Synthesis, spectroscopy and magnetism of diluted magnetic semiconductor nanocrystals

A variety of diluted magnetic semiconductor (DMS) nanocrystals have been synthesized by the solution routes, and their structural, spectroscopic and magnetic properties have been investigated. Ligand-field electronic absorption spectroscopic techniques (electronic absorption and magnetic circular dichroism) have been applied to directly probe transition metal dopant ions in semiconductor nanocrystals. Using Co2+-doped CdS (Co2+:CdS) quantum dots (QDs) as the model system it is found that simple coprecipitation yields predominantly surface bound dopant ions, which are subsequently solvated in the presence of a coordinating solvent. Comparison with Co2+:ZnS QDs revealed that dopant exclusion in the case of Co2+:CdS QDs is associated with the ionic radius mismatch between the dopant and the host cations. An isocrystalline core/shell method has been developed as a convenient and effective way for obtaining high quality internally doped DMS-QDs, especially in cases involving dopant-host mismatches. Co2+:CdS DMS-QDs show large Zeeman splitting of the semiconductor band states, characteristic for true DMS materials, as confirmed by magnetic circular dichroism spectroscopy. Effects of quantum confinement on the properties of dopant ions have also been investigated, showing increased interactions between dopant ions and the semiconductor band structure with increasing nanocrystal sizes.; Following similar synthetic and spectroscopic strategies Co2+ - and Ni2+-doped ZnO DMS-QDs have been successfully synthesized, and internal doping has been confirmed using ligand-field electronic absorption spectroscopy. While these nanocrystals are paramagnetic or superparamagnetic as such, they exhibit distinct ferromagnetism at room temperature when aggregated under reaction limited conditions. The appearance of ferromagnetism in the aggregates has been attributed to an increase in the ferromagnetic domain sizes and to the formation of defects at the nanocrystal interfaces during the aggregation process. Distinct temperature behavior of the hysteresis properties of Ni2+:ZnO aggregates has been observed, and associated with unique ground state electronic structure of the isolated paramagnetic Ni 2+ in ZnO.; Room temperature ferromagnetism has also been observed in Ni2+ :SnO₂ aggregates and thin films prepared from the colloidal nanocrystals. SnO₂ is a particularly promising host lattice since it exhibits a high conductivity in the native form, and therefore relates to the formation of polyfunctional materials at the nanometer scale.