Synthesis and unusual electron paramagnetic resonance spectrum of metastable nanoclusters of ZnO semiconductor crystallites.

ABSTRACT

Metastable nanoclusters of ZnO semiconductor crystallites, 20 to 30 nm diameter, are synthesized by a reconstructive decomposition of a polymer precursor of dispersed Zn2+ cations in poly vinyl alcohol (PVA) polymer molecules. They have EPR (electron paramagnetic resonance) spectrum of distinct excitonic features. Multiple EPR bands appear in prominent intensities in oxygen vacancies VO+ and singly ionized Oi- and Zn(i)+ interstitials. A paramagnetic VO+ vacancy derives from usual diamagnetic O2- vacancy of VO++ (behaves as if doubly charged compared to the lattice) by addition of one electron. The results demonstrate the existence of a surface-interface or surface barrier layer in free-carrier depletion at the crystallite surface in the clusters and its effects on the Oi- and Zn(i)+ ionization states (determine green photoluminescence). Both VO+ and Zn(i)+ are curable by a thermal annealing in O2 gas. A cured sample of equilibrium structure achieved by heating at approximately 550 degrees C has a single EPR in Oi- at g = 1.990. The results are useful in understanding their correlation with EPR and optical properties in ZnO semiconductors and devices.