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Nonlinear optical characteristics of chemically prepared silver selenide (Ag2Se, 3.7 nm) and lead selenide (PbSe, 6.3 nm) colloidal quantum dots (QDs) are investigated using the 1030 nm femtosecond pulses. Ag2Se QDs demonstrated two-photon absorption followed by saturable absorption. PbSe QDs initially showed the saturable absorption at small intensities of 1030 nm probe pulses, followed by reverse saturable absorption at stronger excitation. The difference in the positive nonlinear optical absorption mechanisms in these two QD colloids is attributed to their different bandgaps. The studied QD colloids exhibited the Kerr-induced positive nonlinear refraction. Transient absorption studies of the QDs showed that photoexcitation creates a hot exciton that forms band-edge exciton within 1.2 ps after excitation. Most of the electron-hole pairs further recombine through band-to-band recombination with a time constant of 1490 ps. Furthermore, the results from the transient absorption studies indicate the contribution of trap-assisted recombination occurs on a time scale exceeding that of the experimental time window. [ABSTRACT FROM AUTHOR]

In this paper, we present the results on photoinduced formation of colloidal Ag 2 S quantum dots with sizes of 1.5-3 nm passivated by 2-mercaptopropionic acid (Ag 2 S/2-MPA) in the presence of ethylene glycol. The synthetized colloidal Ag 2 S/2-MPA QDs have NIR recombination luminescence with its maximum near 800 nm. The control of absorption and luminescence properties of the QDs is achieved by photoactivation. It is shown that photoexposure of colloidal solution of Ag 2 S/2-MPA QDs leads to an increase in the QD size and monodispersity along side with the growth of the luminescence quantum yield from 1% to 7.9%. Enhancement of the luminescence quantum yield is accompanied by an increase in the average luminescence lifetime up to 190 ns, which is due to the blocking of the nonradiative recombination channel with the radiative recombination rate being (3-5.5) × 10 5 s -1 . It is shown that the purification of the Ag 2 S/2-MPA solution by a dialysis membrane from regenerated cellulose leads to an increase in the sample monodispersity, as well as stops the photoinduced growth of QDs, and also reduces the degradation of their photoluminescence., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)