Surface Chemistry of InP Quantum Dots, Amine–Halide Co-Passivation, and Binding of Z-Type Ligands
Kim Corinna Dümbgen, Jari Leemans, Vic De Roo, Matthias M. Minjauw, Christophe Detavernier, Zeger Hens
Abstract
Understanding and controlling the surface chemistry of colloidal quantum dots (QDs) are essential steps toward improving their opto-electronic properties and tailoring the material for specific applications. For oleylamine–chloride co-passivated InP QDs synthesized using di-ethylaminophosphine (DEAP), knowledge of possible exchange reactions and their effect on the QD properties is still very limited. In this work, we address this issue by a combination of experimental and computational studies. First, we prove that InP QDs are passivated by a combination of oleylamine (OlNH 2 ) and chloride, bound as L-type and X-type ligands, respectively. By exposure to organic acids such as carboxylic acids or thiols, this L–X combination can be replaced with oleylammonium chloride in an acid–base-mediated ligand exchange reaction that results in the binding of carboxylates or thiolates as X-type ligands. The latter tend to quench the band-edge emission by forming strongly localized mid-gap states on the sulfur atoms of the thiolates. Furthermore, we observe that the binding of ZnCl 2 to the InP QD surface, a process enabled by the prior complexation of this Z-type ligand with OlNH 2, considerably increases the band-edge emission. However, as the resulting photoluminescence efficiency remains modest, we conclude that InP QDs synthesized using DEAP feature a diverse set of surface states, for which passivation depends at least on the elimination of undercoordinated surface phosphorous and the choice of the X-type ligand.