Temperature features of non-radiative energy transfer in hybrid associates of CdS/TGA quantum dots with methylene blue molecules

The paper presents the study results of non-radiative resonance energy transfer of electronic excitation from colloidal CdS quantum dots passivated with thioglycolic acid (CdS/TGA QDs) with an average size of 2.9 nm to methylene blue (MB+) molecules in temperature ranging from 300 to 80 K. In an aqueous MB+ and colloidal CdS/TGA QDs mixture, the increase in (MB+)2 dimers fraction and shift of the absorption band peaks of monomers (MB+) from 662 to 676 nm and dimers (MB+)2 from 606 to 590 nm were found. It is concluded that the hybrid associates of CdS/TGA+MB+ and CdS/TGA+(MB+)2 are formed. Luminescence quenching at CdS/TGA QDs band and MB+ luminescence enhancement under increase in MB+ molecules concentration were observed. A decrease in average lifetime of CdS/TGA QDs luminescence from 25 to 4 ns was found for MB+ concentration of 42 μ M. The Stern–Volmer constant, determined from the data on luminescence quenching, is 0.36 ∗ 106 M− 1. It is concluded that there is non-radiative resonance energy transfer (FRET) from the luminescence center of CdS/TGA quantum dots to MB+ molecule. Decreasing in temperature value to 80 K led to an increase in the luminescence intensity by 12 times. At the same time, the average lifetime of luminescence for pure CdS/TGA QDs increases from 25 to 72 ns. The Stern–Volmer constant increased to 0.64 ∗ 106 M− 1. It is concluded that its increase is due to an increase in average lifetime of CdS/TGA QD luminescence. In the framework of FRET theory, the rate constant of non-radiative energy transfer was estimated for an even distribution of CdS/TGA quantum dots and MB molecules in solution. Its is much smaller (6.7 ∗ 104 s− 1) than the experimentally determined value (2.5 ∗ 108 s− 1). This fact confirms the conclusion about the formation of CdS/TGA+MB hybrid system with effective FRET.