  Effect of ambient gases on photoluminescence (PL) and electrical conductivity
of films prepared using ZnO nanoparticles (NPs) have been investigated. It is
observed that NPs of size below 20 nm kept inside a chamber exhibit complete
reduction in their visible PL when oxygen partial pressure of the surrounding
gases is decreased by evacuation. However the visible PL from ZnO NPs is
insensitive to other major gases present in the ambient air. The rate of change
of PL intensity with pressure is inversely proportional to the ambient air
pressure and increases when particle size decreases due to the enhanced surface
to volume ratio. On the other hand an assembly of ZnO NPs behaves as a complete
insulator in the presence of dry air and its major components like N2, O2 and
CO2. Electrical conduction having resistivity ~102 - 103 {\Omega}m is observed
in the presence of humid air. The depletion layer formed at the NP surface
after acquiring donor electrons of ZnO by the adsorbed oxygen, has been found
to control the visible PL and increases the contact potential barrier between
the NPs which in turn enhances the resistance of the film.
