Resonant nonradiative energy transfer in hybrid associates of thionine molecules and Ag$_{2}$S colloidal quantum dots with different luminescence mechanisms

We have considered the resonant nonradiative energy transfer in hybrid associates of thionine dye (TH$^+$) molecules and Ag$_2$S colloidal quantum dots (QDs) passivated with thioglycolic acid (Ag$_2$S/TGA) and Ag$_{2}$S QDs stabilized with gelatin (Ag$_2$S/Gel). Used samples of Ag$_{2}$S QDs possess luminescence, which arises by the exciton mechanism, as well as by the recombination mechanisms of holes with electrons localized at luminescence centers and of electrons with holes localized at the luminescence centers. The quenching of luminescence of Ag$_{2}$S/TGA QDs (1.8 nm) with a maximum at 630 nm and a decrease in the luminescence lifetime from 13.7 to 6.5 ns, which occurs upon association with TH$^+$ molecules, have been established. On the contrary, for associates of Ag$_{2}$S/TGA QDs (5.5 nm) with TH$^+$ molecules, we have observed the quenching of luminescence of the dye and a decrease in the lifetime of this luminescence from 0.43 to 0.3 ns, as well as an enhancement of luminescence of QDs. In the case of hybrid association with TH$^+$ molecules, the luminescence enhancement of Ag$_2$S/Gel QDs (1.6 nm) has been established, which results from the recombination of free holes with electrons localized at luminescence centers. Based on our analysis of the luminescence kinetics of the dye, we have inferred the occurrence of resonant nonradiative energy transfer from TH$^+$ molecules to centers of recombination luminescence in Ag$_{2}$S/TGA (5.5 nm) and in Ag$_{2}$S/Gel (1.6 nm) QDs with its maxima at 950 and 1205 nm, respectively. For Ag$_{2}$S/TGA QDs (2.2 nm), which luminesce with a maximum at 620 nm by the exciton mechanism, we have observed a significant overlap both between the luminescence spectra of QDs and TH$^+$ and between their absorption spectra. Close parameters of the luminescence kinetics for both the initial components and their associates indicate the energy transfer, which is realized in opposite directions.