Tailoring the photophysical properties and excitonic radiative decay of soluble CdSe quantum dots by controlling the ratio of capping thiol ligand

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

“…We have synthesized, via an aqueous method [15,42], four kinds of colloidal MSA-capped CdSe QDs (Scheme S1). These different QDs were obtained by changing the Cd/MSA ratios of the precursors (1/1, 1/1.5, 1/2 and 1/2.5) during the first step of synthesis.…”

“…We have previously characterized the physical properties of the synthesized QDs by HRTEM and X-ray diffraction [15]. The diameter of the QDs was statistically determined from electronic microscopy images [15] and increased gradually from 3.5 nm to 5.9 nm as a function of the CdSe/MSA ratio of the reactants during synthesis. The measured parameters (size, absorption and emission maxima) and calculated ones (molecular weight and extinction coefficient) are summarized in Table 1.…”

“…The emission bands 2 and 3, of lower energy, therefore involve recombination due to defects in the crystal units at the surface of QDs (trap states) [54−56]. These defects can be favored by the coexistence of both cubic and hexagonal units in the CdSe lattice, modulated by the Cd/MSA ratio [15]. The proportion of recombination from core exciton and from defect trap states is very sensitive [3,52,57] and the observed change of spectral profiles in the absence of metal cations reflects the changes of surface states of the QDs [34,47].…”

“…Here, we aimed at understanding whether the crystal defects at the surface of the QDs are involved in the quenching mechanism induced by the binding of a metal cation and the influence of these defects on the specific response between metals. Our approach was to modulate defects by varying the ratio between the precursors of CdSe and the capping ligand MSA in the first step of synthesis, since the Cdse/MSA ratio influences the growth of QDs, as shown previously [15]. 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. Overall, our results show that the surface defects influence the emission spectrum in the absence of cations.…”