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

Bulk NiO is a well-known antiferromagnet with Neel temperature T-N(infinity) = 524 K and an optical band-gap E-g = 4.3 eV. With decrease in particle size D from 40 nm to 4 nm of NiO, systematic changes of T-N and E-g are observed and discussed here. From magnetic measurements, the changes in T-N with D are found to fit finite-size scaling equation T-N(D) = T-N(infinity) [1 = (xi(o)/D)(lambda)] with lambda = 3.2 +/- 0.5 and xi(o) = 3.260.2 nm, in good agreement with the predictions for a Heisenberg system. The observed blue shifts of E-g with decrease in D reaching E-g = 5.12 eV for D similar to 4 nm are likely due to quantum confinement and non-stoichiometry. (C) 2013 AIP Publishing LLC.