Solvent-Controlled Post-Synthetic Mn Modification and Emission Dynamics in CdSe Nanoplatelets

Post-synthetic doping provides a route to modify the excited-state dynamics of semiconductor nanoplatelets without coupling dopant incorporation to their initial nucleation and growth. Here, we investigate post-synthetic Mn incorporation into preformed CdSe nanoplatelets using manganese (II) diethyldithiocarbamate followed by thermal treatment under inert conditions. Structural and compositional analyses confirm the association of Mn with the nanoplatelets while preserving their platelet-like morphology and zinc-blende CdSe structure. Under the present reaction conditions, DPE-based treatments yield substantially higher Mn incorporation than HB-based treatments, demonstrating the strong sensitivity of the incorporation process to the reaction environment. Importantly, undoped control experiments show that broad visible-to-NIR emission can also arise from pristine CdSe nanoplatelets and can be strongly enhanced by thermal treatment, demonstrating that the spectral appearance of the broad band alone is not a unique signature of Mn-related states. The decisive difference is instead found in the emission dynamics: pristine and thermally treated undoped nanoplatelets, as well as low-Mn HB samples, exhibit broad-band decay on the few-microsecond timescale, whereas the main Mn-rich DPE samples display exceptionally long-lived broadband emission extending into the hundred-microsecond regime. PLE measurements further show that the low-energy emission is populated following excitation of the CdSe host. These results identify the emission kinetics, rather than the presence of broadband photoluminescence itself, as the clearest signature of additional Mn-associated relaxation pathways and demonstrate that post-synthetic reaction conditions provide a means of accessing long-lived excited-state dynamics in CdSe nanoplatelets. A representative sample also retained a comparable PLQY after more than two years of ambient storage, supporting the long-term stability of the optical response.