Electron and Hole Spin Relaxation in CdSe Colloidal Nanoplatelets.

Solution-processed quantum-confined nanocrystals are important building blocks for scalable implementation of quantum information science. Extensive studies on colloidal quantum dots (QDs) have revealed subpicosecond hole spin relaxation, whereas the electron spin dynamics remains difficult to probe. Here we study electron and hole spin dynamics in CdSe colloidal nanoplatelets (also called quantum wells) of varying thicknesses using circularly polarized transient absorption spectroscopy at room temperature. The clear spectroscopic features of transition bands associated with heavy, light, and spin-orbit split-off holes enabled separate probes of electron and hole dynamics. The hole spin-flip occurred within ∼200 fs, arising from strong spin-orbit coupling in the valence band. The electron spin lifetime decreased from 6.2 to 2.2 ps as the platelet thickness is reduced from 6 to 4 monolayers, reflecting an exchange interaction between the electron and the hole and/or surface dangling bond spins enhanced by quantum confinement.