Effect of Ni2+ and O2− vacancies on the electrical and optical properties of nanostructured nickel oxide synthesized through a facile chemical route

• Effects of Ni2+ and O2− vacancies on the electrical and optical response of NiO.
• The concentration of uncompensated Ni2+ vacancies enhances the DC conductivity.
• Use of conductivity in vacuum and air ambience to explain filling up of O2− vacancies.
• The increase of O2− vacancies inhibits intraionic transitions due to optical absorption.
• Effect of the concentration of O2− vacancies on the PL emission.

Nanostructured nickel oxide samples were synthesized through a novel chemical route using nickel chloride and ethanol amine. The DC electrical conductivity of the as prepared sample is found to be enhanced by 3–6 orders of magnitude in comparison to single crystalline NiO. The conductivity increases with increase in the annealing temperature. The dominant conduction mechanism is found to be that due to large polarons associated with holes in the O2p band. Study of DC conductivity in vacuum and air indicate that the oxygen vacancies are filled up on annealing thereby causing an increase in the concentration of uncompensated Ni2+ concentration. The relative concentration of Ni2+ and O2− vacancies influence the optical absorption also. It is observed that the presence of O2− vacancies inhibit the low energy (2–4 eV) intraionic 3d8–3d8⁎ transitions while the increase in the percentage of uncompensated Ni2+ vacancies enhances these transitions. The PL emission in the UV range (2.4–3.4 eV) is unaffected by the O2− vacancy concentration while 490 nm emission due to O2− vacancies decreases with increase in annealing temperature.

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