Surface Charging and Trion Dynamics in CdSe-Based Core/Shell Quantum Dots

Static and time-resolved fluorescence and high- and low-power transient absorption results on CdSe/CdS and CdSe/ZnSe core/shell particles are presented. Three CdS shell thicknesses were examined, and all of the particles had either octadecylamine (ODA) and tributylphosphine (TBP) or just ODA ligands. The results can be understood in terms of a mechanism in which there is a thermal equilibrium between electrons being in the valence band or in chalcogenide localized surface states. Thermal promotion of a valence band electron to a surface state leaves the particle core positively charged. Photon absorption when the particle is in this state results in two valence band holes and a conduction band electron a positive trion. The trion can undergo a fast Auger recombination, making the particle nonluminescent. A lack of TBP ligands results in more empty surface orbitals and therefore shifts the equilibrium toward surface trapped electrons and hence trion formation. Low- and high-power transient absorption measurements give the trion and biexciton lifetimes, and we find that the trion Auger lifetimes are about a factor of 2.2-2.3 greater than those of the biexciton, as expected from the degeneracies of Auger pathways. We also find that the trion lifetimes vary with shell thickness and composition as expected, based on effective volume and electron hole overlap considerations. Taken together, these observations strongly support a ground-state equilibrium between electrons in the valence band and the surface states. The relevance of these results to fluorescence intermittency ("blinking") is discussed.