Time-dependent photoluminescence spectroscopy as a tool to measure the ligand exchange kinetics on a quantum dot surface

The exchange kinetics of native ligands that passivate CdSe quantum dots (hexadecylamine (HDA), trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP)) by thiols is followed in situ. This is realized by measuring, in real-time, the decrease in emission intensity of the QDs upon addition of hexanethiol (HT) which quenches the emission. The effect of adding an excess of native ligands prior to thiol addition on the capping exchange is studied to provide insight in the bond strength and exchange kinetics of the individual surfactants. Temperature-dependent measurements reveal faster kinetics with increasing temperature. A kinetic model to describe the time-dependent measurements is introduced, taking into account the equilibrium between native ligands before thiol addition and describing the evolution of surface coverage by all ligands over time. The model allows us to extract the quenching rate for a single thiol ligand (0.004 ns(-1)) as well as exchange rates, equilibrium constants, activation energies, and changes in Gibbs free energy for replacement of the different native surfactants by HT. The analysis reveals that the substitution half-time of HDA by HT (72 s) is much shorter than for TOP (5 h) or TOPO (2.5 h) under the same conditions. The temperature dependence of the kinetics shows that the activation energy for exchange of HDA/TOPO by hexanethiol (1.6 kJ/mol) is much smaller than for TOP (20.9 kJ/mol).