Helping Observe and Tune Diffusion Using Mn2+ Photophysics in Inorganic Nanocrystals after Growth

Abstract

Doped inorganic crystals exhibit attractive optical, electrical, and magnetic properties that enabled the development of important modern technologies including ruby lasers and silicon microelectronics. The next frontier of doping chemistry is taking place on the nanoscale, where quantum confinement effects dramatically change the photophysical properties of semiconductors. This thesis describes recent advances in the doping of colloidal II–VI semiconductor nanocrystals known as “quantum dots.” Interactions between excitonic excited states and Mn2+, a model magnetic dopant, are investigated, primarily by magnetic circular dichroism (MCD) spectroscopy. These data reveal a new mechanism of nanocrystal doping characterized by thermodynamic addition of cation + anion pairs, followed by diffusional mixing. A broad range of equilibrium compositions are achieved, from less than one to hundreds of impurities per quantum dot, and subsequent cation exchange reactions are utilized to develop a model for dopant diffusion in these nanocrystals. Unique spectroscopic signatures are demonstrated, including nonmonotonic magnetization in the low-doping regime and extraordinarily large (whopping) Zeeman splittings in more highly doped quantum dots. Collectively, these studies provide insight into the fundamental processes of diffusion and magnetic exchange on the nanoscale, and may be useful for future spintronics, spin-photonics, and solar energy conversion technologies.