Size-dependent shifts of the Néel temperature and optical band-gap in NiO nanoparticles.

Bulk NiO is a well-known antiferromagnet with Neel temperature TN(∞) = 524 K and an optical band-gap Eg = 4.3 eV. With decrease in particle size D from 40 nm to 4 nm of NiO, systematic changes of TN and Eg are observed and discussed here. From magnetic measurements, the changes in TN with D are found to fit finite-size scaling equation TN(D) = TN(∞) [1 - (ξo/D)λ] with λ = 3.2 ± 0.5 and ξo = 3.2 ± 0.2 nm, in good agreement with the predictions for a Heisenberg system. The observed blue shifts of Eg with decrease in D reaching Eg = 5.12 eV for D ∼ 4 nm are likely due to quantum confinement and non-stoichiometry. [ABSTRACT FROM AUTHOR]