Characterizing the Influence of TOPO on Exciton Recombination Dynamics in Colloidal CdSe Quantum Dots

Abstract

Surface ligands play a significant role in the carrier recombination dynamics of colloidal quantum dots (QDs). Trioctylphosphine oxide (TOPO) is a ligand used during CdSe QD synthesis, which binds to surface cadmium atoms, but, despite its widespread use, its effect on exciton relaxation rates is poorly understood. In this work, we extract TOPO–CdSe binding equilibria from isothermal titration calorimetry experiments and use them to study the effect of TOPO concentration on CdSe time-resolved photoluminescence. We develop a quantitative kinetic model of CdSe QD exciton relaxation that is based upon an ensemble probability distribution of bound ligands and show that bound TOPO gives rise to band-edge states that participate in carrier recombination dynamics. Optoelectronic applications of QDs necessitate a good understanding of interfacial dynamics so that charge separation, carrier transport, and other phenomena can be optimized. This technique is a promising step toward that goal that is useful for any ligand and compliments Fourier transform infrared, NMR, and other spectroscopic methods.