  This work reports a systematic study of the evolution of charge transport
mechanism in granular ultra-thin films of palladium of thickness varying
between 6nm and 2nm. While the films with thickness > 4nm exhibit metallic
behaviour, that at 3nm thickness undergoes a metal-insulator transition at
19.5K. In contrast, the 2nm thick film remained insulating at all temperatures.
with transport following Mott's variable range hopping. At room temperature,
while the thicker film exhibit resistance decrease on H$_2$ exposure. the
insulating film showed an anomalous initial resistance increase before
switching to a subsequent decrease. The nanostructure dependent transport and
the ensuing H$_2$ response is modeled on a percolation model, which also
explores the relevance of film thickness as a macroscopic control parameter to
engineer the desired system response in granular metal films.
