Magnetic nanocrystals: Synthesis and properties of diluted magnetic semiconductor quantum dots

The chapters in this thesis describe the investigations into the synthesis and magnetic and electronic properties of diluted magnetic semiconductor quantum dots (DMS-QDs), specifically Co2+ and Mn2+-doped ZnO and Co2+:ZnSe. Homogeneous dopant incorporation and substitutional speciation of Co2+ and Mn2+ in ZnO QDs during solution synthesis at room temperature were confirmed by electronic absorption and electron paramagnetic resonance spectroscopy measurements. Post-synthetic methods were shown to eliminate any dopants bound to the nanocrystal surfaces, resulting in high-quality, internally-doped ZnO DMS-QDs. The magneto-optical properties observed by MCD spectroscopy demonstrated the presence of sp-d exchange interactions that are characteristic of DMSs. The sensitivity of this technique allowed clear observation and assisted in the assignment of dopant charge transfer transitions in both materials. Ferromagnetism was activated in both Co2+:ZnO and Mn2+:ZnO nanocrystalline aggregates and thin films by addition of the proper defects, providing significant insight to the understanding of magnetic ordering in ZnO and other DMSs. The influence of post-synthetic treatments on the luminescent properties of ZnO nanocrystals were also investigated since surface defects identified as sources of trap state luminescence of ZnO could also potentially be incorporated into the nanocrystalline aggregates and thin films. Colloidal CO2+:ZnSe QDs were prepared from solution using the "hot injection method". Lower than expected Zeeman splittings in CO2+:ZnSe QDs, as measured by low temperature electronic absorption and MCD spectroscopies, were attributed to the presence of an undoped ZnSe core in the QDs, a result of a dopant-excluding nucleation event during the synthesis of these nanocrystals. Additionally, direct observation of a charge transfer transition by MCD spectroscopy showed experimental evidence that the dopant levels are pinned in energy as the valence and conduction bands of the semiconductor increase their energy gap with increasing quantum confinement. These studies highlight the utility of DMS-QDs as materials for studying DMS properties, and suggest opportunities for the use of these highly processable colloidal magnetic nanocrystals as potential building-block precursors for the construction of more advanced nanoscale DMS architectures.