Resolving carrier recombination in CdS quantum dots: A time-resolved fluorescence study

- At least two distinct pathways for trap fluorescence in CdS quantum dots.
- Electron traps recombine quickly (few tens of ns).
- Hole traps are lower energy and recombine more slowly (hundreds of ns).
- Exciton recombination dynamics affected by both types of trap.
- Evidence for low energy dual (electron + hole) trap state emission.

Trap states in colloidal quantum dots are known to play an important role in exciton recombination dynamics. CdS is an interesting quantum dot system because it exhibits both exciton and trap photoluminescence (PL) at room temperature. Previous ultrafast work has uncovered both electron and hole trapping processes, but the relationship between exciton and trap PL and the nature of the trap states remains poorly understood. In this article we take a detailed look at exciton and trap PL in several CdS samples. By varying temperature as well as excitation and emission energies, we develop a detailed picture of carrier recombination dynamics in these materials and, with the help of dynamical models, assign two distinct recombination pathways involving trapped electrons and holes. These results help us to understand the factors that govern charge carrier trapping in colloidal quantum dots and could help us to optimize their role in future applications.