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

TL;DR

This study investigates how core/shell structures in CdSe nanoplatelets influence excitonic spin coherence lifetimes, revealing significant increases with shell growth and surface passivation, and providing insights into spin dynamics at room temperature.

Contribution

It demonstrates that core/shell morphology and surface passivation substantially enhance spin coherence times in colloidal CdSe nanoplatelets, a novel insight for spintronic applications.

Findings

Shell growth increases spin lifetime to ~49 ps at room temperature.

ZnS shells further extend spin lifetime up to ~100 ps.

Surface termination and passivation critically affect spin coherence.

Abstract

We report spin-polarized transient absorption for colloidal CdSe nanoplatelets as functions of thickness (2 to 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. 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 picoseconds 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 excitonic g-factor.