Ultrafast exciton relaxation dynamics of PbS and core/shell PbS/CdS quantum dots

Optical properties and ultrafast exciton relaxation dynamics in PbS and core/shell PbS/CdS quantum dots (QDs) have been studied using UV-vis absorption and fluorescence spectroscopy as well as femtosecond (fs) transient absorption spectroscopy. The electronic absorption spectrum of the PbS QDs features broad absorption in the entire near IR-vis-UV region with a monotonic increase in intensity towards shorter wavelength. Relative to PbS, the absorption of the core/shell PbS/CdS QDs shows a slight blue shift in the 600-800 nm region, due to the decrease of the PbS crystal size caused by the synthetic process of the core/shell structure, and increased absorption near 400 nm due to the CdS shell. The PL of the PbS/CdS QDs was similar to 2.6 times more intense than that of the PbS QDs, due to surface passivation of PbS by CdS, and blue-shifted, attributable to smaller PbS size and thereby stronger quantum confinement in the core/shell QDs. Fs transient absorption measurements of both systems showed a strong transient absorption feature from 600 to 750 nm following excitation at 750 nm. The transient absorption decays can be fit to a biexponential with time constants of 8 and 100 ps for PbS and 6 and 80 ps for PbS/CdS. The amplitude and lifetime of the fast component were excitation intensity dependent, with the amplitude increasing more than linearly with increasing excitation intensity and the lifetime decreasing with increasing intensity. The fast decay is attributed to exciton-exciton annihilation and it occurs more readily for the PbS/CdS than the PbS QDs, which is attributed to a lower density of trap states in the core/shell QDs, as supported by their stronger PL.