Excitonic Spin-Coherence Lifetimes in CdSe Nanoplatelets Increase Significantly with Core/Shell Morphology

We report spin-polarized transient absorption for colloidal CdSe nanoplatelets as functions of thickness (2-6 monolayer thickness) and core/shell motif. Using electro-optical modulation of co-and cross-polarization pump-probe combinations, we sensitively observe spin-polarized transitions. Core-only nanoplatelets exhibit few-picosecond spin lifetimes that weakly increase with layer thickness. The spectral content of differenced spin-polarized signals indicate biexciton binding energies that decrease with increasing thickness and smaller values than previously reported. Shell growth of CdS with controlled thicknesses, which partially delocalize the electron from the hole, significantly increases the spin lifetime to -49 ps at room temperature. Implementation of ZnS shells, which do not alter delocalization but do alter surface termination, increased spin lifetimes up to -100 ps, bolstering the interpretation that surface termination heavily influences spin coherence, likely due to passivation of dangling bonds. Spin precession in magnetic fields both confirms long coherence lifetime at room temperature and yields the excitonic g factor.

Automated summary

We investigate spin-polarized transient absorption in colloidal CdSe nanoplatelets, varying thickness and core/shell structure. By modulating co-and cross-polarizations, we observe spin-polarized transitions. Core-only nanoplatelets exhibit short spin lifetimes, weakly increasing with thickness. Biexciton binding energies decrease with thickness and differ from previous reports. CdS shell growth, which delocalizes electrons from holes, significantly enhances spin lifetimes at room temperature. ZnS shells, which alter surface termination, further increase spin lifetimes, suggesting surface passivation plays a crucial role.