  The present article describes the size induced changes in the structural
arrangement of intrinsic defects present in chemically synthesized ZnO
nanoparticles of various sizes. Routine X-ray diffraction (XRD) and
Transmission Electron Microscopy (TEM) have been performed to determine the
shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using
positron annihilation spectroscopy reveals the presence of zinc vacancy.
Whereas analysis of photoluminescence results predict the signature of charged
oxygen vacancies. The size induced changes in positron parameters as well as
the photoluminescence properties, has shown contrasting or non-monotonous
trends as size varies from 4 nm to 85 nm. Small spherical particles below a
critical size (~ 23 nm) receive more positive surface charge due to the higher
occupancy of the doubly charge oxygen vacancy as compared to the bigger
nanostructures where singly charged oxygen vacancy predominates. This
electronic alteration has been seen to trigger yet another interesting
phenomenon, described as positron confinement inside nanoparticles. Finally,
based on all the results, a model of the structural arrangement of the
intrinsic defects in the present samples has been reconciled.
