Anomalous Laser-Fluence Dependence of Electron Spin Excitation in CdS Colloidal Quantum Dots: Surface Effects

Electron spin dynamics in CdS quantum dots (QDs) with hole acceptor 1-octanethiol organic molecules are investigated by time-resolved ellipticity spectroscopy. An anomalous dependence of laser fluences on electron spin excitation for the first time is reported. Increasing the laser fluence, the electron spin is switched from one direction to an antiparallel direction (spin direction switching, SDS) when adding enough 1-octanethiol hole acceptors in an air atmosphere. The analysis shows that the electron spin direction changes from heavy hole excitation defined to spin-orbit split hole excitation defined. In as-grown CdS QDs with native ligands, laser-fluence-dependent SDS phenomena are absent. Electron wave function spread into 1-octanethiol molecules is demonstrated to be important for the presence of SDS phenomena. The finding here thus reveals the importance of surface conditions on electron spin excitation processes in semiconductor QDs and that the surface can be used as an important factor to manipulate the spin.