Dual Exciton Polarization in Bipolar CeO_2–x Nanocrystals Controlled by Defect-Based Redox Processes

Abstract

Discovering alternative means to control electronic states in semiconductor nanostructures is the key to the development of new quantum technologies. Controlling the cyclotron motion of free charge carriers in semiconductor nanocrystals using an external magnetic field generates a tunable angular momentum, as a collective electronic degree of freedom, which can be imparted to the electronic band states to achieve complete exciton polarization. The sign of this polarization is determined by the type of majority charge carriers in a given lattice. Using magnetic circular dichroism spectroscopy, we demonstrate a simultaneous polarization of excitonic states in substoichiometric oxygen-deficient CeO 2−x nanocrystals associated with electrons and holes, which can be controlled by the thermal treatment of colloidal nanocrystals in oxidizing or reducing conditions. The presence of both occupied and unoccupied midgap states, due to Ce 3+ 4f and Ce 4+ 4f orbitals, respectively, allows for selective probing of the effect of holes in the valence band (VB → Ce 4+ 4f) and electrons in the conduction band (Ce 3+ 4f → CB). The two transitions show the opposite sign at 300 K due to the opposite angular momenta associated with cyclotron electrons and holes. The ability to manipulate Ce 4f-derived midgap states by defect formation during the synthesis or postsynthesis treatment allows for a range of new technological applications of CeO 2−x nanocrystals in optoelectronics.