Surface and Quantum Confinement Effects in ZnO Nanocrystals

ZnO nanocrystals are studied using theoretical calculations based on the density functional theory. The two main effects related to the reduced size of the nanocrystals are investigated: quantum confinement and a large surface:volume ratio. The effects of quantum confinement are studied by saturating the surface dangling bonds of the nanocrystals with hypothetical H atoms. To understand the effects of the surfaces of the nanocrystals, all saturation is removed and the system is relaxed to its minimum energy position. Several different surface motifs are reported, which should be observed experimentally. Spin-polarized calculations are performed in the nonsaturated nanocrystals, leading to different magnetic moments. We propose that this magnetic moment can be responsible for the intrinsic magnetism observed in ZnO nanostructures.CAPESCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAPESPFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNP

Origin of FM Ordering in Pristine Micro- and Nanostructured ZnO

An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic exptl. and theor. studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepd. samples at low temp. (high temp.) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepd., annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calcns. that a range of magnetic properties in ZnO can result, depending on the sample prepn. and annealing conditions. For example, within the same ZnO sample we have obsd. ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calcns