Defect characteristics of cadmium oxide nanocrystallites synthesized via a chemical precipitation method

In this study, we synthesized cadmium oxide nanocrystallites measuring 16 nm-30 nm via a chemical precipitation method at synthesis temperatures ranging from 400 degrees C to 800 degrees C. X-ray diffraction analysis identified well-defined peaks with no indication of mixed or impurity phases. The samples were also characterized by transmission electron microscopy. The band gap energies measured by optical absorption varied between 2.32 eV and 2.38 eV, thereby indicating quantum confinement effects over the entire range of nanocrystallite sizes. Fourier transform-infrared spectroscopic measurements demonstrated the localized surface plasmonic resonance behavior of the samples. Positron annihilation studies were conducted to identify and quantify the vacancy type defects, sizes, and concentrations. Increased annihilation of positrons occurred on the surfaces of nanocrystallites with dimensions less than the thermal diffusion length of positrons (similar to 25.4 nm). In crystallites larger than this limit, positrons were trapped in the non-stoichiometric defects within the crystallites, as demonstrated by both the positron lifetime and coincidence Doppler broadening measurements. Defects played a major role in determining the favorable properties of these cadmium oxide nanocrystallites because defects and parameters such as the synthesis temperature were strongly correlated.