Controlling Exciton Polarization in Transition Metal Doped Indium Oxide Nanocrystals

Abstract

The simultaneous manipulation of electronic charge and spin in dilute magnetic semiconductors (DMSs) doped with transition metal ions has been long considered a holy grail for advancing spintronics and quantum information processing technologies. Although room temperature (RT) ferromagnetism has been attained in multiple DMSs including DMS oxides (DMSOs), these results have inconsistency in reproducibility. Moreover, RT ferromagnetism has also been observed in several metal oxides in the absence of any transition metal dopants. This phenomenon has been attributed to intrinsic defects in the host lattice, which creates confusion over the nature of exchange interactions that lead to RT ferromagnetism in DMSs. Here, using a carefully designed series of Co2+-doped indium oxide (Co2+:In2O3) nanocrystals (NCs) with different doping concentrations, and employing magnetic circular dichroism spectroscopy, we unravel the dominant mechanism governing spin polarization of charge carriers as a function of doping concentration, and establish the contributions of intrinsic defects and dopants to Zeeman splitting in Co2+:In2O3 NCs. Furthermore, the exchange coupling between the excitonic states and Co2+ is strongly dependent on the NC volume for sizes approaching the quantum confinement regime, but largely independent on the radial position of the dopants for larger NCs due to spatially homogeneous wavefunction of the exciton. Our work provides a critical understanding on why DMSOs are so sensitive to the synthesis methodology and will aid in developing new DMSO NC systems whose magnetic properties can be consistently predicted and reproduced.