Unraveling photoluminescence quenching pathways in semiconductor nanocrystals

Role of surface defects in colloidal cadmium selenide (CdSe) nanocrystals in the specificity of fluorescence quenching by metal cations

“…Accordingly, the Gaussian component 1, of higher energy, was assigned to the core emission (direct exciton recombination) because its central wavelength position follows the QDs size increase (Table 1). This assignment agrees well with spectra which also disclose well separated components [41,45].…”

“…1A) at higher energy (503 nm) which is sharper and clearly distinct from the defect trap emission at 615 nm. This separation of both bands is pronounced for smaller QDs [41,44]. Well separated components appear when using a low concentration of ligand during synthesis because the proportion of surface defects increases with the density of grafted chains.…”

“…The surface emission is more affected than the core emission for three QDs. Only in the extreme case of CdSe(2.5) the core (band edge exciton) emission is altered, which could be indicative of a charge transfer process [41]. However, the quenching due to the binding of cations to the charged carboxylate groups is conveyed by the link between the ligand and the surface defects and cannot be explained by charge transfer [41].…”

“…S1) and the fluorescence emission (Fig. 1) were both shifted to longer wavelengths (Table 1), according to the confinement effect linked to increased size (gaining energy with the increased difference between the conduction and the valence bands as the size decreases) [3,18,41]. The excitonic emission band is identified by considering that its energy position depends on the CdSe QDs size and has a higher energy than the surface defect emission band.…”

“…The resulting QDs were tested for quenching by numerous metal cations and their fluorescence spectra analyzed in terms of individual emission bands, which appeared greatly altered by the metal cations. This protocol allowed to quantify the contributions of the band edge and the surface emissions [41] due to excitonic relaxation. The crystal defects are favored by the coexistence of both cubic and hexagonal units in CdSe the lattice and a proportion of hexagonal units can be created at the surface of MSA-CdSe QDs [15], acting as "trap" states for excitonic relaxation.…”