Surface Chemistry of InP Quantum Dots, Amine–Halide Co-Passivation, and Binding of Z-Type Ligands

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 (OlNH2) 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 ZnCl2 to the InP QD surface, a process enabled by the prior complexation of this Z-type ligand with OlNH2, 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.