Increase of Mn solubility with decreasing grain size in ZnO

Nanograined (grain size 20 nm) ZnO films with various Mn content (from 0 to 47 at%) were synthesized by the novel wet chemistry method. The solubility limit for Mn was determined at 550 °C. The lattice parameter c of the ZnO-based solid solution with wurzite structure ceases to grow at 30 at% Mn. The peaks of the second phase (Mn3O4 with cubic lattice) become visible in the X-rays diffraction spectra at 30 at% Mn. The same second phase appears in the bulk ZnO already at 12 at% Mn. The recently published papers on the structure and magnetic behaviour of Mn-doped ZnO allowed us to obtain the size-dependence of Mn solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Mn solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk solubility limit, the thickness of an Mn-enriched layer is several monolayers in GBs and at least two monolayers in the free surfaces.