Thermally stimulated infrared shift of cadmium oxide optical absorption band edge

The effect of annealing temperature on the optical properties of nanosized cadmium oxide with rock-salt structure synthesized by thermal treatment of cadmium formate Cd(HCOO)(2)center dot 2H(2)O was studied using X-ray diffraction and UV-Vis-NIR spectroscopy methods. The samples were obtained by heating of as-prepared cadmium oxide in air at a temperature successively increasing from 500 to 900 degrees C with an interval of 50 degrees C. The exposure time at each temperature was 1 h. It was established that as a result of annealing in air in this thermal regime the CdO band gap decreased from 1.95 to 1.50 eV with the corresponding change of the powder color from reddish-brown to black. The particle size estimated by the Williamson-Hall method and the density decreased from 85.5 to 79.8 nm and from 8.24 to 8.20 g/cm(3), respectively. With the use of the first-principle calculations of the electronic band structure, optical absorption and vacancy formation energy it was shown that the reason of the band gap narrowing is the appearance of vacancies in the CdO oxygen sublattice, the concentration of which grows with increasing annealing temperature.

Automated summary

The annealing temperature significantly affects the optical properties of nanosized cadmium oxide with a decrease in band gap and particle size as the temperature increases. First-principle calculations indicate that the band gap narrowing is due to the appearance of vacancies in the CdO oxygen sublattice, which increase in concentration with higher annealing temperatures.