Luminescence of Thioglycolic Acid-Passivated PbS Quantum Dots in the Presence of Potassium Iodide

In this paper, we discuss general trends in the IR luminescence of colloidal PbS quantum dots 3 nm in average size, capped with thioglycolic acid molecules (PbS/TGA QDs). Treatment of the PbS/TGA QDs with a KI solution has been shown to cause a shift of a composite luminescence band peaking at 1120 nm to shorter wavelengths, to 1060 nm; an increase in the quantum yield of its shorter wavelength component, related to excitonic emission, from 1 to 10%; and quenching of its longer wavelength component, due to radiative recombination at defect levels. In this process, the cubic structure of PbS undergoes no changes. The average size of the PbS/TGA QDs has been shown to decrease slightly, by 0.2–0.3 nm. The conclusion has been drawn that the increase in the quantum yield of excitonic emission from the PbS/TGA QDs as a result of KI treatment is due to the more efficient passivation of interfacial defects, which act as both recombination luminescence and nonradiative carrier recombination channels. Using thermoluminescence in the temperature range from 80 to 350 K, we have demonstrated the presence of two types of shallow localized states, at 0.17- and 0.25-eV depths, whose density is only slightly sensitive to treatment of the PbS/TGA QDs with a KI solution. We assume that some of the traps identified are due to native defects in the nanocrystals—interstitial lead and sulfur ions—rather than to dangling bonds of surface lead and sulfur atoms.