Probing defects in chemically synthesized ZnO nanostrucures by Positron Annihilation and Photoluminescence Spectroscopy

TL;DR

This study investigates how size affects intrinsic defects in chemically synthesized ZnO nanoparticles using positron annihilation and photoluminescence spectroscopy, revealing size-dependent defect structures and charge states.

Contribution

It provides a detailed analysis of size-induced defect changes in ZnO nanoparticles and proposes a model for their structural defect arrangement based on combined spectroscopic techniques.

Findings

Size-dependent variation in positron parameters and photoluminescence properties.

Presence of zinc vacancies and charged oxygen vacancies identified.

Observation of positron confinement phenomena in smaller nanoparticles.

Abstract

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.