Trap state and exciton luminescence of colloidal PbS quantum dots coated with thioglycolic acid molecules
Abstract
This work presents the results of studying the IR luminescence of colloid PbS quantum dots coated with molecules of thioglycolic acid.
Luminescence of the sample was recorded using the InGaAs image sensor PDF 10C/M (ThorlabsInc., USA) and a diffraction monochromator with 600 mm-1 grating. To study the temperature dependence of luminescence, the sample was cooled in a nitrogen cryostat down to 80 К. A redistribution of the luminescence intensity between two peaks (1100 and 1280 nm) was identified upon a decrease in temperature. It was shown that an exciton absorption peak was present in the excitation
spectrum for the short-wave luminescence peak, and the Stokes shift was ΔEstokes ~ 0.1 eV. On the contrary, the exciton peak was absent in the luminescence excitation spectrum of the long-wave band, and its red boundary was shifted towards the short-wave region, that provided the Stokes shift of more than 0.3 eV.
It was concluded that the short-wave luminescence band appeared as a result of the radiative annihilation of an exciton, while the long-wave band appeared due to the recombination of charge carriers at trap states. Trap state luminescence was effectively excited upon direct absorption of the radiation by the luminescence centre. A three-level diagram was suggested that determined the IR luminescence of colloid PbS quantum dots coated with thioglycolic acid molecules.
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