  ZnO nanoparticles (NP) of size less than around 20 nm inside a chamber
exhibits complete reduction in their visible photoluminescence (PL) intensity
when partial pressure of oxygen in the surrounding gaseous environment is
decreased by evacuation. However the visible PL from ZnO nanoparticles are
insensitive to the gases like N2, He and Ar. The rate of change of PL intensity
with pressure (or the sensitivity) is inversely proportional to the ambient air
pressure. Sensitivity increases when surface to volume ratio increases for
lower particle size. The charged oxygen vacancies are attributed as the
responsible candidates for surface related visible PL from ZnO NP.
