matrix changes following a graphitization
trajectory between amorphous carbon and nanocrystalline graphite. Finally, by
means of AFM measurements we observe a reduction of the volume of the samples
with increasing irradiation time which we attribute to the removal of carbon
molecules.
