Impurity incorporation and exchange interactions in Co²⁺-doped CdSe/CdS core/shell nanoplatelets

The intentional incorporation of transition metal impurities into colloidal semiconductor nanocrystals allows an extension of the host material's functionality. While dopant incorporation has been extensively investigated in zero-dimensional quantum dots, the substitutional replacement of atoms in two-dimensional (2D) nanostructures by magnetic dopants has been reported only recently. Here, we demonstrate the successful incorporation of Co²⁺ ions into the shell of CdSe/CdS core/shell nanoplatelets, using these ions (i) as microscopic probes for gaining distinct structural insights and (ii) to enhance the magneto-optical functionality of the host material. Analyzing interatomic Co²⁺ ligand field transitions, we conclude that Co²⁺ is incorporated into lattice sites of the CdS shell, and effects such as diffusion of dopants into the CdSe core or diffusion of the dopants out of the heterostructure causing self-purification play a minor role. Taking advantage of the absorption-based technique of magnetic circular dichroism, we directly prove the presence of sp-d exchange interactions between the dopants and the band charge carriers in CdSe/Co²⁺:CdS heteronanoplatelets. Thus, our study not only demonstrates magneto-optical functionality in 2D nanocrystals by Co²⁺ doping but also shows that a careful choice of the dopant type paves the way for a more detailed understanding of the impurity incorporation process into these novel 2D colloidal materials.